Direct powder introduction inductively coupled plasma atomic emission spectrometry with a photodiode array spectrometer

Specfrochrmrcu A+ Printed

in Great

Vol. Brttain

46B. No.

11. pp.

149%1515.

1991 Q

05x4-8547191 $3.(K) + 1991 Pergamon Press

00

pk.

Direct powder introduction inductively coupled plasma atomic emission spectrometry with a photodiode array spectrometer*

K. NIMALASIRIDE SILVA Mineral Resources Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Canada KlA OE8

and ROGER GUEVREMONT National Research Council of Canada, Institute for Environmental Canada KlA OR6 (Received

Chemistry, Montreal Road, Ottawa,

16 January 1991: accepted 14 May 1991)

Abstract-A direct powder introduction inductively coupled plasma (DPI-ICP) atomic emission system has been coupled to a PLASMARRAY (LECO Inc.) photodiode array spectrometer. Chelex-100 samples

containing spikes of elements ranging from 11 ppm to 1470 ppm have been used to evaluate the precision of element to element ratios, and for the estimation of the detection limits of 11 elements. Detection limits for Cd (0.11 ppm), Hg (1.7 ppm), SC (2.1 ppm), Y (1.8 ppm) and Cu (3 ppm) were calculated for introduction of dry Chelex-100 powder. The precision of the ratio of emission intensities of Ni, Sn, Co, Mn and Y to the emission intensity of In (considered to be the internal standard element) were 4%, 2%, 2%, 2% and 3% relative standard deviation respectively, for 15 repeat 10 s integration measurements while introducing Chelex-100 into the plasma. Mixtures of a geological reference material (including NIM-D, NIM-G, NIML, NIM-N, NIM-P, or NIM-S) and Chelex-100 (1:3 by weight) were used to evaluate system performance with complex samples. It was considered impractical to carry out measurements on these samples with more than a few slots of the multielement spectral preselection mask open. The relative standard deviation (R.S.D.) of the ratio of elements contained in the geological sample vs those on the Chelex-100 were between 2% and 16%. The R.S.D. of the ratio of elements wherein both were found on the geological material was between 2 and 8%. Detection limits for Ni (7 ppm), Cr (25 to 85 ppm) and Y (1.5 ppm) were estimated for the geological materials (1:3 by weight with Chelex-100).

1. INTROXJCM~N THE DIRECT powder

introduction inductively coupled plasma (DPI-ICP) atomic emission spectrometric analysis offers promise for the rapid determination of trace elements in difficult matrixes including coal, refractory oxides and geological materials. Among the important advantages of this approach are the cost savings realized by reduction of the complexity of sample preparation. Simplified sample preparation also minimizes the possibility of contamination, and loss of analyte. Several approaches for the direct introduction of dry powdered solids for inductively coupled plasma atomic emission spectrometric measurements have been reported. In these methods the solid particles are generally carried into the plasma as a gas/solid aerosol [l-8]. Various methods of forming the aerosol have been described. A fluidized-bed powder delivery system (5, 81 has been described. The device is capable of operation from about l-50 mg/min powder transport rates without alteration of the flow of argon (for example 400 ml/min) to the plasma. This hardware has made it possible to study several important aspects of delivery of solid materials into a plasma. These experimental parameters have included: (a) hardware performance and analyte emission characteristics [5, 81; (b) internal standards approaches for reduction of noise when sampling particles [6]; (c) experimental assessment of the degree of measurement uncertainty arising from sample heterogeneity and from particle statistics at low flow rates of sample [9]; (d) effect of particle size on emission intensity [lo]; and (e) assessment of the particle size segregation inherent to the sampler [ll]. * NRC No 32914, GSC No 11591 1499

1500

K.

NIMALASIRI DE SILVA

and R.

GUEVREMONT

Table 1. Instrumentation and operating conditions Pliotodiode array spectrometer: PLASMARRAY, LECO Inc. Preselection polychromator: 0.5 m focal length, dispersion 3.45 nm/mm Mask slot widths: 0.005 inch Echelle polychromator: 31.6 groove/mm, resolution 0.0090 nm FWHM @ 300 nm Dispersion at photodiode array: 0.0022 nmldiode @ 300 nm Wavelength range: 190 to 415 nm Plasma-Therm HFP-2500 rf generator, 27.12 MHz Rf power: I.8 kW Coolant gas flow: 14 I/min argon Auxiliary gas flow: 1 Umin argon Sampler gas flow: 30 ml/min argon Carrier gas flow: 400 ml/min argon Powder flow rate: l-10 mg/min

