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Influence of sample matrix components on the selection of calibration strategies in electrothermal vaporization inductively coupled plasma mass spectrometry
.• ELSEVIER
SPECTROCHIMICA ACTA PART
B
Spectrochimiea Acta Part B 51 (1996) 1591-1599
Influence of sample matrix components on the selection of calibration strategies in electrothermal vaporization inductively coupled plasma mass spectrometry R . W . F o n s e c a , N.J. M i l l e r - I h l i * United States Department of Agriculture, Agricultural Research Service, Beltsville Human Nutrition Research Center, Food Composition Laboratory, Beltsville, MD 20705, USA Received 20 February 1996; accepted 8 July 1996
Abstract
Quantification of both digested and slurry samples were studied using ultrasonic slurry electrothermal vaporization inductively coupled plasma mass spectrometry (USS-ETV-ICP-MS). The results of external calibration using aqueous standards, method of additions, and In as an internal standard were compared. The elements studied include: M_n, Ni and Cu and the materials analyzed include: NIST SRM 1548 total diet and SRM 1549 milk powder. Palladium was used as a physical carder and oxygen ashing was used to remove the organic part of the slurry matrix. Different degrees of matrix suppression effects were observed when different skimmer cones were employed. Aging of the skimmer cone and consequent loss of its original circular symmetry and decrease in orifice size resulted in differences in sampling of the ion beam and changes in the degree of matrix effects were observed as the skimmer cone was rotated. The presence of matrix suppression effects is evidenced by strong suppressions in the Ar2, C and analyte signals. When matrix suppression effects were present, the method of external calibration provided low recoveries (average accuracy 73 +_ 12%), therefore it was necessary to use the method of additions to compensate for these problems, providing an average accuracy of 108 - 13%. When matrix effects were absent, the external calibration method resulted in an average accuracy of 101 +-- 16%.
Keywords: Electrothermal vaporization inductively coupled plasma mass spectrometry; Analyte transport; Direct solids analysis; Ultrasonic slurry sampling; Oxygen ashing
1. Introduction
Although pneumatic nebulization has been commonly used for the introduction of liquid samples into inductively coupled plasmas (ICP), alternative sample introduction techniques such as electrothermal vaporization (ETV) provide attractive advantages. Sample introduction using ETV has been used for analysis by inductively coupled plasma atomic * Corresponding author. Published by Elsevier Science B.V. PII S 0 5 8 4 - 8 5 4 7 ( 9 6 ) 0 1 5 7 2 - 8
emission spectrometry (ICP-AES, e.g. [1-4]) as well as inductively coupled plasma mass spectrometry (ICP-MS, e.g. [1,5-7]). Some of the benefits of ETV compared to solution nebulization include: improved sensitivity, small sample size requirements and the capability for solids analysis. Another advantage of ETV is the possibility of separating the analyte from some of the matrix components by using an appropriate temperature-time program before the analyte is transported into the ICP source [8]. As mentioned earlier, ETV is also suitable for
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direct solids analysis especially when combined with techniques such as ultrasonic slurry sampling (e.g. [9-12]). Bendicho and de Loos-Vollebregt [19] provide an extensive review on solid sampling in ETAAS including a discussion of the advantages of ultrasonic slurry sampling. More recently, this approach has been extended to ETV-ICP-MS [8,13,14]. Compared to traditional sample preparation methods such as acid digestion and dry ashing, slurry sampling offers several benefits including reduced sample preparation time, reduced possibility of sample contamination, and decreased probability of analyte loss prior to analysis. In addition, slurry sampling utilizes conventional liquid sample handling apparatus such as autosamplers combining the benefits of solid and liquid sampling. Slurry sampling generates very little acid waste and this technique is less affected by inhomogeneity problems than direct solids analysis [2,3,9,12]. Although no literature reports were found regarding systematic analyte transport studies using slurry sampies, the results by GrEgoire et al. [13] suggest that a difference in analyte transport is observed when comparing slurry samples and aqueous standards. The latter is evidenced by the high results obtained for slurry samples when quantified using aqueous calibration standards. Although it is possible that these high results could be a consequence of matrix effects which produce signal enhancements, it is postulated that the results of Gr6goire et al. [13] are related to a difference in analyte transport between slurries and standards caused by the presence of slurry matrix. This difference may be alleviated by adding a physical carrier or by removing part of the slurry matrix prior to the vaporization step to ensure that the analyte transport for slurries and standards is the same, facilitating quantitation using aqueous standards. Oxygen ashing has been successfully used in ETAAS to remove carbonaceous material present in sample matrices [15,16]. Fonseca and Miller-Ihli [14] have also extended this approach to USS-ETV-ICP-MS, and reported accurate results using a combination of 1/.tg Pd, added as a physical carrier, and a 40 s oxygen ashing cycle. Non-spectroscopic matrix interferences are commonly observed in ICP-MS when analyzing "real world" samples [17-24]. These types of interferences are generally observed as signal enhancements or
suppressions due to the presence of high concentrations of matrix components, such as salts. Signal suppressions are the most common, and are attributed to the presence of space charge effects taking place in the mass spectrometer. Previous works (e.g. [24]) indicate that space charge effects are related to changes in the electrostatic field in an ion beam produced by an excess of charged particles of one polarity over the other. Since the plasma is essentially neutral, charge separation is expected, especially after the ion beam passes the skimmer cone and approaches the ion optics. Under regular ICP-MS conditions, the ion beam becomes positively charged, and if the space charge field is sufficiently strong, electrostatic repulsion between positive charges takes place causing the ion beam to defocus resulting in reduced ion transmission efficiencies [24]. This study investigates the influence of space charge effects on quantitation due to the presence of high concentration concomitants in "real samples". It also evaluates the usefulness of the external calibration method using aqueous standards and the method of additions to quantify the concentration of different analytes in both digested samples and slurry samples. Palladium (1 /xg) was used as a physical carrier and oxygen ashing was used to remove organic matrix components in the case of the slurry samples.
2. Experimental 2.1. A p p a r a t u s
A Perkin-Elmer (Concord, Ontario) Elan 5000A ICP mass spectrometer equipped with an HGA600MS electrothermal vaporizer was employed. Pyrolytically coated graphite tubes with no platform were used. The transfer line consisted of a piece of P'ITE tubing (62 cm long, 6 mm I.D.). The sample introduction system included a Model AS-60 autosampler equipped with a USS-100 ultrasonic slurry sampler. Teflon autosampler cups were used. The only modification to the AS-60 was the replacement of the capillary with a larger diameter PTFE capillary (I.D. 0.889 mm) to facilitate accurate sampling of larger diameter particle sizes. The USS-100 was set at 25 W and 45% power with a 45 s mix time of the slurries prior to injection of a 20 #1 sample volume for
R.W. Fonseca,N.J. Miller-lhli/SpectrochimicaActa PartB 51 (1996) 1591-1599
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Table 1 Experimental conditions Step
T/°C
Rarnp/s
Hold/s
ETV controller gas flow/ ml/min
Argoncarrier gas flow through ETV/ml/min
1
110 1200 110 750 2O 2600 2700 2O
5 5 5 20 1 0 0 1
25 20 25 40 50 7 8 10
300 300 300 300 (Air) 300 0 0 0
0 0 0 300(Air) 0 958 958 958
2 3 4 5 6 7 8 ICP-MS Radio frequencypower/W Coolant Ar flow-rate/1/min Intermediate Ar flow-rate/1/min Carrier Ar flow-rate/1/min Sampler/skimmer cones Transfer line Length/cm Material l.D./mm Data acquisition Dwell time/ms Scan mode Isotopes monitored Signal measurement mode
1100 15.0 0.859 0.958 Platinum 62 Teflon 6 lO Peak hopping 13C, 5SMn' 6°Ni' 63Cu, S°AI-2, l°4pd' 115in Integrated
analysis. The experimental conditions for the ICP-MS and ETV are described in Table 1. The temperature program used was similar to that used for graphite furnace atomic absorption spectrometry and includes a preliminary 1200°C thermal pretreatement step to reduce the palladium and an air ashing step to remove any carbonaceous matrix. There is an air ashing step which is followed by a 50 s low temperature step during which time any residual oxygen is purged from the system prior to the vaporization step. The isotopes monitored were 13C, 55Mn, 6°Ni, 63Cu, 8°mr2, X°4pd and 115In. The reference materials studied included: National Institute of Standards and Technology (NIST, Gaithersburg, MD) SRM 1548 total diet, and SRM 1549 milk powder.
