Chapter 3 Electrothermal vaporization sample introduction for inductively coupled plasma-mass spectrometry

Chapter 3

Electrothermal vaporization sample introduction for inductively coupled plasma-mass spectrometry D. C o n r a d Gr~goire

3.1 INTRODUCTION Four decades have passed since electrothermal vaporization (ETV) was first introduced [1] as a means of producing analyte vapour for analytical atomic spectrometry. During this time, graphite furnace atomic absorption spectrometry (GFAAS) has become firmly established and has reached maturity as a technique capable of determining about 50 elements at trace levels in a wide variety of sample types. One of the attractive features of GFAAS is the very small sample size requirement where only microlitre volumes can be successfully analysed. In 1974, Nixon et al. [2] coupled an ETV to an inductively coupled plasma-atomic emission spectrometer (ICP-AES). The promise of truly multi-element analysis using ETV did result in a significant number of publications, but the full potential of the technique was not realized and popularization of the technique has been slow. The widespread use of ETV-ICP-AES has been hindered by the difficulty conventional spectrometers have with accurately measuring transient signals, especially when background correction is required. As well, limits of detection for most elements were not competitive with those available by GFAAS. The advent of solid-state array detectors for ICP-AES spectrometers allows for the m e a s u r e m e n t (with background correction) of fast transient signals, and m a y result in renewed interest in this technique. Inductively coupled p l a s m a - m a s s spectrometry (ICP-MS) offered a second opportunity for ETV sample introduction to successfully couple 347

with argon plasmas. Although quadrupole based ICP-MS is not a simultaneous technique, it is a fast sequential technique capable of scanning spectra on the ms time scale, thus allowing for the accurate measurement of transient signals. The high sensitivity of ICP-MS, which rivalled GFAAS in detection power, also offered the capability of measuring isotope ratios of individual elements. This combination of attributes set the stage for the development of a new technique which could significantly extend the reach of the analytical chemist, especially for the trace analysis of micro-samples. Shortly after the introduction of ICP-MS, Gray and Date [3] pointed out that the origin of most of the "permanent" background peaks in ICP-MS originate from water and result in high hydrogen and oxygen populations in the plasma which are typically 108 times greater than that of analyte ions. These authors suggested that ETV could eliminate water from the sample stream and diminish these interferences. Detection limits of 1 to 12 pg (absolute) were quoted [3] and the measurement of Cd isotope ratios was demonstrated. In a later paper Gray [4], by w a y of background spectra, showed how ETV could be used to reduce or eliminate the interference of 56ArO÷ on 5~Fe+. The first comprehensive application of ETV-ICP-MS was published by Date and Cheung [5] who reported on the use of the technique for the determination of Pb isotope ratios. These authors showed that Pb isotope ratios could be determined with a precision of about 1% in NBS 981 (Pb isotope standard reference material) and a series of Pb mineral concentrates. Good agreement was obtained for these ratios when compared with values obtained by thermal ionization mass spectrometry. In a series of papers, Park et al. [6-11] reported on the use of a custom designed ETV device based on the use of a metal (W, Re) filament or a small fiat graphite platform (1.1 × 0.4 cm) as the vaporization surface. The relevance of vaporizer design and its influence on analyte mass transport efficiency [7] was used to construct an ETV, oriented vertically, and having a 5 ml volume above the vaporization surface. This design, along with a tangentially oriented argon carrier gas flow, was reported to maximize the transport of analyte vapour to the argon plasma resulting in transport efficiencies on the order of 80%. The analytical potential for this device was demonstrated [6,8] by the successful determination of As, Cu, Mn, Pb, Rb, V, Zn and Ag in orchard leaves (NBS SRM 1571) and oyster tissue (NBS SRM 1566) as 348

well as m e a s u r e m e n t of Pb isotope ratios in NBS SRM 981. The determination ofMo, W [10] and T1 [11] by isotope dilution in a number of geological reference materials was also reported. A comparison of the Park-design ETV unit [12] with the direct insertion device showed that ETV was more versatile and amenable to the analysis of samples high in salt content. ETV-ICP-MS was found useful for the determination of the platinum group elements in rocks [13] and in a series of 27 [14] geological reference materials including silicates, iron formation rocks and ores. Gr6goire [15] determined Os isotope ratios by ETV-ICP-MS and found that the addition of Te as a chemical modifier increased the sensitivity of the determination by a factor of 18. Osmium isotope ratios were measured with a precision of 0.5% compared to a precision of 0.2% for solution nebulization sample introduction. Gold was determined in seawater by Falkner and Edmond [16] who reported an ETV-ICP-MS limit of detection of i fM in one litre of sample. Imakita et al. [17] determined trace amounts of Bi in iron and steel using ETV-ICP-MS. Femtogram limits of detection were reported [18] for the determination of Pu and U in urine using ETV-ICP-MS. Two papers have appeared on the analysis of semiconductor materials by ETV-ICP-MS. Trace amounts of U and Th in silicon and silicon dioxide [19] and trace impurities of As, Fe, Sb, Mg, Mo, K, Na and V [20] were determined in semiconductor grade HC1. N e w m a n et al. [21] determined Te in biological fluids with an absolute limit of detection of 57 pg (5.7 ng ml 1) Te in blood. Whittaker et al. [22] determined Fe isotope ratios in unprocessed h u m a n blood as part of a metabolic study. The analysis of petroleum products (C5-C~2 alkanes) for Hg was reported by Osborne [23]. A limit of detection of 3 ng g-1 for Hg was obtained. In a novel application of the technique, Richner and Wunderli [24] used ETV-ICP-MS to differentiate between organic and inorganic chlorine for the determination ofpolychlorinated biphenyls in waste oils. Organic chlorine was vaporized at a temperature of 400°C whilst the inorganic chlorine containing compounds were volatilized at 2650°C. Detection limits between 0.5 and 10 ~g-1 PCB in oils were reported. A series of papers on the use of a tungsten electrothermal vaporization device was published by Tsnkahara and Kubota [25] and by Shibata et al. [26-28]. Extensive studies were carried out on the optimization of instrumental parameters [25] and the use of hydrogen mixed with the argon carrier gas [27]. Hydrogen [27] not only prevented the oxidation of 349

the W filament, but also produced a rise in the excitation temperature and an increase in the electron number density at the interface, resulting in enhanced ionization of the analyte. This system [26] was applied to the determination of the rare-earth elements (REE) with absolute limits of detection ranging from 2 fg (10 -~ g) for La to 12 fg for Gd. Shibata et al. [28] published an extensive study of REE oxide formation in ETV-ICP-MS. Their findings were applied to the analysis of high-purity REE oxide samples for traces of other REEs. Under optimized conditions, Tb and Lu impurities were determined at concentration levels of 0.01 ~g-~ in high-purity Gd203. Shen et al. [29] and Carey et al. [30] used a modified HGA-300 ETV system for the determination o f P b (d.l.= 10 fg), o f F e (d.l.= 0.2 pg) and As (d.l.= 1.5 pg). The system incorporated a specialized coolant argon flow designed to isolate the analyte vapour from the vaporizer walls, thus promoting efficient transport to the argon plasma. Gr~goire et al. [31] compared and contrasted GFAAS and ETV-ICPMS. These authors pointed out that these two techniques are complementary in nature and that data obtained from each can be combined, by repeating the same experiment on both systems, to provide fundamental information on vaporization and atomization processes and mechanisms of matrix modification. This work was followed up by two papers by Byrne et al. [32,33] in which the mechanism of chloride interferences in GFAAS was elucidated by combining information obtained from GFAAS and ETV-ICP-MS. Also studied was the mechanism of chemical modification of ascorbic acid on the elimination of the interference of magnesium chloride on Mn in GFAAS [33] as well as the pyrolysis of magnesium chloride itself [34]. The virtues of ETV-ICP-MS have been reported in a number of papers by several authors [31,35-42] and the advantages of the technique will be discussed in detail later in the chapter. A number of reports have appeared on fundamental aspects ofETV-ICP-MS including the role of chemical modifiers on analyte sensitivity [43-45], transport loss [46-50], particle size distribution of analyte aerosol produced in an ETV [51], the background spectral features of ETV mass spectra [52] and the formation of oxides in ETV-ICP-MS [28]. Studies have also been published on the mechanism of vaporization of B [53], W [54], Ra and other alkaline earth elements [55], Cr, [56], Y [57], the rare earth elements (REE) [57,58], U [59], A1 [60], the platinum group elements [61], P [62] and S [63] in ETV devices as well as the effect of acids on analyte signals [64]. 350

Applications of the technique have appeared on the determination of isotope ratios in rocks and minerals [65] and biological tissue [66], the determination of trace elements in Arctic snow [67], the determination of REE in single zircons [68], the direct analysis of solids and solids introduced as slurries [69]. A more complete listing of published applications is given at the end of this chapter. F u r t h e r information on ETV-ICP-MS can be obtained from a book chapter written by Williams [70]. An excellent literature review covering publications to 1992 on both ETV-ICP-AES and ETV-ICP-MS was published by Carey and Caruso [71] and is recommended as companion reading.

3.2 INSTRUMENTATION Research on the development of an 'ideal' electrothermal vaporization device for use in plasma spectrochemistry is unfolding in much the same manner as did research on the development of the best atomization device for GFAAS during the early 70s. A number of configurations have been investigated including filament or platform [9,13,22] based systems as well as tube-type [20,52,67,72,73] devices and ETV systems based on the insertion of a metal wire loop into a tube-type [29] ETV and an electrically heated wire loop [74] placed directly into the argon plasma. Vaporization substrates have included Re [8,73], W [25,73], Ta [15,73], Mo [73] and a variety of graphites ranging from pyrolytically coated crystalline graphite [14,32] to glassy carbon [15]. Low voltage high current power supplies are generally used to resistively heat the metal or graphite vaporizer. Those using metal vaporization surfaces for ETV work report that these are less likely (than graphite) to react with analyte to form refractory compounds. However, there are reports [75] on the formation of involatile intermetallic compounds between V [76], Mo [77] and Ni [76] with hot Ta metal. Metal filaments or strips can lose their ductility (W) once heated to incandescence and are prone to contamination. A major shortcoming of metal vaporization surfaces is the rate of evaporation of metal at high temperatures. Microgram quantities of metal entering the plasma can result in loading effects and serious matrix effects, both spectroscopic and non-spectroscopic [26,73]. Gr6goire [52] has shown, however, that carbon volatilized at 2600°C 351

from a graphite tube did not result in the suppression of analyte signals. Carbide formation can be a problem when using graphite, but the judicious selection of vaporization temperature along with the use of chemical modifiers can minimize the effects of carbide formation for most elements [53,54]. Today, most commercially available systems use graphite as the vaporization substrate and approximately 75% of the ETV-ICP-MS literature is based on the use of a tube-type system. No publication has appeared reporting on a comprehensive comparison of the analytical performance of filament or platform systems with tube-type ETV devices. An 'ideal' ETV system for ICP-MS would incorporate design features that promote complete vaporization of analyte followed by the efficient (100%) transport of analyte vapour from the ETV to the argon plasma. Other desirable features include versatile and accurate temperature control, an inert vaporization surface capable of being rapidly heated to high temperatures (3000°C), ease of automation and low cost. Early work in ETV-ICP-MS [3,5,36] was carried out on a device similar to the system developed by Gunn et al. [78] which used a carbon rod containing a 5 pl sample volume cavity. The graphite rod was enclosed in a glass envelope of 1 1 volume. The first ETV device specifically designed for use as a sample introduction system for ICPMS was the filament type device built by Park et al. [9], shown in Fig. 3.1. The main performance criterion behind this design was the isolation of vaporized sample from surfaces (where condensation could take place) to ensure good analyte transport efficiency. The introduction of carrier argon gas (1 1 min -1) at a tangent to the filament or platform support base resulted in a sheathing effect which isolated analyte vapour from the surface of the glass cover long enough to allow for condensation of analyte into solid particles. These particles could then be transported to the plasma with a reported efficiency of up to 80%. The effective volume above the vaporization surface was only 5 ml. This reduced analyte dilution by argon gas and also served to promote collisions of analyte vapour/aerosol particles during condensation resulting in the formation of easily transported particulates. A novel spray chamber assembly [79] was designed for use with this system to accommodate fast conversion from ETV to solution nebulization (SN) sample introduction without plasma shut-down. The filament shown on the ETV in Fig. 3.1 can easily be substituted for a pyrolytic graphite coated platform. Later designs of the platform 352

TolCP

Metalfilarnent~

G.op.,te

Quartz cover

inlet ~

Coppertubing Fig. 3.1. Filament or platform type electrothermal vaporizer (from Ref. [7] with permission). incorporated a small depression in the centre [13] to prevent spreading of the analyte solution during the drying step. When highly acidic solutions or organic solvents were used as sample solutions, the sample tended to spread towards (and at times into) the contact electrodes. This situation became more serious as the surface of the platform aged and became more porous. Control of the problem was achieved by using small volumes (5 ~1) and relatively low drying temperatures (80°C). Two manufacturers have developed ETV systems based on the Park design. T s u k a h a r a and Kubota [25] have characterized the Seiko I n s t r u m e n t s Micro Sampling (ETV) System which uses a W ribbon in which a small cavity has been impressed to contain up to 50 ~1 of sample solution. The glass chamber covering the W ribbon has a volume of 300 ml. The second platform-type ETV device [80] is the EV2000 System and is marketed by Hewlett-Packard Co. This device uses a graphite platform contained in a glass chamber 90 ml in volume. Gas flows are carefully controlled to provide laminar flow conditions preventing analyte loss on the chamber walls. 353

Pre-vaporization Graphite SealingProbe

Internal

Sampling ~ ==

Gas ~

Valve ~

~ Internal ExternaIGas Gas /

•

\

I

OplicalsenMaXP°Wersor

• ..

Gas

ICP-MS

ETV

Vaporization Internal

ICP-MS

Internal

ETV

Fig. 3.2. Perkin-Elmer HGA-600MSelectrothermalvaporization system. Two tube-type ETV systems for ICP-MS are currently commercially available: the Perkin-Elmer HGA-600MS (Fig. 3.2) and the VG Elemental MicroTherm system. Both instruments use graphite tubes of approximately the same dimensions (28 x 8 mm o.d., 6 mm i.d.) which can be obtained pre-coated with pyrolytic graphite. These tubes are identical to those which are standard for GFAAS applications. The MicroTherm System uses an Ar sheath gas which flows between the graphite tube and a quartz envelope surrounding the tube. 354

The sheath gas protects the graphite from air-oxidation at high temperatures. Using either manual or automated pipetting, a sample of up to 100 ~1 in volume can be deposited into the graphite tube through a hole cut in the quartz envelope which is aligned with the dosing hole. During the temperature pre-treatment steps (dry, pyrolysis), water and other waste vapours are removed from the dosing hole of the graphite tube by an argon carrier flow of about 1 1 rain -1 During the high-temperature or vaporization step a graphite rod is placed into the dosing hole of the graphite tube essentially sealing the tube and thus allowing analyte-containing vapour to be swept into the argon plasma. The Perkin-Elmer Sciex HGA-600MS (Fig. 3.2) uses a modified HGA system originally developed for GFAAS. Argon sheath gases continuously flow over the exposed graphite parts to prevent oxidation. During the dry and pyrolysis steps, opposing argon flows originating from both ends of the graphite tube remove water and other vapours through the dosing hole of the graphite tube. An argon flow of about 300 ml m i n 1 is generally used during the dry and pyrolysis steps. During the high-temperature or vaporization step, a graphite probe is pneumatically activated to seal the dosing hole. Once the graphite tube is sealed, a valve located at one end (downstream) of the HGA workhead directs the carrier argon flow (800-1100 ml min-1), which originates from the far end of the graphite tube, directly to the argon plasma. The HGA-600MS is also equipped with an autosampler which can deliver sample volumes of from 2 lul to 100 ~1, which is the practical maximum capacity of the graphite tube. Two laboratory-built tube-type ETV systems that incorporate features not offered by commercial suppliers are those reported by Shen et al. [29], Carey et al. [30] and Lamoureux [81] and Lamoureux et al. [82,83]. The device built in Caruso's laboratory [29,30] was a standard HGA furnace incorporating specialized gas flows to help isolate analyte vapour from any surfaces on leaving the graphite tube. A tungsten loop [29] or graphite platform [30] set within the graphite tube were used to contain sample. The second system built in Gr6goire's laboratory [81-83] also used a modified HGA system. Unlike other tube-type configurations, all vapours produced are carried out through the dosing hole rather that out of one of the ends of the graphite furnace. This was done by installing a quartz ring to surround the graphite tube. The quartz ring was fitted with a ball-joint which was connected via a length of PTFE tubing directly to the argon 355

plasma. The opening in the quartz ball joint was aligned directly above the dosing hole of the graphite tube. Argon flows could be carefully controlled and laminar flow conditions were obtained for analyte vapour leaving the graphite tube. This ETV device features both the vapour isolating conditions characteristic of filament-type systems and the advantages of a graphite tube ETV. The interface between each of these ETV devices and the argon plasma was generally a i m (or less) length of plastic or PTFE tubing. The diameter of the tubing can be an important parameter, but in general, a diameter equal to the inner diameter of the graphite tube is used to minimize turbulent flow. Comparison of the performance of the ETV systems discussed in this section is difficult. Because the ETV is only a sample introduction device, reported limits of detection contain information on both the performance of the ETV and the ICP-MS. A valid comparison of different ETV designs and types can only be done if a single ICP-MS instrument is interfaced with different ETVs and operated under identical experimental conditions. Table 3.1 contains reported limits of detection for eight elements determined using a variety of ETV and ICP-MS systems. Although the limits of detection for individual elements can vary by two orders of magnitude, virtually every system TABLE

3.1

Comparison

of absolute limits of detection in picograms As

Co

Cu

Fe

Mn

Ni

Pb

Zn

Tube-type ETV systems Ref. 66-68

-

0.14

0.42

-

0.120

0.47

0.023

0.27

Ref. 39

-

0.030

0.020

-

0.020

0.070

0.001

-

Ref. 83

-

0.075

0.169

-

0.072

-

-

0.617 -

Ref. 29

1.5

-

-

0.20

-

-

0.010

Ref. 84*

-

0.025

0.50

1.0

0.10

0.50

-

0.50

Ref. 37

0.005

.

0.002

-

.

.

.

.

F i l a m e n t or platform type ETV systems Ref. 8

0.1

.

0.93

0.3

0.4

Ref. 25,26

-

0.02

-

0.3

-

-

0.008

-

Ref. 80

-

0.14

0.42

-

0.120

0.47

0.023

0.27

*Calculated

356

.

.

using 50 pl sample volume.

.

listed r e p o r t s t h e l o w e s t l i m i t of d e t e c t i o n for a t l e a s t one e l e m e n t . T a k i n g into c o n s i d e r a t i o n t h a t m o r e t h a n one g e n e r a t i o n of I C P - M S i n s t r u m e n t a t i o n w a s u s e d to p r o d u c e d t h e s e d a t a a n d w i d e l y different e x p e r i m e n t a l a n d d a t a collection p a r a m e t e r s w e r e also used, t h e overall i m p r e s s i o n is t h a t all s y s t e m s can, u n d e r c e r t a i n conditions, p e r f o r m e q u a l l y well. T h e r e a l t e s t for e a c h of t h e s y s t e m s d e v e l o p e d t h u s f a r will be in t h e i r u s e in a c t u a l a p p l i c a t i o n s for t h e a n a l y s i s of complex matrices.