In this communication we describe DPI-ICP emission measurements with a photodiode array spectrometer. In previous work we have reported some of the major drawbacks of analysis of powders by photomultiplier-based direct reading spectrometers. The most important drawback has been the inability to simultaneously measure the peak and off-peak background emission of analyte elements. In addition to problems with quantitative measurement, this also leads to difficulty in collection of wavelength spectra in the vicinity of emission lines of analyte and internal standard elements (to assess potential interference problems). The latter can be difficult to acquire if the flow rate of sample is non-uniform. In general, a spectrometer based on array detection [12-141 is an ideal sensor for emission measurements in DPI-ICP. Since existing hardware for transport of the powder to the ICP cannot deliver the sample with the flow rate uniformity achieved with liquid nebulization techniques, the accuracy and precision of measurements made on a powder can only approach that of liquid introduction if peak emission and offpeak background emission measurements are recorded simultaneously. The versatility of the measurement system is further enhanced if the emission spectrum in the vicinity of several analyte and internal standard elements can be recorded simultaneously. With existing technology the photodiode array is less sensitive than a photomultiplier. Although the sensitivity will fall short of that of a photomultiplier-direct-reading spectrometer when samples are introduced by conventional liquid nebulization, this reduced sensitivity is not necessarily disadvantageous in DPI-ICP. The total mass of sample carried into the plasma with powder introduction can be considerably higher than is possible with conventional liquid introduction methods. Note, however, that the total mass of solvent plus sample in conventional liquid introduction is comparable (or higher) than the total mass of sample introduction in DPI-ICP. Good sensitivity for analyte detection therefore can be achieved with direct introduction of powders by virtue of elimination of the sample dissolution step. High efficiency of volatilization of the solid is possible if: (a) the total load of material transported to the plasma remains lower than that used in conventional liquid nebulization; (b) the particle size is below 20 pm; and (c) the flow rate of powder is uniform.

2. EXPERIMENTAL 2.1. Spectrometer Emission measurements were made with a photodiode array spectrometer (PLASMARRAY, LECO Inc.) viewing an inductively coupled plasma source (Plasma Therm, Kresson, NJ, U.S.A.). The characteristics of the photodiode array spectrometer and the instrumental operating conditions appear in Table 1.

Powder sampling ICP PHOTODIODE

1501

ARRAY

photodiode array spectrometer. A single emission wavelength is shown to appear on the photodiode array in two locations (consecutive

Fig. 1. Schematic of the optics of a PLASMARRAY orders).

The optical design of a photodiode array spectrometer is illustrated in Fig. 1. With a multielement spectral preselection mask installed, such as MEM-24 (LECO Inc.), narrow wavelength regions (Table 2) are transmitted through the slots in the mask, recombined into quasi-white light, further dispersed by an echelle grating, and fall onto the photodiode array. The echelle grating operates in the high orders, and several images of each wavelength range may fall onto the array if consecutive orders are not separated more widely than the length of the array.

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Photodiodr Pixel Fig. 2. Pixels of the photodiode array exposed through each of the preselection slots in LECO mask MEM24. The range of exposed array was evaluated experimentally with fluidized-bed DPI-ICP of a 1:3 (by weight) mixture of geological material NIM-P and Chelex-100. Analyte emission wavelengths nominally isolated by this preselection mask appear in Table 2.

K. NIMALASIR~ DE SILVAand R. GUEVREMONT

1502

Table 2. Elements and wavelengths selected by the multielement spectral preselection mask MEM24 Element

Wavelength

Zn Pb Cd Cd/As Ni

213.856 220.353 226.502 228.807 231.604

As Si Hg Mn Fe

234.984 251.611 253.652 257.610 259.940

Cr Mg Sn In Ca

267.716 279.553 283.999 303.936 317.933

(nm)

Element CU Na co Ar SC

Y Mg Ca Al

Wavelength

(nm)

324.754 330.237 345.350 355.431 361.384 371.030 383.826 393.366 396.152

Ranges of the photodiode array which are exposed by individual slots in the mask during fluidized-bed introduction of geological material NIM-P appear in Fig. 2.

A multielement preselection mask may include slots cut for several analyte emission lines. Usually the simultaneous use of more than a few wavelength regions cannot be accommodated without partial overlap on the photodiode array. Figure 2 illustrates the photodiode array pixels which are exposed when various of the preselection mask (MEM-24) slots are open. This aspect of the spectrometer performance is considered in Section 4.1. 2.2. Powder sample delivery system The operation of the fluidized-bed powder delivery system, and preliminary evaluation for determination of trace elements in geological samples has been described [8]. This device is capable of uniform flow (l-50 mg/min) of dry solid particulate to a plasma. Moreover, any flow rate of powder within this range is available at a constant, selected (for example 400 ml/min), total gas flow to the plasma. During the design of the sampler it was considered important to separate the processes of formation of the aerosol from the process of transport of the aerosol to the ICP. This was achieved by independent control of the “sampler gas flow” rate and the “carrier gas flow” rate. In addition, the operations of aerosol formation and mixing of the aerosol with a carrier gas occur in different locations within the sampler. With this approach, for example, powder sample flow rates could be adjusted from below 1 mg/min to above 50 mg/min, with a constant total gas flow to the plasma. The sampler was also designed in a manner which ensured that the plasma was in a constant, stable operating state before powder was introduced. It was considered important that the flow of powder should be started and stopped via an electrical control, rather than by alteration in the flow rates of gas. In the present fluidized-bed sampler the aerosol formation only occurs during mechanical motion of the sample holder. For practical operation, the gas flows through the sampler and to the plasma are allowed to stabilize, after which the powder flow is initiated by computer control. This ensures that all of the experimental measurements are made under stable, reproducible conditions. The device for the delivery of powder samples and the associated gas flow controls are illustrated in schematic form in Fig. 3. Gas entry and exit ports to the device are labelled (A) through (E). The powder sample is contained in a removable glass vial (6), 1.5 cm internal diameter. The steps necessary to exchange sample vials include: (a) open gas port (D) which vents the gas cylinder (10) to allow chamber (9) to move away from the o-ring seal (5); (b) slip the sample vial (6) from the holder (7) to which it was held by o-rings; (c) put a new sample vial over the holder; and (d) introduce gas into port (D) whereupon the chamber (9) again moves into contact with o-ring (5) thus forming a gas-tight seal. Argon is introduced into the sampler through ports (A) through (C). Gas flow (C) can be