(Aldrich, Milwaukee, Wisconsin) in a mixture of 1.5 ml sub-boiling distilled HNO3 (Seastar, Seattle, WA) and 0.42 ml HCI and diluting to a final volume of 30 ml with 18 M~ ultra pure water. Aqueous standards were prepared daily by serial dilution from 10.0 #g/ml stock solutions using 0.8 mold HNO3. The concentrations of the standards were 1, 10 and 100 ng/ml.
2.2. Reagents
2.4. Preparation of slurries
Palladium solutions were obtained by dilution from a stock solution prepared by dissolving 0.125 g Pd
Reference material samples were weighed directly into Teflon autosampler cups and a 1.0 ml aliquot of
2.3, Preparation of Digested Samples Approximately 0.5 g of the reference material samples were digested using a wet ashing sample preparation procedure [25]. After digestion, samples were diluted to a final volume of 11-13 ml and stored in acid-cleaned polyethylene test tubes until analyzed.
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R.W. Fonseca, N.I. Miller-IhlilSpectrochimicaActa Part B 51 (1996) 1591-1599 2,5~,0~-
diluent consisting of 0.8 mol/1 H N O 3 and 0.005% Triton X-100 was added to the samples using an Eppendorf pipette. The following masses of the reference materials were used to prepare 1 ml slurries: NIST SRM 1548 total diet (20 mg), SRM 1549a milk powder (20 mg).
55Mn
2,0~,0~1,5~,0~-
/ /~_./3C
e~ -I O
SOAr
1,0~,0~5~,0~-
3. Results and discussion
0 0.0
3.1. Suitability of slurry sampling ETV-ICP-MS
1.0
2.0 3.0 time (s)
4.0
5.0
Fig. 1. ETV-ICP-MSsignal pulses for Mn, Ni and Cu in NIST SRM 1548 total diet (20 #1 of a 20 mg/ml slurry preparation). The signal pulses for S°Arz and 13C are also shown. A skimmer cone which provided low matrix suppression effects was used.
Previous studies have illustrated the usefulness of slurry sampling ETV-ICP-MS as a tool for direct solids analysis [8,13,14]. Gr6goire et al. [13] evaluated the method of external calibration and the method of additions for the analysis of a variety of reference materials. They observed that the results had a tendency to be high when calibration against aqueous standards was used, however the method of additions produced accurate results in good agreement with certified values. Fonseca and Miller-Ihli [14] obtained good agreement between experimental and certified values when external calibration was carded out together with the use of oxygen ashing in combination with the addition of Pd. These authors also observed a tendency for the results to be high if external calibration was not used in combination with oxygen ashing and concluded that the organic material present in the slurry matrix was acting as a physical carrier improving analyte transport efficiency between the graphite furnace and the ICP-MS. The use of oxygen ashing results in the removal of the organic matrix present in the slurry samples facilitating accurate quantitation with external calibration
using aqueous standards. Table 2 shows the results for Mn, Ni and Cu for two reference materials NIST SRM 1548 total diet and NIST SRM 1549 milk powder using 20 m g / l m l slurry preparations. The experimental results shown in Table 2 are in good agreement with the certified values and also with the results reported by Fonseca and Miller-Ihli [14]. Matrix suppression effects in ETV-ICP-MS are commonly evidenced by sharp analyte signal suppressions which are often so pronounced that the result is a double peak. The presence of signal suppressions in the argon dimer signal has been used by different authors as an indicator of matrix effects [13,26,27]. Fig. 1 shows typical ETV-ICP-MS signal pulses for Mn, Ni and Cu in an SRM 1548 Diet slurry sample. The ETV-ICP-MS signal pulses for C and Ar2 are also shown in Fig. 1. The signal pulses shown in Fig. 1 do not indicate any signal suppression indicating that matrix suppression effects are absent under the experimental conditions used. The absence of matrix
Table 2 Results for NIST SRM 1548 total diet and NIST SRM 1549 milk powder (20 mg/ml slurries) using external calibration Concentration/t.tg/g Mn
Ni
Cu
Total diet Certified rangea
5.7 --- 0.3 (3) 5.2 --+0.04
0.44 + 0.03 (3) (0.41)
2.9 - 0.2 (3) 2.6 --+0.3
Milk powder Certified rangea
0.26 --- 0.01 (3) 0.26 --- 0.06
0.063 --+0.004 (3) [0.09 --- 0.03]
0.66 --- 0.03 (3) 0.7 + 0.1
Data obtained by USS-ETV-ICP-MS,the numbers in parentheses refer to the number of determinations. "Values with uncertainties represent certified value, values in parentheses correspond to NIST information values, and values in square brackets correspond to concentrations determined by ICP-AES after acid digestion and determination.
R.W. Fonseca, NJ. Miller-lhli/Spectrochiraica Acta Part B 51 (1996) 1591-1599
1,000,000
suppression effects was expected since the external calibration method using aqueous standards provided accurate results (Table 2).
SSMn 800,000
"~ 3.2. Influence of matrix suppression effects on signal pulses and on quantitation
1,0~,0~
•~, 750,000"~ ~
SSMn
/ e3Cu /
eOAr
,
_o
•-~
250,000 T
0.0
,
1.0
,
2.0
,
3.0
,
4.0
5.0
time (s)
Fig. 2. ETV-ICP-MSsignalpulses for Mn, Ni and Cu in NISTSRM 1548 total diet (20/xl of a 20 mg/ml slurrypreparation).The signal pulses for 8°Ar2and 13Care also shown. A skimmercone which providedhigh matrix suppressioneffectswas used.
/ \~
~
600,000-
~
400,000
~== ~==
Although, matrix suppression effects appear to be very common, especially in solution nebulization ICP-MS work, no evidence was observed in this laboratory until the skimmer cone was replaced with a new one. Fig. 2 shows the ETV-ICP-MS signals for Mn, Ni, Cu, C and Ar2 in NIST SRM 1548 total diet (20#1, 20 mg/ml). Interestingly, the replacement of the skimmer cone resulted in an increase in the magnitude of the matrix suppression effects as observed in Fig. 2. Please note that the signal intensities in Fig. 2 may not be compared with those shown in Fig. 1 as the ion lens voltages were different. The strong At2 signal suppression as well as the signal suppressions for Mn and Cu observed in Fig. 2 are most likely due to space charge effects in the mass spectrometer due to the presence of high concentrations of matrix components. Since carbon has a low mass-to-charge ratio (m/ z = 12), its ETV-ICP-MS signal pulse is also expected to be affected by matrix suppression effects and this is evidenced in Fig. 2. Space charge effects appear to be caused by high concentrations of practically any type of positive ions going into the mass spectrometer [17-24,28]. Real world samples such as foods usually contain high
1595
1
0.0
63
Cu
/ ~
1.0
/
"~-/ ,t,
I
so-
/Ar
2
',i\\~ -~-"--~-~/ \ _
2.0 3.0 time (s)
4.0
5.0
Fig. 3. ETV-ICP-MS signal pulses for Mn, Ni and Cu in a simulated total diet sample (concentrations of Mn, Ni, Cu, Na, K and Ca correspond to those expected in a 20 mg/ml sample of a NIST SRM 1548 total diet). The signal pulses for 8°Ar2 and 13C are also shown. A skimmer cone which provided low matrix suppression effects was used.