3.3 ANALYTICAL DOMAIN OF ETV-ICP-MS M o s t a n a l y t i c a l t e c h n i q u e s h a v e a s s o c i a t e d w i t h t h e m s o m e applications w h i c h no o t h e r t e c h n i q u e c a n h a n d l e as well or as efficiently. A l e g i t i m a t e q u e s t i o n is w h e t h e r or not E T V - I C P - M S h a s a u n i q u e d o m a i n in a n a l y t i c a l c h e m i s t r y . E l e c t r o t h e r m a l v a p o r i z a t i o n I C P - M S is a m i c r o a n a l y t i c a l t e c h n i q u e c a p a b l e of a n a l y s i n g pl v o l u m e s of s a m p l e solution. T a b l e 3.2 lists s o m e of t h e a p p l i c a t i o n s w h i c h a r e well s u i t e d to E T V - I C P - M S . G r a p h i t e f u r n a c e AAS, h o w e v e r , is also a m i c r o a n a l y t i c a l t e c h n i q u e a n d is, in fact, v e r y s e n s i t i v e w i t h l i m i t s of detection for m a n y e l e m e n t s t h a t c o m p a r e well w i t h solution nebuliTABLE 3.2 Applications well suited to ETV-ICP-MS 1. Analysis of small samples - - blood, body fluids - - rare materials - - pre-concentrates 2. Analysis of radioactive and toxic samples 3. Analysis of difficult matrices - - organic samples - - high-salt samples - - solids and slurries 4. Isotope ratio determinations - - B r , S , Se - - tracer studies - - isotope dilution calibration

357

zation ICP-MS [85]. Although both ETV-ICP-MS and GFAAS can handle applications listed in the first three categories of Table 3.2, only ETV-ICP-MS is practical for the measurement of isotope ratios. This inherent capability of mass spectrometric techniques allows for the measurement of isotope ratios important to geochemical studies [65], h u m a n and animal metabolic studies [66] and the application of isotope dilution calibration [11,13,14] for quantitative analysis. Table 3.3 compares absolute limits of detection obtained by both ETV-ICP-MS and GFAAS. With the exception of Cr and Zn, limits of detection for all elements determined by ETV-ICP-MS are better t h a n those reported by GFAAS. Except for the transition metals for which ETV-ICP-MS is about 0.3 (Cr) to 150 times more sensitive, ETV-ICPMS surpasses GFAAS in detection power by four or more orders of magnitude. Another noteworthy feature is t h a t for most elements, ETV-ICP-MS limits of detection are in the low femtogram (10 -15 g) range and do not show the very large variations in detection limits across the periodic table as does GFAAS. The ETV-ICP-MS data contained in Table 3.3 was obtained using a Perkin-Elmer ELAN 5000. Modern ICP-MS instruments are from 10 to 100 times more sensitive t h a n the ELAN 5000 giving even better limits of detection for ETVICP-MS t h a n those reported here. The comparison of limits of detection can be taken a step further if we examine Table 3.4 which lists sensitivities for certain elements determined by GFAAS. Sensitivities are expressed as the number of pg required to give 1% absorbance. For elements B through Y, these sensitivities range from 240 to over 26,000. Elements from S to Zr in practice cannot be determined at all by GFAAS. Only under extraordinary experimental conditions [86] have some of these elements been determined by GFAAS. Most elements successfully determined by GFAAS have sensitivities ranging from 0.3 to 30 pg. All of the elements listed in Table 3.4 can be determined with fg to pg sensitivities using ETV-ICP-MS. Clearly, ETV-ICP-MS not only has superior sensitivity when compared to GFAAS, this technique can be used for the determination of analytes, including the rare earth elements, refractory platinum group elements, non-metals and elements such as B, Th, U, W and Zr which are difficult or impossible to handle by GFAAS. The added capability of ICP-MS for multi-element analysis and the determination of isotope ratios of individual elements promises to make ETV-ICP-MS one of the most sensitive and versatile analytical techniques available. 358

TABLE 3.3 Comparison of limits of detection Element

ETV-ICP-MS1

GFAAS2

Cd

0.230 pg

0.3 pg

Ce

0.023 pg

>50 ng

Co

0.140 pg

2.0 pg

Cu

0.420 pg

1.0 pg

Cr

3.200 pg

1.0 pg

Cs

0.057 pg

5.0 pg

La

0.018 pg

=20 ng

Mn

0.120 pg

1.0 pg

Nd

0.013 pg

=13 ng

Ni

0.470 pg

10.0 pg

Pb

0.023 pg

5.0 pg

Sc

0.044 pg

=1.2 ng

Sm

0.036 pg

=3.3 ng

Th

0.038 pg

-

T1

0.029 pg

10.0 pg

U

0.016 pg

-

V

0.360 pg

20.0 pg

Y

0.030 pg

=4.8 ng

Yb

0.004 pg

=38 pg

Zn

0.27 pg

0.1 pg

1This laboratory. 2Ref. [86] W h e n c o m p a r e d to s o l u t i o n n e b u l i z a t i o n I C P - M S , e l e c t r o t h e r m a l vaporization sample introduction has many advantages. The removal of w a t e r a n d o t h e r s o l v e n t allows for t h e m e a s u r e m e n t at low levels of s e v e r a l i m p o r t a n t e l e m e n t s i n c l u d i n g F e a n d S. T h e a d v e n t o f m i c r o concentric nebulizers has, in some cases, made this device more 359

TABLE 3.4 Elements not easily determined by GFAAS (Ref. [86]) Element B

Gd Ir La Nd Os P Sm U* Y S Ce Pr Tb Ho Lu Th W Zr

Sensitivity(pg ~r 0.0044 A-s) 700 11000 250 26000 1800 1400 4200 240 12000 13000

Note: For most other elements, sensitivities range from 0.3 to 30 pg for 0.0044 A.s. *Ref. [86]. practical for the analysis of small v o l u m e solutions. However, microconcentric n e b u l i z e r s are p r o n e to clogging and all h a v e a s a m p l e solution dead v o l u m e g r e a t e r t h a n 0. As t h e r e m a i n i n g sections of this c h a p t e r will show, ETV-ICP-MS h a s u n i q u e capabilities for the analysis of difficult samples (Table 3.2) including slurries a n d solids. By u s i n g selective v a p o r i z a t i o n of analytes, u n r e s o l v a b l e (by q u a d r u p o l e m a s s s p e c t r o m e t r y ) spectral i n t e r f e r e n c e s can be avoided. E T V - I C P - M S is a fully q u a n t i t a t i v e t e c h n i q u e which s t a n d s r e a d y to t a k e a d v a n t a g e of a n y d e v e l o p m e n t s in t h e field of m a s s s p e c t r o m e t r y , including i n s t r u m e n t s h a v i n g inc r e a s e d sensitivity, resolution, the ability to r e m o v e (by gas p h a s e collisions and/or reactions) spectral i n t e r f e r e n c e s a n d to m e a s u r e t h e e n t i r e mass s p e c t r u m simultaneously. 360

3.4 PRINCIPLE OF OPERATION OF ETV-ICP-MS S a m p l e i n t r o d u c t i o n by E T V t e c h n i q u e s is b a s e d on t h e conversion of solid s a m p l e (dried solution, slurry, etc.) to v a p o u r u s i n g heat, followed by t r a n s p o r t of t h e s a m p l e u s i n g a n argon s t r e a m , to the argon plasma. A l t h o u g h this process m a y a p p e a r to be trivial, complex chemical reactions a n d physical effects can t a k e place m a k i n g application of this t e c h n i q u e difficult for q u a n t i t a t i v e chemical analysis. A m o r e detailed e x a m i n a t i o n of some of t h e s e factors will serve to i l l u s t r a t e the point. T h e h e a t i n g cycle d u r i n g which liquid s a m p l e is c o n v e r t e d to analyte v a p o u r comprises several s e p a r a t e steps. E a c h of t h e steps is designed to p e r f o r m a specific function, b u t at t h e s a m e time, o t h e r u n d e s i r a b l e effects m a y occur. Table 3.5 lists h e a t i n g steps in a n E T V TABLE 3.5 Processes occurring during ETV heating steps A.

Drying step

Evaporation of water Evaporation of acid Evaporation of organic solvent

B.

Pyrolysis step

Loss of water of hydration Loss of residual acids Loss of hydrolysis products Volatile matrix removal Pre-vaporization analyte loss Chemical modification Analyte-matrix interaction Analyte-surface interaction

C.

Vaporization step

Loss of matrix components Decomposition of analyte salt Carbon reduction of analyte salt Volatilization of analyte Analyte-matrix interaction Analyte-surface interaction Gas-phase reactions Vapour-phase chemical modification

D.

Cleaning step

Removal of residual matrix components Removal of residual analyte

361

program and some of the processes that are associated with each of these. The lowest temperature step is the drying step which is used to drive off solvent including acids and volatile organics. Loss of sample solution and hence analyte is possible during the drying step if the rate of heating or the maximum temperature is too high. This generally results in explosive boiling of the sample causing a physical loss or redistribution of sample material within the graphite tube. Low sensitivity, poor reproducibility and broad or multiple analyte peaks indicate too vigorous a drying step. The pyrolysis step can vary in temperature from 100°C to 1800°C depending upon the specific element being determined and the composition of the matrix. The pyrolysis step is used to complete solvent removal, to volatilize some of the matrix components especially carbon and volatile salts and to convert analyte to a form that can be easily volatilized during the high temperature vaporization step. The maxim u m pyrolysis temperature that can be used for an element is limited by the volatility of the element and/or the volatility of the compound containing analyte. Table 3.6 contains the maximum pyrolysis ternperature for a number of elements, some of which have been thermally stabilized by the addition of a chemical modifier. The maximum pyrolysis temperatures given were obtained from GFAAS studies, and it is likely that many of these will also apply to ETV-ICP-MS work. Measurement of pyrolysis temperatures is done by determining the temperature beyond which a measurable loss in analytical signal is obtained during the subsequent high temperature atomization step. Since losses occurring during a GFAAS heating step will also occur during an ETV heating step (these are essentially identical except for internal argon gas flow), it follows that the temperatures given in Table 3.6 can be used at least as an upper limit for ETV-ICP-MS studies. The wide range of maximum pyrolysis temperature imposes a limitation on ETV-ICP-MS when used for multi-element analysis. It may not be possible to determine both volatile and involatile elements during a single ETV run under optimum pyrolysis conditions for a given matrix type. Several other processes can occur during the pyrolysis step. One that has been explored in detail is the co-volatilization of Mn in the presence of hydrolysable salts such as magnesium chloride. Magnesium chloride is common in saline samples such as seawater and brines and would be present as a major component. Using ETV-ICP-MS 362

TABLE 3.6 Maximum

pyrolysis temperatures

for elements determined Element

by GFAAS

Element

Modifier

Pyrolysis temperature (°C)

Modifier

Pyrolysis temperature (°C)

Ag

Pd+Mg(NO3) 2

1000

Mn

Mg(NO3) 2

1400

AI

Mg(NQ) 2

1700

Mo

-

1800

As

Pd+Mg(NO3) 2

1300

Na

-

900

Au

Pd+Mg(NO3) 2

1000

Nd

-

1500

B

Ca

1000

Ni

-

1400

Ba

-

1200

Os

-

200

Be

M g ( N O 3 ) 2)

1500

P

Pd+Ca(NO3) 2

Bi

Pd+Mg(NO3) 2

1200

Pb

PO 4

700

Ca

-

1100

Pd

-

900

Cd

Pd+Mg(NO3) 2

850

Pt

-

1300

Co

Mg(NQ) 2

1400

Rb

-

800

Cr

Mg(NO3) 2

1650

Rh

-

1300

Cs

H2SO 4

900

Ru

-

1400

1350

Cu

-

1000

Sb

Pd+Mg(NQ)

2

1100

Dy

-

1500

Se

Pd+Mg(NO3) 2

1100

Er

-

1700

Si

-

1400

Eu

-

1300

Sm

-

1400

Fe

Mg(NO3) 2

1400

Sn

Pd+Mg(NO3) 2

1400

Ga

Pd+Mg(NQ)2

1200

Sr

-

1300

Gd

-

1600

Te

Pd+Mg(NQ)

Ge

Pd+Mg(NO3) 2

1500

Ti

-

Hg

Pd+Mg(NO3) 2

250

T1

Pd+Mg(NO3) 2

1000

In

Pd+Mg(NO3) 2

1200

Tm

-

1700

Ir

-

1300

U

-

1000

2

1200 1400

K

-

950

V

Mg(NOs) 2

1100

La

-

1600

Y

-

1400

Li

-

900

Yb

-

1300

Mg

-

900

Zn

Mg(NOs) 2

700

From Refs. [86,87].

363

techniques, Byrne et al. [32] showed that during the pyrolysis step at temperatures above 700°C, Mn is co-volatilized with HC1 which is produced as a reaction product from the hydrolysis of magnesium chloride. A similar loss of Mn would not occur for aqueous reference standards volatilized alone illustrating the importance of proper method development in ETV-ICP-MS. These authors also reported that the addition of ascorbic acid can control the hydrolysis reaction and hence the pre-vaporization loss of analyte. The high-temperature vaporization step can range from 800°C to a maximum of 2700°C. The maximum temperature is limited by the thermal stability of the vaporization surface. For graphite, temperatures above 2800°C result in the vaporization of large quantities of C which can overload the plasma and cause premature wear of the graphite tube. The optimum pyrolysis and vaporization temperatures for many elements determined by ETV-ICP-MS have been reported in individual papers studying the mechanism of vaporization of elements. Generally, the optimum vaporization temperature is the lowest ternperature that provides complete vaporization of analyte or analytecontaining compound, while at the same time minimizing unwanted reactions such as carbide formation. When a complex sample matrix is present in relatively large quantities, co-volatilization occurs and the vaporization characteristics of the analyte can take on the vaporization properties of the matrix. Well defined atomization temperatures are known for GFAAS, but these are not generally transferable to ETV-ICPMS because for GFAAS, the atomization of analyte is a primary requirement, whereas for ETV-ICP-MS only vaporization of analyte or analytecontaining molecules is necessary. Literature reports to date indicate that temperatures significantly lower than those used in GFAAS are optimum for ETV-ICP-MS work. The reasons for this will be more fully discussed in the section on vaporization mechanisms in this chapter. During the vaporization step, analyte-containing compounds are heated to reactive temperatures resulting in the thermal decomposition of these compounds, carbon reduction and volatilization. One of the undesirable chemical reactions that can also occur is the interaction of analyte with the metal filament or graphite tube to form refractory alloys and/or compounds. For graphite-based ETV systems, the formation of carbides is perhaps most important particularly for elements such as B, U, Th and the REEs. Most of the elements listed in Table 3.4 as being not easily determinable by GFAAS are in this category because of their propensity to form involatile carbides. 364

TABLE 3.7 Typical ETV-ICP-MSheating program Heating step Temperature Ramp time Hold time Internal gas flow Valve (°C) (s) (s) (ml min 1) setting Drying Pyrolysis Vaporization Cooling Cleaning Cooling

100 300 2500 20 2700 20

10 10 0 1 -

30 20 6 10 5 20

300 300 1000 1000 1000 1000

vent vent plasma plasma plasma plasma

Chemical modifiers and physical carriers are also used during the vaporization step. Improvement in sensitivity by orders of magnitude is possible, particularly for volatile elements. The final ETV heating step is the cleaning step which is normally a maximum temperature (2700°C) step. Matrices high in organic compounds such as blood, or those containing refractory compounds such as dissolved zircons require high temperatures to get rid of residual quantities of these materials from the graphite tube. Solid material left behind in the graphite tube can affect the drying characteristics of subsequent samples injected into the graphite tube or m a y provide active sites for undesirable chemical reactions such as those discussed above. The cleaning step is not always required and should only be used when necessary because this step is tantamount to an additional tube firing and contributes to shortening tube lifetime. Table 3.7 gives a typical heating program for the determination of trace metals by ETV-ICP-MS using the Perkin-Elmer HGA-600MS system. The entire cycle takes about 2 rain to complete. Including the time required to load sample into the ETV, approximately one determination can be completed every 3 min or about 20 h -1. A cooling step is required after the vaporization step to prevent overheating of the graphite tube during the cleaning step. A second cooling step following the cleaning step is needed to allow graphite parts to cool while still being protected by the argon sheath gas. This is done because the graphite probe is heated to incandescence following the cleanup step. Withdrawing the probe before cooling allows rapid air oxidation of the probe and eventual failure of the seal between the probe and the dosing hole. The sealing probe is left in place sealing the dosing hole during 365

200000

8

o

150000 100000Il~l/I][ ~ 50000 It 0 1

I

I

r

205TI 33118pb

2 3 4 Timseconds e,

Fig. 3.3. ETV-ICP-MSsignals for 20 pg T1, 30 pg Cs and 50 pg Pb. the cleaning step to remove any material that may have condensed onto the tip of the probe during previous heating steps. Internal gas flows should be relatively low during the drying and pyrolysis steps to prevent expulsion of solution or analyte-containing salt crystals and to avoid the excessive build-up of back-pressure within the graphite tube which could blow out the argon plasma or significantly change the effective sampling depth at the interface. The internal gas flow can also be a mixture of gases incorporating air during the pyrolysis step to promote oxidation of organic matrices or added freon during the vaporization step which acts as a chemical modifier. Typical ETV-ICP-MS signals for several elements are shown in Fig. 3.3. Signals were obtained using conditions given in Table 3.7 and are about I to 2 s in width. The relative temporal variation in the appearance of analyte signals is a reflection of the different volatility characteristics of each element being determined. Because of the transient nature of ETV-ICP-MS signals, it is important that the analyte signal intensity be monitored frequently during the high temperature vaporization step. Too few readings results in an inaccurate measurement of the shape of the analyte signal profile leading to error whether peak-height or area counts are recorded. Most practitioners restrict the number of isotopes monitored to 366

about 5 and use a dwell time of from 10 ms to 50 ms depending upon the number of isotopes monitored and the intensity of the analyte signal. Spectra are generally collected in peak-hopping mode with one m e a s u r e m e n t point per m / z .

