Water, NIST 1573a Tomato Leaves, NIST 1566a Oyster. Tissue, NIST 2704 Buffalo River Sediment and Bio-Rad. Reference Urine Level 2). A spectral resolution ...
Fresenius J Anal Chem (1999) 364 : 521–526
© Springer-Verlag 1999
O R I G I N A L PA P E R
Ashley T. Townsend
The determination of arsenic and selenium in standard reference materials using sector field ICP-MS in high resolution mode
Received: 28 December 1998 / Accepted: 9 February 1999
Abstract Sector field ICP-MS was used to analyse As and Se in a range of standard reference materials (NIST 1643d Water, NIST 1573a Tomato Leaves, NIST 1566a Oyster Tissue, NIST 2704 Buffalo River Sediment and Bio-Rad Reference Urine Level 2). A spectral resolution of m/∆m = 7500 enabled 75As and 77Se to be separated from problematic ArCl interferences. Following microwave acid digestion, solid samples were typically diluted 1 + 99 prior to analysis, while the urine sample was diluted 1 + 9. The water sample was analysed undiluted and diluted 1 + 9. Despite near baseline spectral separation, 75As and 77Se were still found to be influenced by ArCl at high Cl concentrations, the effect being most pronounced for 77Se. When necessary 82Se was also monitored to determine the accuracy of the 77Se results. Detection limits (LOD, based on 3σ of 10 replicates) for 75As, 77Se and 82Se in ultra-pure water, 1% (w/w) HNO3 and 1% (w/w) HCl were ~ 0.1, ~ 0.2 and ~ 0.5 ng g–1, respectively. Although signal intensities when using high resolution were ~ 1% of that found when using low resolution mode (m/∆m = 300), measured As concentrations and certified values were found to agree to within ± 11% for all samples analysed. The concentration of Se in NIST 1566a Oyster Tissue, NIST 2704 Buffalo River Sediment and Bio-Rad Reference Urine were found to be in agreement with certified values to within ± 15– 20%, as measured by 77Se. However, closer agreement (± 5%) was found when these samples were analysed using 82Se. The Se concentration in NIST 1643d Water was found to agree to within ± 5% of the certified value (depending on dilution factor). Due to the low concentration of Se in NIST 1573a Tomato Leaves, quantitation was not possible (below LOQ, 10σ). As a consequence of the lower ion transmission when using resolution 7500, analytical precisions were found to be elevated over that normally observed using low resolution mode, typically ± 5–20% (depending on analyte concentration and isotopic abundance).
A. T. Townsend Central Science Laboratory, University of Tasmania, GPO Box 252-74, Hobart, TAS, Australia 7001
Introduction The determination of As and Se by ICP-MS is problematic for many samples as the isotopes of both elements typically suffer from severe spectral interferences [1–3]. This is most apparent for samples with high chloride concentrations, as ArCl interferes with both 75As and 77Se [4]. Other Se isotopes (76Se, 78Se, 80Se and 82Se) are also subject to interference from Ar dimer based species [4, 5]. A number of approaches have been used to help minimise interference effects when these two elements are analysed using ICP-MS. Perhaps the most commonly used method is hydride generation, as both elements readily form volatile hydrides [6–8]. This has proven to be an effective technique as the analyte is separated from the sample and digestion matrix before analysis. Other studies have utilised chromatographic methods [9–12] or ETV [2, 13] for separation of the analyte of interest, while other workers have performed mathematical corrections to compensate for the presence of interferences [2, 14]. Since the introduction of magnetic sector ICP-MS instruments [15, 16] many isotopes prone to overlap from polyatomic species are able to be analysed with no spectral interference using higher resolution settings [3, 17–19]. Examples include the separation of 45Sc from 29Si16O, 51V from 35Cl16O, and 56Fe from 40Ar16O. Sector based instruments typically employ a number of resolution settings (low resolution (m/∆m) = 300–500; medium = 3000– 4000; high resolution = 7500–10,0000) with a drop in ion transmission associated with increased resolution [20]. Many studies have now been reported in the literature using low and medium resolution modes (for examples see the review by Becker and Dietze [21]). However, little has been presented specifically focussing on the use of high resolution mode (m/∆m ≥ 7500) [6, 22], which is required for relatively interference free analysis of As and Se [3, 17]. It was the purpose of this study to ascertain whether a standard magnetic sector ICP-MS in high resolution mode could be routinely used to determine the concentration of
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As and Se in a range of standard reference materials (NIST SRM 1643d Water, NIST 1573a Tomato Leaves, NIST 1566a Oyster Tissue, NIST 2704 Buffalo River Sediment and Bio-Rad Reference Urine Level 2), thus avoiding additional complication and expense associated with hydride generation, chromatographic or ETV accessories.