concentrations of elements such as sodium, calcium, potassium, etc. For this reason, a test solution was prepared using the approximate concentrations of the concomitants Na, Ca and K and the analytes Mn, Ni and Cu found in a 20 mg/ml NIST SRM 1548 total diet slurry sample. Fig. 3 shows the ETVICP-MS signal pulses for the analytes as well as the argon dimer and the carbon signals. Although the dip observed in the argon dimer signal for the real sample (Fig. 2) is more pronounced, the general trend observed for the signal suppression for the different masses monitored is in good agreement with the test solution. Table 3 presents a comparison of the quantitative results for this simulated sample as well as a real slurry and a real digested sample. Table 3 shows the individual and collective effects of K (120 #g/ml), Na (120 #g/ml) and Ca (40 /~g/ml) on quantitation. Interestingly, K did not cause any significant deterioration of the analytical results and there was very good agreement between experimental and expected values. On the other hand, Na and Ca both resulted in apparent low recoveries due to matrix suppression effects. The collective effect of these concomitants produce results which are in very good agreement with the results for real slurry and digested samples for all the elements studied. Therefore these results support the idea that concomitants present in high concentrations cause space charge effects which produce signal attenuations. Since these concomitants are not present in the aqueous standards, direct
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Table 3 Results for test solutions containing approximately the same concentrations of the analytes (Mn, Ni and Cu) and the concomitants (K, Na and Ca) found in a 20 mg/ml NIST SRM 1548 total diet slurry sample Concentration/#g/g Mn
Ni
Cu
Test diet samples K only Na only Ca only K, Na and Ca
5.16 + 0.03 (3) 4.7 ± 0.3 (3) 4.2 ± 0.2 (3) 3.8 ± 0.3 (3)
0.42 0.32 0.35 0.33
(3) (3) (3) (3)
2.7 _ 0.1 (3) 2.5 -+ 0.1 (3) 2.3 ± 0.t (3) 2.10 ± 0.09 (3)
Diet samples Slurry Digest
3.61 ± 0.05 (3) 3.66 ± 0.05 (3)
0.34 --- 0.01 (3) 0.37 ± 0.01 (3)
2.02 _ 0.07 (3) 1.92 --- 0.04 (3)
Certified range a
5.2 ± 0.04
(0.41)
2.6 ± 0.3
± ± ±
0.03 0.02 0.02 0.06
Results for a diet slurry sample an a digested sample are also shown, the numbers in parentheses refer to the number of determinations. Values with uncertainties represent certified value, values in parentheses correspond to NIST information values, and values in square brackets correspond to concentrations determined by ICP-AES after acid digestion and determination.
comparison of samples and calibration standards generate low results. It is also important to note that the results obtained for slurry and digested samples are virtually identical. Previous studies have shown that the magnitude of matrix effects could be affected by different experimental variables such as sampler-skimmer separation [29] and sampler-skimmer orifice size [30-32]. In addition, changes in variables such as the distance between the skimmer cone sampling orifice and the first ion lens or the alignment between sampler and 2,5~,000-
SSMn 1.5oo,ooo l,OOO,ooo
"C~ "~, S°Ar2 I
..~
~o
500,000o 0.0
1.0
2.0 3.0 time (s)
4.0
5.0
Fig. 4. ETV-ICP-MS signal pulses for Mn, Ni and Cu in NIST SRM 1548 total diet (20/~1 of a 20 mg/ml slurry preparation) corresponding to the same conditions used in Fig. 1, but after a slight counterclockwise rotation of the skimmer cone. The signal pulses for S°Ar2 and 13C are also shown.
skimmer sampling orifices could also affect the degree of matrix effects observed. It is possible that the exact volume of the ion beam which is sampled could have an impact on the resulting matrix suppression effects. Fig. 4 shows the ETV-ICP-MS signal pulses for Mn, Ni, Cu, C and Ar2 using the same sample, graphite furnace and skimmer cone used to obtain the data shown in Fig. 1. In Fig. 4, the skimmer cone was slightly rotated counterclockwise, resulting in a larger degree of matrix suppression effects than in Fig. 1, however the signal attenuations were not as drastic as those observed with the newer skimmer cone (Fig. 2). Previous studies indicate that in many cases analytes are not distributed symmetrically in the plasma [33]. Because the sampling orifice in the older skimmer cone is no longer circularly symmetrical, rotation of the skimmer cone could result in changes in the sampling of the ion beam. Although one would expect sampling orifices to get larger with aging, the older skimmer cone used in this work presented a sampling orifice which had collapsed toward the center reducing its sampling diameter. From inspection under a microscope with subsequent imaging and measurement of the area of the orifice, the sampling orifice in the older skimmer cone appeared to be approximately 50-60% smaller than the newer one. The operating pressure for the older skimmer cone (0.6 × 10-5 torr) was lower than for the newer skimmer cone (1.0 x 10 -5 torr) which further supports the
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idea that the older skimmer cone had a smaller sampling orifice.
4. Evaluation of different calibration strategies As shown in Table 3, when specific experimental conditions favor the presence of matrix suppression effects, quantification of real world samples results in apparent low recoveries if the samples contain high concentrations of concomitants. As mentioned earlier, the problem is that if external calibration is used, matrix suppression effects will affect the samples but not the calibration standards since the standards do not contain the same matrix. The use of other calibration strategies such as method of additions [13] and use of an internal standard [19] have proven useful in compensating for matrix suppression problems. Table 4 compares the results obtained for Mn, Ni and Cu in NIST SRM 1548 total diet and NIST SRM 1549 milk powder (20 mg/ml slurry preparations) using three calibration strategies: external calibration with aqueous standards, method of additions and use of 0.2 ng In as an internal standard. When matrix effects were absent, external calibration using aqueous standards resulted in an average accuracy of 101 - 16%, however when the system was prone to
matrix suppression effects, the results for both sample types were low compared to the expected values (average accuracy 73 - 12%). In general it was observed that the SRM 1549 milk powder (99 /xg Na/ml, 338 ttg K/ml, 260/xg CaJml) samples resulted in more pronounced matrix suppression effects than SRM 1548 total diet (125/~g Na/ml, 121/zg K/ml, 35 /~g Ca/ml) which would be expected due to the higher concentrations of the different concomitants present in SRM 1549 milk powder. This was also reflected in the lower recoveries obtained for SRM 1549 milk powder compared to SRM 1548 total diet when the external calibration method was used. The internal standard method, using In ( m / z -~ 115, 10 ng/ml) as the internal standard, resulted in noticeable improvements in the recovery (average accuracy 83 _ 19%), but the results were still low especially for the SRM 1549 milk powder samples. The use of an internal standard is helpful in compensating for some of the space charge effect problems, but there is a compromise in the selection of the internal standard since it is very difficult if not impossible to match exactly all the properties of the different analytes such as ionization potential, mass to charge ratio, volatility, etc. Using multiple internal standards to obtain a better "correction factor" might be a possibility in regular solution nebulization, however in ETV-ICP-MS due to the
Table 4 Comparisonof results using three calibration strategies: external calibration using aqueousstandards, method of additions, and the use of 0.2 ng In as internal standard Calibration strategy
Concentration/~g/g Mn
Ni
Cu
3.61 --_0.05 (3) 5.2 +--0.4 (3) 4.9 - 0.2 (3)
0.34 --- 0.01 (3) 0.45 --- 0.03 (3) 0.39 + 0.03 (3)
2.02 --- 0.07 (3) 2.8 --- 0.2 (3) 2.6 +--0.1 (3)
5.2 _+0.04
(0.41)
2.6 +--0.3
0.20 _+0.02 (3) 0.28 -+ 0.04 (3) 0.23 -+ 0.03 (3)
0.059 --- 0.001 (3) 0.13 -+ 0.05 (3) 0.046 +- 0.001 (3)
0.36 --- 0.01 (3) 0.75 -+ 0.2 (3) 0.49 -+ 0.04 (3)
0.26 _+0.06
[0.09 +- 0.03]
0.7 -+ 0.1
Total diet Ext. calibration MOA Internal std. Certified rangea Milk powder Ext. calibration MOA Internal std. Certified rangea
Results for NIST SRM 1548 total diet and NIST SRM 1549 milk powder (20 mg/ml slurries). Data obtained by USS-ETV-ICP-MS,the numbers in parentheses refer to the number of determinations. Values with uncertainties represent certified value, values in parentheses correspond to NIST information values, and values in square brackets correspond to concentrations determined by ICP-AES after acid digestion and determination.
