Evaluation of a modified commercial graphite furnace for reduction of isobaric interferences in argon inductively coupled plasma mass spectrometry

Specrrochumca Acta, Vol Prmted m Great Bntam

468. No

13, pp 1711-1721.

1991 0

0584-a547/91 $3 Ml + .oo 1991 Pergamon Press plc

Evaluation of a modified commercial graphite furnace for reduction of isobaric interferences in argon inductively coupled plasma mass spectrometry JEFFREYM. CAREY,E. HYWEL EVANSand JOSEPHA. CARUSO* Department

of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172, U.S.A.

and WEI-LUNG SHEN National Forensic Chemistry Center, U.S. Food & Drug Administration, Cincinnati, OH 45202, U.S.A.

1141 Central Parkway,

(Received 8 May 1991; accepted 18 June 1991) Abstract-A modified Perkin-Elmer HGA 300 graphite furnace has been investigated for sample introduction into inductively coupled plasma mass spectrometry (ICP-MS). Its application for the reduction of matrix related isobaric interferences at m/z 56 (ArO’ and the major isotope of Fe) and m/z 75 (ArCI+ and monoisotopic As) was studied. Optimization of operating conditions for the ETV instrument, including carrier flow, drying temperature and coolant flow, was performed. Absolute detection limits were found to be 1.5 pg for As and 0.2 pg for Fe, with reproducibility between 5 and 8% RSD for ten 10 ~1 sample injections of 10 ng g-l Fe and As. Linear responses were found to be three orders of magnitude for Fe and four orders of magnitude for As.

1. INTRODUCTION USE of inductively coupled plasma mass spectrometry (ICP-MS) has become increasingly popular for trace element determinations due to its excellent detection capability and selectivity for most elements. Due to these features it has found widespread use in environmental, geological, toxicological, physiological and biochemical research. Pneumatic nebulization is most commonly used for sample introduction due to its relative simplicity, low cost, good stability and capability for rapid sample throughput [l]. However, pneumatic nebulization has some disadvantages which make it less than ideal. First, transport efficiencies of less than 10% are typical [2, 31. Second, because analyte and matrix are introduced simultaneously into the plasma mass spectrometry system, several other problems can occur. These problems include: clogging of the MS interface and nebulizer when solutions containing high quantities of dissolved solids are aspirated; formation of polyatomic ions resulting in matrix related interferences; relatively large sample requirements (> 1 ml); nonspectroscopic matrix induced effects on the analyte signal; and differing transport efficiencies depending upon the physical properties of the solution being introduced (e.g. viscosity) [2, 4-91. Although ultrasonic nebulization results in higher transport efficiencies, its higher cost, complexity and low reliability make it undesirable for many routine analyses [lo]. Recently, HORLICK et al. [ll-131 have addressed the problem of transport efficiency with a direct insertion device. Another possibility as a means of sample introduction is electrothermal vaporization (ETV). Electrothermal vaporization devices have found application in the field of atomic emission spectrometry since 1974 [14]. Electrothermal vaporization provides several advantages as a means of sample introduction into an ICP-MS system [15-231. First, the analysis of small sample sizes (10 ~1 or less) is possible. This is particularly useful when analyzing biological samples when sample volume is often limited. Second, higher transport efficiencies are obtained, typically 20-80%, and thus detection limits are improved. Thirdly, ETV allows for the analysis

THE

* Author to whom correspondence

should be addressed. 1711

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J. M. CAREY et al.