Powder sampling ICP

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Fig. 3. Schematic of the fluidized-bed powder sampler. Typical sampler gas flows (port B) and carrier gas flows (port A) are 30 and 400 mlimin. Port C is closed during sample delivery. Pressurized gas fed to chamber IO via port D keeps the sampler closed during sampte delivery.

used to flush out chamber (9) during the sample change procedure, but no gas is introduced to (C) while the sample is being delivered to the plasma. Argon (about 30 ml/mm) is introduced into port (B) during sample delivery, and since no other exits for the gas are open in chamber (9) the gas must flow up the capillary tube (4) (internal diameter about 0.3 mm). The capillary passes through a gas tight, but removable, rubber plug (3). Sample introduction begins when the solenoid (8) is activated (oscillation frequency between 1 and 10 Hz). The sample is suspended in the gas by the shaking motion, and a small portion carried up the capillary (4) by the gas flow. The gas flow entering port (B) and which consequently carries the sample particles out through capillary (4) is referred to as the “sampler gas flow”. An argon gas flow of approximately 400 mUmin (adjusted for optimum operation of the plasma) is introduced into port (A). This gas is mixed with the gas/powder aerosol from capillary (4), and flows out through the capillary (1) (diameter about 2 mm). This gas flow maintains the material in suspension, and carries the powder to the plasma. This gas flow is referred to as the “carrier gas flow”. The powder sampler and its gas control system was designed to eliminate the necessity for any mechanical flow-diverting valves along the flow path of the gas/powder mixture, and yet to allow the sampler to be opened for change of sample without disruption of the operation of the plasma. During the period of time which the sampler is opened to the atmosphere, the gas flow entering port A will divide into two streams, one of which flows toward the plasma, and the other which flows through the capillary (4) and into the sample vial (6). This isolates the plasma from the sample change process, although the flow rate of gas to the plasma will decrease during the sample change period. Gas flows entering ports B, and C help to minimize the entry of air into the sampler. Since the flow of gas out of the chamber (9) during the delivery of powder to the plasma is about 10% of the total carrier gas flow, effects caused by contamination of the sampler with air have been observed to be negligible. 2.3. Sample preparation 2.3.1. Chefex-100 samples. The operating parameters and detection limits of the system were evaluated with synthetic samples. With these materials it was possible to evaluate the precision of element to element ratios (for internal standard calculation). It was also possible to evaluate detection limits; (a) with samples wherein the blank and standards are based on a common matrix; and (b) with samples wherein both the blank and standards contain a set of common, added internal standard elements at constant concentration. Chelex-100 (Bio-Rad Laboratories, Richmond, CA) particle size ZOO-400 mesh (hydrated form) was extracted with acid. The sample was rinsed thoroughly with water and converted to the ammonium form with ultrapure ammonium hydroxide (Ultrex Ultrapure Reagent, J. T. Baker, Phillipsburg, NJ). The excess ammonia was rinsed from the Chelex-100, and the powder divided into three portions. The first portion (sample “A”) was retained for measurement of the blank. The second portion (sample “B”) was reacted with Ni, Co, Mn, Y, Sn and In. The third portion (sample “C”) was reacted with Hg, Pb, SC, Cd, Zn, Cu and Cr.

1504

K. NIMALASIRI DE SILVAand R. GUEVREMONI Table 3. Composition of Chelex-100 samples “B” and “C”, and of aqueous samples “B”aq and “C”aq Sample “B”

Element

Concentration (ppm)

Chelex-100 samples Y 65.5 Mn 192 co 614 In 1473 Sn 1470 Ni 592 Aqueous solutions Y

6

Mn co In

20 60 150

Sn

1.50

Ni

60

Sample “c”

Element

Cr cu Hg Cd Pb Zn SC Cr

cu Hg Cd Pb Zn SC

Concentration (ppm)