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transient nature of the signals, the number of masses that can be monitored in each firing is commonly set to less than 10 due to counting statistic limitations [8]. Therefore the selection of multiple internal standards would not be practical in ETV-ICP-MS analyses since each mass used to monitor an internal standard will decrease the available number of analyte masses that could be used in quantification. The results for the method of additions are also shown in Table 4. The method of additions may be thought of as a more ideal method for the correction of matrix reduction effects since each analyte is essentially used as its own internal standard. In general, the results obtained using the method of additions provided good agreement between experimental and expected values. The average accuracy for method of additions was 108 _+ 13% for the three elements and the two different matrices studied.
being simpler and faster than other calibration strategies. USS-ETV-ICP-MS is a good method for direct solids analysis. Comparison of the results for digested and slurry samples for two different reference materials indicated excellent agreement using both external calibration as well as method of additions. Slurry sampling eliminates the sample preparation step, resulting in shorter analysis time and less likelihood for sample contamination.
6. Disclaimer Mention of trademark or proprietary products does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply their approval to the exclusion of other products that may also be suitable.
5. Conclusions Acknowledgements Different degrees of matrix suppression effects were observed when different skimmer cones were used. Changes in parameters such as the distance between the skimmer cone sampling orifice and the first ion lens, the separation between the skimmer and sampler cone sampling orifices, and the precise section of the ion beam which is sampled by the skimmer cone could be affecting the degree of space charge effects. The primary source of space charge effects in this work may be attributed to the difference in ion beam density as a function of the orifice size and orientation of the skimmer cone. Matrix suppression effects due to high concentrations of matrix constituents going into the mass spectrometer are evidenced by strong signal suppressions in the argon dimer, carbon and analyte signals. When matrix suppression effects are present, a calibration strategy such as the method of additions is preferable to external calibration. The use of In as an internal standard compensated for some of these effects but it did not provide the same type of performance as the method of additions. Isotope dilution is also expected to produce accurate results and will be the subject for future work. When no indication of matrix suppression effects was observed, external calibration provided accurate results, with the advantage of
The authors thank the Perkin-Elmer Corporation for providing the instrumentation used for this research. The authors gratefully acknowledge the assistance of Mrs. F. Ella Greene.
References [1] J.M. Carey and J.A. Caruso, Crit. Rev. Anal. Chem., 23 (1992) 397. [2] P. Verrept, R. Dams and U. Kurf-iirst, Fresenius J. Anal. Chem., 346 (1993) 1035. [3] V. Karanassios, J.M. Ren and E.D. Salin, J. Anal. Atom. Spectrom., 6 (1991) 527. [4] J.M. Ren and E.D. Salin, Spectrochim. Acta, Part B, 49 (1994) 555. [5] D.C. Gr6goire, M. Lamoureux, C.L. Chakrabarti, S. AI-Maawali and J.P. Byrne, J. Anal. Atom. Spectrom., 7 (1992) 579. [6] D.C. Gr6goire, S. AI-Maawali and C.L. Chakrabarti, Spectrochim. Acta, Part B, 47 (1992) 1123. [7] R.D. Ediger and S.A. Beres, Spectrochim. Acta, Part B, 47 (1992) 907. [8] S. Beres, R. Thomas, E. Denoyer and P. Briickner, Spectroscopy, 9 (1994) 20. [9] C. Bendicho and M.T.C. de Loos-Vollebregt, J. Anal. Atom. Spectrom., 6 (1991) 353. [10] N.J. Miller-Ihli, J. Anal. Atom. Spectrom., 4 (1989) 295. [11] N.J. Miller-Ihli, At. Spectrosc., 13 (1992) 1.
R.W. Fonseca, N,I. Miller-lhli/Spectrochimica Acta Part B 51 (1996) 1591-1599
[12] N.J. MiUer-Ihli, J. Anal. Atom. Spectrom., 9 (1994) 1129. [13] D.C. Gr6goire, N.J. Miller-lhli and R.E. Sturgeon, J. Anal. Atom. Spectrom., 9 (1994) 605. [14] R.W. Fonseca and N.J. Miller-lhli, Appl. Spectrosc., 49 (1995) 1403. [15] D.K. Eaton and J.A. Holcombe, Anal. Chem., 55 (1983) 946. [16] D.K. Eaton and J.A. Holcombe, Anal. Chem., 55 (1983) 1821. [17] J.A. Olivares and R.S. Houk, Anal. Chem., 58 (1986) 20. [18] G.R. Gillson, D.J. Douglas, J.E. Fulford, K.W. Halligan and S.D. Tanner, Anal. Chem., 60 (1988) 1472. [19] J.J. Thompson and R.S. Houk, Appl. Spectrosc., 41 (1987) 801. [20] D. Beauchemin, J.W. McLaren and S.S. Berman, Spectrochim. Acta, Part B, 42 (1987) 467. [21] S.H. Tan and G. Horlick, J. Anal. Atom. Spectrom., 2 (1987) 745. [22] B.S. Ross and G.M. Hieftje, Spectrochim. Acta, Part B, 46 (1991) 1263. [23] M.R. Tripkovic and I.D. Holclajtner-Antunovic, J. Anal. Atom. Spectrom., 8 (1993) 349.
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[24] S.D. Tanner, L.M. Cousins and D.J. Douglas, Appl. Spectrosc., 48 (1994) 1367. [25] N.J. Miller-IMi, J. Agr. Food Chem., in press. [26] D.C. Gr6goire and R.E. Sturgeon, Spectrochim. Acta, Part B, 48 (1993) 1347. [27] F. Vanhaecke, G. Galb~ics, S. Boonen, L. Moens and R. Dams, J. Anal. Atom. Spectrom., 10 (1995) 1047. [28] A. Montaser and D. W. Golightly, Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd Edn., VCH, New York, 1992, pp. 638-640. [29] J.W.H. Lam and G. Horlick, Spectrochim. Acta, Part B, 45 (1990) 1327. [30] J.S. Crain, R.S. Houk and F.G. Smith, Spectrochim. Acta, Part B, 43 (1988) 1355. [31] D.J. Douglas and L.A. Kerr, J. Anal. Atom. Spectrom., 3 (1988) 749. [32] M.A. Vaughan and G. Horlick, Spectrochim. Acta, Part B, 45 (1990) 1289. [33] M.P. Dziewatkoski, J.W. Oiesik and T. Bryden, Talk No. 582, Conference of the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS XXII), Cincinnati, OH, October 15-20, 1995.