3.5 BACKGROUND SPECTRAL FEATURES When solution nebulization sample introduction is used for ICP-MS measurements, large quantities of water and water vapour are generally introduced into the argon plasma. The presence of liquid aerosol can have a profound effect on the excitation characteristics of the plasma itself [88] in addition to contributing to the formation of polyatomic ions which may interfere with the determination of certain elements [89]. For example, 4°Ar160+ interferes with the determination of the major isotope of iron, 5~Fe+, which has an abundance of 91.52% [3,22]. In the presence of C1, 35Cl1~O÷interferes with the determination of ~IV which is nearly monoisotopic with an abundance of 99.76%. Studies on the background spectral features of ETV-ICP-MS and polyatomic ions produced from the vaporization of common matrix components have been published [52,90]. Figure 3.4 shows the background spectra obtained using both SN and ETV sample introduction techniques. The virtual elimination of water from the sample results in an ETV background spectrum which is qualitatively simpler than the SN background spectrum. Marked reductions in the background signal at m / z = 19,32,33,34 and 56 allow for the determination of 19F (100% abundance), 32S (95.02%), 33S (0.75%). 3'S (4.22%) and 56Fe (91.52%) by ETV-ICP-MS. In a paper on the application of ICP-MS to the determination of isotope ratios important in geochemical studies, Gr5goire [65] has shown that the use of a dry plasma allows for the determination of isotope ratios of S, C1 and Se at trace levels and aids in the determination of Br isotopes. For example, using SN, the background spectrum for the three S isotopes free of interference from other elements gave count rates (Cs -1) which were overrange for 3~S+, 142,400 for 33S+ and 45040 for ~4S+. Using ETV, count rates were reduced to 60,300 for *2S+, 600 for ~3S+ and 4100 for 34S+. Interfering species eliminated or reduced using ETV-ICP-MS compared to SN include oxygen and hydroxide species as well as oxides and hydrides of Ar which combine to form a fairly large number of well documented polyatomic ions [89]. 367

[

250000

Solution Nebulization

200000 150000 100000 50000 0 10

20

30

40

50

60

70

8O

m/z

300000

Electrothermal Vaporization

200000

o

100000

i

50000 0 10

20

30

40

50

60

70

80

m/z

Fig. 3.4. Backgroundspectra for solutionnebulizationand electrothermalvaporization sample introduction. When using ETV, the major ionic species other t h a n argon found in the plasma are nitrogen and oxygen entrained from the air surrounding the torch compartment. When heating carbon at temperatures up to 2700°C, sublimation occurs and carbon becomes a fourth species found in abundance in the plasma. Table 3.8 lists the four major elemental sources of ions involved in formation of polyatomic ions in the argon plasma using electrothermal vaporization sample introduction. It is fortuitous t h a t three of these four elements are nearly monoisotopic whereas the minor carbon isotope is only 1.11% TABLE3.8 Isotopes originating from graphite electrothermal vaporizer, plasma and atmospheric gases Carbon

Nitrogen

Oxygen

Argon

leC (98.89)* 13C(1.11)

14N(99.6) 15N(0.4)

160 (99.76) 1SO(0.20) 1~O(0.04)

40Ar(99.6) ~6Ar(0.34) 3sAr (0.06)

*Percent natural abundance. 368

abundant. However, since the ion concentration of these elements in the plasma is high, the influence of the minor isotopes cannot be ignored. The net result of the introduction of vaporized carbon into the argon plasma is evident when comparing the two spectra on Fig. 3.4. The appearance of several new intense peaks on the ETV background spectra w a r r a n t further discussion. Table 3.9 lists the polyatomic ions formed as a result of the introduction of carbon into the plasma as well as other major species present as a result of Ar flowing through the graphite tube. The quantity of carbon vaporized from the surface is dependent on the type of graphite used as well as the physical condition of the tube surface and, at times, the composition of the sample. For a given graphite tube, the temperature of its surface is the most significant parameter influencing the absolute quantity of carbon reaching the plasma since the vapour pressure of carbon increases by 25 orders of magnitude between 1000 and 3000 K. Therefore, it is important to study the background spect r u m as a function of steady-state temperature. Table 3.10 lists the possible polyatomic ions produced when the graphite tube is heated to temperatures ranging from 20°C to 2500°C. Polyatomic ions not observed or remaining at background noise levels (such as C3÷) were omitted from this table. A number of polyatomic ions not associated with carbon but which are included for information and possible comparison with intensities obtained using solution nebulization sample introduction are given in parentheses. Minor isotopes of C and O were monitored to obtain values t h a t were on scale. Ion intensity levels for all species not containing carbon were relatively insensitive to vaporization temperature. It is interesting to note, however, t h a t levels of 02 ÷ did not decrease with the introduction of C, indicating t h a t any scavenging effect of C on oxygen either took place primarily with O ÷ (which decreased somewhat with increasing temperature) or t h a t diffusion of 02 from the surrounding atmosphere was rapid enough to replace any oxygen scavenged by carbon. The argon dimer (8°Ar2+) is a species produced only in the plasma (and/or interface region of the mass spectrometer) and therefore can be used as a diagnostic tool to monitor the plasma for loading and/or interference effects. Additionally, it has been used [91] as an internal standard reference isotope to correct for non-spectroscopic or matrix interferences. Although the intensity of the argon dimer does not quantitatively reflect the severity of matrix effects for all ions across the entire mass range, it was useful in this study (see below) as a 369

TABLE 3.9 Count rate for ions produced during heating of graphite vaporizer at steady-state temperatures Mass

Ion(s)

Temperature (°C) 20

13 [14]* [18] 24 26 28 29

18C+ 14N+ 1SO+ 12C12C+ 12C14N+ 14N14N+,12C160+ 14N14NIH+,12C1601H+ 13C160+

30 [31] [32] [33] 44

14N160+,13C16OIH+ 14N1601H+ 160160+ 1601601H+ 12C160160+

48

12C36Ar+

52 53 [54] [80]

12C40Ar+ 13C40Ar+ 4OAF14N+ 4°Ar4°Ar+

7,730 1,140,000 1,390,000 75 50 14,000 1,450 10,000 75 44,500 100 2,050 -

200 1,500 963,200

800 26,600 1,128,900 1,430,700 100 100 38,100 10,500 21,900 100 40,200 200 4,700 -

500 1,500 930,900

1500

2500

177,700 432,200 1,075,300 1,045,600 1,151,000 1,202,800 200 300 300 1,900 101,700 186,000 7,500 12,800 17,800 200 42,600 100 5,800 190

6,400 50 1,500 963,800

17,200 500 60,300 600 7,050 790

26,200 300 1,500 945,300

*Masses in parentheses arise from the presence of plasma and/or atmospheric gases and are included for comparison purposes. g e n e r a l i n d i c a t o r of p l a s m a loading, p a r t i c u l a r l y for t h e l i g h t e r a n a l y t e s studied. T a b l e 3.9 s h o w s t h a t t h e ion i n t e n s i t y for t h e a r g o n d i m e r did not c h a n g e t h r o u g h o u t t h e r a n g e of v a p o r i z a t i o n t e m p e r a t u r e s s t u d i e d i n d i c a t i n g no serious p l a s m a l o a d i n g effects f r o m carbon. I t is t h e r e f o r e u n l i k e l y t h a t c a r b o n c a u s e d t h e s u p p r e s s i o n of a n a l y t e ion signals e v e n a t t h e h i g h e s t v a p o r i z a t i o n t e m p e r a t u r e of 2500°C. M o s t c a r b o n c o n t a i n i n g p o l y a t o m i c ion i n t e n s i t i e s i n c r e a s e r a p i d l y w i t h t e m p e r a t u r e a l t h o u g h t h e g r e a t e s t i n c r e a s e occurs for p o l y a t o m i c ions c o n t a i n i n g b o t h O a n d C. T h e c a r b o n d i m e r i n c r e a s e d only slightly w i t h t e m p e r a t u r e in accord w i t h t h e fact t h a t t h e v o l u m e c o m p o s i t i o n of t h e v a p o u r in e q u i l i b r i u m over solid c a r b o n is only a b o u t 0.5% C2 at 370

3000 K, and the likelihood that this species is very reactive toward entrained oxygen to form CO. This supposition was supported by an experiment in which particulate activated carbon was injected into the carrier argon flow to the argon plasma. The addition of large quantities of carbon to the gas stream resulted in the production of relatively small signal intensities for C ÷ compared to massive signals for CO + despite the lower ionization energy of C. Higher mass carbon ion clusters involving three or more carbon atoms were not observed. The growth of carbon ion count rate with temperature closely followed the t e m p e r a t u r e - t i m e curve of the graphite tube as measured by optical pyrometry. Thus, the repeatability of the heating program and/or the condition of the graphite tube could be assessed by monitoring the carbon ion intensity during the vaporization step. It is interesting to note that the growth of the C ion count rate on increasing the temperature of the graphite tube to 2500°C was orders of magnitude less than predicted by the vaporization characteristics of C. This implies that only a fraction of the C volatilized in the graphite tube is converted to ions and detected by the mass spectrometer. Several mechanisms [92] can account for this observation including the formation and deposition of soot within the graphite tube or at the contact cones, the deposition of C on the transfer line between the ETV and the plasma and the traverse of unionized carbon and carbon particles through the argon plasma. The deposition of C on the surface of the transfer line is evident by the gradual build-up of a carbon deposit over several hundred firings. The carbon deposit is visible particularly on the first 5 cm of PTFE tubing located at the exit and downstream of the graphite tube. Table 3.10 lists the background ions observed at a graphite tube temperature of 2500°C along with analyte isotopes which are isobaric with these polyatomic ions. Table 3.11 summarizes the analytical importance of these interferences expressed as background equivalent mass of analyte. Primarily light elements were interfered by carboncontaining polyatomic ions. These elements included Mg, Si, Ti, Ca and Cr. Magnesium: Two Mg isotopes are isobaric with carbon-containing ionic species leaving 25Mg (10.11%) available for analytical purposes. However, the BEM for 24Mg(78.6%) was only 0.15 pg which may make this isotope attractive for certain applications. Silicon: All three Si isotopes are interfered with carbon-containing polyatomic ions making this element difficult to determine at trace 371

TABLE 3.10 Spectral interferences arising from graphite electrothermal vaporizer heated to 2500°C Mass

Background ion(s)

24 26 28

12C12C+

29

14N14N1H÷,12C1601H+ 13C1GO+

Si(4.7)

30

14N160+,13C16OIH+

[31]* [32] [33]

14N1601H+ 160260+ 1601601H+

Si(3.1) P(100) S(95.0) S(0.8)

44 48 52 53 [54] [80]

12C1~O1~O+ 12C36Ar+ 12c4°nr+ 13C4°Ar+ 4°Ar14N+ 4°Ar4°Ar+

Ca(2.1) Ca(0.18),Ti(73.5) Cr(83.8) Cr(9.6) Cr(2.4),Fe(5.9) Se(49.97),Kr(2.3)

12C14N+ 14N14N+,12C160+

Analyte (% abundance) Mg(78.6) Mg(11.3) Si(92.2)

*Masses in parentheses arise from the presence of plasma and/or atmospheric gases and are included for comparison purposes.

levels using ETV-ICP-MS. Note t h a t a significant proportion of the BEM is due to nitrogen containing polyatomic ions which interfere with the determination of Si by all modes of sample introduction. Titanium: 48Ti (73.5%), which is isobaric with a minor Ca isotope, was interfered by a carbon argide polyatomic ion. For the determination of levels of Ti approaching an absolute mass of 0.42 pg, use of 47Ti (7.75%) or 49Ti (5.51%) is recommended. Calcium: 4SCa (0.18) is isobaric with a carbon argide species with a BEM of 171 pg. 44Ca (2.13%) was isobaric with CO2+ with a BEM of 122 pg. 42Ca (0.64%) and 43Ca (0.13%) are free of interferences. The major calcium isotope (4°Ca) is isobaric with the massive 4°Ar peak and cannot be used for analysis. Chromium: 52Cr (83.3%) and 53Cr (9.6%) are both interfered by carbon argides leaving only 54Cr (2.38%) which was free of carbon argide b u t isobaric with 54Fe (5.9%) and argon nitride. Since most real 372

TABLE 3.11 Background equivalent analyte mass for spectral interferences arising from graphite electrothermal vaporizer heated to 2500°C Analyte 24Mg 26Mg

Background equivalent mass (Pg) 0.15 6.6

288i 29Si 3°Si

839 1127 2296

31p

2.0

32S 33S

150" 150"

4STi

0.42

44Ca 48Ca

129 171

52Cr 53Cr 54Cr

12.3 1.2 24.4

54Fe

9.9

*Nanograms.

samples will contain significant quantities of Fe, the use of 53Cr with a BEM of 1.2 pg is recommended for use. Each of the elements discussed above is relatively involatile and therefore will be vaporized from the graphite surface at a temperature near the pre-programmed m a x i m u m or steady-state temperature. Consequently, the seriousness of interferences from carbon-containing polyatomic ions will be dependent on the actual appearance and peak temperatures of the analyte and thus it is important to consider the temporal characteristics of the background species. The appearance temperature is defined as the temperature at which the analyte signal can be detected above the baseline noise and the peak temperature is 373

30000

1

J

J

J

L _ _

25000 S2CAr+

15000

0

10000

5000

1

2

3

4

5

6

Time, seconds

Fig. 3.5. ETV-ICP-MSsignals for chromium and carbon argide 52Cr: 20 pg (from Ref. [52] with permission). the temperature at the maximum of the analyte signal. For example, Fig. 3.5 shows the temporal characteristics ofCr ÷ relative to that of the carbon argide species produced during rapid heating of the HGA600MS to a maximum temperature of 2500°C. Clearly, the Cr and the carbon argide signals are nearly temporally resolved and if a means could be found to volatilize Cr at a lower temperature (with a chemical modifier) thus shifting the Cr peak to the left (more volatile), this element could then be easily determined using the major chromium isotope (52Cr). Alternatively, the use of peak-height measurements rather than integrated signals will reduce the BEM significantly for the determination of 52Cr, especially in view of the very broad signal produced by the carbon argide. For all elements interfered by carbon-containing polyatomic ions, optimum analytical conditions should include the use of the lowest possible vaporization temperature combined with peak-height measurements and chemical modification to increase analyte volatility. Integration of analyte signals will only be practical if the analyte signal can be totally resolved from that of the interfering polyatomic ion. Since the metal filaments used for ETV studies have a relative high atomic mass (e.g. Re = 186.2; W = 183.8; Ta = 180.95), few important 374

polyatomic species are formed that can interfere with the determination of elements of m / z lower than that of the metal itself, with the possible exception of doubly-charged ionic species. Argides of each of these three elements do not interfere with any element of the periodic table. Oxides, however, can form with oxygen entrained into the argon plasma and serious interference could result. For example, the oxides of Ta are isobaric with 196Pt (35.3%), 196Hg (0.15%) and 197Au, which is monoisotopic. Tungsten oxides are isobaric with 19sPt (7.21%) and three isotopes of Hg: 199Hg (16.84); 2°°Hg (23.13%); 2°1rig (13.22%). Rhenium oxides interfere with the determination of 2°~Hg (13.22%) and 2°~T1 (29.5%). Interferences on Hg isotopes, however, would not be severe since this element is volatile and would vaporize at temperatures well below the appearance temperatures of W or Re. Although the effect of evaporated metal ETV filament material on analyte signals has been reported [8,25,73], no systematic study of possible filament metal oxide spectral interferences has been completed with the exception of a study by Karanassios and Horlick [93]. These authors report on the presence of metal ions and their oxide species in the background spectrum of direct insertion devices made from graphite, Mo, Ta and W. A tantalum oxide at m / z 197 was observed when using a Ta filament and significant contaminant metals such as Sn and Ta in Mo cups and Zr and Cs in W wire loops were also reported.

3.6 MECHANISM OF VAPORIZATION OF ANALYTE When sample material deposited within a graphite tube is heated during the vaporization step, analyte can be converted to vapour via several pathways. Understanding the mechanism of analyte vaporization is important in determining optimum experimental conditions for quantitative analysis. As discussed earlier in this chapter, both GFAAS and ETV-ICP-MS use graphite tubes to produce analyte vapour. Dnring the past 30 or more years, much information has been published in the GFAAS literature concerning the mechanisms of atomization of a number of elements. An extensive review of this literature has been published by Styris and Redfield [94] and should be consulted by anyone conducting research in this field. In an earlier paper, Sturgeon and Chakrabarti [95] proposed that there are four potential routes by which atomic vapour (Mg) is produced in the graphite furnace: 375

A.

MO~ + C

> M~ + COg > Mg >MMg >2 M~

B.

MO~

>Mg + Og

C.

MO~

> MOg

> Mg+ Og

D.

MX~

> MXg

> Mg+X~

In mechanism A, analyte atoms are produced by the reduction of analyte-containing salt by C followed by the volatilization of metal from the graphite surface. An alternative pathway involves the sublimation of metal dimers which further dissociate within the graphite tube into monomeric species. Mechanism B represents the thermal dissociation of analyte oxide followed by the sublimation of its decomposition products from the graphite surface. The vaporization of oxide to the gaseous state to produce analyte atoms by thermal dissociation of the vapour-phase oxide is represented in mechanism C. The last mechanism describes the volatilization of analyte in another chemical form such as a chloride which, on reaching the gas-phase, thermally dissociates. Table 3.12 lists some of the elements which are atomized via each of mechanisms A, B and C. Although much information of use to ETV-ICP-MS mechanistic studies can be gleaned from the GFAAS literature, there are important differences between these two techniques t h a t m a y change the nature of the vaporization and atomization process. This is especially true for reaction mechanisms which do not occur on the surface of the graphite TABLE 3.12 Mechanisms ControllingAtomizationin GFAAS(fromRef. [86]) A.

Vaporizationof the Metal: Ag, Au, Co, Cu, Fe, Hg, Ni, Pb, Sn, Pt, Pd, Ru, Ir, Rh, Os

B. Thermal Dissociationof the Oxide: A1, Ba, Be, Ca, Cd, Cr, Mg, Mn, Sr, Zn C. Thermal Dissociationof the Carbide: Hf, Mo, Nb, Ta, Ti, Th, V, Zr 376

tube. Clearly, surface reactions such as carbon reduction will be similar in either GFAAS or ETV-ICP-MS systems. A detailed comparison of GFAAS and ETV-ICP-MS has been published by Gr~goire et al. [31]. During the vaporization (atomization) step in GFAAS, no argon gas is normally allowed to flow within the graphite tube. This is done to increase the vapour-phase temperature of volatilized analyte and analyte species and to help confine atomic vapour within the graphite tube for as long as possible. Ideal conditions for GFAAS include 100% vaporization and atomization of analyte and long residence times for analyte atoms within the analytical volume where atomic absorption takes place. Conditions within the graphite tube during an ETV-ICP-MS experiment are quite different than for GFAAS work. The most important parameter change for ETV-ICP-MS is the use of a carrier gas flow of about 1 1 min -1 which continually flushes out the graphite tube of vaporized sample. The requirement for complete vaporization of analyte is the same for ETV-ICP-MS as it is for GFAAS, but for ETVICP-MS the analyte can be in any form including atoms and molecules. Whereas the m e a s u r e m e n t (atomic absorption) step occurs within the graphite tube in GFAAS, in ETV-ICP-MS sample-containing aerosol is atomized and ionized at a remote location (argon plasma) as much as a metre away from the graphite tube. The argon stream continuously passing through the plasma during the vaporization step is constantly cooling and removing vaporized material from the graphite surface. Reduction reactions and solid state thermal decomposition will occur and atoms released from the surface may quickly re-condense as dimers, clusters or in polyatomic form as oxides and chlorides. As is shown for reactions C and D (above), the vaporization of analyte oxides and chlorides as well as carbides can become important. Many elements have volatile chlorides and oxides and the vaporization of these within the graphite tube is likely the dominant mechanism by which analyte-containing vapour is produced. Table 3.13 lists the boiling point of elements and their chlorides, oxides and carbides. Once volatilized and carried to the argon plasma by the gas stream, oxides and carbides of all of the elements listed in Table 3.13 are easily dissociated. Table 3.14 lists the bond strengths for these oxides and carbides. The strongest bonds listed are for the carbides of W and U and experiment [54,59] has shown that vaporized oxides of these elements resulted in UO + and WO ÷ ion levels well below those observed when nebulizing aqueous U and W solutions. For ETV-ICP-MS, it is 377

T A B L E 3.13 B o i l i n g p o i n t s o f e l e m e n t s a n d t h e i r c h l o r i d e s , o x i d e s a n d c a r b i d e s ( f r o m Ref. [96]) Element

Boiling point (°C)

Boiling point of c h l o r i d e (°C)

Boiling point of o x i d e (°C)

Boiling point of c a r b i d e (°C) -

Ag

2212

1550

d 230

A1

2467

d 262

2980

d 2200

As

s 613

130

s 193

-

Au

2808

s 265

-

-

B

2550

12.5

~1860

>3500

Ba

1640

1560

=2000

-

Be

2970

520

=3900

-

Bi

1560

447

1890

-

Ca

1484

>1600

2850

2300

Cd

765

140

d 950

-

Ce

3443

1727

-

-

Co

2870

1049

-

-

Cr

2672

s 1300

4000

3800

Cs

669

1290

-

-

Cu

2567

1490

-

Dy

2567

1500

mp 2340

-

Er

2868

1500

-

-

Eu

1527

>2000

-

-

Fe

2750

=290

mp 1594

m p 1837

Ga

2403

535

-

-

Gd

3273

-

mp 2330

-

Ge

2830

84

s 710

-

Hf

4602

s 319

=5400

mp 3890

Hg

357

302

d 500

-

Ho

2700

1500

-

-

In

2080

600

~850

-

Ir

4130

-

d 1100

-

760

s 1500

d 350

La

K

3464

>1000

4200

-

Li

1342

1350

1200

-

Lu

3402

s 750

-

-

Mg

1107

1412

3600

-

Mn

1962

1190

-

-

Mo

5560

268

s 1155

mp 2692

Na

883

1413

s 1275

-

378

Element

Boiling point (°C)

Boiling point of chloride(°C)

Boiling pointof oxide(°C)

Boiling point of carbide(°C)

Nb

5127

254

-

mp 3500

Nd

3074

1600

mp =1900

-

Ni

2370

s 973

mp 1984

-

Os

>5300

d =550

130

-

180

173

-

P

280

Pb

1740

950

mp 886

-

Pd

2970

d 500

-

-

Pr

3520

1700

-

-

Pt

3827

d 581

d 550

-

Rb

686

1390

d 400

Re

~5627

>500

d 400

-

Rh

3727

s 800

-

-

Ru

3900

S

445

d 500

d 108

-

-

-

-

Sb

1750

283

s 1550

-

Sc

2836

s 800

-

-

Se

684

d 130

s 340

125

Si

2355

57

2230

s 2700

Sm

1794

-

-

-

Sn

2270

652

-

-

Sr

1384

1250

=3000

-

Ta

5425

242

mp 1872

5500

Tb

3230

-

-

-

Te

1390

327

1245

-

d 928

4400

=5000

3287

d 475

=2750

4820

T1

1457

720

1865

-

Tm

1950

1440

-

-

Th Ti

-

U

3818

792

mp 2878

4370

V

3380

-

d 1750

3900

W

5660

-

s 1430

6000

Y

3338

1507

mp 2410

-

Yb

1196

1900

-

-

732

mp 1975

-

d 350

=5000

5100

Zn

907

Zr

4377

s = sublimes; d = decomposes; mp = melting point.