Experimental Reagents, standards and reference materials High purity HNO3, HCl, HF (all Mallinckrodt, Paris, US) and H2O2 (Univar, Sydney, Australia) were used as received for sample digestion. Milli-Q deionised water (≥ 18 MΩ) was further purified in a quartz sub-boiling still prior to use. ICP-MS calibration was performed using standards prepared from a 100 µg g–1 multi element solution (QCD Analysts-Environmental Science Solutions, Spring Lake, US). Indium was used as an internal standard and was prepared from a 1000 µg g–1 single element solution (High Purity Standards, Charleston, US). NIST SRM 1643d Water, 1573a Tomato Leaves, 1566a Oyster Tissue and 2704 Buffalo River Sediment (all Gaithersburg, MD, USA) were used to evaluate the accuracy of the analytical method, and were not treated prior to digestion. A Bio-Rad reference urine (Lyphochek® Urine Metals Control Level 2–69022, Munich, Germany) was reconstituted with 25 mL ultra-pure water as recommended by the manufacturer. Calibration Aqueous standards covering the concentration range 0–100 ng g–1 were used for external calibration. Five standards were used for each Table 1 Typical instrument and measurement settings
Instrument settings: Instrument: Available resolutions (m/∆m): Rf Power: Gas flows: Torch: Nebuliser: Spray chamber: Cones: Sample uptake: Instrument tuning: Ion transmission: Ion sampling depth: Ion lens settings: Measurement parameters: Resolution utilised: Isotopes considered: Scan type: Sample sweeps: Mass window: Dwell time per acquisition point: No of acquisition points per mass segment: Segment duration per scan: Magnet settle time: Detector mode: Integration type:
element, providing correlation coefficients in excess of 0.995. All standards were prepared in 1% (w/w) HNO3. Sample preparation A Milestone MLS-1200 Mega Microwave Digestion System (Sorisole, Italy) with medium pressure vessels was used to digest NIST Tomato Leaves, Oyster Tissue and Buffalo River Sediment, in the presence of acid or acid/peroxide. The microwave procedure suggested by the manufacturer for each sample type was followed and typically consisted of three 5 minute power stages (250W, 400W and 600W). Digestion using sealed microwave vessels ensured there was no loss of volatile analytes. For the Tomato Leaves and the Oyster Tissue, 3 mL HNO3 and 1 mL H2O2 were added to 0.25 g of sample for digestion. After cooling, the resulting solution was diluted to 25 g (i.e. a dilution factor of 1 + 99). Buffalo River Sediment (0.25 g) was digested in the presence of 3 ml HNO3 and 2 or 3 mL HF. Again the sample was diluted to 25 g after microwave digestion. The Bio-Rad reference urine was diluted 1 + 9 prior to analysis, which has been found to be adequate for accurate analysis in other work in our laboratory [23]. The NIST SRM 1643d Water was analysed as received and after dilution 1 + 9. Instrumentation Measurements were carried out using an ELEMENT High Resolution ICP-MS (Finnigan MAT, Bremen, Germany). This instrument has predefined resolution settings (m/∆m at 10% valley definition) of 300 (low), 3000 (medium) and 7500 (high). Typical instrument and method settings used are outlined in Table 1. A standard Meinhard nebuliser and Scott double pass water cooled spray chamber were employed in this study. Instrument tuning and optimisation was performed daily using a 10 ng g–1 multi-element solution con-
ELEMENT (Finnigan MAT) Low = 300, Medium = 3000 and High = 7500 1250 W Plasma gas: 12 –13 L min–1 Auxiliary: 0.9–1 L min–1 Sample gas: 1.0–1.2 L min–1 Fassel type Meinhard Scott-type (double pass), cooled to 3.5–5 °C Ni sampler (1.1 mm orifice i.d.) & skimmer (0.8 mm) Pumping via a Spetec peristaltic pump Performed daily using a 10 ng g–1 multi-element solution ~100,000 counts s–1 per ng g–1 In using resolution 300 Adjusted to obtain maximum signal intensity Adjusted to obtain maximum signal intensity and optimum resolution 7500 75As, 77Se, 82Se
and 115In Magnet jump with E-scan over small mass range 40 100–150% 10 ms 25 375 ms 200 ms Counting Average
523 taining the elements of interest. Further details concerning the ELEMENT have been reported previously [17–20, 23].