39 362 1188 13 674 11.2 187 4

40 120 1.5

70 1.0 20

The supernatant solutions were analysed by emission spectrometry to determine the efficiency of chelation to the Chelex-100 resin. In addition, a portion of each Chelex-100 sample was extracted with acid, and the extract analysed to provide the concentration of analytes on the resin. The composition of samples B and C are summarized on Table 3. From the samples of Chelex-100 described above, three mixed samples were prepared. First, two types of “blanks” were prepared. These were 1:l mixtures by weight of samples “A” mixed with “B” (sample “AB”), and of “A” mixed with “C” (sample “AC’). Similarly, sample “BC” was prepared by 1:l mixture of “B” and “C”. The detection limit of several elements in aqueous solution was measured with an experiment analogous to that used for the Chelex-100. The compositions of the aqueous solutions “B”aq and “C’aq are shown in Table 3. The samples “AB”aq and “AC”aq were prepared by 1:l dilution of “B”aq and “C’aq with deionized water. The mixed sample “BC”aq was prepared by 1:l mixture of solutions “B”aq and “C’aq. As a consequence of the above preparation method, the detection limit (in Chelex-100 or aqueous solution) of the “B” group of elements could be calculated using the sample “AC” as blank and sample “BC” as the standard for a single point calibration for estimation of the measurement sensitivity. The “C” group of elements was common to both samples, and served to provide a selection of internal standard elements (if needed). Similarly, the detection limits of the “C” group were calculated using emission intensity measurements for the “AB” and “BC” samples. 2.3.2. Geological materials. Geological materials NIM-N (norite), NIM-D (dunite), NIM-L (lujavarite), NIM-P (pyroxenite), NIM-S (syenite) and NIM-G (granite) were obtained from the National Institute of Metallurgy, South Africa. The geological materials were mixed 1:3 by weight with Chelex-100 (sample “C”, described above). The Chelex-100 served two purposes [8]. First it served as a carrier for the micron sized particles of minerals. The finely divided powders are not free flowing, but can be fluidized readily when mixed with free flowing dry Chelex-100. Secondly, the Chelex-100 served as a substrate for addition of internal standard elements. The measurements of the emission of internal standards are used in analytical calculations to avoid the requirement for quantitative assessment of the rate of flow of powder into the plasma. The NIM geological materials are complex, and contain very high concentrations [15, 161 of potentially interfering elements including zirconium, chromium, nickel and iron. These samples have been chosen for this investigation because they are unusually difficult geological matrixes. Table 4 summarizes the literature values for the concentrations of several elements in these geological samples.

Powder sampling ICP Table 4. Concentrations NIM-D Percent SO, TiOz Al,03 Fe,O, Fe0 MnO MgO ppm co Cr cu Ni Pb V Y Zn Zr

38.96 0.02 0.3 0.71 14.63 0.22 43.51

210 2900 10 2050 7 40

1505

of selected elements in the NIM geological materials* NIM-G

75.70 0.09 12.08 0.58 1.30 0.02

4 12 12 8 40

90

NIM-L

52.40 0.48 13.64 8.74 1.13 0.77 0.28 8 10 13 11 43 81 25 400 11000

NIM-N

52.64 0.20 16.50 0.8 7.30 0.18 7.50 58 30 14 120 220 6 68 23

NIM-P

51.10 0.20 4.18 1.02 10.59 0.22 25.33 110 24000 18 560 6 230 6 100 30

NIM-S

63.63 0.04 17.34 1.07 0.30 0.01 0.46 3 12 19 7 5 10 20 10 33

* Values from Abbey [15].

3.

RESULTS

AND DISCUSSION

3.1. Spectrometer performance The photodiode array spectrometer offers the opportunity for simultaneous measurement of analyte emission line intensity, and off-peak background emission intensity. For the material transferred into the plasma during an integration period, a quantitative measurement of emission intensity is available. Although repeat measurements may result in emission intensities due to varying quantities of powder, each measurement provides the peak and off-peak emission for the powder which flowed during the integration time. It is this property which suggests that the combination of powder sampling (by any one of several hardware approaches) and the photodiode array spectrometer offers significant new possibilities for analytical procedures which do not involve sample dissolution. We consider some of the properties of the photodiode array spectrometer in the following paragraphs. The PLASMARRAY spectrometer is equipped with an exchangeable spectral preselection mask, into which one or more slots are cut. Each slot permits a limited wavelength range to be transmitted through to the echelle grating, and hence to the photodiode array. Individual wavelength regions, and groups of wavelengths can be selected by either changing the mask, or by temporarily altering the selection of opened slots by covering the remaining unused slots with tape. The beam intensity of major elements can be attenuated, as needed, by reduction of the height of the slot by covering the ends of the slots with tape. Reduction of the height of the slots has the optical consequence of improving the apparent resolution of the spectrometer. Detection limits were measured using the preselection mask slots entirely open. With an ideal mask in place, only the wavelengths immediately adjacent to the analyte line should be transmitted. The ranges of diodes exposed by opening each slot in multielement mask MEM24 was experimentally determined using fluidized-bed introduction of a mixture of geological material NIM-P and Chelex-100. The ranges of diodes exposed in this way are summarized in Fig. 2 (wavelengths appear in Table 2). The origins of various structural features in areas of the array other than those directly within the expected windows, were not further investigated. The direct overlap of analyte emission lines is avoided in the design of multielement mask MEM24. Nevertheless weak emission lines from concomitant major elements

Fig. 4. ICP emission falling on photodlode array pixels 70 through 220 (expected Cu 324.754 line at pixel 112) during introduction of geological material NIM-P. Spectra (b-h) were acquired individually (singie slot of the preselectian mask open) and spectrum (a) with 22 slots open. Traces include (b) Ar 355.431 nm, (c) Cr 267.716 nm, (d) Sn 283,999 nm, (e) Hg 253.652 nm, (f) Cd/As 228.807 am, (g) Co 324.754 nm and (h) Ni 231504 nm. Other slots (see Fig. 2) contributed only slightfy on the emission scale shown. Traces were offset from each other for clarity.