379

TABLE 3.14 Bond strengths of diatomic molecules (from Ref. [96]) Oxides

Bond strength (eV)

Carbides

Bond strength (eV)

A1-O B-O Ba-O Be-O Ca-O Cd-O Cr-O Mg-O Mn-O Sr-O U-O

5.3 8.4 5.8 4.5 4.2 2.4 4.5 3.8 4.2 4.4 7.9 7.0 <2.8

B-C Hf-C Mo-C Nb-C Ta-C Ti-C Th-C U-C V-C W-C Zr-C

4.6 5.6 5.0 5.9 4.4 4.7 4.7 4.4 5.8

W-O

Zn-O

possible t h a t o b s e r v e d oxide to m e t a l r a t i o s (<0.2%) o r i g i n a t e d f r o m t h e m a s s s p e c t r o m e t e r i n t e r f a c e region r a t h e r t h a n f r o m u n d i s s o c i a t e d oxide f r o m t h e p l a s m a . B e c a u s e c a r b i d e s a r e r e l a t i v e l y involatile, t h e v o l a t i l i z a t i o n of t h e s e c a n n o t be relied u p o n as a m e a n s of p r o d u c i n g useful a n a l y t e v a p o u r , b u t r a t h e r t h e f o r m a t i o n of t h e s e should be avoided by selecting a v a p o r i z a t i o n t e m p e r a t u r e a n d e x p e r i m e n t a l conditions t h a t f a v o u r t h e p r o d u c t i o n of t h e m o r e volatile oxide or h a l i d e species. F o r s o m e e l e m e n t s s u c h as t h e p l a t i n u m g r o u p e l e m e n t s [61] w h i c h a r e k n o w n to be r e d u c e d b y g r a p h i t e to m e t a l l i c form, t h e v e r y h i g h boiling p o i n t of s o m e of t h e s e e l e m e n t s r e q u i r e s t h a t m e a n s be t a k e n to e i t h e r p r e v e n t r e d u c t i o n or p r o m o t e t h e f o r m a t i o n of volatile c o m p o u n d s w i t h t h e u s e of c h e m i c a l modifiers [13,61]. W h e n selecting E T V o p e r a t i n g conditions for t h e d e t e r m i n a t i o n of a n y e l e m e n t , r e f e r e n c e should be m a d e to k n o w n v a p o r i z a t i o n (atomization) p a t h w a y s a n d to a n a l y t e p h y s i c a l c o n s t a n t s s u c h as t h o s e g i v e n in T a b l e 3.14. R e c e n t s t u d i e s h a v e a p p e a r e d on t h e E T V - I C P - M S v a p o r i z a t i o n m e c h a n i s m s of m o r e t h a n 30 e l e m e n t s [53-63]. One of t h e m a i n 380

conclusions that can be drawn from these studies is that it is almost always advantageous to vaporize analyte in the combined form as an oxide or a halide while at the same time avoiding the production of very refractory compounds such as carbides which tend to remain incompletely vaporized from the graphite surface and can also result in a memory or carryover effect. To illustrate how the mechanism of vaporization of an analyte can be elucidated, the vaporization characteristics of two refractory elements, B and W are summarized below. 3.6.1 V a p o r i z a t i o n of b o r o n in ETV-ICP-MS The sensitivity for the determination of B by GFAAS is very poor. One explanation for the low sensitivity for B implicates formation of nonvolatile refractory carbides or the pre-atomization loss of volatile B203 [86]. Following up on thermodynamic calculations published by Frech et al. [97], Byrne et al. [53] investigated the mechanism of B vaporization using both GFAAS and ETV-ICP-MS. Graphite furnace AAS was used to obtain data on atomic species produced within the graphite tube and ETV-ICP-MS could be used to detect the volatilization of both polyatomic and atomic B species. The thermodynamic equilibrium calculations by Frech [97] predicted that a number of volatile B polyatomic species should be formed in the graphite furnace at temperatures well below the appearance temperature of B atoms (-- 2200°C). These species include B203(~), HBO2(g), B202(g ) and BO(g) as well as involatile B4C(~). According to Frech's calculations these species should form in various temperature ranges below 2300°C. The species listed above are in the order of increasing temperature of formation, i.e. the first gaseous species listed, H B Q forms between 900°C and 1600°C, while solid boron carbide is not predicted to form until about 2000°C, just prior to the appearance of B atoms (GFAAS). If this is the case, then B should begin to vaporize and be lost from the graphite tube as gaseous polyatomic species at temperatures above 900°C. This being so, then the determination of B by ETV-ICP-MS should be possible and not limited in sensitivity as is GFAAS. The volatilization of B species at temperatures below that required for the formation of B4C may provide a means of sensitive detection for this element by ETV-ICP-MS. Figure 3.6 shows the ETV-ICP-MS vaporization curve as well as the GFAAS atomization curve for B. Above this figure is shown the various 381

I-1-BO 2 (g)

BO

(g)

B 4 C (s)

18000 16000 14000

g

ro

025

•

.

~

0.20

12000

v

0.15.~

10000

<

eta

b3

8000 ~

o.io "3

6000 L 4000

0.05 ~

2000 600

800

1'000 1200 1400 t600

1800 2000 2200 2J400 2600 2800

Vaporization Temperature ( ° C) Fig. 3.6. ETV-ICP-MS vaporization curve for 2.5 ng of boron and GFAAS atomization curve for 25 ng of boron. The temperature regions in which molecular species are predicted to form are shown above the figure (from Ref. [53] with permission).

boron species that are predicted to form with lines drawn to indicate the temperature range over which these species occur. Data on vaporization and atomization curves were obtained by increasing the vaporization/atomization temperature and recording either the integrated ion intensity (ICP-MS) or the peak-height absorbance (GFAAS). The most striking feature of Fig. 3.6 is the marked difference in the appearance temperature for boron in the GFAAS and ETV-ICP-MS experiments. In GFAAS, where only boron atoms are detected, boron is not observed below 2200°C. In the ETV-ICP-MS experiments, the boron signal appears at a vaporization temperature of about 800°C. Since atomic boron is not detected by GFAAS below 2200°C, the boron observed by ETV-ICP-MS at vaporization temperatures between 800 and 2200°C clearly originates from boron polyatomic species. These would be vaporized and lost from the graphite furnace at some stage prior to atomization, either in the pyrolysis step or the atomization ramp normally employed in GFAAS. This pre-atomization loss, commencing at temperatures as low as 800°C, accounts for the low sensitivity for boron analysis in GFAAS. 382

40000 35000 oa ¢:1

30000 2500020000

o

7~ L~

15000 10000 5000 0 0

2

3

4

5

T i m e (s)

Fig. 3.7. ETV-ICP-MS signal for 2.5 ng of boron vaporized at 1600°C (from Ref. [53] with permission).

With regard to ETV-ICP-MS, these results show that the optimum vaporization temperature for the determination of boron is about 1800°C. Above this temperature the boron peak areas decrease, presumably because of the formation of polyatomic species, either BO or BC. Figure 3.7 shows an ETV-ICP-MS signal obtained for the vaporization of 2.5 ng of B at 1600°C. The signal is narrow and shows no tailing effect from the vaporization of less volatile species later in the vaporization cycle. A 10-fold increase in sensitivity for B was also reported [53] when Ni (0.5 rxg) was used as a chemical modifier.

3.6.2 Vaporization o f t u n g s t e n in ETV-ICP-MS The vaporization mechanism of W in ETV-ICP-MS was reported by Byrne et al. [54]. A similar approach was used as for the B study except t h a t GFAAS data was not obtainable. Tungsten is one of the elements (Table 3.4) not determinable by GFAAS because of the formation of the refractory carbide (Table 3.13) and the loss of volatile W polyatomic species from the graphite furnace prior to the high temperature m e a s u r e m e n t step. The latter effect would not hinder the determination of W by ETV-ICP-MS because W oxide species can be atomized and ionized on reaching the argon plasma. 383

12

10

2

8

(/)

4

2

o

700

I

I

I

I

1000

1300

1600

1900

Temperature,

I

2200

I

2500

2800

°C

Fig. 3.8. ETV-ICP-MS vaporization curve for 250 pg of t u n g s t e n (from Ref. [54], with permission).

Figure 3.8 shows the ETV-ICP-MS vaporization curve for tungsten which, as discussed earlier, is a plot of the integrated signal intensity for ls4W+ as a function of vaporization temperature. In order to interpret the vaporization process for tungsten, this curve should be considered in conjunction with the series of ETV-ICP-MS signals shown in Fig. 3.9. The vaporization curve of Fig. 3.8 shows that tungsten begins to volatilize from the graphite surface at temperatures around 800°C. The amount of tungsten vaporized increases with increasing temperature up to 1000°C. Since the melting and boiling points of tungsten metal are extremely high, atomic tungsten could not be volatilized at these low temperatures. However tungsten oxide, W Q , which would be expected to form when a tungstate solution is dried and heated, is much more volatile and has a vapour pressure of 9.1 torr at 980°C and is reported to sublime at around 1100°C. Thus the single peak observed at a vaporization temperature of 1000°C (Fig. 3.9a) could be attributed to the volatilization of W Q . As the vaporization temperature is raised above 1000°C the ETVICP-MS signal intensity, rather unexpectedly, begins to decrease and then levels off at around 20,000 counts when the vaporization tempera384

6O

(a)

(b)

1000°C

1300°C

50 4O

A

30 m 10 0

•

i

6O (C)

2300°C

5O 40

2O 10

0

1

2

3

i

i

i

,

4

5

6

7

8

2

3

4

5

6

7

8

Time/s

Fig. 3.9. ETV-ICP-MS signals for 250 pg of W vaporized at various temperatures: (a) 1000°C; (b) 1300°C; (c) 2300°C; (d) 2700°C (fromRef. [54] with permission). ture reaches 1400°C (Fig. 3.8). This decrease can be explained by the onset of tungsten carbide formation which is reported to commence at about 850°C and be complete at 1410°C. As the temperature of the graphite surface rises above 1000°C any residual WO3 which has not been vaporized will be converted to refractory WC which should not be then released until much higher vaporization temperatures. The amount of WO3 converted to WC remains approximately constant for vaporization temperatures between 1400 and 2200°C. This is reflected by the flat portion of the vaporization curve in this temperature region and by the fact t h a t the ETV-ICP-MS signal consists of a single peak throughout this temperature range. The peak shape at 1300°C, shown in Fig. 3.9b, is typical for this temperature range. At a temperature around 2300°C the vaporization curve of Fig. 3.8 begins to rise as more tungsten is released from the graphite surface. This coincides with the appearance of a small shoulder on the trailing edge of the ETV-ICP-MS signal at 2300°C (Fig. 3.9c). As the temperature is further increased to 2700°C the vaporization curve rises steeply and a large second peak occurring between 2 and 8 s now appears in the ETV-ICP-MS signal (Fig. 3.9d). This second peak can be 385

explained by the volatilization of WC at temperatures above 2500°C. Also, WC melts to form a eutectic with W2C at 2525°C and then decomposes at 2600°C to form a W-rich liquid and solid carbon. Thus when tungsten is vaporized from a graphite substrate there are two distinct vaporization processes. At low temperatures WO3 is volatilized to give a single peak in ETV-ICP-MS; at temperatures above about 2500°C this signal becomes double peaked as WC is released at the higher temperature. Associated with this second and much larger carbide peak is the problem of memory effect. Figure 3.9d (second smaller signal starting at 1.8 s) shows a typical blank signal from a second heating of the graphite tube at a temperature of 2700°C. For this blank firing there is no memory effect associated with the early WO3 peak between 1 and 2 s, but the memory effect from the larger carbide peak, between 2 and 8 s, amounts to about one third of the signal for 250 pg of tungsten. Double peaks and memory effects pose practical problems for the analysis of tungsten by ETV-ICP-MS. Possible strategies to overcome these problems include: (i) prevention of formation of carbide and use of suitable chemical modifiers, and (ii) use of lower vaporization temperatures which release only WO~ giving a single but smaller peak without memory effects. Reported limits of detection for W by ETV-ICP-MS were 0.51 pg using a vaporization temperature of 1100°C.

3.7 CHEMICAL MODIFICATION, PHYSICAL CARRIERS AND MASS TRANSPORT EFFICIENCY

3.7.1 Chemical modifiers and physical carriers Chemical modification refers to the deliberate alteration of the thermal properties of the analyte, matrix components or the vaporization surface by the addition of a foreign substance. Within the context of ETV-ICP-MS, chemical modification also refers to the alteration of analyte vapour transport properties. Thus chemical modifiers can have both chemical effects and physical effects, each improving conditions for the determination of an analyte in a specific matrix. It is at times beneficial to add modifiers to stabilize the analyte to allow for the elimination of more volatile matrix components, to improve the temporal separation of analyte and matrix during the volatilization step and to reduce losses prior to the high temperature 386

step by preventing hydrolysis reactions which can expel analyte by covolatilization with hydrolysis reaction products. On the other hand, modifiers can be added to increase the volatility of the analyte to promote the vaporization of analyte before matrix components and to prevent or reduce the interaction of analyte with graphite to form refractory compounds. As well, chemical modification can be useful in converting analyte to a single chemical form, eliminating double signals which can complicate quantitative analysis. Chemical modification is generally accomplished by the addition of a foreign substance to the sample solution or the introduction of a gas other than argon to the transport (Ar) gas flow. Modifiers in solution are added just prior to or at the same time as sample solutions are added to the graphite tube and gas-phase modifiers are used during the high-temperature vaporization step. Chemical modifiers used in GFAAS have been reviewed in a paper by Tsalev et al. [87]. The quantities used range from 25 to 200 ~ag depending on the application (Table 3.6). Modifiers which improve the mass transport efficiency of analyte from the graphite tube to the argon plasma are called physical carriers and will be discussed later in this part of the chapter. A significant number of papers have been published which report on the use of chemical modifiers. The use of Freon to increase the volatility of W vaporized on a carbon surface was first reported by Kirkbright and Snook [98] for ETV-ICP-AES studies and was later applied [10] to the determination of W in geological materials. Ediger [99] reported that the use of Freon could dramatically improve the sensitivity, especially for the determination of refractory elements such as U, Th and the REE. For the determination of the platinum group metals, Gr6goire [13] demonstrated that the addition of Ni chemical modifier could improve the sensitivity by up to a factor of 9.4. Table 3.15 shows the effect of adding a variety of chemical modifiers on the determination of Os by ETV-ICP-MS. Osmium is reduced to metal on the graphite surface during heating and remains on the surface as a refractory metal. The boiling point of Os is greater than 5300°C and it therefore has a relatively low vapour pressure at the maximum vaporizer temperature of 2700°C. The presence of Ni, Se or Te likely results in the formation of relatively more volatile alloys or compounds [13, 61]. An improvement of 17-fold in Os sensitivity was realized when 2 l~g of Te was used as chemical modifier. Byrne et al. [32] reported that the use of ascorbic acid as a chemical modifier reduced analyte loss during the pyrolysis heating step by slowing down the hydrolysis of magnesium 387

TABLE3.15 Effect of matrix modifier on Os (0.25 ng) ion count rate (fromRef. [15]with permission) Matrix modifier (2 ~g) -

Sodium chloride Thiourea 8-Hydroxyquinoline Nickel Selenium Tellurium

1920S+Ion count rate (cs-1) 36,450 36,800 50,100 63,400 176,900 183,700 653,000

chloride thus preventing loss of analyte (Mn) by co-volatilization with released HC1. This example illustrates the use of a chemical modifier to stabilize matr ix components. The influence of the condition of the graphite substrate on analyte signal was reported by Majidi and Miller-Ihli [100]. These authors concluded t h a t the use of an oxygenated pyrolytic graphite surface and chemical modifiers gave the best precision for ETV-ICP-MS signals. Ediger and Beres [46] and Gr6goire et al. [47] reported on the use of NaC1 and Pd/Mg chemical modifiers. In a comprehensive paper by Hughes et al. [43], the use of seawater stripped of trace metals was investigated as a universal mixed chemical modifier. Other papers report on the use of mannitol as a modifier for B [101], halogencontaining compounds for Zn, Cd and Pb [44] and a series of ten modifiers ranging from HC1 to freon for the determination of B, La and U [102]. Eleven modifiers were studied by Gr~inke et al. [45] and applied to the determination of Mn, Cu, Zn, Cd and Pb. Fonseca and Miller-Ihli [103] reported on the influence of sample m at ri x components on analyte signals and on the selection of calibration strategy, particularly as applied to the analysis of slurries. The use of a physical carrier is essential if aqueous external standards are to be used for quantitative analysis of multi-component sample solutions [104]. For a physical carrier to work, it m u s t be present in the gas-phase at the same time (and place) as the analyte vapour. Any single substance used as a physical carrier will have a limited t e m p e r a t u r e range over which it will volatilize, and therefore it 388

is unlikely that a physical carrier could be found that would work equally well for a volatile element such as Cd and relatively involatile elements such as the REE. Therefore, a mixed physical carrier of known composition and of high purity is required. A mixed physical carrier that could meet all of the criteria mentioned above was proposed by Gr6goire and Sturgeon [52] and Hughes et al. [43]. These authors suggested that seawater be used as a mixed physical carrier. A standard reference seawater (NASS-3) used in trace analysis studies and available from the National Research Council of Canada was selected. NASS-3 reference seawater is readily available and is well characterized for its trace metal content. Before use, trace metals were stripped from the NASS-3 by chelation on a column of immobilized ligand and diluted 500-fold with ultra-pure water. The resulting solution provides ~-0.07 pg ~l1-1 dissolved solids of which 0.055 lag laF1 is NaC1, 0.011 pg la1-1 is MgC12, 0.0022 lag tl1-1 is CaCI~ and 0.0015 lag la1-1 is KC1. Figure 3.10 shows ETV-ICP-MS signals for a number of analytes (parts a and b) and for the major components of NASS-3 (part c). A comparison of these signals shows that individual components of NASS-3 volatilize at different times and together span the time during which all of the analytes in parts a and b (Fig. 3.10) are being volatilized. Significant (> × 10) sensitivity enhancements using NASS-3 physical carrier were reported [67] for several analytes studied. In separate study, Hughes et al. [43] reported on the mechanism by which NASS-3 improves the mass transport of ana]yte from the graphite furnace to the argon plasma. The determination of 10 elements using NASS-3 as a physical carrier as well as studies using separate salts (NaC1 and MgC12) in different acids (HC1 and HNO3) were reported. Table 3.16 summarizes results obtained for differing masses of physical carrier using HC1 and H N Q as diluent. The results show that for both volatile and relatively involatile elements, the greatest enhancements (increase in mass transport efficiency) in integrated counts were obtained using MgC12. For elements of intermediate volatility, NaC1 gave the best results. Elements of high volatility (e.g. Cd) benefited most from the use of HC1 and it was concluded that HC1 itself is a powerful physical carrier. This acid may promote the formation of simple chlorides of analytes which are volatilized from the graphite surface in elemental form. Single atomic particles or small clusters of atoms are reported to have a low mass transport efficiency [105] over distances greater than a few centimetres. 389

i

6000

i

/'~

5000

i

i

i

F Aqueous

,.

/ 150000

i Standard

205 TI

VY~ ]

4000.

(b) ......

/~ - ~

133 Cs

8

8 3000-

~'-t % 1

g 8

i

200000

Aqueous Standard

100000

,,c~ 0

2000

60000. 1000

O1

2 Time

3

/~Z. ~ ~ F

4

1

/ seconds

,

2 Time

3

4

/ seconds

1500000

/~,

1250000.

/

Seawater Carrier

~

1000000.