Results and discussion Signal intensity decrease with increased spectral resolution There is a decrease in ion transmission and sensitivity when using higher resolution settings with magnetic sector ICPMS instruments, resulting from a reduction in entrance and exit slit width [20]. Table 2 shows typical intensity values measured over a 24 month period for 75As, 77Se
Table 2 Sensitivity for 75As, 77Se and 115In using magnetic sector ICP-MS in low, medium and high resolution modes Resolutiona
Sensitivity (counts s–1 per ng g–1) 75As
300 3000 7500
1,500–2,500 110 – 130 15 – 20
a Resolution
77Se
115In
400–500 20– 30 4– 5
100,000–150,000 6,000– 10,000 700– 1,500
refers to m/∆m at 10% peak height
and 115In using each available resolution. In our laboratory the signal response for In is measured on a daily basis using a 10 ng g–1 tune solution. When using resolution 3000, the 115In signal drops to ~7–10% of that measured using resolution 300, while the signal measured using resolution 7500 is only ~1% of that recorded in low resolution. Similar values have also been reported in other studies [20, 22]. This trade off between decreased signal intensity and increased resolution may limit the use of high resolution (m/∆m = 7500) for the analysis of As and Se in some sample types. Table 2 also highlights the poorer response of the non-metals As and Se (in comparison with the metal In), attributable to their higher ionisation energies [1, 24].
As and Se interferences Arsenic is a mono-isotopic element and is prone to interference from ArCl species [both 40Ar35Cl (major) and 38Ar37Cl (minor)], ArK and CaCl (Table 3), with ArCl known to be the most serious [2, 3]. A spectral resolution of ~7500 is sufficient to overcome a number of these interferences. Figure 1 shows the mass spectrum of 100 ng g–1 As in 5% (w/w) HCl solution. Although complete baseline separation is not possible using the highest resolution
Table 3 Isotopes of interest and some potential spectral interferences when analysing As and Se by ICP-MS Isotope
% Abundance
Interferencea
75As
100
59Co16O 38Ar37Cl 40Ar35Cl 36Ar39K 40Ca35Cl 60Ni14NH
76Se
9.0
Ge 60Ni16O 42Ca34S 44Ca32S 64Zn12C 40Ar36S 39K37Cl 40Ca36S 40Ar36Ar 40Ca36Ar 41K35Cl
77Se
7.6
39K38Ar 42Ca35Cl 65Cu12C 40Ar37Cl 40Ca37Cl 41K36Ar 63Cu14N 40Ar36ArH
a This
Resolutionb 11498 10644 7773 7761 7614 3730 34571 11686 10429 9104 7638 7405 7298 7259 7081 6947 6619 11776 10175 9762 9182 8966 8123 6029 4310
Isotope
% Abundance
Inteferencea
78Se
23.6
Kr 62Ni16O 44Ca34S 40Ar38Ar 40Ca38Ar 66Zn12C 41K37Cl 64Ni14N 64Zn14N 63Cu14NH 31P 16O 2
25192 13081 12879 9970 9718 8916 7476 5671 5223 3359 3101
80Se
49.7
Kr 64Ni16O 44Ca36Ar 64Zn16O 40Ar 2 68Zn12C 40Ar40Ca 66Zn14N 32S16O 3
549253 12562 12276 10601 9688 9594 9455 6348 1985
82Se
9.2
44Ca38Ar
54612 25392 19316 19069 7308 3455 2283
interference list is not exhaustive and should be used as a guide only required for separation of the analyte of interest from the interference shown
b Resolution
Kr 66Zn16O 42Ca40Ar 68Zn14N 40Ar H 2 2 34S16O 3
Resolutionb
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Fig. 1 Mass spectrum of (A) 75As and (B) 40Ar35Cl interference measured in a solution containing 100 ng g–1 As and 5% (w/w) HCl Fig. 3 The influence of HCl concentration (as a source of Cl–, as w/w) on 75As and 77Se signals in solutions of original concentration 10 and 100 ng g–1
The effect of Cl– interference on As and Se
Fig. 2 Mass spectrum of (A) 77Se and (B) 40Ar37Cl interference measured in a solution containing 1000 ng g–1 Se and 5% (w/w) HCl
available with our ICP-MS instrument, As quantitation should be possible in the presence of Cl– if a mass window that excludes the spectral interference is chosen. Selenium has 6 isotopes available for analysis. Some of these are listed in Table 3. Many of the most abundant isotopes of Se suffer severe interference from Ar and Ca polyatomics, and in particular Ar dimer species (e.g. 40Ar2 on 80Se), requiring resolutions in excess of ~ 9000 for separation. Although less abundant, 77Se (7.6%) avoids many of the Ar dimer interferences noted above, although it is subject to spectral overlap from 40Ar37Cl and 40Ca37Cl. A resolution of ~ 9000 is also required to resolve these polyatomics from 77Se. It should also be noted that 74Se (least abundant isotope, 0.9%), 76Se, 80Se and 82Se all suffer isobaric interference from 74Ge (36.5%), 76Ge (7.8%), 80Kr (2.25%) and 82Kr (11.6%), respectively. Both 77Se and 82Se have been the isotopes typically used for analysis in other studies [6, 7, 12, 25]. In this work 77Se was selected for analysis thus avoiding interference from major Ar2 polyatomics. Figure 2 shows the mass spectrum of 1000 ng g–1 Se in 5% (w/w) HCl solution. As for As, quantitation using 77Se should be possible, even in the presence Cl–. 82Se was also monitored when necessary to confirm 77Se analysis results. Using resolution 7500 guarantees that previously noted [26] interferences from S found when analysing 82Se in biological samples would not be observed in this work.