will pass through various slots in the mask, and appear on the photodiode array (consider Figs 2 and 4). The emission from such lines may contribute to the background, or may interfere with analyte lines. Any analytical work with an unfamiliar matrix will require a careful in~~esti~ation of the emission spectrum characteristic to each slot in the multielement mask. Clearly, in addition to those spectral interferences documented in the ICP literature [17-191, overlap of portions of spectra originating from slots transmitting other wavelength regions can complicate the interference problems immensely. Fur the ICP measurements of Chelex-100, up to seven slots were open simult~~ously~ For the work involving geological materials, as shown in Fig. 4, it was considered impractical to open more than three slots. In some cases the overlaps of emission lines with the off-peak zones due to other slots in the mask can be tolerated. First, the background emission at longer wavelengths may be weak, and therefore does not pose a problem when overlapped with other analyte lines. Secondly, a unique possibility for quantitative separation of overlapped orders may exist in a few cases. Since the emission spectrum due to a given slot may fall twice on the photodiode array, the second copy of the spectrum could be used to compensate for overlaps in the first spectrum (and vice versa). This operation could be carried out automatically with the appropriate software. Note, however, that: (a) the photodiode array is physically short, and the wavelength regions often fall on the array once; (b) the emission intensity of consecutive orders is not identical (nor is sensitivity uniform across the region displayed); and (c) no practical overlap subtraction software is available at this time. The limited dynamic range of the photodiode array may present some difficulty. With the software and hardware presently installed in the PLASMARRAY instrument between 2 s and 5 s are required to transfer the data from the spectrometer to the

Powder sampling ICP

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5 MEASUREMENT

10 NUMBER

(b) I L

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15

I

5 MEASUREMENT

10 NUMBER

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Fig. 5. Emission peak areas (off-peak background subtracted) of selected elements for replicate measurements of Chelex-100 samples AB and AC: (a) sample AB, A Sn; Cl Y; 0 Co; and + Ni; (b) sample AC, + Hg; LI Cd; 0 Pb and 0 SC. The Hg emission was divided by 5 (to use the same scale as the other elements).

system. In practice this imposes a limit upon computer controlled extension of the dynamic range. In principle, the addition of several repeat measurements of short integration times could significantly extend the dynamic range of the instrument, but for the present discussion the system is limited to only a single measurement. The maximum peak height signal before photodiode saturation is about 16000 counts. A signal should not be below 50 counts to realize better than 2% (theoretical) relative measurement uncertainty. The maximum dynamic range (at a fixed integration time) therefore covers only i6000/50, i.e. a factor of about 320. Although one may repeat the measurement with several different integration times, the advantages of internal standard compensation are lost if either the analyte line or the internal standard line saturates the photodiode array or falls below detectability. data

3.2. Measurement precision and detection limits for Chelex-100 Each of the mixed Chelex-100 samples (AB, AC and BC) described in Section 2.3.1. were introduced into the plasma and emission intensity recorded for a minimum of 15 replicate 10 s integrations. Figure 5 shows replicate (off-peak background subtracted) emission measurements of several elements in the samples AB and AC. No attempt was made to introduce the powder at a uniform rate, rather, where necessary (e.g. Fig. 5b) the flow rate was altered by modifying the ratio of sampler gas and carrier gas flows to increase the range of emission intensity measurements. The data shown in Fig. 5 was further processed to yield the linear least squares correlation between several pairs of elements, as illustrated in Fig. 6.

K. NIMALASIRI DE SILVA and R. GUEVREMON?

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(4

6000

0 6000

12000

16000

In Emission

20000

24000

Intensity

1

04 ‘oooo /

7500 * c E f 5000 0' i .?! E w

2500

A

0

2500

5000

Cu Emission

7500

10000

Intensity

Fig. 6. Correlation of emission intensity of selected elements to an internal standard element with direct introduction of Chelex-100: (a) sample AB, A Sn; 0 Y; 0 Co; and + Ni vs internal standard indium. (b) Sample AC, + Hg; A Cd vs internal standard copper. The Hg emission was divided by 5 (to use the same scale as the other elements).

The precision of analytical measurements based on direct introduction of solid materials will be limited by the precision of element to element ratios, where internal standards will be used to correct variations in the flow rate of sample. Table 5 summarizes the correlation between several elements in the Chelex-100 samples for 15 repeat 10 s integration periods. A linear correlation was calculated between the emission intensities of pairs of elements, taking one element as the analyte, and the second as the internal standard element. The R.S.D. in the dependent variable (analyte element) was calculated from the standard error in this dependent variable (i.e. scatter about the regression line), divided by the average value of the dependent variable. If the least squares regression is given by Eqn 1, then the standard error of the estimate s,, of the dependent variable (y) is given by Eqn 2 where Y is a measured data point, and Y,,, is calculated from Eqn 1. N is the number of degrees of freedom; N is 8 for 10 measurements.

Y=

a,, +

apx.

(1)

1509

Powder sampling ICP Table 5. Precision of the ratio of analyte element intensity to internal standard element intensity in Chelex-100 samples “AB” and “AC” Percent R.S.D.$ Element

Internal Standard Elements

Sample AB Ni* Ni+ Sn* Snf In* Int co Mll Y

Ill 3.4 4.1 2.1 1.5 0.7 2.1 1.7 3.4

Ni 1.8 4.7 4.9 3.8 4.1 4.2 3.6 3.7

Mn 3.4 3.6 2.2 1.7 1.6 1.9 2.8 3.1

Sample AC CU* Cllt Hg* Hgt Cd Pb* Pbt SC

CU 0.7 4.7 4.6 3.1 3.4 2.7 6.0

Hg

Cd 3.0 3.3 4.4 4.3 1.6 2.3 7.5

4.1 4.1 0.4 4.0 4.7 3.7 8.8

* First (low pixel) appearance on photodiode array. t Second appearance on photodiode array. $ 15 measurements, %RSD see text.