750000

500000

250000

0 1

2 Time

3

4

/ seconds

Fig. 3.10. Comparison of ETV-ICP-MS signals derived from analyte (a and b) and NASS-3 seawater components (c) (from Ref. [67] with permission).

The mechanisms proposed by Hughes et al. [43] define three zones of action to explain the effect of NASS-3 on analyte signals. Figure 3.11 schematically illustrates these three zones and also shows the elements whose appearance temperatures are within these zones. The appearance temperature is defined as the temperature (of the graphite surface) at which the analyte signal becomes measurable above the background noise. In the first zone, which is the lowest temperature zone, hydrolysis of magnesium chloride occurs. A reaction product of hydrolysis is HC1 390

TABLE 3.16 Effect of acids, physical carrier and mass of physical carrier on integrated analyte (20 pg) signal (from Ref. [43]) Analyte Acid (1%)

NASS-3 (pg)

ll4Cd

HNQ HC1

2°5T1 2°spb 133Cs

0.07

NaC1 (pg)

0.35

1.05

0.07

0.35

2.9

3.2

2.5

2.2

1.9

1.2

4.2

8.5

1.0

0.4

HNO 8

1.8

1.6

1.5

2.0

1.7

HC1

0.8

0.7

0.8

0.9

HNO 3

1.8

1.5

1.3

1.8

HC1

1.0

0.9

0.9

1.0

0.8

MgC12 (i~g) 1.05

0.07

0.35

1.05

1.8

1.8

4.5

6.1

0.2

3.5

8.5

11.5

1.6

0.8

2.1

2.4

1.0

1.0

0.9

1.8

2.6

1.7

1.6

1.6

1.9

2.2

0.7

1.7

1.2

1.1

HNQ

1.3

1.2

1.2

1.1

1.2

1.2

1.0

1.1

1.3

HC1

2.6

2.2

2.1

2.5

2.2

2.2

1.6

2.5

2.8

HNO 3

1.3

1.1

0.9

1.2

1.1

1.0

1.0

1.0

1.2

HC1

1.3

0.9

0.8

1.2

0.9

0.9

1.1

1.6

1.8

HNO 3

1.8

1.1

1.0

2.1

1.4

1.3

0.9

1.1

1.6

HC1

1.2

1.1

0.9

1.3

1.0

1.0

1.0

0.9

1.1

1°TAg

HNO 3

1.5

0.7

0.5

1.4

0.6

0.5

1.0

1.1

1.5

HC1

1.3

0.7

0.5

1.2

0.7

0.5

1.0

1.2

1.8

115In

HNO 3

1.7

1.3

0.9

1.7

1.5

1.2

1.3

1.2

0.9

HC1

0.8

0.9

1.0

0.9

0.8

1.0

1.0

1.0

1.0

HNO 3

1.2

0.8

0.7

1.3

1.0

0.8

0.8

0.7

0.7

HC1

1.2

1.0

0.9

1.3

1.0

0.8

0.9

0.7

0.7

HNO 3

1.4

1.2

1.0

1.3

1.2

1.1

1.2

1.1

1.1

HC1

1.4

1.2

1.0

1.1

0.9

0.8

1.2

1.1

1.1

S~Rb 2°9Bi

~9Ga 59Co

w h i c h a c t s as a n e f f e c t i v e c a r r i e r f o r v o l a t i l e e l e m e n t s . T h e s e c o n d z o n e , w h i c h is o f i n t e r m e d i a t e t e m p e r a t u r e , s p a n s t h e t i m e d u r i n g w h i c h m o s t o f t h e N a C 1 is b e i n g v o l a t i l i z e d a n d s e r v e s a s a p h y s i c a l carrier. In t h e t h i r d a n d h i g h e s t t e m p e r a t u r e zone, r e s i d u a l M g O r e m a i n i n g f r o m t h e h y d r o l y s i s r e a c t i o n w h i c h o c c u r r e d i n z o n e 1 is v o l a t i l i z i n g . S e v e r a l p h e n o m e n a could be r e s p o n s i b l e for s i g n a l enh a n c e m e n t i n z o n e 3. F i r s t , v o l a t i l i z e d M g O c o u l d s i m p l y a c t as physical carrier and interact with analyte only in the gas phase. Secondly, M g O could occlude a n a l y t e w i t h i n its m a t r i x a n d co-volatilize w i t h t h e a n a l y t e a n d f i n a l l y , M g O c a n s e r v e as a n e f f e c t i v e b a r r i e r 391

1400OOO

12OO0OO A

c

B

10O000O

?'"'..

800000

~ 600000

J

400000 200000

i!

-

~ \ t \

1.5

2

-

0 0.5

1

~ x. %-.:-._. 2.5

3

Time, s

Fig. 3.11. ETV-ICP-MS signals for major ions derived from NASS-3 seawater carrier. Zone A: Carrier HC1; volatile elements: Cd, Ag, T1, Cs, Rb, Pb, Zn. Zone B: Carrier NaC1; medium volatility elements: Bi, In, Ga. Zone C: Carrier Mg/MgO, Ca/CaO; low volatility elements: Co, Ni, Dy, Eu,Tm, Yb.

preventing interaction of analyte with the graphite surface inhibiting the formation of refractory compounds such as carbides. In a related study, Gr~goire et al. [64] reported on the vaporization properties of HC1 and HNO~ vaporized under various experimental conditions. Drying step temperatures of 140°C (50 s) and pyrolysis step temperatures of 400°C (10 s) were effective in volatilizing most of the chloride from a 10 pl 1% (v/v) HC1 solution, however, a small amount (40 ng) of acid was retained on the graphite even after pyrolysis at 400°C. Under the same experimental conditions, H N Q was completely volatilized from the graphite tube. The effect of a range of concentrations ofHC1, HNO3, H2SO 4 and H3PO 4 o n analyte signals for Co, Cu, Ag, Cs, Pb, Bi and U were studied. Analyte signals were enhanced by as much as a factor of two in the presence of 1% (v/v) H N Q and g 2 s o 4. Phosphoric acid suppressed analyte signals for Ag and Bi and the use of HC1 resulted in relatively small changes in analyte sensitivity. The use of a pyrolysis step in the heating program reduced effects associated with acid matrices, but at the expense of signal intensity. NASS-3 reference seawater carrier attenuated the matrix effects associated with H2SO 4 and HsPO4 and essentially eliminated them for HNO3 and HC1. Table 3.17 shows the effect of these acids at two different concentrations and Table 3.18 illustrates the power of NASS-3 to control these differences. As a consequence of the above studies, one can 392

TABLE 3.17 Effect of acid concentration on integrated analyte signal pulses (50 pg) (from Ref. [64], with permission) Integratedsignal(counts) Co

Cu

Ag

Cs

0.05%HNO 3

16275

7144

12507

163790

RSD % (n=5) 1.0%HNO~

2.5 19659

6.4 11417

3.8 22294

4.5 196151

RSD % (n=5)

5.6

8.1

9.9

3.6

0.05% HC1

19833

10530

20060

187167

Pb

Bi

U

122529

174329

136369

7.6 196731

8.5 263292

7.7 225123

7.7

5.6

5.4

250100

212667

106140

RSD % (n=5)

1.1

3.6

1.00

0.9

0.70

6.1

6.3

1.0% HC1

19616

10155

19863

187006

248335

199736

99589

RSD % (n=5)

4.8

4.7

3.1

3.6

6.3

4.3

12.0

0.05%H2SO 4

25211

15720

13497

1 2 2 5 7 5 129097

136053

116543

RSD % (n=5)

6.5

6.5

6.3

5.1

5.7

4.2

5.3

1.0%H2SO 4

34887

21455

18065

268796

239324

368462

217413

RSD % (n=5)

5.1

5.6

5.0

8.8

7.0

7.4

8.4

0.05%H3PO 4

45040

-

23254

188548

-

206831

335407

RSD % (n=5)

9.7

-

9.9

12.6

-

5.6

10.1

1.0% H3PO4

27139

-

13225

371608

-

192756

239306

RSD % (n=5)

10.6

-

2.5

9.4

-

4.0

6.4

c o n c l u d e t h a t t h e u s e of HC1 r a t h e r t h a n H N O 3 is r e c o m m e n d e d for E T V - I C P - M S w o r k a n d i n p a r t i c u l a r for t h e d e t e r m i n a t i o n of v o l a t i l e e l e m e n t s . T h i s is i n m a r k e d c o n t r a s t to S N - I C P - M S w h e r e H N Q is t h e p r e f e r r e d acid. T h e f o r m a t i o n of a n a l y t e c h l o r i d e s i n t h e a r g o n p l a s m a h o w e v e r , r e m a i n s a c o n c e r n for b o t h E T V a n d S N w o r k w h e n s a m p l e s high in chloride concentration are being analysed. In a related study, t h e r e t e n t i o n of w a t e r a n d h y d r o g e n f r o m g r a p h i t e t u b e s a n d i t s effect o n a n a l y t e s i g n a l s for A1, V, C u a n d P b w a s r e p o r t e d b y G r f i n k e e t al. [106]. T h e v e r y h i g h s e n s i t i v i t y of E T V - I C P - M S a n d i t s u s e for m u l t i element analysis places some requirements on the chemical modifiers 393

TABLE 3.18 Effect of acid concentration on integrated analyte signal pulses (50 pg) using NASS-3 as a physical carrier (from Ref. [64] with permission) Integratedsignal(counts) Co

Cn

Ag

Cs

Pb

Bi

U

0.05%HNO 3

30382

10148

29399

125492

220425

373365

53410

RSD % (n=5)

9.4

8.1

3.3

8.1

5.4

2.4

6.3

1.0%HNO 3 RSD % (n=5)

28464 7.9

11165 3.3

27839 4.1

115206 3.3

225277 7.3

366476 2.8

48990 5.4

0.05%HC1

23085

9297

24848

1 4 9 9 0 3 256772

223700

30720

RSD % (n=5)

2.3

10.4

5.5

7.9

4.7

4.2

8.3

1.0% HC1

21648

9409

19402

144490

256704

181660

48030

RSD % (n=5)

2.9

5.6

1.3

5.4

6.9

5.1

8.4

0.05%H2804 40437 RSD % (n=5)

3.42

29286 4.1

37877 7.8

322094 3.5

284412 6.0

300002 3.1

58469 5.3

1.0%H2SO 4

42471

30117

27204

394135

381171

480804

114642

RSD % (n=5)

5.34

4.3

7.0

3.7

1.6

6.7

6.8

0.05%HsPO 4

46581

-

31027

181483

-

209078

469163

RSD % (n=5)

9.1

-

4.6

5.9

-

17.8

9.5

1.0%H3PO 4

27581

-

13626

419118

-

389964

188118

RSD % (n=5)

5.6

-

2.0

4.4

-

5.0

13.6

u s e d . F o r e x a m p l e , m o d i f i e r s m u s t b e of t h e h i g h e s t p u r i t y , free of elements that could interfere with target analytes. The modifier should b e of b e n e f i t to t h e l a r g e s t n u m b e r of a n a l y t e s a v o i d i n g t h e u s e of s e v e r a l d i f f e r e n t m o d i f i e r s for m u l t i - e l e m e n t s c h e m e s . A n i m p o r t a n t c o n s i d e r a t i o n w h e n u s i n g m o d i f i e r s s u c h as t h o s e m e n t i o n e d a b o v e , is t h e c o n t r i b u t i o n t h a t m i c r o g r a m q u a n t i t i e s of t h e s e s u b s t a n c e s c a n h a v e to t h e b a c k g r o u n d s p e c t r u m . F o r e x a m p l e , i f s u f f i c i e n t q u a n t i t i e s of a n e l e m e n t a r e v a p o r i z e d i n t o t h e p l a s m a , i t is k n o w n t h a t p o l y a t o m i c i o n s c a n b e f o r m e d s u c h a s a r g i d e s , oxides, n i t r i d e s a n d for E T V , c a r b i d e s . G r 6 g o i r e a n d S t u r g e o n [52] a n d also B j S r n e t al. [90] a d d r e s s e d t h i s p r o b l e m a n d r e p o r t e d o n t h e p o l y a t o m i c 394

TABLE 3.19 Isotopes from graphite and sodium chloride, magnesium nitrate, nickel nitrate and palladium nitrate chemical modifiers (from Ref. [52] with permission) Carbon

Sodium

Magnesium

Chlorine

12C (98.89)* 13C (1.11)

2~Na (100)

24Mg (78.6) 25Mg (10.11)

3~C1(75.4) 35C1(24.6)

26Mg (11.29) Nickel ~SNi (67.77) ~°Ni (26.16)

Palladium l°6Pd (27.1) l°sPd (26.7) l°sPd (22.6) 11°Pd (13.5) l°4Pd (9.3) l°2Pd (0.8)

*Percent natural abundance.

ions formed in the plasma from the vaporization chemical modifiers. The results obtained by GrSgoire and Sturgeon [52] using ascorbic acid, sodium chloride, magnesium nitrate, nickel nitrate and palladium nitrate are summarized below. These modifiers were selected for study because these are the most commonly used for chemical modification in ETV-ICP-MS studies. For each modifier, the impact of the polyatomic ions formed on the determination of individual elements is discussed and alternative analyte isotopes are suggested for use. Table 3.19 lists the source isotopes (carbon and modifiers) from which interfering polyatomic ions can be formed in the argon plasma. Gr~goire and Sturgeon [52] report that for all of the matrix modifiers studied, no carbides or nitrides were detected in the plasma and therefore the formation of these polyatomic ions will not be considered further. 3.7.1.1 S o d i u m chloride

Table 3.20 lists the possible oxide and argide polyatomic ions that can be formed when NaC1 chemical modifier is vaporized into an argon plasma. The detection of sodium oxide was not possible since this polyatomic ion at a m/z of 39 daltons is overlain by the wings of the intense 4°Ar ion. Sodium oxide (m/z = 39), should it exist, would be 395

TABLE 3.20 Possible major interfering polyatomic ions produced when using sodium chloride chemical modifier (from Ref. [52] with permission) m/z of Polyatomic ion

Isobaric analyte isotopes (abundance)

Sodium oxide

39

K (93.1)

Chlorine oxides

51

V (99.8)

53

Cr (9.6)

Sodium argide

63

Cu (69.1)

Chlorine argides

75

As (1O0)

77

Se (7.5)

isobaric with the major isotope of K (93.1%), an element not generally determinable at trace levels by ICP-MS. Copper: No sodium argide was observed at masses below 10 llg of added sodium. Sodium argide is isobaric with 6SCu (69.1%), the major Cu isotope. The BEMs for 63Cu resulting from the addition of 10 IJg and 20 pg of Na were 1.5 pg and 3.6 pg, respectively. Arsenic and Selenium: The use of up to 10 l~g of NaC1 did not result in the production of any carbides, nitrides, oxides or argides of chlorine. At elevated levels of C1 (20 pg) only the chlorine argides are observed, resulting in BEMs of 0.4 pg and 1.1 pg for 75As and 77Se, respectively. The use of such high levels of C1 as NaC1 chemical modifier is unnecessary [46] and therefore this interference would not normally be encountered. However, it is possible t h a t such high levels of C1 m a y exist in a real sample solution or slurry. Correction for these polyatomic ions may be necessary, particularly for As which is monoisotopic and also for 7~Se (7.5%) which is the only Se isotope t h a t is free of isobaric interferences from isotopes of other elements. The formation of natrides with analyte ions was also investigated. Copper natrides at m/z of 86 and 88 daltons were not detected even when 5 lag of NaC1 were vaporized with 1000 pg of Cu. The corresponding copper chlorides were also not observed during vaporization of 8 tlg of C1 as sodium chloride.

3. 7.1.2 Magnesium nitrate Table 3.21 lists the oxide and argide polyatomic ions t h a t can be formed when magnesium nitrate chemical modifier is vaporized into an argon 396

TABLE 3.21 Possible major interfering polyatomic ions produced when using magnesium nitrate chemical modifier (from Ref. [52] with permission)

Magnesium oxides

Magnesium argides

m/z of polyatomic ion

Isobaric analyte isotopes (abundance)

40 41 42 64 65 66

Ar (99.6), K (0.01); Ca (96.9) K (6.9) Ca (.64) Ni (1.2), Zn (49.8) Cu (30.9) Zn (27.8)

TABLE 3.22 Background equivalent analyte mass for spectral interferences produced when vaporizing 5 ~g of magnesium nitrate chemical modifier (from Ref. [52] with permission) Analyte

Background equivalent mass (pg)

42Ca 64Ni aSCu 64Zn 66Zn

149 28.5 0.14 0.68 0.18

p l a s m a . W h e n h e a t e d , m a g n e s i u m n i t r a t e d e c o m p o s e s to f o r m t h e oxide w h i c h is s u b s e q u e n t l y v a p o r i z e d f r o m t h e g r a p h i t e s u r f a c e during t h e h i g h t e m p e r a t u r e step. M a g n e s i u m oxides a n d a r g i d e s w e r e d e t e c t e d w h e n 5 ~g of M g as n i t r a t e w e r e v a p o r i z e d in t h e H G A - 6 0 0 M S . T h e B E M s for t h e s e polya t o m i c ions for t h e affected a n a l y t e s a r e g i v e n in T a b l e 3.22 a n d a r e d i s c u s s e d below. Calcium: M a g n e s i u m oxides occur a t t h r e e m/z r a n g i n g f r o m 40 to 42 daltons. T h e oxides at m/z 40 a n d 41 a r e isobaric w i t h 4°Ar÷ a n d 4°ArlH+ a n d h e n c e d e t e c t i o n of t h e s e oxides w a s not possible. H o w e v e r , a m a g n e s i u m oxide isobaric w i t h 42Ca (0.64%) w a s d e t e c t e d giving a 397

BEM of 149 pg Ca for this isotope. 44Ca (2.13%) is isobaric with the small but significant CO2+ ion (see background spectrum and ascorbic acid discussions) and 4SCa (0.18%) is isobaric with the major isotope of Ti. The only Ca isotope free from isobaric interference was 43Ca with an abundance of 0.13%. Nickel: ~4Ni (1.2%) is isobaric with the major zinc isotope and also isobaric with a magnesium argide. The three remaining Ni isotopes including 6°Ni (21.16%) are free of isobaric interferences from magnesium argide polyatomic ions. Copper: ~SCu (30.91%) is isobaric with a magnesium argide giving a BEM of 0.14 pg. However, the major copper isotope (63Cu) was free of magnesium argide polyatomic ions. Zinc: Both 64Zn (49.8%) and 66Zn (27.8%) are isobaric with magnesium argides giving BEMs of 0.68 pg and 0.18 pg, respectively. Of the remaining Zn isotopes, ~4Zn (48.9%) is isobaric with a minor Ni isotope and 7°Zn is isobaric with a Ge isotope leaving 67Zn (4.1%) and 68Zn (18.56%) free of interference. 3. 7.1.3 Nickel nitrate Table 3.23 lists the oxide and argide polyatomic ions that can be formed when nickel nitrate chemical modifier is vaporized into an argon plasma. When heated, nickel nitrate decomposes to form the oxide which is subsequently either thermally decomposed to elemental nickel or rapidly reduced by carbon during the high temperature step. The formation of nickel oxide in the plasma produced isobaric interferences with two isotopes each of Ge and Se and nickel argides produced isobaric interferences with two isotopes each of Mo and Ru. Background equivalent masses for these isobaric interferences are given in Table 3.24. Germanium: A nickel oxide polyatomic ion is isobaric with 74Ge (36.53%), the major Ge isotope. This isotope is isobaric with a minor Se isotope of 0.96% abundance. ~SGe (7.76%) which was interfered by a nickel oxide is also isobaric with a minor Se isotope of 9.12% abundance. Of the three remaining Ge isotopes, ~2Ge (27.43%) and 7~Ge (7.76%) were free of isobaric interferences. 72Ge, having the higher isotopic abundance is recommended for use when determining this element in the presence of nickel. Selenium: A nickel oxide is isobaric with 743e (0.96%) which is also isobaric with the major Ge isotope of 36.53% abundance. A second nickel oxide interfered with 768e (9.12%) which is isobaric with 7GGe 398

TABLE 3.23 Possible major interfering polyatomic ions produced when using nickel nitrate chemical modifier (from Ref. [52] with permission) m/z of Polyatomic ion

Isobaric analyte isotopes (abundance)

Nickel oxides 74 76

Ge (36.5), Se (.96) Ge (7.8), Se (9.1)

Nickel argides 98

Mo (23.8), Ru (2.2)

100

Mo (9.6), Ru (12.7)

TABLE 3.24 Background equivalent analyte mass for spectral interferences produced when vaporizing i pg of nickel nitrate chemical modifier (from Ref. [52] with permission) Analyte

Background equivalent mass (pg)

74Ge

0.22

76Ge

0.40

74Se

8.4

768e

0.34

9SMo

43.1

l°°Mo

41.9

98Ru

464

l°°Ru

31.6

(7.76%). Of the four remaining isotopes of Se, only 778e (7.5%) is free of isobaric interferences. S°Se (49.97%), the major Se isotope, is isobaric with the intense argon dimer. Both Se isotopes at m/z of 78 and 82 daltons are isobaric with Kr isotopes. Krypton is often present as a contaminant in the Ar supply with intensities ranging to hundreds of 399

counts/s for the more a b u n d a n t isotopes. Since 7SSe is more a b u n d a n t (23.61%) t h a n S2Se (8.84%) and since 7SKr is only 0.35% abundant compared to 11.56% for S2Kr, then 7SSe is a second isotope which can be used for the determination of this element in the presence of Ni. Molybdenum: Nickel argides are isobaric with 9SMo (23.78%) and l°°Mo (9.63%). Both of these isotopes are isobaric with minor Ru isotopes. Of the remaining five Mo isotopes, only 95Mo (15.72%) and 91Mo (9.46%) are free of isobaric interferences. Ruthenium: 9SRu (2.22%) and l°°Ru (12.7%) are isobaric with nickel argides. Both of these Ru isotopes are also isobaric with Mo isotopes. Of the remaining five Ru isotopes, 99Ru (12.81%) and l°lRu (16.98%) are free of isobaric interferences. The nickel oxides observed from the use of nickel nitrate chemical modifier were likely formed in the plasma and not vaporized as oxides from the surface of the graphite tube. This was verified by increasing the charring temperature from 300°C to beyond 1000°C, a temperature at which carbon reduction of the oxide readily takes place. No change in the nickel oxide intensity was observed with increasing charring.