SPECTROCHIMICA ACTA PART
B
Spectrochimiea Acta Part B 51 (1996) 1591-1599
Influence of sample matrix components on the selection of calibration strategies in electrothermal vaporization inductively coupled plasma mass spectrometry R . W . F o n s e c a , N.J. M i l l e r - I h l i * United States Department of Agriculture, Agricultural Research Service, Beltsville Human Nutrition Research Center, Food Composition Laboratory, Beltsville, MD 20705, USA Received 20 February 1996; accepted 8 July 1996
Abstract
Quantification of both digested and slurry samples were studied using ultrasonic slurry electrothermal vaporization inductively coupled plasma mass spectrometry (USS-ETV-ICP-MS). The results of external calibration using aqueous standards, method of additions, and In as an internal standard were compared. The elements studied include: M_n, Ni and Cu and the materials analyzed include: NIST SRM 1548 total diet and SRM 1549 milk powder. Palladium was used as a physical carder and oxygen ashing was used to remove the organic part of the slurry matrix. Different degrees of matrix suppression effects were observed when different skimmer cones were employed. Aging of the skimmer cone and consequent loss of its original circular symmetry and decrease in orifice size resulted in differences in sampling of the ion beam and changes in the degree of matrix effects were observed as the skimmer cone was rotated. The presence of matrix suppression effects is evidenced by strong suppressions in the Ar2, C and analyte signals. When matrix suppression effects were present, the method of external calibration provided low recoveries (average accuracy 73 +_ 12%), therefore it was necessary to use the method of additions to compensate for these problems, providing an average accuracy of 108 - 13%. When matrix effects were absent, the external calibration method resulted in an average accuracy of 101 +-- 16%.
Keywords: Electrothermal vaporization inductively coupled plasma mass spectrometry; Analyte transport; Direct solids analysis; Ultrasonic slurry sampling; Oxygen ashing
1. Introduction
Although pneumatic nebulization has been commonly used for the introduction of liquid samples into inductively coupled plasmas (ICP), alternative sample introduction techniques such as electrothermal vaporization (ETV) provide attractive advantages. Sample introduction using ETV has been used for analysis by inductively coupled plasma atomic * Corresponding author. Published by Elsevier Science B.V. PII S 0 5 8 4 - 8 5 4 7 ( 9 6 ) 0 1 5 7 2 - 8
emission spectrometry (ICP-AES, e.g. [1-4]) as well as inductively coupled plasma mass spectrometry (ICP-MS, e.g. [1,5-7]). Some of the benefits of ETV compared to solution nebulization include: improved sensitivity, small sample size requirements and the capability for solids analysis. Another advantage of ETV is the possibility of separating the analyte from some of the matrix components by using an appropriate temperature-time program before the analyte is transported into the ICP source [8]. As mentioned earlier, ETV is also suitable for
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direct solids analysis especially when combined with techniques such as ultrasonic slurry sampling (e.g. [9-12]). Bendicho and de Loos-Vollebregt [19] provide an extensive review on solid sampling in ETAAS including a discussion of the advantages of ultrasonic slurry sampling. More recently, this approach has been extended to ETV-ICP-MS [8,13,14]. Compared to traditional sample preparation methods such as acid digestion and dry ashing, slurry sampling offers several benefits including reduced sample preparation time, reduced possibility of sample contamination, and decreased probability of analyte loss prior to analysis. In addition, slurry sampling utilizes conventional liquid sample handling apparatus such as autosamplers combining the benefits of solid and liquid sampling. Slurry sampling generates very little acid waste and this technique is less affected by inhomogeneity problems than direct solids analysis [2,3,9,12]. Although no literature reports were found regarding systematic analyte transport studies using slurry sampies, the results by GrEgoire et al. [13] suggest that a difference in analyte transport is observed when comparing slurry samples and aqueous standards. The latter is evidenced by the high results obtained for slurry samples when quantified using aqueous calibration standards. Although it is possible that these high results could be a consequence of matrix effects which produce signal enhancements, it is postulated that the results of Gr6goire et al. [13] are related to a difference in analyte transport between slurries and standards caused by the presence of slurry matrix. This difference may be alleviated by adding a physical carrier or by removing part of the slurry matrix prior to the vaporization step to ensure that the analyte transport for slurries and standards is the same, facilitating quantitation using aqueous standards. Oxygen ashing has been successfully used in ETAAS to remove carbonaceous material present in sample matrices [15,16]. Fonseca and Miller-Ihli [14] have also extended this approach to USS-ETV-ICP-MS, and reported accurate results using a combination of 1/.tg Pd, added as a physical carrier, and a 40 s oxygen ashing cycle. Non-spectroscopic matrix interferences are commonly observed in ICP-MS when analyzing "real world" samples [17-24]. These types of interferences are generally observed as signal enhancements or
suppressions due to the presence of high concentrations of matrix components, such as salts. Signal suppressions are the most common, and are attributed to the presence of space charge effects taking place in the mass spectrometer. Previous works (e.g. [24]) indicate that space charge effects are related to changes in the electrostatic field in an ion beam produced by an excess of charged particles of one polarity over the other. Since the plasma is essentially neutral, charge separation is expected, especially after the ion beam passes the skimmer cone and approaches the ion optics. Under regular ICP-MS conditions, the ion beam becomes positively charged, and if the space charge field is sufficiently strong, electrostatic repulsion between positive charges takes place causing the ion beam to defocus resulting in reduced ion transmission efficiencies [24]. This study investigates the influence of space charge effects on quantitation due to the presence of high concentration concomitants in "real samples". It also evaluates the usefulness of the external calibration method using aqueous standards and the method of additions to quantify the concentration of different analytes in both digested samples and slurry samples. Palladium (1 /xg) was used as a physical carrier and oxygen ashing was used to remove organic matrix components in the case of the slurry samples.
2. Experimental 2.1. A p p a r a t u s
A Perkin-Elmer (Concord, Ontario) Elan 5000A ICP mass spectrometer equipped with an HGA600MS electrothermal vaporizer was employed. Pyrolytically coated graphite tubes with no platform were used. The transfer line consisted of a piece of P'ITE tubing (62 cm long, 6 mm I.D.). The sample introduction system included a Model AS-60 autosampler equipped with a USS-100 ultrasonic slurry sampler. Teflon autosampler cups were used. The only modification to the AS-60 was the replacement of the capillary with a larger diameter PTFE capillary (I.D. 0.889 mm) to facilitate accurate sampling of larger diameter particle sizes. The USS-100 was set at 25 W and 45% power with a 45 s mix time of the slurries prior to injection of a 20 #1 sample volume for
R.W. Fonseca,N.J. Miller-lhli/SpectrochimicaActa PartB 51 (1996) 1591-1599
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Table 1 Experimental conditions Step
T/°C
Rarnp/s
Hold/s
ETV controller gas flow/ ml/min
Argoncarrier gas flow through ETV/ml/min
1
110 1200 110 750 2O 2600 2700 2O
5 5 5 20 1 0 0 1
25 20 25 40 50 7 8 10
300 300 300 300 (Air) 300 0 0 0
0 0 0 300(Air) 0 958 958 958
2 3 4 5 6 7 8 ICP-MS Radio frequencypower/W Coolant Ar flow-rate/1/min Intermediate Ar flow-rate/1/min Carrier Ar flow-rate/1/min Sampler/skimmer cones Transfer line Length/cm Material l.D./mm Data acquisition Dwell time/ms Scan mode Isotopes monitored Signal measurement mode
1100 15.0 0.859 0.958 Platinum 62 Teflon 6 lO Peak hopping 13C, 5SMn' 6°Ni' 63Cu, S°AI-2, l°4pd' 115in Integrated
analysis. The experimental conditions for the ICP-MS and ETV are described in Table 1. The temperature program used was similar to that used for graphite furnace atomic absorption spectrometry and includes a preliminary 1200°C thermal pretreatement step to reduce the palladium and an air ashing step to remove any carbonaceous matrix. There is an air ashing step which is followed by a 50 s low temperature step during which time any residual oxygen is purged from the system prior to the vaporization step. The isotopes monitored were 13C, 55Mn, 6°Ni, 63Cu, 8°mr2, X°4pd and 115In. The reference materials studied included: National Institute of Standards and Technology (NIST, Gaithersburg, MD) SRM 1548 total diet, and SRM 1549 milk powder.