of samples containing organic, high solid, and high acid matrices. Finally, selective volatilization of the analyte from its matrix can lead to a reduction of matrix related as well as isobaric interferences on the analytical signal. Despite the advantages of ETV over pneumatic nebulization, ETV has not found widespread use for several reasons. First and foremost, the condensation of the analyte vapor upon transport to the plasma commonly causes poor precision and memory effects from run to run. Secondly, the ETV instruments which have been designed specifically for use with plasmas are often complex and are not readily available. Finally, the difficulties associated with data acquisition for multi-element determination and measurement of isotope ratios as well as slow sample throughput have been major detractors for ETV development. These drawbacks have greatly retarded the acceptance of ETV as a feasible alternative sample introduction method. The difficulty encountered with condensation of the analyte in the transfer line has been addressed in two different manners. The most common method proposed has been to shorten the transfer distance from the furnace to the plasma [24, 251. This is often accomplished by interfacing the exit of the furnace directly to the base of the plasma torch, which adds to the complexity of the system. Recently, another solution to this problem has been presented [21, 221. It was proposed that the analyte vapor, if cooled immediately after exiting the furnace, would condense to form micro-particles. These micro-particles, which consist of small clusters of atoms, can be transported to the plasma without losses due to condensation. The latter technique may alleviate one of the disadvantages associated with the use of ETV for sample introduction into the ICP, by offering improved transport efficiency without greatly adding to the complexity of the system. The use of a commercially available graphite furnace with minor modifications should increase the availability and simplicity of the instrumentation. The design of such an instrument has been reported recently [26]. This design facilitates the formation of micro-particles by the introduction of a flow of argon just after the exit of the graphite tube, to cool the analyte vapor and cause condensation into micro-particles. In the present study, the use of this furnace, with further minor modifications, has been evaluated for the determination of As and Fe. The use of ETV should result in improved detection limits in comparison with solution nebulization for these elements, due to its ability to remove matrix constituents containing chlorine and oxygen and thus greatly reducing the isobaric interferences of ArCl’ on As+ (m/z 75) and ArO’ on Fe+ (m/z 56).

2. EXPERIMENTAL 2.1. Instrumentation The ICP mass spectrometer used in this work was a VG PlasmaQuad (VG Elemental, Winsford, Cheshire, U.K.). The ETV device used was a modified graphite furnace unit (HGA 300, Perkin-Elmer, Norwalk, CT, U.S.A.). The initial modifications to this furnace have been described previously [26]. Further modifications were made by the inclusion of a drying vent and an alteration to the method of sample introduction. The modifications are shown in Fig. 1. The vent was added by modifying the rear adaptor used previously (Fig. 1). The rear adaptor was constructed with a $” Swagelock fitting and a 1” piece of stainless steel tubing was connected to the adaptor through this fitting. A two-way valve was attached to the end of this tube. This modification was necessary to facilitate the removal of the interfering matrix components during the drying and ashing stages. This vent was left open during the drying and ashing stages and manually closed prior to the volatilization of the analyte. A major draw back of the design previously described [26] was the means of sample introduction. The W-loop/Ta-tube setup described previously caused problems with contamination, and was abandoned and replaced with a L’vov platform (Perkin-Elmer). Sample introduction onto the platform was accomplished by inserting the tip of a pipette (Eppendorf 4710, Brinkmann Instruments, Westbury, NY, U.S.A.) into the furnace through the opening for the rear adaptor and depositing 10 u1 of solution onto the platform. The rear adaptor was then inserted and the

1713

Furnace for isobaric interferences

Comer

flow

Rear adaptor

I To

ICP

Fig. 1. Design of ETV furnace indicating the modifications performed. was allowed to purge for a short period of time to remove any air present. This procedure was necessary because the sample introduction hole in the graphite tube was plugged as described in the previous work [26]. furnace

2.2. Solutions All solutions were prepared using distilled, deionized water. The Fe solutions were prepared in 1% doubly distilled nitric acid (GFS Chemicals, Columbus, OH, U.S.A.). Arsenic solutions were prepared in a 2% solution of this nitric acid and contained 1% reagent grade ammonium hydroxide (Fisher Scientific, Fairlawn, NJ, U.S.A.) as well as 400 pg g-l nickel nitrate prepared from ultrapure Ni(N0&*6H20 (Puratronic, Johnson Matthey, Danvers, MA, U.S.A.). As and Fe solutions were prepared using 1000 kg g-l reference solutions (Fisher Scientific). Blank solutions for both Fe and As were prepared in the same manner and differed only by the absence of the analyte. The As solutions, as previously mentioned, contained two matrix modifiers. The nickel nitrate was added to reduce the volatility of the As by forming a Ni-As complex, thereby preventing its loss during the drying stages. This was especially important since high drying temperatures were used in this study to remove chloiine containing substances prior to the volatilization of the analyte. Removal of the chlorine is facilitated by the addition of the NHIOH to promote the formation of relatively volatile NH&l, which is more readily removed during the drying stages.