The percent

R.S.D. in the dependent

variable was calculated using Eqn 3 where

YaV, is the average value of the dependent %R.S.D.

variable for set of the measurements. = lOO+

d”e

.

(3)

In the limit of a regression line which passes through the origin, this is comparable to the relative standard deviation in the ratios of the intensities of the two elements. Regression analyses were carried out with the QUATTRO PRO (Borland International, CA, U.S.A.) spreadsheet program on an IBM PC compatible computer. The correlation coefficient R2 shown in Figs 6 and 7 has been calculated using Eqns 4 and 5.

(JY=

Jw-

Ya,J2 ~--____ N

R2+$. Y

The data on Table 5 suggests that R.S.D. below 5% should be routinely possible, and that it may be possible to achieve 2% R.S.D. Note also that the R.S.D. for measurements of the same wavelength but on consecutive orders on the photodiode is about 1%.

1510

Powder sampling ICP Table 6. Detection limits in dry Chelex-100 powder, and in aqueous solution Chelex-100

Aqueous solution

El

Det limit(ppm)

El

Det limit(ppm)

El

Det limit(ppm)

El

Det limit(ppm)

Ni Sn In co Mn Y

10* 33 4 19 4 1.8

CU Hg Cd Pb SC

3 1.7 0.11 10 2.1

Ni Sn In Co Mn Y

1.3t 0.3 2.3 0.16 0.0004 0.007

CU Hg Cd Pb SC

0.007 1.7 0.17 0.84 0.0006

* ppm in dry, sieved Chelex-100. Detection limit calculated from 3x standard deviation of the noise (at analyte wavelength, off-peak background subtracted, using the Chelex-100, “AB” or “AC”, which does not contain the analyte). t ppm in aqueous solution. Detection limit calculated from 3~ standard deviation of the noise (at analyte wavelength, off-peak background subtracted, using the aqueous sample, “AB”aq or “AC”aq, which does not contain the analyte).

Table 6 summarizes the detection limits (three times the S.D. of the blank) for the direct fluidized-bed introduction of Chelex-100. All measurements were off-peak background subtracted. The S.D. of measurements of the blank (absence of analyte element) was taken using the Chelex-100 mixture which lacked the particular element (sample AB or AC). The background subtraction was taken using the same peak, and off-peak points as were used for the sample which contained the analyte. The sensitivity for a given analyte was assessed by introduction of the Chelex-100 sample which contained both compliments of elements (sample BC). Since the sample flow rate was not uniform, the most intense signal observed during the introduction was used for the sensitivity measurement. For comparison, the detection limit for several elements in aqueous solution is included in Table 6. The detection limits were calculated using the same experimental and calculation procedures described above for Chelex-100. The compositions of the aqueous solutions “B”aq and “C’aq are shown in Table 3 and sample preparation in Section 2.3.1. The maximum number of preselection mask slots open during measurements of the aqueous solutions was 11 (the available slots appear in Table 2). Note that the detection limit for each analyte was determined using a sample which contained high concentrations of elements other than the analyte. In the particular case of the diode array spectrometer, because of the overlaps of the spectral regions (Fig. 2), the emission from other parts of the spectrum may contribute to the background emission, and to the standard deviation of the “blank”. In general, the detection limit of the diode array spectrometer can be degraded by an increase in the complexity of the sample, and by an increase in the number of spectral preselection mask slots. The detection limits on Table 6 are conservative. Since, for most elements the magnitude of background noise (absence of analyte) was weakly correlated with sample introduction rate, this term in the detection limit calculation was relatively constant. On the other hand the sensitivity measurement can be altered in a great many ways. For example, the integration period can be extended, or the flow of sample may be increased, or the plasma rf power may be increased. For comparison, detection limits were evaluated using internal standard calculations. This improved the precision of replicate measurements; however it degraded the ultimate detection limit for most elements. The S.D. of the noise (blank uncertainty) is considerably increased when ratioed to the emission from an internal standard element. The correlation between the background emission (not necessarily related to rate of sample introduction) and the emission from the internal standard element (closely related to rate of sample introduction) is very poor.

K. NIMALASIRI DE SILVA and R. GUEVREMONT

3.3.