3.7.1.4 Palladium nitrate Table 3.25 lists the polyatomic ions t h a t can be formed when palladium nitrate chemical modifier is vaporized into an argon plasma. The relatively large number of species t h a t can arise is due to the large number of Pd isotopes each having significant natural abundances. Therefore, use of palladium nitrate as a chemical modifier can have a potentially considerable impact on a fairly large number of analytically useful isotopes. No oxides of palladium were detected when vaporizing 1 ~g and 10 pg quantities of Pd (as nitrate) chemical modifier. Pd argide polyatomic ions, however were detected and these are isobaric with six isotopes of Nd, three isotopes of Sin and one Ce isotope. The BEMs determined for these polyatomic ion interferences are given in Table 3.26. Neodymium: All but one of the seven Nd isotopes is interfered by a palladium argide. 143Nd (12.14%) is free of argide isobaric interferences and also free of isobaric interference from isotopes of other elements. A second isotope, 142Nd, has an abundance of 27.4% and a relatively low BEM of 0.27 pg. 142Nd can be used for Nd determinations in the presence of Pd only if sample concentration levels are well above concentrations corresponding to 0.27 pg in 20 ~1 of sample solution. 400

TABLE 3.25 Possible major interfering polyatomic ions produced when using palladium nitrate chemical modifier (from Ref. [52] with permission) m/z of Polyatomic ion

Isobaric analyte isotopes (abundance)

Palladium oxides

118 120 121 122 124 126

Sn (24.1) Sn (33), Te (0.09) Sb (57.3) Sn (4.7), Te (2.5) Te (4.6), Sn (5.98) Te (18.7), Xe (0.09)

Palladium argides

142 144 145 146 148 150

Ce (11.1), Nd (27.1) Nd (23.8), Sm (3.1) Nd (8.3) Nd (17.3) Nd (5.7), Sm (11.2) Nd (5.8), Sm (7.4)

TABLE 3.26 Background equivalent analyte mass for spectral interferences produced when vaporizing 1 pg of palladium nitrate chemical modifier (from Ref. [52] with permission) Analyte

Background equivalent mass (pg)

142Nd 144Nd 145Nd 146Nd 148Nd

0.27 3.6 25.0 14.4 43.0

15°Nd

22.0

1443m 1483n~ 15°Sm

27.0 21.6 16.4

142Ce

0.65

401

Samarium: Three of the seven Sm isotopes are isobaric with Pd argides. 1473m (14.97%) and 1493m (13.8%) are free of Pd argide interference and are not isobaric with isotopes from other elements. 152Sm (26.26%) is the major Sm isotope and is also free of Pd argide interference, however, this isotope is isobaric with 152Gd(0.2%). For samples low in Gd (relative to Sm), 152Sm can be used for analysis providing appropriate Gd corrections are made. Cerium: 142Ce(11.1%) is isobaric with a Pd argide. 14°Ce (88.48%) is free of isobaric interference and as the major Ce isotope is the analyte isotope of choice. It is clear from the recent ETV-ICP-MS literature t h a t the judicious selection and use of chemical modifiers is key to the successful application of this technique. Most of the elements which suffer interference from modifier polyatomic ions have suitable isotopes t h a t are free from any isobaric interferences or have at least one isotope with a BEM close to the limit of detection. However, the loss of available free isotopes may affect the determination of isotope ratios or the use of isotope dilution as a calibration strategy. The above discussion shows t h a t the use of microgram quantities of a variety of chemical modifiers results in the formation of only oxide and argide polyatomic ions. The concentration levels of these interfering species correspond in most cases to relative concentrations of analyte in the parts-per-trillion range assuming the use of a 50 pl sample aliquot. The use of in-situ pre-concentration techniques such as repetitive sampling (and drying) prior to vaporization or the deposition of gaseous hydrides in the graphite furnace m a y reduce even further the influence of these polyatomic ions. It is well known t h a t the selection of plasma parameters and to some extent instrumental operating conditions of the quadrupole mass spectrometer can influence the degree of oxide and argide formation. Therefore, it is possible t h a t instrumental operating conditions other t h a n those used to obtain the above results [52] could reduce (or increase) the BEM values for isobaric interferences reported for each chemical modifier.

3.7.2 Analyte transport efficiency Electrothermal vaporization as a means of sample introduction for ICP mass spectrometry has been successfully used for over fifteen years. It was clear, even in early studies, t h a t the mass transport efficiency was

402

250

200

Curve b

w

~

150

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100

50

C.~ea

0 ~

0

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Analyte Mass, ng Fig. 3.12. Effect of physical carrier on ETV-ICP-MS calibration curves. Curve a: analyte alone. Curve b: analyte with carrier.

not quantitative and that a significant fraction of vaporized analyte remained within the ETV system or transfer tubing and did not reach the plasma. Electrothermal vaporization ICP-MS signals increased in intensity when nanograms of analyte were co-vaporized with microgram quantities of an added salt, such as NaC1. A second effect, observed and reported almost simultaneously in the literature for both filament type [47] and tube-type [46] ETV systems, was the non]inearity of calibration curves when analyte was vaporized alone. Upon the addition of a salt, linear calibration curves were obtained along with a general enhancement of signal ranging from 20% to about a factor of ten, depending upon analyte and experimental conditions. This effect is illustrated in Fig. 3.12. The similarity of results obtained from the two different vaporizer systems indicated that incomplete analyte transport was not a problem specific to an ETV design, but 403

rather an effect present (to various degrees) in all ETV systems. Ediger and Beres [46] interpreted the non-linearity of calibration curves and incomplete mass transfer using a model derived by K~intor [105] to describe similar effects in ETV ICP atomic emission studies. Kfintor [105] first used the theoretical prediction (Eq. 1) to describe the transport of analyte vapour from the ETV surface to the argon plasma as being dependent on the formation of stable nuclei of size exceeding a critical diameter whose value is given by the equations: d p = 4(~kV?n S

(1)

where, dp is the critical diameter, (~ is the surface tension of the liquid droplet, Vm, is the molecular volume of the vapour species, k is the Boltzmann constant, T is the temperature in Kelvin and S is the saturation ratio. The saturation ratio is a measure of the magnitude of analyte supersaturation and is given by: S - Pwp

(2)

P(T)

where Pvap, is the partial vapour pressure (atm) and pe(T) is the equilibrium vapour pressure at the temperature of nucleation. For ETV sample introduction, as the vaporization surface increases in temperature and sample vaporization takes place, the partial vapour pressure of the analyte in the argon carrier gas stream increases. The vapour pressure of the analyte can be approximated by the following relationship:

p~p = NsT~R / V~t~

(3)

where Ns is the number of moles ofvapour from the evaporated sample, Tg is the initial temperature of the carrier gas (in Kelvin), R is the gas constant (cm 8 atm K -1 tool-l), Vg is the flow rate (cm 3 s -~) of the carrier gas at the initial temperature Tg, and tv is the mean evaporation time (s) of the major fraction of the sample. The above equations show that, as the partial vapour pressure of the analyte increases, the saturation ratio increases and the critical size shifts to smaller particles. This results in an increase in the concentration of clusters and stable nuclei. This increase in stable nuclei is much steeper than a linear increase in S; therefore, as the mass of the analyte in the vapour is increased, the formation of stable 404

particles increases non-linearly, which accounts for the shape of the analytical calibration curve observed when no carrier is present. In other words, at very low analyte mass, the concentration of atoms or clusters in the vapour state is insufficient to promote self-nucleation or homogeneous condensation. In this case, particles are lost to cold containment or conduit surfaces before they can be transported to the argon plasma or other m e a s u r e m e n t device. At higher analyte vapour concentrations, the efficiency of formation of transportable (by the argon stream) particles increases at a faster rate than the vapour state concentration of analyte, resulting in non-linear analytical calibration Curves.

The presence of a foreign substance, such as a matrix component, also serves to provide nucleation and condensation sites (heterogeneous nucleation) for analyte atoms or molecules and thus foreign substances improve the transport efficiency of analyte by providing a 'carrier' for the analyte. The presence of sufficient concentrations of analyte or matrix component vapour provides for the production of adequate numbers of transportable particles capable of carrying all or most of the analyte to the argon plasma. Kfintor [105] concluded that the efficiency of formation of dry aerosols is one of the major contributing factors determining the mass transport efficiency of analyte from the vaporization surface to the argon plasma. A number of physical carrier systems have been suggested for use in ETV-ICP-MS, including NaC1 [46] palladium [49] and diluted seawater [67] stripped of trace metals. A detailed study of the use of mixed carriers, including seawater and its components, was completed by Hughes et al. [43], wherein it was shown that the addition of carrier not only served to linearize calibration curves, but also normalized analyte transport from the ETV to the plasma. Once a very small quantity of carrier was added (i.e., micrograms), analyte signal was enhanced by up to a factor of 5. Further additions of carrier did not result in corresponding enhancements and analyte signal remained constant until a point was reached when additional carrier resulted in ion signal suppression in the mass spectrometer system. Once the initial carrier was added, calibration curves remained linear even when analyte signal was suppressed at higher carrier additions. These results indicate that vaporized analyte is likely occluded (co-precipitated) within condensed particles of the carrier material. Analyte transport efficiency in ETV-ICP-MS was first reported by Park et al. [9] using a platform-type ETV device. Solutions containing 405

1000 ~g m1-1ofCu, Pb, Cd, Fe and Ni were vaporized from the graphite ETV surface and collected in a 60 ml bottle containing 1.5 g of loosely packed cotton wool. Fifteen replicate valorizations of 2 111 of analyte solution were completed for each analytical run. The average analytical recovery of analyte found in the cotton wool was about 84%, which corresponds to the amount of analyte that would reach the plasma under the same experimental conditions. Using a tungsten furnace ETV device, Nonose et al. [44] concluded that analyte signal enhancement observed when analyte was vaporized in the presence of halidecontaining modifiers was due to increased ionization of analyte within the plasma and was not a result of any physical or carrier effects. In more recent studies, Sparks et al. [50,51] examined both the particle size distribution of sample transported from a tube-type ETV device to the argon plasma [51] and the retention of sample in the transport tubing [50]. These authors [51] used laser scattering techniques to count particles and concluded that the majority of particles produced were in the 0.1 t~m to 0.2 l~m range and that the number of particles produced increased by about 25% when NaC1 was used as a carrier. The use of a laser system necessitated the application of instrumental conditions, in particular, gas flows, which are not typical of ETV-ICP-MS analytical conditions. Additionally, the majority of particles measured were emitted from the graphite tube at a time when all analyte would be completely vaporized and had been swept out to the argon plasma. Using Ag as a test element, Sparks et al. [50] reported that about 10% of the analyte was retained on the walls of the transfer tubing. For these experiments, from 10 to 60 lal of 10 lJg m1-1Ag was vaporized from the ETV surface, which is a quantity of metal orders of magnitude greater than would ordinarily be encountered in ETV-ICP-MS experiments. Langer et al. [107] reported that vaporization of high masses (20 pl of 100 l~g m1-1) of Ag from a graphite ETV system resulted in the agglomeration of micro-particles, suggesting homonucleation as the primary particle formation mechanism in ETVICP-MS. Particles were captured from the Ar carrier gas using a thermophoretic collection technique. Fonseca et al. [108] used oxygen ashing and Pd modifier to improve analyte transport efficiency, possibly by increasing the number of carbon particles produced during the vaporization step, which could act as a physical carrier for analyte. Kfintor [109] recently reported on the efficiency of graphite furnace ETV sample introduction into argon plasmas. By dissolving deposited sample material from the transport tube, a transport efficiency 406

between 67% and 76% for the medium volatility elements (Cu, Mg, Mn) and between 32% and 38% for the volatile elements Cd and Zn was found. Using a modified commercially available ETV system (with electrostatic collection of the aerosol vaporized), Buchkamp and Hermann [110] found that the transport efficiency for Pb increased with increasing mass of vaporized Pb up to about 6 ng. For a sample containing greater than 6 ng of Pb, the transport efficiency was about 20%. Transport losses were measured by Barth et al. [111] for analytes vaporized as slurries using a tungsten coil ETV device. These authors used radio-tracers to determine the fate of vaporized elements and showed that transport losses ranged from 7% for La to 54% for Cu. For the 12 elements studied, transport losses decreased with increasing boiling point of the element or the oxide. A more recent study by Sch~iffer and Krivan [112] measured transport efficiencies for eight elements in an ETV-ICP-AES device using a mixture of radiotracers. The authors reported transport efficiencies ranging from 26% for Cr to 40% for K. However, as for many studies discussed above, analyte (radiotracer) masses used were orders of magnitude greater than normally determined by ETV-ICP-MS and were high enough to act as physical carriers. Gr~goire and Sturgeon [48] reported results for the quantitative determination of absolute transport efficiency in electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICPMS) for the Perkin-Elmer HGA-600MS electrothermal vaporizer. The absolute transport efficiencies for Mo, In, T1 and Bi were determined using experimental conditions typical of those applied to real analysis by ETV-ICP-MS. Experiments using an on-line filter trapping apparatus indicated that particles produced by the ETV device were smaller than 0.1 pm in diameter. The nature and condition of the ETV graphite surface, the length of the transfer tube, and the effect that diluted seawater and palladium matrix modifiers have on analyte transport efficiency were investigated. Transport efficiency was comparable for all elements studied and was enhanced with previously used, rather than new, graphite tubes and when seawater and palladium carriers were present. When analyte was vaporized without carrier from a new graphite tube, the transport efficiency to the plasma was approximately 10%. About 70% of the total amount of analyte vaporized was deposited within the ETV switching valve, 19% onto the transfer tubing and 1% onto the components comprising the torch assembly. These conditions however, represent the 'worst case scenario', with 407

analyte transport to the plasma increasing to about 20% or more with the addition of carrier. Analyte transport losses in ETV-ICP-MS are substantial, amounting to approximately 75% of the analyte vaporized from the graphite tube. These losses, however, do not limit the applicability of the technique to quantitative analysis. The published literature currently contains in excess of 70 papers detailing fully quantitative applications of ETV-ICP-MS to the analysis of real samples. Careful selection of experimental conditions, including the use of carriers and chemical modifiers, normalizes analyte transport for both samples and standards, making accurate calibration possible.

3.8 NON-SPECTROSCOPIC INTERFERENCES Non-spectroscopic or so-called matrix interferences are well known in ICP-MS [113]. Since electrothermal vaporization is only an alternative source of analyte vapour/aerosol for the ICP-MS, clearly the same factors affecting matrix interferences in ICP-MS will also affect ETVICP-MS. In fact, the situation may be worse in ETV-ICP-MS because of the higher sample transport efficiency characteristic of the technique. For example, a comparison of the amount of salt reaching the plasma for SN and ETV from a 1000 1Jg m1-1 illustrates this point. If for SN, a sample uptake rate of 2 ml min -1, a mass transfer efficiency of 2% and a sampling time of 2 s is assumed, then 667 ng of NaC1 will reach the argon plasma. If for ETV a 10 pl sample is used and a mass transfer efficiency of 25% and a vaporization time of 2 s is assumed, then a total of 2500 ng of NaC1 reaches the plasma. On the surface it would appear that the ETV loads the plasma with salt by a factor of 3.8 greater than does SN, however in reality the situation is somewhat worse than this. Electrothermal vaporization produces a transient signal and the maximum rate of vaporization occurs at the centre of this signal and for a period of time much shorter than the total signal time. Because of this, the amount of NaC1 reaching the plasma during the evolution of the central part of the vaporization signal is perhaps 20 times greater than during the equivalent period for a steady-state sample introduction technique such as SN. Therefore, matrix effects can be expected to be serious and to change in severity during the lifetime of the analyte signal. Gr6goire et al. [31] showed that when Os is determined in the presence of NaC1 (Table 3.27), loss of analyte signal intensity was more 408

TABLE 3.27 Effect of NaC1 on 192Osion count rate (from Ref. [31] with permission); concentration of 192Os= 100 ng g-1 NaC1 concentration (pg gl) -

lOOO 2000 3000 5000 7000

Solution nebulization

Electrothermal vaporization

1.oo 1.o0 0.95 0.91 0.83 0.73

1.o0 0.96 0.82 0.53 0.37 0.34

severe for ETV t h a n for SN sample introduction, when both systems were operated u n d e r optimum conditions. For example, when Os was determined in the presence of 5000 ~g m1-1 NaC1, the SN signal was suppressed by only 17%, whereas for the ETV, Os in the same solution was suppressed by 63%. P a r k et al. [8] reported on the effect of added Na, Cr, Ca and Se on ETV-ICP-MS signals for As, Cd and Cu. These authors found that 1 #g

of concomitant vaporized with ng masses of analyte resulted in significant signal suppression. The argon dimer is a polyatomic ion produced within the argon plasma or the interface region of the ICP-MS and is known to behave in a similar m a n n e r to analyte ions with respect to non-spectroscopic interferences [114]. The argon dimer has found some use as an internal s t a n d a r d reference ion [91] in the de t e r m i na ti on of trace elements in marine sediments. Alary and Salin [115] also found t h a t the argon dimer as well as other argon polyatomic ions could be used for plasma diagnostics in ETV-ICP-MS. Figure 3.13 illustrates the effect of added concomitant salts on the shape of the Ar dimer ETV-ICP-MS signal. The concomitant compounds are ar r an ged in order of increasing atomic mass to show the effect of concomitant mass on non-spectroscopic interferences. For NaC1, the volatilization of up to 20 ~tg did not appreciably change the shape of the argon dimer signal. W hen using Mg(NQ)2, masses in excess of 10 micrograms caused an e n h a n c e m e n t in the argon dimer signal. Nickel n itra t e caused a similar e n h a n c e m e n t w hen more t h a n 0.5 pg were used. At masses g r e a t e r t h a n 3 l~g of Ni(NQ)2, serious 409

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5

6

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Time, Seconds

Fig. 3.13. Effect of matrix component mass on argon dimer signal (from Ref. [52] with permission).

argon dimer signal suppression was observed. The use of Pd, the concomitant with the greatest atomic mass, resulted in argon dimer signal suppression for all masses above 0.5 pg and complete suppression of the signal at a mass of 10 pg. Clearly, both the atomic mass of the analyte and the absolute quantity of concomitant present during the vaporization step have an important influence on observed matrix effects. Although these curves can only show in a qualitative manner the effect of concomitants on analyte signals, the trends observed may be useful in predicting the quantity of concomitant that ETV-ICP-MS can tolerate. Figure 3.14 shows the analyte signal obtained for 10 pg of Sn 410

2500

2000

1500 0

1000

500

I

I

I

i

1

2

3

4

Time,

5

Seconds

Fig. 3.14. ETV-ICP-MSsignal for 10 pg of Sn vaporizedwith 10 pg ofPd. Arrowindicates location of signal maximumfor Pd (fromRef. [52] with permission). vaporized in the presence of 10 pg of Pd. The arrow in Fig. 3.14 indicates the location of the peak of the Pd ETV-ICP-MS signal (not shown) where the Sn intensity is completely suppressed. As the temperature of the vaporization surface increased during the high temperature ETV step, Sn was volatilized first, followed by Pd. As the vaporization step progressed, the quantity of Pd entering the gas-phase increased and caused the Sn signal to be suppressed resulting in what appears to be a double-peak. Care should be exercised when interpreting the n a t u r e of a double analyte signal especially when concomitants are present during vaporization. These results also show t h a t in most cases no more t h a n 0.5 pg to 1 pg of chemical modifier or physical carrier should be added to sample. This conclusion is supported by the results of Hughes et al. [43] who showed t h a t with the addition of more t h a n I pg of a mixed physical carrier, analyte signals for m a n y elements were suppressed. A non-spectroscopic interference of importance to ETV-ICP-MS is related to the expansion of gases within the ETV during the high temperature vaporization step. The ETV is essentially a flow-through cell with an argon flow restricted by the diameter of the injection tube within the plasma torch. The diameter at the exit of the injection tube 411

350000

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'--- J '" (.)