3. RESULTS AND DISCUSSION 3.1. Effect of solvent removal As shown in Fig. 2, the effect of removing the solvent from the plasma, dry plasma vs wet plasma (i.e. with aqueous nebulization), was a reduction in the background signal at m/z 56 and m/z 75. It is thought that the background signal at m/z 75 was due to Cl contaminants present in the reagents used to make the solutions. Since the effect of using an ETV for sample introduction into the ICP is to create a dry plasma, the interferences on the major isotope of Fe at m/z 56 and monoisotopic As at m/z 75 should be reduced. This may result in improved detection limits compared with solution nebulization for these elements. 3.2. Optimization of ICP-MS conditions When using an ETV device for sample introduction there is no continuous flow of analyte to the plasma, therefore it is difficult to tune the ion lenses on a particular mass of interest. In order to tune on the specific mass to be studied it would be

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et al.

I

(a)

ArO+/Fe+ m/z =56

J

5

;; -8 za

0 250

42

45

46

51

54

57

ArCL+/As+ m/z = 75 T

60

63

65

66

71

AAn 77

I

E ‘= -

74

(b)

200

100 ArCI+/As+ m/z = 75

ArO+/Fe+ m/z = 56 50

01 42

45

46

51

54

57

60

63

65

66

71

74

77

m/z

Fig. 2. Comparison of signal obtained from: (a) wet plasma and; (b) dry plasma.

necessary to perform multiple ETV runs changing the lens settings slightly each time until a maximum signal was obtained. Because of the length of time taken to perform this optimization, it was considered impractical, and therefore tuning was performed on some element with a high vapor pressure (i.e. mercury) or on a spectral feature present in the dry plasma (i.e. the argon dimer at m/z 80 chosen for this study due to its proximity to the mass to charge ratios of interest). The RF forward power was also optimized by determining the forward power which yielded the greatest peak area signal for Fe and As. The optimum power is shown in Table 1. The optimum conditions were found to be the same for both elements. Other operating conditions for the ICP-MS instrument are shown in Table 1.

Table 1. ETV-ICP-MS

operating conditions

ICP Forward power Reflected power Inner Ar flow Outer Ar flow

1 kW less than 5 W 1.5 llmin 15 i/min

MS Lenses 1st Stage pressure 2nd Stage pressure 3rd Stage pressure

Tune on Ar, (mlz=80) 1.4 mbar
ETV Coolant flow Carrier flow

530 ml/min 110 ml/min

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36Oti/mln

44

650mL/man

9

8

01234567

JO

11

12

73

13

14

15

16

Time Is) *For

Fig. 3. Optimization

3.3.

Optimization

presentai~on only

Peaks

not sequentially

obtained

of coolant flow in ETV furnace. Each peak is for a 10 p,I injection of 1 ngg-’ Fe.

of ETV gas flows

The coolant and carrier gas flows were optimized utilizing a univariate optimization for both Fe and As. The optimum conditions were found to be the same for both elements. Since the design of the instrument is such that the coolant flow should cool the analyte vapor into micro-particles to improve transport efficiency, it should have a dramatic effect on the signal obtained. This was found to be the case as shown in Fig. 3. It can be seen that the signal was less at lower coolant flows until a maximum signal was reached at a flow of 530 ml min- l. It was at this point that the condensation of the micro-particles was the most efficient. When coolant flows greater than this maximum were used the peak shape was severely degraded. This was due to the carrier gas flow not being high enough to ‘puncture’ the coolant gas sheath and carry the micro-particles to the plasma. A carrier gas flow of 110 ml min-’ was found to be optimum, and was the minimum flow necessary to ‘puncture’ the sheath formed by the coolant flow. A flow greater than this did not allow enough time for complete condensation of the micro-particles, nor did it allow sufficient residence time in the plasma for efficient ionization. The optimum flow rates used in this investigation are shown in Table 1. 3.4. ETV controller program The furnace controller program used is shown in Table 2. The stages were for the following purposes: Steps 1 and 2 were for drying the sample and step 3 was the ashing Table 2. ETV controller program Ramp time (s) Step Step Step Step Step Step

1 2 3 4 5 6

10 10 10 0 1 1

Temperature (“C)’ 90 170 1200 2400 1000 2650

Hold time (9 35 35 30 10 3 5

*Temperatures are those read from the controller display and were not actually measured.

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al.