It has already been noted that the determination of 75As and 77Se may be compromised in the presence of Cl–. Although a spectral resolution of 7500 provides some separation of the analytes of interest from nearby polyatomics (Figs. 1 and 2), this may not always be sufficient for accurate quantitation at high Cl– concentrations. To ascertain the effect of Cl– concentration on the determination of As and Se, various amounts of HCl (source of Cl–) were added to 10 and 100 ng g–1 solutions of As and Se, and the apparent concentration of these two isotopes measured. Data is presented in Fig. 3. Accurate quantitation of both elements was observed to a HCl concentration of ~2% (w/w). Above this level the measured concentrations of As and Se were found to be elevated, particularly for Se. This effect was most noticeable for Se in the apparent 10 ng g–1 sample. These results suggest that despite the chosen resolution offering a large degree of separation of the analyte from the interference, high Cl– concentrations may interfere with As and Se determinations using 75As and 77Se. Detection limits The detection limits (LOD) for As and Se using resolution 7500, based on 3σ of 10 consecutive blank measurements, are shown in Table 4 (values as ng g–1). The blanks analysed consisted of ultra-pure water, 1% (w/w) HNO3 and 1% (w/w) HCl (all with added In internal standard). Detection limits for As were measured as ~ 0.1 ng g–1, while values of ~ 0.2 and ~ 0.5 ng g–1 were recorded for 77Se and 82Se, respectively. It is important to note that these detection limits were measured using typical acquisition parameters noted in Table 1. Although Wildner [24] has shown that lower detection limits can be obtained for non-metals like As and Se using high resolutions when longer acquisition times are employed (up to 100 s), such
525 Table 4 Measured limits of detection and quantitation for As and Se (ng g–1) using resolution 7500 Matrix description
Water 1% (w/w) HNO3 1% (w/w) HCl a Limit b Limit
75As
77Se
82Se
LODa
LOQb
LODa
LOQb
LODa
LOQb
0.076 0.095 0.094
0.252 0.317 0.312
0.144 0.172 0.219
0.48 0.573 0.73
0.419 0.662 0.347
1.396 2.207 1.157
of detection, based on 3σ of 10 consecutive blank measurements of detection, based on 10σ of 10 consecutive blank measurements
Table 5 Measured As and Se concentrations in standard reference materials. Reference or target values are shown in parentheses Sample description
Dilution factor
NIST SRM 1643d Water
none 1 in 10
NIST SRM 1573a Tomato Leaves
1 in 100
na
Units
75As
77Se
82Se
8 8
ng g–1 ng g–1 ng g–1
54.43 ± 2.21 56.26 ± 3.59 [56.02 ± 0.73]
12.39 ± 1.22 11.05 ± 1.52 [11.43 ± 0.17]
b
12
µg µg g–1
0.122 ± 0.022 [0.112 ± 0.004]
c
c
[0.054 ± 0.003]
[0.054 ± 0.003]
g–1
g–1
b
[11.43 ± 0.17]
NIST SRM 1566a Oyster Tissue
1 in 100
16
µg µg g–1
14.1 ± 1.5 [14.0 ± 1.2]
2.58 ± 0.28 [2.21 ± 0.24]
2.25 ± 0.34 [2.21 ± 0.24]
NIST SRM 2704 Buffalo River Sediment
1 in 100
10
µg g–1 µg g–1
20.9 ± 3.0 [23.4 ± 0.8]
0.94 ± 0.20 [1.12 ± 0.05]
1.07 ± 0.20 [1.12 ± 0.05]
BIORAD Level 2-69012 Urine
1 in 10
10
ng g–1 ng g–1 ng g–1
161 ± 4 [152] [121–182]d
240 ± 20 [198] [148–247]d
201 ± 26 [198] [148–247]d
a Number
c Below
b Not
d Suggested
of digestions (where appropriate) and individual analyses analysed
conditions cannot be considered routine. Limit of quantitation values (LOQ, 10σ of 10 consecutive blank measurements) are also shown in Table 4, and are important in subsequent discussion when deciding on the applicability of HR-ICP-MS using resolution 7500 for the determination of As and Se in various sample types. Analysis of standard reference materials Five standard reference materials covering a range of sample types were digested and analysed regularly over a 6 month period. Average As and Se values are shown in Table 5, each value being the average of 8–16 individual digestions (where appropriate) and analyses. Certified and target values are also shown. A number of dilution factors were used depending on the nature of the sample and the expected concentration of As and Se. 77Se was the isotope used for Se determinations, with 82Se also analysed when further confirmation was required or interference suspected on 77Se. As 75As and 77Se both suffer from interference from ArCl, it is important to be aware of the chloride concentration present within the reference materials to be analysed. Concentrations supplied for the reference materials were 8290 µg g–1 for Oyster Tissue, 6600 µg g–1 (not certified)