(Aldrich, Milwaukee, Wisconsin) in a mixture of 1.5 ml sub-boiling distilled HNO3 (Seastar, Seattle, WA) and 0.42 ml HCI and diluting to a final volume of 30 ml with 18 M~ ultra pure water. Aqueous standards were prepared daily by serial dilution from 10.0 #g/ml stock solutions using 0.8 mold HNO3. The concentrations of the standards were 1, 10 and 100 ng/ml.
2.2. Reagents
2.4. Preparation of slurries
Palladium solutions were obtained by dilution from a stock solution prepared by dissolving 0.125 g Pd
Reference material samples were weighed directly into Teflon autosampler cups and a 1.0 ml aliquot of
2.3, Preparation of Digested Samples Approximately 0.5 g of the reference material samples were digested using a wet ashing sample preparation procedure [25]. After digestion, samples were diluted to a final volume of 11-13 ml and stored in acid-cleaned polyethylene test tubes until analyzed.
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R.W. Fonseca, N.I. Miller-IhlilSpectrochimicaActa Part B 51 (1996) 1591-1599 2,5~,0~-
diluent consisting of 0.8 mol/1 H N O 3 and 0.005% Triton X-100 was added to the samples using an Eppendorf pipette. The following masses of the reference materials were used to prepare 1 ml slurries: NIST SRM 1548 total diet (20 mg), SRM 1549a milk powder (20 mg).
55Mn
2,0~,0~1,5~,0~-
/ /~_./3C
e~ -I O
SOAr
1,0~,0~5~,0~-
3. Results and discussion
0 0.0
3.1. Suitability of slurry sampling ETV-ICP-MS
1.0
2.0 3.0 time (s)
4.0
5.0
Fig. 1. ETV-ICP-MSsignal pulses for Mn, Ni and Cu in NIST SRM 1548 total diet (20 #1 of a 20 mg/ml slurry preparation). The signal pulses for S°Arz and 13C are also shown. A skimmer cone which provided low matrix suppression effects was used.
Previous studies have illustrated the usefulness of slurry sampling ETV-ICP-MS as a tool for direct solids analysis [8,13,14]. Gr6goire et al. [13] evaluated the method of external calibration and the method of additions for the analysis of a variety of reference materials. They observed that the results had a tendency to be high when calibration against aqueous standards was used, however the method of additions produced accurate results in good agreement with certified values. Fonseca and Miller-Ihli [14] obtained good agreement between experimental and certified values when external calibration was carded out together with the use of oxygen ashing in combination with the addition of Pd. These authors also observed a tendency for the results to be high if external calibration was not used in combination with oxygen ashing and concluded that the organic material present in the slurry matrix was acting as a physical carrier improving analyte transport efficiency between the graphite furnace and the ICP-MS. The use of oxygen ashing results in the removal of the organic matrix present in the slurry samples facilitating accurate quantitation with external calibration
using aqueous standards. Table 2 shows the results for Mn, Ni and Cu for two reference materials NIST SRM 1548 total diet and NIST SRM 1549 milk powder using 20 m g / l m l slurry preparations. The experimental results shown in Table 2 are in good agreement with the certified values and also with the results reported by Fonseca and Miller-Ihli [14]. Matrix suppression effects in ETV-ICP-MS are commonly evidenced by sharp analyte signal suppressions which are often so pronounced that the result is a double peak. The presence of signal suppressions in the argon dimer signal has been used by different authors as an indicator of matrix effects [13,26,27]. Fig. 1 shows typical ETV-ICP-MS signal pulses for Mn, Ni and Cu in an SRM 1548 Diet slurry sample. The ETV-ICP-MS signal pulses for C and Ar2 are also shown in Fig. 1. The signal pulses shown in Fig. 1 do not indicate any signal suppression indicating that matrix suppression effects are absent under the experimental conditions used. The absence of matrix
Table 2 Results for NIST SRM 1548 total diet and NIST SRM 1549 milk powder (20 mg/ml slurries) using external calibration Concentration/t.tg/g Mn
Ni
Cu
Total diet Certified rangea
5.7 --- 0.3 (3) 5.2 --+0.04
0.44 + 0.03 (3) (0.41)
2.9 - 0.2 (3) 2.6 --+0.3
Milk powder Certified rangea
0.26 --- 0.01 (3) 0.26 --- 0.06
0.063 --+0.004 (3) [0.09 --- 0.03]
0.66 --- 0.03 (3) 0.7 + 0.1
Data obtained by USS-ETV-ICP-MS,the numbers in parentheses refer to the number of determinations. "Values with uncertainties represent certified value, values in parentheses correspond to NIST information values, and values in square brackets correspond to concentrations determined by ICP-AES after acid digestion and determination.
R.W. Fonseca, NJ. Miller-lhli/Spectrochiraica Acta Part B 51 (1996) 1591-1599
1,000,000
suppression effects was expected since the external calibration method using aqueous standards provided accurate results (Table 2).
SSMn 800,000
"~ 3.2. Influence of matrix suppression effects on signal pulses and on quantitation
1,0~,0~
•~, 750,000"~ ~
SSMn
/ e3Cu /
eOAr
,
_o
•-~
250,000 T
0.0
,
1.0
,
2.0
,
3.0
,
4.0
5.0
time (s)
Fig. 2. ETV-ICP-MSsignalpulses for Mn, Ni and Cu in NISTSRM 1548 total diet (20/xl of a 20 mg/ml slurrypreparation).The signal pulses for 8°Ar2and 13Care also shown. A skimmercone which providedhigh matrix suppressioneffectswas used.
/ \~
~
600,000-
~
400,000
~== ~==
Although, matrix suppression effects appear to be very common, especially in solution nebulization ICP-MS work, no evidence was observed in this laboratory until the skimmer cone was replaced with a new one. Fig. 2 shows the ETV-ICP-MS signals for Mn, Ni, Cu, C and Ar2 in NIST SRM 1548 total diet (20#1, 20 mg/ml). Interestingly, the replacement of the skimmer cone resulted in an increase in the magnitude of the matrix suppression effects as observed in Fig. 2. Please note that the signal intensities in Fig. 2 may not be compared with those shown in Fig. 1 as the ion lens voltages were different. The strong At2 signal suppression as well as the signal suppressions for Mn and Cu observed in Fig. 2 are most likely due to space charge effects in the mass spectrometer due to the presence of high concentrations of matrix components. Since carbon has a low mass-to-charge ratio (m/ z = 12), its ETV-ICP-MS signal pulse is also expected to be affected by matrix suppression effects and this is evidenced in Fig. 2. Space charge effects appear to be caused by high concentrations of practically any type of positive ions going into the mass spectrometer [17-24,28]. Real world samples such as foods usually contain high
1595
1
0.0
63
Cu
/ ~
1.0
/
"~-/ ,t,
I
so-
/Ar
2
',i\\~ -~-"--~-~/ \ _
2.0 3.0 time (s)
4.0
5.0
Fig. 3. ETV-ICP-MS signal pulses for Mn, Ni and Cu in a simulated total diet sample (concentrations of Mn, Ni, Cu, Na, K and Ca correspond to those expected in a 20 mg/ml sample of a NIST SRM 1548 total diet). The signal pulses for 8°Ar2 and 13C are also shown. A skimmer cone which provided low matrix suppression effects was used.