\x

12OO’C

8OO~C SOOT

0

2

4

6

6

Time

10

12

,1000--c

14

16

(s)

Fig. 4. Illustration of the removal of matrix during the drying and ashing stages at m/z 56. Graph (a) is for a 10 ~1 injection of H,O with the indicated drying temperatures and (b) is for 10 ~1 injections of 1 ng g-’ Fe.

step for removing less volatile matrix elements present; Step 4 was for volatilization of the analyte; Steps 5 and 6 were ‘cleanout’ steps for removing any residual sample present in the furnace. The conditions used were the same for both As and Fe since it was found that sufficient vaporization for both analytes occurred at the temperature used in this study. The vaporization temperature, equivalent to the atomization temperature in GFAAS, is only required to volatilize the analyte, while in GFAAS it is necessary for atomization to occur. Since the purpose of this investigation was to evaluate the utility of this furnace design for the removal of matrix related isobaric interferences, the drying and ashing stages were critical to obtain complete separation of the matrix from the analyte of interest. Figure 4 illustrates the effect of the dying and ashing temperatures on the signal obtained at m/z 56 during the respective stages of the furnace cycle. Most of the matrix (i.e. H20) was removed during the low temperature drying stages. However, it was necessary to include a third higher temperature stage to remove any residual matrix which was presumably absorbed into the graphite tube, or condensed on the cooler tube ends. Figure 4(b) shows that no appreciable loss of analyte occurred during the higher temperature ashing step. Figure 5(a) and (b) illustrate the reduction of the interference signal from the matrix components present in the blank. It is important to note that although the signal was reduced significantly, some interference still remained. A further increase in the ashing temperature yielded a further reduction in the interference. However, the signal due to the analyte became very irreproducible indicating that some of the analyte was lost at this higher temperature. The effect of the different drying temperatures on the signal at m/r 56 and m/z 75 is shown in Fig. 6(a) and (b) respectively. The removal of the residual matrix material by the higher temperature resulted in the elimination of one of the peaks. This ‘split

1717

Furnace for isobaric interferences

;E

30

(b)

20

15

10 25:

/?\?; '1.__.\ .

5 200

I _1_._ 400

,. .~ _l600

800

1000

Final dryw-q temperature

1200

1400

PC)

Fig. 5. Effect of ashing temperature pm peak area signal at: (a) m/z 56 and (b) m/z 75 for 10 ~1 injections of the blanks.

peak’ is the result of two different materials being detected by the mass spectrometer. The removed peak is believed to be due to the ArO’ and ArCl+ interferences from matrix components containing oxygen and chlorine, respectively. The chlorine containing matrix elements are believed to originate from the reagents used in making the solutions. 3.5. Analytical figures of merit The analytical figures of merit for this study are shown in Table 3. The detection limits were based on 3u of five replicate injections of 10 ~1 of blank for As and Fe. As can be seen this technique offers very low detection limits for both elements, with good linearity, R* values near 1 and log-log slopes near 1. The linearity for As was determined for 10 (*l injections of solutions containing 1 ng g-l-10 pg g-i As, four orders of magnitude. The linearity for Fe was determined for 10 l.~l injections of solutions containing 100 pg g-‘-100 ng g-l, and was three orders of magnitude. The reproducibility is reasonable and was obtained for ten 10 (~1injections of 10 ng g-l solutions of both As and Fe. When comparing detection limits obtained in this investigation with those found in the literature, with electrothermal vaporization sample introduction into the ICP-MS system, it was found that the detection limits were similar utilizing the modified commercial furnace (Table 4). The detection limit for As was an order of magnitude greater than that reported. However, the advantages of the current furnace design previously described would seem to offset this loss in detectability. These detection limits are superior to those obtained with solution nebulization ICP-MS, ETV or solution nebulization ICP-AES, and graphite furnace atomic adsorption (GFAAS). Once again the detection limits for As in the current study did not provide a large

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6 5 4 3 2

0

12

3

4

5

6

Time *For

Fig. 6. Signals

obtained

presentotlon

only

Peaks

8

9

1011

not

sequentlalty

obtalned

for 10 ~1 injections of Fe (a) and As (b) with different temperatures.