limits of quantitation and detection concentration range
for Tomato Leaves and < 100 µg g–1 (not certified) for the Buffalo River Sediment. No data were provided for the trace metals in water sample, while chloride was measured in the Bio-Rad Urine sample using ion chromatography as 2200 µg g–1. Good agreement between measured and certified values was found for As in all samples. Determined concentrations were all within 11% of certified values, with appreciable differences being noted for the Buffalo River Sediment and Tomato Leave samples. Incomplete digestion with HF/HNO3 acid mixture can account for the As inaccuracy associated with the Buffalo River sample, while the (relatively) low As concentration of 0.112 ng g–1 in the Tomato Leaves may explain the discrepancy between the two values. Arsenic at this concentration is below the limit of quantitation for this element. Generally, the difference between certified and measured values became noticeably smaller as the As concentration increased. For the analysis of 1643d Water the difference between found and measured As concentrations was only 1.7% and 0.4% in the neat and 1 + 9 diluted samples. A slightly elevated As concentration was noted for the Bio-Rad urine sample. However, the measured As concentration was still found to lie within the suggested acceptable range. The concentrations of Se in Oyster Tissue, Buffalo River Sediment and Bio-Rad urine were found to agree with
526
certified values to within ± 15–20% when 77Se was used for quantitation. Closer agreement with certified values to within ~5% was noted when 82Se was used, suggesting that the elevated Cl– (or perhaps Ca and K) concentrations in these samples may interfere with the analysis of the 77Se isotope. Unfortunately, the concentration of Se in Tomato Leaves could not be determined as the Se level was below the levels of detection and quantitation (Table 5). Selenium in NIST 1643d Water was measured in both undiluted and 1 + 9 diluted samples. Agreement with certified values to within 8.4% and 3.3% was found. As a result of the lower ion transmission when working with resolution 7500, short term precisions were found to be elevated over that normally observed using low resolution mode [22–24]. Precisions ranging from ± 5–20% from 8–10 consecutive sample measurements were found, depending on the isotope and concentration under consideration (Table 5).
Conclusion Arsenic and Se were determined in a wide range of standard reference materials by magnetic sector ICP-MS using a nominal spectral resolution of m/∆m = 7500. Despite the lower ion transmission associated with this spectral resolution, accurate quantitation of As and Se in a range of sample types was possible provided sample concentration levels were above the measured detection limits. Some interference on 75As and 77Se was found for samples with Cl– levels far in excess of the analyte concentration. 82Se was found to provide greater accuracy for such samples. Based on preliminary reports from the manufacturer, it is anticipated that the new generation of magnetic sector ICP-MS instruments offering even greater resolution and ion transmission will make the direct determination of elements such as As and Se easier. Acknowledgements The facilities and support of the Central Science Laboratory, University of Tasmania, are gratefully acknowledged. Alison Featherstone is thanked for her useful comments on the manuscript.
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