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Geological materials

Figure 4 illustrates the measured emission intensity falling on photodiode array pixels 70 to 220 (expected Cu 324.754 line at pixel 112) during the introduction of geological material NIM-P. Emission measurements (b-h, Fig. 4) were made with one preselection mask slot open at a time, and (a, Fig. 4) with 22 of the 24 slots in MEM24 simultaneously open. Two slots, namely Mg(279.553 nm) and Ca(393.366 nm), remained covered. Operation of the spectrometer with more than a few slots open during ICP measurements on complex geological materials would appear to be impractical. For the measurements based on geological materials reported here, several combinations of three open slots were used. All of the combinations included Mn (since Mn was found in all of the geological materials used in this study. Each of the geological materials was mixed 1:3 by weight with Chelex-100 (sample “C” described above). This Chelex sample provided several elements as potential internal standards, including Cd and Cu. A set of three slots in the multielement mask MEM24 was opened, and the mixture of geological material and Chelex transported into the plasma. A minimum of 10 replicate measurements of 10 s integration were acquired. Without changing the sample, the multielement mask was altered for a second set of three open slots and the data acquisition repeated. Similarly, acquisition was repeated for a third set, and a fourth set of three open slots. The four sets of open slots were: (a) Ni, Cd, Mn; (b) Cr, Cd, Mn; (c) Cu, Co, Mn; and (d) Cu, Y, Mn. The wavelengths thus selected, and regions of the photodiode array exposed, can be found on Table 2 and Fig. 2. The emission data for the sets of measurements described above were processed using the PLASMARRAY software to yield background corrected peak areas. The data was transferred to a spreadsheet program (QUATTRO PRO, Borland International, CA, U.S.A.), for further calculations. Table 7 presents the precision of element to element correlation for several of the elements. The R.S.D. was calculated from the standard error of the dependent variable (analyte) divided by the average of this variable. Two types of correlation are of interest. Table 7 includes several examples of the correlation between an element contained in the Chelex-100 powder and an element in the geological material and, in addition, the correlation between pairs of elements found in the geological materials. Figure 7 illustrates the correlations; (a) Mn vs Cd; (b) Y vs Mn, wherein both are limited to the geological material: and (c) Y vs Cd; wherein one element is found on the Chelex and the other in the geological material. We note from Table 7 that in the three cases considered in Fig. 7, the R.S.D. of the analyte emission (standard error in the analyte emission divided by the average analyte emission intensity) ranges from 1.8% for Mn vs Cd, to 4.4% for Y vs Cu. Figure 8 illustrates the wavelength region near the Ni 231.604 nm line for the six NIM geological materials. The Ni concentration in NIM-G, NIM-S, and NIM-L (each below 10 ppm) is near, or below the detection limit. In each case the background spectrum is consistent (despite large differences in each of the materials), and the appearance of emission from Ni relatively easily detected. NIM-N contains about 120 ppm and the Ni signal is well above the detection limit. The S.D. of the background corrected signals for the samples with low Ni are approximately 22 counts (NIM-G, NIM-S and NIM-L) whereas the background corrected emission for NIM-N is 1090 counts. The detection limit for Ni in these geological samples is estimated to be approximately 7 ppm. The approach discussed above was also used to calculate approximate detection limits for Cr and Y. Detection limits were calculated on the basis of three times the standard deviation of the blank noise. All measurements were off-peak background subtracted. For detection limit calculations, the measurements were always taken at the wavelength of the analyte line, and uncertainty was determined by replicate measurements of a sample which contained no detectable analyte. The detection limit for Cr is a function of the complexity of the geological material. Using the Cr emission from NIM-D (2900 ppm Cr) which yields peak areas of

K.

1512

NIMALASIRI

DE SILVAand R. GUEVREMONT

J

R’=O.OOS

21

: = f

14000

A

t

5 ‘si on E w 4

10000 -

ilii 3000

2000

Cd Emlsslon

4000

lntrnslty

04 25000 1 NIM-G + CHELEX-100 20000

t”

P 15000 f

htn Emission (c)

Intensity

25000

NIM-G + CHELEX-100

0

4000

6000

Cu Emlsslon

12000

16000

Intensity

Fig. 7. Fluidized-bed introduction of 1:3 geological material NIM-G and Chelex-100 with added Cd and Cu. (a) Mn vs Cd, relative standard deviation (%RSD) of the ratio is 1.8%, (b) Y vs Mn, RSD = 4.2% and (c) Y vs Cu, RSD = 4.4%.

1513

Powder sampling ICP Table 7. Precision of emission intensity ratios for several elements in 3:l mixtures of Chelex-100 and geological reference materials Internal Std Element

Element

(a) Elements in geological in Chelex-100 Ni (NIM-N) Ni (NIM-P) Ni (NIM-D)

Relative Standard Deviation* (%)

samples ratioed to elements Cd Cd Cd

4.1 5.6 8.9

Mn Mn Mn Mn Mn Mn

(NIM-L) (NIM-S) (NIM-G) (NIM-N) (NIM-P) (NIM-D)

Cd Cd Cd Cd Cd Cd

5.0 6.0 1.8 3.3 7.5 4.6

Mn Mn Mn Mn Mn Mn

(NIM-L) (NIM-S) (NIM-G) (NIM-N) (NIM-P) (NIM-D)

cu CU CU cu cu cu

7.6 6.6 4.6 15.6 3.4 11.4

Y

(NIM-G)

cu

4.4

(b) Elements in geological in the geological material Ni (NIM-N) Ni (NIM-P) Ni (NIM-D)

material ratioed to elements Mn Mn Mn

2.0 7.6 2.2

Cr Cr

(NIM-P) (NIM-D)

Mn Mn

5.9 2.1

Y

(NIM-G)

Mn

4.2

* 10 replicate measurements,

%RSD.

approximately 57000 counts, the detection limit for Cr in NIM-L (standard deviation of the background corrected emission at the Cr wavelength of 164 counts) would be approximately 25 ppm. Similarly, however, the detection limit in NIM-G (standard deviation of 555 counts) would be about 85 ppm. The detection limit of Cr in NIM-S would also be about 65 ppm. Based on these estimates, the NIM-N sample which contains about 30 ppm Cr, should be at or below the detection limit. Cr could not be unambiguously detected in NIM-N. The geological material NIM-G contains 145 ppm Y, and the peak area for this emission line was 17900 counts. The standard deviation for background subtracted blank measurements using NIM-L was 55 counts, and using NIM-S was 60 counts. The detection limit for Y is estimated to be about 1.5 ppm. This is comparable to the detection limit calculated using introduction of Chelex-100 (Table 6). Unfortunately, because of the complexity of the spectra in the vicinity of its emission line, Co was not above the detection limit in any of the geological materials used here.