Table 3. Analytical

figures

Iron Reproducibility Detection limit* Linear range RZ log-log slope

7 (s)*

7.63%

of merit Arsenic

RSD

4.88%

20 Pg g-’ (0.2 PP) 0.001 ng to 1 ng 0.99963 0.95707

*Detection limit calculated blank solution.

final drying

based

upon

RSD

150 pg gg’ (1 5 Pg) 0.01 ng to 100 ng 0.99996 0.93875 3a of five replicate

improvement with respect to solution nebulization; however, better detectability in more complex matrices. The values these techniques, along with other ETV-ICP-MS (utilizing a for As and a tungsten furnace for Fe) and those obtained shown in Table 4a and b.

runs of 10 ~1

the ETV should provide for detection limits with rhenium filament furnace in the current study are

3.6. Isotope ratios for Fe It has been shown [26] that, utilizing the current system and the fast scanning capability of the quadrupole, it is possible to obtain isotope ratios. The isotope ratios for Fe (m/z 54, 56, 57) were determined and the results obtained are shown in Table 5. The isotope at m/z 58 was not determined due to interference from erosion of the Ni sampling and skimmer cones. It is clear that the errors are not as low as would be desired. This was probably due to the narrow peaks obtained when using the fast

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Table 4a. Comparison with reported detection limits for As

Detection limit (ng g-Y Graphite EW-ICP-MS Neb. ICP-MS Neb. ICP-AES Rhenium ETV-ICP-MS ETV-ICP-AES GFAAS

Detection limit (pg)

0.150 0.400 60 N/A N/A N/A

1.5 N/A N/A 0.1 200-loo00 10

Ref.

This study [51 [271

PO1

Table 4b. Comparison with reported detection limits for Fe

Graphite ETV-ICP-MS Neb. ICP-MS Neb. ICP-AES Tungsten ETV-ICP-MS ETV-ICP-AES GFAAS

Detection limit (ng g-‘)

Detection limit (PP)

Ref

0.020 0.200 6 N/A N/A N/A

0.20 N/A N/A 0.50 20 300

This study

151

(271

WI

[291 [II

Table 5. Isotope ratios for Fe

m/z = 54 m/z = 56 ml2 = 57

Calculated percent

Accepted percent

Percent error

6.37 91.13 2.17

5.90 91.52 2.25

7.97 (0.43) (3.56)

vaporization ramp (0 s, see Table 2) as seen in Fig. 7. The large positive error for m/z 54 may also be due, in part, to ArN+ interference. This emphasizes the difficulties encountered with multi-element and isotope ratio determinations with ETV, where transient signals are being monitored. A trade off between the number of elements that can be determined, sensitivity, and peak resolution is necessary due to the constraints of a sequential, albeit very rapid, method of data acquisition compared with truly simultaneous methods. However, the results do indicate that the ArO+ interference has been significantly reduced, since the magnitude of the signal at m/z 56 relative to mlz 54 and 57 was close to the accepted value based on the natural isotope abundances for Fe.

4. CONCLUSIONS The results obtained in this investigation indicate that the modified commercial ETV device facilitated the reduction of matrix related isobaric interferences in argon ICP-MS. This device offered similar detection limits for As and Fe compared with furnaces utilizing the same concept of cooling the analyte vapor into micro-particles. However, the current design should allow for greater availability of the instrumentation by making use of simple modifications to a commercially available instrument. The simplicity of the current design makes possible the rapid interchange of sample introduction devices to the ICP-MS instrument while still increasing the transport efficiency of the analyte. The problems mentioned in previous work [26] including the

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15

18

~

0 4 22

4

58

76

9 4 11 13

17

Time

20

22

24

26

27

29

31

33

35

(~1

Fig. 7. Isotope ratio signals at indicated m/z for Fe.

need for the addition of a drying vent have been addressed. The applicability of obtaining isotope ratios has been demonstrated. Another difficulty detailed in the previous paper, namely that of the impurities in the tungsten wire, has been corrected by the addition of a L’vov platform. Future work is in progress to simplify sample introduction onto the L’vov platform, and to investigate the utility of the device for multi-element determinations in real samples. Acknowledgements-The authors wish to acknowledge CHARLES STORY for the use of his data manipulation program. Furthermore, they would like to extend their gratitude to the National Institute of Environmental Health Sciences for providing research support through grants numbered ES-03221 and ES-04908 as well as the NIH-BRS Instrument Program for providing the VG PlasmaQuad Instrument through grant SlORR02714.

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