concentrations of elements such as sodium, calcium, potassium, etc. For this reason, a test solution was prepared using the approximate concentrations of the concomitants Na, Ca and K and the analytes Mn, Ni and Cu found in a 20 mg/ml NIST SRM 1548 total diet slurry sample. Fig. 3 shows the ETVICP-MS signal pulses for the analytes as well as the argon dimer and the carbon signals. Although the dip observed in the argon dimer signal for the real sample (Fig. 2) is more pronounced, the general trend observed for the signal suppression for the different masses monitored is in good agreement with the test solution. Table 3 presents a comparison of the quantitative results for this simulated sample as well as a real slurry and a real digested sample. Table 3 shows the individual and collective effects of K (120 #g/ml), Na (120 #g/ml) and Ca (40 /~g/ml) on quantitation. Interestingly, K did not cause any significant deterioration of the analytical results and there was very good agreement between experimental and expected values. On the other hand, Na and Ca both resulted in apparent low recoveries due to matrix suppression effects. The collective effect of these concomitants produce results which are in very good agreement with the results for real slurry and digested samples for all the elements studied. Therefore these results support the idea that concomitants present in high concentrations cause space charge effects which produce signal attenuations. Since these concomitants are not present in the aqueous standards, direct
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Table 3 Results for test solutions containing approximately the same concentrations of the analytes (Mn, Ni and Cu) and the concomitants (K, Na and Ca) found in a 20 mg/ml NIST SRM 1548 total diet slurry sample Concentration/#g/g Mn
Ni
Cu
Test diet samples K only Na only Ca only K, Na and Ca
5.16 + 0.03 (3) 4.7 ± 0.3 (3) 4.2 ± 0.2 (3) 3.8 ± 0.3 (3)
0.42 0.32 0.35 0.33
(3) (3) (3) (3)
2.7 _ 0.1 (3) 2.5 -+ 0.1 (3) 2.3 ± 0.t (3) 2.10 ± 0.09 (3)
Diet samples Slurry Digest
3.61 ± 0.05 (3) 3.66 ± 0.05 (3)
0.34 --- 0.01 (3) 0.37 ± 0.01 (3)
2.02 _ 0.07 (3) 1.92 --- 0.04 (3)
Certified range a
5.2 ± 0.04
(0.41)
2.6 ± 0.3
± ± ±
0.03 0.02 0.02 0.06
Results for a diet slurry sample an a digested sample are also shown, the numbers in parentheses refer to the number of determinations. Values with uncertainties represent certified value, values in parentheses correspond to NIST information values, and values in square brackets correspond to concentrations determined by ICP-AES after acid digestion and determination.
comparison of samples and calibration standards generate low results. It is also important to note that the results obtained for slurry and digested samples are virtually identical. Previous studies have shown that the magnitude of matrix effects could be affected by different experimental variables such as sampler-skimmer separation [29] and sampler-skimmer orifice size [30-32]. In addition, changes in variables such as the distance between the skimmer cone sampling orifice and the first ion lens or the alignment between sampler and 2,5~,000-
SSMn 1.5oo,ooo l,OOO,ooo
"C~ "~, S°Ar2 I
..~
~o
500,000o 0.0
1.0
2.0 3.0 time (s)
4.0
5.0
Fig. 4. ETV-ICP-MS signal pulses for Mn, Ni and Cu in NIST SRM 1548 total diet (20/~1 of a 20 mg/ml slurry preparation) corresponding to the same conditions used in Fig. 1, but after a slight counterclockwise rotation of the skimmer cone. The signal pulses for S°Ar2 and 13C are also shown.
skimmer sampling orifices could also affect the degree of matrix effects observed. It is possible that the exact volume of the ion beam which is sampled could have an impact on the resulting matrix suppression effects. Fig. 4 shows the ETV-ICP-MS signal pulses for Mn, Ni, Cu, C and Ar2 using the same sample, graphite furnace and skimmer cone used to obtain the data shown in Fig. 1. In Fig. 4, the skimmer cone was slightly rotated counterclockwise, resulting in a larger degree of matrix suppression effects than in Fig. 1, however the signal attenuations were not as drastic as those observed with the newer skimmer cone (Fig. 2). Previous studies indicate that in many cases analytes are not distributed symmetrically in the plasma [33]. Because the sampling orifice in the older skimmer cone is no longer circularly symmetrical, rotation of the skimmer cone could result in changes in the sampling of the ion beam. Although one would expect sampling orifices to get larger with aging, the older skimmer cone used in this work presented a sampling orifice which had collapsed toward the center reducing its sampling diameter. From inspection under a microscope with subsequent imaging and measurement of the area of the orifice, the sampling orifice in the older skimmer cone appeared to be approximately 50-60% smaller than the newer one. The operating pressure for the older skimmer cone (0.6 × 10-5 torr) was lower than for the newer skimmer cone (1.0 x 10 -5 torr) which further supports the
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idea that the older skimmer cone had a smaller sampling orifice.
4. Evaluation of different calibration strategies As shown in Table 3, when specific experimental conditions favor the presence of matrix suppression effects, quantification of real world samples results in apparent low recoveries if the samples contain high concentrations of concomitants. As mentioned earlier, the problem is that if external calibration is used, matrix suppression effects will affect the samples but not the calibration standards since the standards do not contain the same matrix. The use of other calibration strategies such as method of additions [13] and use of an internal standard [19] have proven useful in compensating for matrix suppression problems. Table 4 compares the results obtained for Mn, Ni and Cu in NIST SRM 1548 total diet and NIST SRM 1549 milk powder (20 mg/ml slurry preparations) using three calibration strategies: external calibration with aqueous standards, method of additions and use of 0.2 ng In as an internal standard. When matrix effects were absent, external calibration using aqueous standards resulted in an average accuracy of 101 - 16%, however when the system was prone to
matrix suppression effects, the results for both sample types were low compared to the expected values (average accuracy 73 - 12%). In general it was observed that the SRM 1549 milk powder (99 /xg Na/ml, 338 ttg K/ml, 260/xg CaJml) samples resulted in more pronounced matrix suppression effects than SRM 1548 total diet (125/~g Na/ml, 121/zg K/ml, 35 /~g Ca/ml) which would be expected due to the higher concentrations of the different concomitants present in SRM 1549 milk powder. This was also reflected in the lower recoveries obtained for SRM 1549 milk powder compared to SRM 1548 total diet when the external calibration method was used. The internal standard method, using In ( m / z -~ 115, 10 ng/ml) as the internal standard, resulted in noticeable improvements in the recovery (average accuracy 83 _ 19%), but the results were still low especially for the SRM 1549 milk powder samples. The use of an internal standard is helpful in compensating for some of the space charge effect problems, but there is a compromise in the selection of the internal standard since it is very difficult if not impossible to match exactly all the properties of the different analytes such as ionization potential, mass to charge ratio, volatility, etc. Using multiple internal standards to obtain a better "correction factor" might be a possibility in regular solution nebulization, however in ETV-ICP-MS due to the
Table 4 Comparisonof results using three calibration strategies: external calibration using aqueousstandards, method of additions, and the use of 0.2 ng In as internal standard Calibration strategy
Concentration/~g/g Mn
Ni
Cu
3.61 --_0.05 (3) 5.2 +--0.4 (3) 4.9 - 0.2 (3)
0.34 --- 0.01 (3) 0.45 --- 0.03 (3) 0.39 + 0.03 (3)
2.02 --- 0.07 (3) 2.8 --- 0.2 (3) 2.6 +--0.1 (3)
5.2 _+0.04
(0.41)
2.6 +--0.3
0.20 _+0.02 (3) 0.28 -+ 0.04 (3) 0.23 -+ 0.03 (3)
0.059 --- 0.001 (3) 0.13 -+ 0.05 (3) 0.046 +- 0.001 (3)
0.36 --- 0.01 (3) 0.75 -+ 0.2 (3) 0.49 -+ 0.04 (3)
0.26 _+0.06
[0.09 +- 0.03]
0.7 -+ 0.1
Total diet Ext. calibration MOA Internal std. Certified rangea Milk powder Ext. calibration MOA Internal std. Certified rangea
Results for NIST SRM 1548 total diet and NIST SRM 1549 milk powder (20 mg/ml slurries). Data obtained by USS-ETV-ICP-MS,the numbers in parentheses refer to the number of determinations. Values with uncertainties represent certified value, values in parentheses correspond to NIST information values, and values in square brackets correspond to concentrations determined by ICP-AES after acid digestion and determination.