4. CONCLUSIONS In general, a spectrometer based on array detection is ideally suited to emission measurements for direct powder introduction ICP analysis. With present technology: (a) the flow of solid material cannot be maintained at a constant rate; and (b) it is inconvenient to transport known masses of material, therefore DPI-ICP measurements

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K. NIMALASIRI DE SILVAand R. GUEVREMONT

Fig. 8. Wavelength regions near the Ni 231.6 nm line (pixel 124) for geological samples (a) NIM-D, (b) NIM-P, (c) NIM-N, (d) NIM-G, (e) NIM-L and (f) NIM-S. The traces have been offset for clarity. The detection limit of Ni based on these samples is 7 ppm.

will require the measurement of the emission of an internal standard element to compensate for changes in the flow rate and in the total delivered mass of sample. In order to maximize the precision of this measurement, both the peak emission and offpeak background emission for each element must be measured simultaneously. The photodiode array spectrometer provides this capability. The precision of element to element ratios for both Chelex-100 samples and geological samples appear to be below 5%, and in some cases this RSD was as low as 2%. The regressions were linear. In some cases, however, for unidentified reasons, the uncertainty may rise as high as 20%. The detection limits of several elements have been calculated for direct introduction of Chelex-100 and for introduction of mixtures of geological materials and Chelex-100. It is anticipated that as the mechanism for transport of the samples to the plasma, particularly the development of combinations of geological materials with carriers like Chelex-100, are improved, the detection limits will be further reduced. The LECO PLASMARR.AY photodiode array spectrometer suffers from two limitations identified in this work. First, the device has limited multielement capability when used with complex samples. Naturally this limitation applies both to introduction of solid materials and to conventional liquid nebulization of digests of the same samples (assuming the relative elemental composition is retained). This problem can be overcome if the wavelength “windows” selected by the mask become sufficiently narrow that between 5 and 10 such windows can fall on the photodiode array without overlap. In addition, computer methods which can unscramble overlapped, repeated portions of the spectrum (consecutive orders) would enhance the versatility of the instrument. Secondly, the photodiode array appears to have a limited dynamic range. This requires additional effort, and care, by the operator. The user must ensure that the measured emission is in fact linearly related to integration time over the range of integration times which are needed to cover the range of anticipated sample concentrations. It is additionally necessary that both the analyte emission and the internal standard emission remain within the available measurement range.

Powder sampling ICP

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REFERENCES [l] R. M. Dagnall, D. J. Smith, T. S. West and S. Greenfield, Anal. Chim. Acfa 54, 397 (1971). [2] H. C. Hoare and R. A. Mostyn, Anal. Chem. 39, 1153 (1967). [3] K. C. Ng, M. Zerezghi and J. A. Caruso, Anal. Chem. 56, 417 (1984). (41 G. M. Allen and D. M. Coleman, Appl. Spectrosc. 41, 381 (1987). [5] K. N. De Silva and R. Guevremont, Spectrochim. Acfa 41B, 865 (1986). [6] R. Guevremont and K. N. De Silva. Specfrochim. Acta 41B. 875 (1986). [7] P. E. Pfannerstill, J. A. Caruso and K. Willeke. Appl. Specfrosc. 43, 626 (1989). [8] K. N. De Silva and R. Guevremont, Spectrochim. Acta 45B. 997 (1990). [9] K. N. De Silva and R. Guevremont, Speclrochim. Acta. 45B, 1013 (1990). [lo] R. Guevremont and K. N. De Silva, Spectrochim. Acta. 46B, 67 (1991). [ll] R. Guevremont and K. N. De Silva, Specrrochim. Acta 46B, 1149 (1991). [12] K. C. Lepla and G. Horlick. Appl. Specrrosc. 43, 1187 (1989). [13] V. Karanassios and G. Horlick, Appl. Specfrosc. 40, 813 (1986). [14] G. M. Levy, A. Quaglia, R. E. Lazure and S. W. McGeorge, Specfrochim. Acfa 42B, 341 (1987). (151 S. Abbey, Studies in “standard samples” for use in the General Analysis of Silicate Rocks and Minerals, Part 6: 1979 Edition of “usable” values. Geological Survey of Canada Paper 8&14 (1980). [16] S. E. Church, Geosrand. News/en. 5, 133 (1980). [17] M. L. Parsons, A. Forster and D. Anderson, An Ailas of Spectral Interferences in ICP Spectroscopy. Plenum Press, New York (1980). [18] R. K. Winge, V. A. Fassel, V. J. Peterson and M. A. Floyd, Inductively Coupled Plasma-Atomic Emission Spectroscopy, An Atlas of Spectral Information. Elsevier, Amsterdam (1985). [19] P. W. J. M. Boumans, Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry. Pergamon Press, Oxford (1984).