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i

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1

2

3

4

5

6

Time, seconds

Fig. 3.15. Effect of sample matrix on argon dimer signal: (a) no sample; (b) organic sample; (c) high salt sample.

is generally about 2 m m whereas the diameter of the graphite tube and the P T F E tr an s f er line is about 6 mm. During rapid heating of the ETV gas expansion occurs and a pressure wave is t r a n s m i t t e d t hrough to the argon plasma. The net effect of this is a change in the back-pressure within the P TF E t r a n s f e r line which increases the velocity of gas leaving the injector tube which in t u r n changes the sampling depth at the plasma mass spectrometer interface. This process occurs during the first 0.5 s to 3 s into the vaporization step. After this period of time, the system recovers to the normal or pre-set argon carrier gas flowrate. Similar gas expansion effects can also occur during the vaporization of matr ix components. Figure 3.15 shows argon dimer signal profiles [69] obtained for samples vaporized as solids. Curve a was obtained for a blank (no sample) vaporization; curve b was for a sample high in organics (lobster hepatopancreas) and curve c was for an organic sample high in salt (total diet reference material). Curve a shows a normal argon dimer profile whose intensity is changing along with the Ar gas pressure inside of the graphite tube. Curve b shows a large m a x i m u m at about 2.5 s at which time the organic matrix is volatilized 412

and curve c shows an initial drop in sensitivity caused by suppression from NaC1 vaporization followed by a net enhancement of signal, which coincides with the vaporization of the organic components of this sample. This figure shows that the nature of the sample matrix can also have a profound effect on the sensitivity of ETV-ICP-MS.

3.9 APPLICATIONS There are currently over 80 published papers detailing applications of ETV-ICP-MS. Table 3.28 contains a list of these applications and also includes information on the analytes determined and the sample preparation method used. A cursory examination of Table 3.28 clearly shows the versatility of ETV-ICP-MS to solve many analytical problems that cannot be easily handled by other techniques. Of particular interest are the applications dealing with direct solids analysis or the analysis of solids introduced into the ETV as a slurry. Nearly 60% of ETV-ICP-MS applications involve the direct analysis of samples with little or no sample preparation required. Banman [185] and Voellkopf [186] discussed some of the advantages and report on the direct analysis of solids by ETV-ICP-MS and the field was recently reviewed by Darke and Tyson [187]. The work of the Ghent University group [132,134,145,161,177,180,182] on the direct analysis of solids is noteworthy and is recommended reading especially for those interested in the direct solids analysis of biological materials. These researchers have written an extensive book chapter on solid sampling ETV [188] as applied to ICP-AES and MS. A recent review on the use of hydride generation techniques in ETV-ICP-MS was published by Olson et al. [189]. A new book edited by Montaser [190] on ICP mass spectrometry contains detailed information on the principles and applications of the technique including sections on electrothermal vaporization sample introduction. The remainder of this part of the chapter is devoted to a more detailed presentation of four applications from this laboratory. For all of these studies a Perkin-Elmer HGA-600MS ETV interfaced with a Perkin-Elmer ELAN 5000a ICP mass spectrometer was used. The methods discussed were selected to show the versatility of ETV-ICP~ MS in several areas of specialized application including ultra-trace analysis [67], the analysis of small samples [68], solids introduced as slurries [69] and the determination of isotope ratios [66]. 413

TABLE 3.28 Applications of electrothermal vaporization ICP mass spectrometry

Sample type

Analyte(s)

Sample preparation mixed acid, fusion

Ref.

Geological materials

Pb isotope ratios

Biological materials

As,Cu,Mn,Pb,Rb,V, Zn,Ag mixed acid

8

Geological materials

Mo,W

fusion

10

Geological materials

TI

mixed acids, fusion, solvent extraction

11

Geological materials

Pt,Pd,R, Ir

nickel sulphide fusion

13

Blood plasma

Fe isotope ratios

direct determination

22

Geological materials

Ru,Pd,Ir

mixed acid, ion exchange

14

Silicon dioxide

U,Th

mixed acids

19

Blood plasma, urine

Te

direct determination

21

Iridosmines

Os isotope ratios

vapour generation, fusion

15

Petroleum

Hg

direct determination

23

Iron and steel

Bi

mixed acids

17

Snow

T1,Cs,Pb,Mn,Co,V, Cu,Ni,Cd,Cr

direct determination

67

Waste oils

C1 as polychlorinated biphenyls

dilution with xylene

24

Biological materials, coal

Ni,Cr,Cu,Pb,Mn,Co

direct determination, slurry sampling

69

Zircons

U,Th, rare earth elements HF digestion in sealed bombs 68

Blood, liver, kidney

Cd and Zn isotope ratios

mixed acid digestion, ion exch.

66

Fresh waters

Pd,Pt

activated charcoalpre-conc.

116

Photoresist

Na,Fe

dilution with organic solvent 117

Photoresist

Na,Fe,Mn,Ni,Cu

dilution with organic solvent 118

Organic materials, sediments

extractable organic chlorine

solvent extraction

119

Blood plasma

Pb

direct determination

120

Semiconductor materials

AI,Ca,Cd,Cr,Cu,Fe, Ga,In,Mg, Mn,Ni,Pb,Si,Sn,Zn

direct determination

121

Semiconductor materials

A1,Cu,Fe,Na,Ni,Pb, Zn

direct determination

122

Basalt

Hg,Pb,Cu,Ag

direct determination

123

414

5

Sample type

Analyte(s)

Sample preparation

Ref.

Silicon wafer surface

Na,Mg,A1,Ti,Cr,Mn ,Fe,Ni,Co, Cu,Zn

HF/peroxide surface etch

124

Fly ash, total diet

As,Cd,Pb

direct determination

125

Biological materials

As,Sb,Se,Bi,Sn

hydride generation/in-situ trapping

126

Urine

As,Bi,Te

hydride generation]in-situ trapping

127

Rain and river waters

A1,V,Mn,Fe,Ni,Cu, Zn,As,Se,MoCd,Sb, Pb

direct determination

128

Trichlorosilane

P,As

CuC1 treatment, evaporation 129

Sludge

Hg

direct determination

130

Biological materials

Cd

direct determination

131

Plant samples

As

direct determination

132

Plant samples

As

direct determination

133

Sea lettuce Silicon wafer surface

As Cu,Zn,Pb

direct determination HF etching

134 135

Water, basalt

As,Be,Cd,Cr,Cu,Rh ,Sb

direct (water) mixed acid (basalt)

136

Pure acids

Fe,Cu,Ni,Zn

direct determination

137

Steel

S

acid digestion, solvent extraction

138

Calcite, seawater Water

V 99Tc,23Su,236U,232Th, 2 3OTh,226Ra

acid digestion, ion exchange 139 direct determination 140

Lubricating oils

metallo-organic A1,Mg,Fe,Y

direct determination

Seawater

Co,Cu,Mn,Ni,V

direct determination

Sediments

Cu,Cd,Pb

Biological materials Plants, soils Airborne particles

Se,As Cd Cr,Fe,Mn,Cu,Zn,Sr ,Cd,Sb,Ba, Pb 9°Sr

Lake sediment

141

142, 143 direct determination, slurry 144 sampling direct determination direct determination direct determination

145 146 147

aqua regia digestion

148

Blood plasma

A1,Ti,V

direct determination

149

Biological materials

Cu,Cd,Pb

nitric acid

150 continued

415

TABLE 3.26 (continuation) Sample type

Analyte(s)

Sample preparation

Ref.

Fresh and saline waters

Hg,Bi

vapour generation/in-situ trapping

151

Silver alloy

Au,Bi,Cd,Pb,Sb,Sn, Zn

in-tube acid digestion

152

Waters

As,Sb,Se

Fresh and saline waters Seawater

As

direct determination direct determination

153 154 155

Seawater

V,Mn,Fe,Co,Ni,Cu, Zn,Mo,Sb

complexation/solvent extraction complexation/solvent extraction

Fish samples

Hg

Biological tissues

direct determination/slurry 158 sampling direct determination/slurry 159 A1,Ba,Co,Cr,Cu,Fe, Li,Mg,Mn, Na,Pb,Sr,U,Zn sampling

Quartz

V,Mn,Fe,Co,Ni,Cu, Zn,Mo,Sb

156

direct determination/slurry 157 sampling

inorganic and total Hg

direct determination direct determination

160 161

As,Sb

hydride generation/in-situ trapping

162

Water, urine

As,Se,Pb

High-purity iron

P

163 164

Geological materials

Ra

Sulfamic acid Urine

Li,Fe,Cd,Pb

direct determination acid dissolution/solvent extraction direct determination/slurry sampling direct determination direct determination acid/peroxide digestion

168

column chromatography direct determination

169 170

Nickel alloys

Bi,Pb,Te

Botanical samples, sludge

Cd,Se

Synthetic samples

Body fluids, tissue

Hg lanthanide elements

Seawater

Cu,Cd,Pb,Bi,Se(IV)

Seawater Urine Biological materials

Mn,Mo,U

Soils

416

Cd,Pb As,Se,Te,Ag,Cr,Cu, V,Ni,Mn,Co,Cs,Pb, Sb,Sn,Bi Cd,Hg,Pb

165 166 167

direct determination 171 direct determination/slurry 172 sampling direct determination/slurry sampling

173

Sample type

Analyte(s)

Sample preparation

Ref.

Fish tissue

Cu,Zn,Cd,Pb

174

Aluminium oxide Fish tissue

Cr,Cu,Ga,Fe,Mg,M n,Na,V, Zn As,Se,Hg

Biological materials, soils Serum Sediments

thermal decomposition products Se Se

direct determination/slurry sampling direct determination/slurry sampling direct determination/slurry sampling direct determination

Photographic materials Waters, biological materials Plastics Plant materials

Ru U, Th

Waters

Mo. U and B

Pd Ge, As, Se, Cd and Pb

direct determination acid dissolution, citric acid modifier direct determination acid dissolution, fusion, online solid phase extraction direct determination slurry sampling direct determination direct determination

175 176 177 178 179 180 181 182 183 184

3.9.1 U l t r a t r a c e a n a l y s i s o f A r c t i c s n o w H e a v y m e t a l c o n c e n t r a t i o n s of n e a r s u r f a c e Arctic snow t y p i c a l l y r a n g e f r o m 0.5 (Cd) to 20 (Pb, Zn, Ni, Cu) p g g-1 T h e s e levels pose cons i d e r a b l e c h a l l e n g e s for t h e r e p r e s e n t a t i v e s a m p l i n g of this m a t e r i a l as well as for t h e a n a l y s i s for t h e m e t a l s of i n t e r e s t . S n o w s a m p l e s a n a l y s e d in this s t u d y w e r e collected f r o m a n ice flow n o r t h of E l l e s m e r e I s l a n d , N.W.T., C a n a d a . All s a m p l e s w e r e collected f r o m t h e s u r f a c e snow l a y e r a n d t a k e n f r o m s m a l l 'pits' a p p r o x i m a t e l y 30 cm deep. A p r e - c l e a n e d p l a s t i c scoop w a s u s e d to t r a n s f e r s n o w f r o m t h e s a m p l i n g face into t h e s a m p l e c o n t a i n e r s , w h i c h w e r e two-litre wide m o u t h s c r e w c a p p e d p o l y e t h y l e n e bottles. All s a m p l e p r e p a r a t i o n o p e r a t i o n s w e r e c o n d u c t e d in class 100 clean r o o m facilities e q u i p p e d w i t h class 10 f u m e c u p b o a r d s . T h e frozen snow s a m p l e s w e r e t h a w e d u n d e r a h e a t l a m p in t h e i r original bottles, acidified to p H 2 b y addition of h i g h p u r i t y H N Q a n d weighed. An aliquot of e a c h s a m p l e w a s t h e n t r a n s f e r r e d to a 30 m l p r e - c l e a n e d p o l y e t h y l e n e s c r e w c a p p e d bottle for s u b s e q u e n t a n a l y s i s b y E T V - I C P 417

TABLE 3.29 Analysis of SLRS-2 standard reference water by ETV-ICP-MS (from Ref. [67] with permission) Concentration (ng g-l) Element

Determined

Certified Value

Pb Cd Cu Ni Mn Co Cr V Cs T1

0.123 _+0.004 0.035 + 0.005 2.77 _+0.19 0.93 _+0.03 11.5 + 0.4 0.071 + 0.004 0.465 _+0.008 0.308 _+0.013 0.016 _+0.003 0.0028 _+0.0013

0.129 + 0.011 0.028 _+0.004 2.76 _+0.17 1.03 + 0.10 10.1 _+0.3 0.063 _+0.012 0.45 _+0.07 0.25 _+0.06 -

MS. The r e m a i n i n g portions were reweighed, t r a n s f e r r e d to precleaned P F A b e a k e r s a n d e v a p o r a t i v e l y c o n c e n t r a t e d (73-fold on average) in a class 10 hood to a p p r o x i m a t e l y 10 ml volumes. This w a s accomplished over a period of 6-12 h u s i n g sub-boil conditions. O p e r a t i n g conditions for the E T V - I C P - M S were as given in Table 3.7. Five pl of purified N A S S - 3 (diluted 1:500) m i x e d physical c a r r i e r were a d d e d to all s a m p l e s a n d s t a n d a r d s . D a t a acquisition w a s done u s i n g p e a k - h o p p i n g mode w i t h a dwell time of 20 m s per isotope w i t h one r e a d i n g per p e a k (m/z). Five isotopes (analytes) were d e t e r m i n e d d u r i n g a single E T V signal m e a s u r e m e n t cycle. The n u m b e r of isotopes m o n i t o r e d was limited to a s s u r e a c c u r a t e m e a s u r e m e n t of t h e a n a l y t e signal profile. Table 3.29 c o m p a r e s results o b t a i n e d for t h e d e t e r m i n a t i o n of 10 a n a l y t e s in N a t i o n a l R e s e a r c h Council of C a n a d a Reference River W a t e r , SLRS-2. Excellent a g r e e m e n t b e t w e e n certified v a l u e s a n d d e t e r m i n e d v a l u e s were o b t a i n e d for all elements. To f u r t h e r verify t h e a c c u r a c y of E T V - I C P - M S for the a n a l y s i s of snow, t h e s e a u t h o r s [67] used G F A A S w i t h w h i c h to c o m p a r e t h e i r results. Table 3.30 r e p o r t s on t h e d e t e r m i n a t i o n of nine e l e m e n t s in Arctic snow by both G F A A S a n d ETV-ICP-MS. A n a l y s i s of t h e s e m a t e r i a l s by G F A A S could only be done u s i n g c o n c e n t r a t e s w h e r e a s 418

TABLE 3.30 Analysis of Arctic snow by ETV-ICP-MS (from Ref. [67]) Sample

1 2 3 4 5

Concentration (ng g-l) Mn

Cd

0.212 (0.18) 0.087 (0.13) 0.139 (0.17) 0.054 (0.093) 0.0008 (0.0013)

0.018 0.09 (0.015) (0.12) 0.024 0.053 (0.0056) (0.069) 0.008 0.066 (0.0083) (0.084) 0.0014 0.0022 (0.0018) (0.030) 0.0006 0.014 (0.0017) (0.022)

Cu

Pb

Ni

V

0.321 (0.270) 0.161 (0.035) 0.241 (0.20) 0.034 (0.042) 0.029 (0.031)

0.053 (0.041) 0.040 (0.030) 0.038 (0.037) 0.005 (0.014) 0.013 (0.007)

0.084 0.0070 0.0022*

T1

Cs 0.0041"

0.053 0.0057 0.051 0.0007 0.0018 0.0021" (0.0013)* 0.005 0.0014 0.00024 0.0006* (0.00027)* 0.004 0.0011 0.00028

Values in parentheses obtained from GFAAS analysis of concentrates (x72). *Data from analysis of concentrates (x72). w i t h few exceptions, all E T V - I C P - M S d e t e r m i n a t i o n s w e r e completed on s a m p l e s at t h e i r n a t u r a l levels. Table 3.30 shows excellent agreem e n t b e t w e e n E T V - I C P - M S a n d G F A A S r e s u l t s for m o s t elements. For V, Co, Cs a n d T1 in some samples, G F A A S did not h a v e a d e q u a t e s e n s i t i v i t y even for c o n c e n t r a t e s . R e p o r t e d limits of detection for E T V - I C P - M S were in the fg (10 -15 g) r a n g e : 29, 57, 86, 120, 140, 360, 420, 470, 870 a n d 3200 for T1, Cs, Pb, Mn, Co, V, Cu, Ni, Cd a n d Cr, respectively.