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transient nature of the signals, the number of masses that can be monitored in each firing is commonly set to less than 10 due to counting statistic limitations [8]. Therefore the selection of multiple internal standards would not be practical in ETV-ICP-MS analyses since each mass used to monitor an internal standard will decrease the available number of analyte masses that could be used in quantification. The results for the method of additions are also shown in Table 4. The method of additions may be thought of as a more ideal method for the correction of matrix reduction effects since each analyte is essentially used as its own internal standard. In general, the results obtained using the method of additions provided good agreement between experimental and expected values. The average accuracy for method of additions was 108 _+ 13% for the three elements and the two different matrices studied.
being simpler and faster than other calibration strategies. USS-ETV-ICP-MS is a good method for direct solids analysis. Comparison of the results for digested and slurry samples for two different reference materials indicated excellent agreement using both external calibration as well as method of additions. Slurry sampling eliminates the sample preparation step, resulting in shorter analysis time and less likelihood for sample contamination.
6. Disclaimer Mention of trademark or proprietary products does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply their approval to the exclusion of other products that may also be suitable.
5. Conclusions Acknowledgements Different degrees of matrix suppression effects were observed when different skimmer cones were used. Changes in parameters such as the distance between the skimmer cone sampling orifice and the first ion lens, the separation between the skimmer and sampler cone sampling orifices, and the precise section of the ion beam which is sampled by the skimmer cone could be affecting the degree of space charge effects. The primary source of space charge effects in this work may be attributed to the difference in ion beam density as a function of the orifice size and orientation of the skimmer cone. Matrix suppression effects due to high concentrations of matrix constituents going into the mass spectrometer are evidenced by strong signal suppressions in the argon dimer, carbon and analyte signals. When matrix suppression effects are present, a calibration strategy such as the method of additions is preferable to external calibration. The use of In as an internal standard compensated for some of these effects but it did not provide the same type of performance as the method of additions. Isotope dilution is also expected to produce accurate results and will be the subject for future work. When no indication of matrix suppression effects was observed, external calibration provided accurate results, with the advantage of
The authors thank the Perkin-Elmer Corporation for providing the instrumentation used for this research. The authors gratefully acknowledge the assistance of Mrs. F. Ella Greene.
References [1] J.M. Carey and J.A. Caruso, Crit. Rev. Anal. Chem., 23 (1992) 397. [2] P. Verrept, R. Dams and U. Kurf-iirst, Fresenius J. Anal. Chem., 346 (1993) 1035. [3] V. Karanassios, J.M. Ren and E.D. Salin, J. Anal. Atom. Spectrom., 6 (1991) 527. [4] J.M. Ren and E.D. Salin, Spectrochim. Acta, Part B, 49 (1994) 555. [5] D.C. Gr6goire, M. Lamoureux, C.L. Chakrabarti, S. AI-Maawali and J.P. Byrne, J. Anal. Atom. Spectrom., 7 (1992) 579. [6] D.C. Gr6goire, S. AI-Maawali and C.L. Chakrabarti, Spectrochim. Acta, Part B, 47 (1992) 1123. [7] R.D. Ediger and S.A. Beres, Spectrochim. Acta, Part B, 47 (1992) 907. [8] S. Beres, R. Thomas, E. Denoyer and P. Briickner, Spectroscopy, 9 (1994) 20. [9] C. Bendicho and M.T.C. de Loos-Vollebregt, J. Anal. Atom. Spectrom., 6 (1991) 353. [10] N.J. Miller-Ihli, J. Anal. Atom. Spectrom., 4 (1989) 295. [11] N.J. Miller-Ihli, At. Spectrosc., 13 (1992) 1.
R.W. Fonseca, N,I. Miller-lhli/Spectrochimica Acta Part B 51 (1996) 1591-1599
[12] N.J. MiUer-Ihli, J. Anal. Atom. Spectrom., 9 (1994) 1129. [13] D.C. Gr6goire, N.J. Miller-lhli and R.E. Sturgeon, J. Anal. Atom. Spectrom., 9 (1994) 605. [14] R.W. Fonseca and N.J. Miller-lhli, Appl. Spectrosc., 49 (1995) 1403. [15] D.K. Eaton and J.A. Holcombe, Anal. Chem., 55 (1983) 946. [16] D.K. Eaton and J.A. Holcombe, Anal. Chem., 55 (1983) 1821. [17] J.A. Olivares and R.S. Houk, Anal. Chem., 58 (1986) 20. [18] G.R. Gillson, D.J. Douglas, J.E. Fulford, K.W. Halligan and S.D. Tanner, Anal. Chem., 60 (1988) 1472. [19] J.J. Thompson and R.S. Houk, Appl. Spectrosc., 41 (1987) 801. [20] D. Beauchemin, J.W. McLaren and S.S. Berman, Spectrochim. Acta, Part B, 42 (1987) 467. [21] S.H. Tan and G. Horlick, J. Anal. Atom. Spectrom., 2 (1987) 745. [22] B.S. Ross and G.M. Hieftje, Spectrochim. Acta, Part B, 46 (1991) 1263. [23] M.R. Tripkovic and I.D. Holclajtner-Antunovic, J. Anal. Atom. Spectrom., 8 (1993) 349.
1599
[24] S.D. Tanner, L.M. Cousins and D.J. Douglas, Appl. Spectrosc., 48 (1994) 1367. [25] N.J. Miller-IMi, J. Agr. Food Chem., in press. [26] D.C. Gr6goire and R.E. Sturgeon, Spectrochim. Acta, Part B, 48 (1993) 1347. [27] F. Vanhaecke, G. Galb~ics, S. Boonen, L. Moens and R. Dams, J. Anal. Atom. Spectrom., 10 (1995) 1047. [28] A. Montaser and D. W. Golightly, Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd Edn., VCH, New York, 1992, pp. 638-640. [29] J.W.H. Lam and G. Horlick, Spectrochim. Acta, Part B, 45 (1990) 1327. [30] J.S. Crain, R.S. Houk and F.G. Smith, Spectrochim. Acta, Part B, 43 (1988) 1355. [31] D.J. Douglas and L.A. Kerr, J. Anal. Atom. Spectrom., 3 (1988) 749. [32] M.A. Vaughan and G. Horlick, Spectrochim. Acta, Part B, 45 (1990) 1289. [33] M.P. Dziewatkoski, J.W. Oiesik and T. Bryden, Talk No. 582, Conference of the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS XXII), Cincinnati, OH, October 15-20, 1995.