3.9.2 Trace a n a l y s i s o f s i n g l e z i r c o n s for REE, U a n d Th The t r a c e e l e m e n t composition of zircons is of i n t e r e s t to geologists as a n aid in i d e n t i f y i n g the source of t h e s e d i m e n t a r y rock in w h i c h t h e y were formed. Single zircons typically h a v e m a s s e s in the p g range. The m u l t i - e l e m e n t q u a n t i t a t i v e a n a l y s i s of s a m p l e s this small is a chal]enge p a r t i c u l a r l y for a n a l y t e s o c c u r r i n g in t h e n g g-1 c o n c e n t r a t i o n range. Zircons were p r e - c l e a n e d by a i r - a b r a s i o n w i t h p y r i t e to r e m o v e t h e o u t e r l a y e r s of t h e crystals. The clearest c r a c k a n d inclusion-free crystals were selected for a n a l y s i s by h a n d - p i c k i n g in ethyl alcohol, a n d t h e n leached for a b o u t 30 m i n in w a r m h i g h - p u r i t y 3 M H N Q . P r i o r to weighing, the crystals were r i n s e d in h i g h - p u r i t y w a t e r followed by 419

acetone and then dried. The zircons were weighed in aluminum boats on a Mettler micro balance (_+ 0.2 pg) and then loaded into clean PTFE microcapsules. Samples (0.1 mg) of powdered zircon reference material BCS 388 were weighed into identical microcapsules. Approximately 0.2 ml of high purity 28.9 M H F and 0.02 ml of high purity 16 M H N Q were added to all of the microcapsules which were then placed in a Parr Instrument Co. (Moline, Illinois) steel-jacketed PTFE bomb. Highpurity H F and H N Q (10:1, v/v; total volume about 5 ml) were also added to the bomb to equalise the vapour pressure between the bomb and the microcapsule. The bombs were placed in an oven and heated at 220°C for two days. After cooling, the samples were evaporated to dryness prior to adding about 0.2 ml of 3.1 M HC1 to the microcapsules. Five ml of 3.1 M HC1 were also added to the bomb before replacing in an oven at 210°C for about 12 hours. The clear HC1 solution was then poured onto clean 50 ml polymethyl pentene beakers which had been equilibrated with double-distilled 8 M HC1 and rinsed with high-purity water. The beakers were then placed on a warm hotplate and the HC1 solution allowed to dry. When dry, the beakers were removed from the hotplate and allowed to cool. The samples were then taken up in 0.3 M H N Q prior to analysis by ETV-ICP-MS. For micro-sample analysis, the dissolved and dried down single zircon crystals and the BCS 388 sample were taken up to a volume of 500 1~1.For the 100 mg sample of BCS 388, the sample was dissolved in a similar fashion to the single zircons. Optimization of plasma and mass spectrometer experimental conditions was accomplished using solution nebulization sample introduction. Only small adjustments in the argon carrier gas flow (_+ 50 ml min -1) were required to optimize signal output in the ETV mode. The peak hopping mode of data acquisition was used with a l-ms dwell time for each isotope monitored. Single (one m / z per mass) sequential measurements were made for each analyte isotope on a continuous basis from the start of the vaporization cycle until the signal returned to baseline values. Because of the significant tailing effect exhibited for several elements (La, Ce, Nd, Y, U), peak height measurements were recorded for each ETV-ICP-MS signal produced. Table 3.31 summarises analytical results for the analysis of BCS 388 zircon reference material. Concentration values obtained for microsample (150 ~g) analysis (first 3 rows), the analysis of a bulk sample (row 4) and reference values (row 5) for comparison purposes are 420

TABLE 3.31 Analysis of BCS-SRM No. 388 Zircon (from Ref. [68] with permission) Technique

Sample Concentration (~g/g) mass Y La Ce

Nd

Sm

Yb

Th

U

ETV-ICP-MS 150~g (ext. calib.) ETV-ICP-MS 150~g (std. addition) FI-SN-ICP-MS 150 pg (micro sample) SN-ICP~MS 100 mg (bulk sample)

973_+30 24.9_+0.8 63-+1 30.5_+0.513_+3

234_+10 173_+13 337_+17

937_+24 24.7_+0.7 61_+1 29.9_+0.513-+3

226_+10 165_+17 322-+16

1083_+33 37_+1

95_+3 39_+1

Reference value*

1071+71 30_+10

80_+1040_+10 10-+10 300_+100158_+18 288_+34

-

954_+28 26.0_+0.8 65-+2 30.5-+0.913.0_+0.4 231_+7 165_+5 319_+10 12A_+0.4227_+7 165_+5 334_+10

*Bureau of Analyzed Samples Ltd., Newham Hall, Middlesborough, England. Revised certificate, October, 1992. Standard error on reference values for La, Ce, Nd, Sin, Yb taken as _+smallest significant figure reported (concentrations expressed as percent on certificate); Values for Y, Th and U are certified values; ETV = electrothermal vaporization; FI = flow injection; SN = solution nebulization sample introduction.

p r e s e n t e d . B C S 388 is t h e o n l y r e f e r e n c e z i r c o n m a t e r i a l a v a i l a b l e for which there are reported trace-element data. F o r m i c r o - s a m p l e a n a l y s i s , t w o c a l i b r a t i o n s t r a t e g i e s for E T V - I C P M S w e r e u s e d a s w e l l a s a flow i n j e c t i o n m e t h o d ( F I ) for s o l u t i o n nebulization ICP-MS analysis of the remaining sample solution from t h e E T V d e t e r m i n a t i o n s . T h e flow i n j e c t i o n t e c h n i q u e i n v o l v e d t h e nebulization of 400 pl (entire sample solution). Data acquisition commenced one second before the sample entered the spray chamber and w a s t e r m i n a t e d 10 s e c o n d s a f t e r t h e f i n a l v o l u m e o f s a m p l e s o l u t i o n reached the nebuliser. T h e r e s u l t s i n T a b l e 3.3 1 s h o w e x c e l l e n t a g r e e m e n t b e t w e e n v a l u e s obtained by both external calibration and the method of standard addition indicating no matrix effects on the analyte signal intensity c a u s e d b y t h e p r e s e n c e o f Zr. E x c e l l e n t a g r e e m e n t b e t w e e n t h e s e values and the FI-SN-ICP-MS were also obtained which provides an i n d e p e n d e n t c h e c k o n t h e E T V r e s u l t s for t h e a n a l y s i s o f m i c r o samples. D a t a for t h e b u l k s a m p l e (100 m g ) d e t e r m i n e d b y S N - I C P - M S a g r e e w i t h m i c r o - a n a l y s i s v a l u e s w i t h i n t h e s t a t e d e r r o r s for Y, Yb, T h a n d U 421

b u t are not in a g r e e m e n t w i t h La, Ce, N d a n d Sin. This m a y be a r e s u l t o f i n h o m o g e n e o u s distribution of some of t h e e l e m e n t s in the r e f e r e n c e zircon powder. Note t h a t all of the a n a l y t e s for which micro and b u l k v a l u e s agree are those of h i g h e s t a b u n d a n c e i n d i c a t i n g a g r e a t e r degree of h o m o g e n e i t y for t h e s e elements. C o n c e n t r a t i o n v a l u e s o b t a i n e d by SN-ICP-MS for t h e b u l k s a m p l e c o m p a r e well w i t h the r e f e r e n c e l i t e r a t u r e v a l u e s for all of t h e e l e m e n t s d e t e r m i n e d . This provides f u r t h e r evidence of i n h o m o g e n e i t y at the pg sample size scale as well as v a l i d a t i n g t h e r e s u l t s r e p o r t e d for t h e b u l k s a m p l e a n a l y s e d by SN-ICP-MS. A selection of single zircons from suites previously d a t e d u s i n g highprecision U-Pb geochronology w e r e p r e p a r e d for analysis as described above a n d a n a l y s e d u s i n g ETV-ICP-MS. The ICP-MS was c a l i b r a t e d using a curve p r o d u c e d from aqueous s t a n d a r d s to which was a d d e d NASS-3 (5 pl) physical carrier. Results for t h e analysis of single zircons are given in Table 3.32 S a m p l e m a s s e s r a n g e d from 7.7 pg (Sample 4) to 75.8 pg (Sample 6). Limits of detection for the m e t h o d w e r e a d e q u a t e for all of t h e e l e m e n t s studied w i t h the exception of L a on t h e low m a s s samples. Gr~goire et al. [68] r e p o r t e d t h a t a n a l y t e c o n c e n t r a t i o n s w e r e m e a s u r e d w i t h a precision of a b o u t 6%. Limits of detection for the analysis o f a 10pg zircon w e r e 150 ng g-1 for Y, 90 ng g-1 for La, 115 ng g-i for Ce, 65 ng g-1 for Nd, 180 ng g-i for Sm, 22 n g g-1 for Yb, 190 n g E i T h and 80 n g g-1 for U. Absolute limits of detection for a 10 pl solution aliquot r a n g e d from 4 to 36 fg. TABLE 3.32 Analysis of single zircons by ETV-ICP~MS(from Ref. [68] with permission) Sample

1 2 3 4 5 6

422

Mass (pg) 19.8 12.4 11.6 7.7 8.2 75.8

Concentration (pg g-i) Ce

La

Sm

Nd

Th

Y

Yb

U

21.4 30.9 11.9 10.3 3.3 2.5

<0.05 <0.07 <0.08 <0.12 <0.11 0.07

1.7 2.4 0.84 2.3 1.4 1.6

1.1 1.5 0.54 2.1 0.81 1.7

163 227 149 57.1 33.4 606

371 473 361 393 281 379

115 148 150 92.0 71.3 130

284 273 270 192 68.0 890

3.9.3 D i r e c t a n a l y s i s o f s o l i d s i n t r o d u c e d as s l u r r i e s

The direct analysis of solids and/or slurries by any analytical technique offers advantages over more conventional sample preparation. Among these advantages are the reduced sample preparation time, the reduced possibility of sample contamination, increased sensitivity (no dilution), decreased likelihood of analyte loss through volatilization prior to analysis and the selective analysis of micro-amounts of solids. Slurry sampling offers additional advantages for samples that occur naturally as slurries such as milk and blood and for the completion of surveys where the analysis of large numbers of samples such as contaminated soils is required. Further, slurry sampling combines the benefits of solid and liquid sampling and permits use of conventional liquid sample handling apparatus such as autosamplers. Gr~goire et al. [69] reported on the direct analysis of solid (as slurries) materials including coal, lobster hepatopancreas and total diet reference materials and this application will be summarized here. The same experimental conditions as for the previous two applications were used for these experiments with the exception that the drying step time was increased to 50 s and the pyrolysis temperature changed to 400°C (for 50 s). Slurries were prepared by taking a weighed portion of reference material (sample) and placing it in a 20 ml plastic centrifuge tube. A diluent consisting of 5% Ultrex (Baker) H N Q containing 0.005% Triton X-100 was added to the sample and mixed on a vortex mixer. An aliquot of sample slurry was then immediately withdrawn from the centrifuge tube using an Eppendorf pipette and placed into a clean Teflon auto-sampler cup. In preparing the slurries prior to analysis, care was taken to ensure that an appropriate ratio of sample/diluent was used by taking into account the particle size and the density of the reference material in order to minimize effects arising from poor sampling statistics. The coal (NIST 1632a) slurry was prepared by mixing 4 mg of sample per 4 ml of diluent. The density of the coal was approximately 0.7 g/cm s and the particle size was apparently less than 50 ~m. A 20 pl aliquot of sample slurry would require a minimum of 0.08 mg/ml of coal to give 50 particles, ensuring a representative sampling of the bulk slurry. The NIST diet (SRM 1548) was prepared in a similar fashion except that 400 mg of sample was suspended in 4 ml of diluent. With a density of about i g/cm 3 and a particle size of less than 250 ~m, only 20 mg/ml 423

w a s r e q u i r e d to e n s u r e t h a t e a c h 2 0 p l a l i q u o t c o n t a i n e d 50 p a r t i c l e s . T h i s c r i t e r i o n w a s m e t w i t h a c o n c e n t r a t i o n of 100 m g / m l . The NRC LUTS-1 lobster hepatopancreas reference material was p r o v i d e d as 10.3 g of h o m o g e n e o u s s l u r r y . T h e m a t e r i a l w a s d i l u t e d b y a d d i n g ( i n a v o l u m e t r i c f l a s k ) d i l u e n t to a t o t a l v o l u m e of 100 ml. T h e s u s p e n s i o n w a s s o n i c a t e d for 30 r a i n i n a n u l t r a s o n i c b a t h a n d , f o l l o w i n g m i x i n g o n a v o r t e x m i x e r , a i m l a l i q u o t of t h e d i l u t e d L U T S slurry was removed using an Eppendorfpipette and placed in a Teflon a u t o s a m p l e r cup. A l t h o u g h n o d a t a a r e a v a i l a b l e c h a r a c t e r i z i n g t h e a c t u a l p a r t i c l e size of t h e L U T S m a t e r i a l , w e b e l i e v e i t to be less t h a n 50 ~ m a n d c e r t a i n l y less t h a n 100 p m . L o o k i n g a t t h e g u i d e l i n e s for m i n i m u m m a s s , o n l y 1.3 m g / m l w a s n e e d e d for 100 p m p a r t i c l e s of a d e n s i t y of 1 g/cc. to p r o v i d e t h e m i n i m u m 50 p a r t i c l e s p e r 20 p l analytical sample aliquot.

TABLE 3.33 Analysis of Reference Materials by Ultrasonic Slurry Sampling ETV-ICP-MS (from Ref. [69] with permission) Concentration (pg/g) Cu

Cr

Ni

Pb

Mn

N.I.S.T. Coal SRM 1632a Ext. Cal.

16.8 ± 0.1

22.7 ± 2.8

20.0 ± 3.5

11.0 ± 0.5

30.8 ± 2.2

Certified

16.5 ± 1.0

34.4 ± 1.5

19.4 ± 1.0

12.4 ± 0.6

28_+2

0.59 ± 0.03

0.013 ± 0.001

N.R.C. LUTS-1 lobster hepatopancreas Ext. Cal.

0.17 _+0.02

0.45 _+0.07

M. of Addn.

0.048_+ 0.006 0.081 ± 0.013

0.159_+ 0.009

0.007 ± 0.001

Certified

0.051 ± 0.006 0.079 ± 0.012

0.200 ± 0.034

0.010 ± 0.002

0.75 ± 0.02

0.025 + 0.005

N.I.S.T. Total Diet SRM 1548 Ext. Cal.

2.1 ± 0.2

0.28 ± 0.02

M. of Addn. Certified

2.9 ± 0.2 2.6 ± 0.3

0.11 ± 0.02 0.29 ± 0.02 0.094 ± .0141 (0.41)2 0.30 ± 0.013

0.045 ± 0.006 (0.05)

1Acid digestion, GFAAS determination. 2Certificate information values. 3Acid digestion, solution nebulization ICP-MS determination. 424

Table 3.33 summarizes the results obtained for the analysis of ultrasonic slurries by ETV-ICP-MS. For coal, excellent agreement between found and certified values was obtained with the exception of Cr. The authors [69] report that even the application of the method of standard addition did not give the correct result indicating that Cr remained either locked in the coal matrix or was not totally volatilized from the graphite tube. For both the lobster hepatopancreas and the total diet samples, excellent agreement was obtained between found and certified values. Because of the large quantities of matrix components (organic or salt) present during the high temperature vaporization step, suppression of analyte ion signals occurred. For these two reference materials, the use of the method of standard addition was required (except for Pb in lobster hepatopancreas) to obtain accurate results. These results were in excellent agreement with certified values. Lead in the lobster reference material could be determined using external ca]ibration because Pb volatilized before the sample matrix components avoiding plasma/interface suppression effects. Using reported ETV-ICP-MS limits of detection (section 3.3) for the analytes studied, the following limits of detection, in ng/g, were calculated for a slurry sample size of 2 mg: Co, 0.070; Cu, 0.21; Cr, 3.2; Mn, 0.060; Ni, 0.24 and Pb, 0.0212. 3.9.4 D e t e r m i n a t i o n o f Cd i s o t o p e r a t i o s in sheep's b l o o d a n d organ tissue Cadmium is a non-essential metal which because of its toxic properties has been widely studied in small animals and humans. However, the few studies reported for ruminants are based on the intake of Cd at concentrations much higher than those encountered by grazing animals. The main impediment to the completion of more realistic studies involving whole body infusion of enriched stable isotopes has been the lack of adequate analytical techniques capable of measuring Cd isotope ratios in small samples at concentration levels below 0.2 ng/g. This problem was addressed by Gr~goire and Lee [66] using ETV-ICP-MS. Plasma or red cells (7 g) were prepared by wet digestion with a minimum quantity (10 ml) of concentrated HNO3. Digestions were performed in a Milestone MLS 1200 microwave oven system. After bringing the digest to near dryness on a hotplate, the sample was made up to 10 ml with 1% HNO3. Liver and kidney samples were prepared in an analogous manner using 0.4 g of freeze-dried sample. 425

Prior to the determination of Cd isotope ratios by ETV-ICP-MS, both analytes were separated from matrix components using off-line column chromatography. The acidity of the prepared blood or organ tissue digest was adjusted to a pH of 6.0 with ammonium hydroxide before being passed through a I cm ~ column containing silica-immobilized 8-hydroxyquinoline. The resin was eluted with 10 ml of 1 M HNO3-0.1 M HC1 and evaporated to dryness in the microwave oven. The residue was dissolved in 0.2 ml of 1% H N Q . For the determination of Cd isotope ratios, a pre-concentration factor of 35 was achieved increasing the concentration of Cd in solution from approximately 0.2 ng/g in plasma to 7 ng/g in the analytical sample. The fast-transient nature of the ETV-ICP-MS signal requires a relatively short duty cycle to ensure good precision of the isotope ratio measurement. Using a dwell time of 20 ms, approximately 40 intensity readings were obtained for each isotope during the high temperature vaporization step. For ETV-ICP-MS determinations, 10 pl of NASS-3 (diluted 1 in 500) physical carrier was added to both standard and sample solutions. I n s t r u m e n t a l mass discrimination for both ETV and solution nebulization work was reported to be generally less t h a n the precision of the isotope ratio m e a s u r e m e n t itself. Correction for mass discrimination was accomplished using as reference the mean of ten separate ETVICP-MS isotope ratio measurements for a pure aqueous standard of Cd. Comparison of this value with the accepted value for the n a t u r a l abundance ratio was used to calculate the appropriate correction factor. Table 3.34 summarizes analytical results for the determination of Cd isotope ratios in a standard reference solution. Isotope ratios were determined on 200 pg of Cd in the presence of NASS-3 physical carrier. Isotope pairs were selected to include both the reference and spike isotope used in the actual sheep study. A third isotope for each element was also selected such t h a t a comparison could be made of the precision of measurement for isotope ratios ranging from unity to some higher value. For Cd, peak-height measurements gave poorer precision t h a n ratios calculated using area counts. The isotope ratio for 111Cd/ll°Cd (1.035) was measured with a precision which was about 1% better t h a n the precision of the lnCd/l°6Cd (10.54) isotope ratio. Tables 3.35 summarizes results for the determination of lnCd/l°6Cd isotope ratios in a number of representative plasma, red cell and organ 426

TABLE 3.34 Precision of Cd isotope ratio m e a s u r e m e n t by ETV-ICP-MS (from Ref. [66] with permission) Run

111Cd/11oCd

111Cd/l°6Cd Integrated

Peak height

Integrated

Peak height

1

10.33

10.37

1.013

1.007

2.

10.56

10.06

1.038

1.027

3.

10.75

10.97

1.034

1.049

4.

10.45

10.64

1.033

1.012

5.

10.90

10.69

1.019

1.007

6.

10.27

10.94

1.011

1.044

7.

10.07

9.84

1.053

1.051

8.

10,58

10.38

1.057

1.065

9.

10.94

11.03

1.060

1.051

10.

10.55

10.53

1.030

1.033

Mean

10.54

10.54

1.035

1.035

s.d.

0.27

0.39

0.017

0.021

RSD

2.61%

3.71%

1.67%

2.03%

TABLE 3.35 Cadmium isotope ratios in sheep's blood and organ tissue (from Ref. [66] with permission Sample

111Cd/l°6Cd

Plasma (2 h)

1.531 _+0.028

Plasma (4 h)

1.263 _+0.045

Plasma (8 h)

0.987 _+0.014

Plasma (17 h)

0.758 _+0.045

Plasma (24 h)

0.691 _+0.003

Plasma (50 h)

0.624 _+0.018

Plasma (56 h)

0.621 + 0.030

Plasma (120 h)

0.615 _+0.017

Red cells

1.639 _+0.033

Kidney

0.313 _+0.003

Liver

0.070 _+0.001

427

tissue samples. These results demonstrated t h a t the 111Cd/1°6Cdisotope ratio in blood and organ tissues varied from 0.070 to 1.531 during the course of the infusion study. For plasma samples, the Cd isotope ratio decreased by 60% during the study. A measurement precision of 2% for the illCd/l°6Cd isotope ratio was more t h a n adequate to accurately track the change in this ratio with time. A limit of detection for Cd in blood of 0.35 pg g l was reported.

3.10 CONCLUDING REMARKS Today, ETV-ICP-MS is a well developed and proven fully quantitative technique. The contents of this chapter clearly demonstrate the potential and the promise of this technique for solving some of the most challenging analytical problems. Since electrothermal vaporizers are attachments to ICP mass spectrometers, the capabilities of ETV-ICP-MS will grow along with new developments in ICP-MS instrumentation such as the development of high resolution systems based on magnetic sector mass spectrometers with multi-collector detectors and systems based on time-of-flight mass spectrometers [191,192]. These will provide the opportunity for ETV-ICP-MS to be used for the simultaneous multielement determination of large numbers of elements in very small samples and for the high-precision measurement of isotope ratios. Because of its versatility and its potential, I am confident t h a t ETVICP-MS will take its place among other spectroscopic techniques [193] and be widely used by analysts everywhere.

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