Determination of arsenic and vanadium in airborne related reference materials by inductively coupled plasma–mass spectrometry
Introduction
Arsenic and vanadium are considered the major “marker elements” of air pollution emitted from residual oil and coal combustion, respectively. Both elements have been released increasingly into the atmosphere during recent periods of industrial development. The determination of these elements in air particulates is therefore important not only in identifying their concentration levels in the air and origins from various sources, but also for gaining insights into their biochemical cycling [1].
A two-step closed vessel acid mixture digestion procedure (HNO3+HClO4/HF) has been proposed in our previous work for multielement analysis of the airborne particles 2, 3, 4, 5. However, inherent problems arise with the V and As determinations during the acid digestion process. It is known that both As and V may be volatile in the halide form, which is formed primarily by interaction with digestion agents such as HClO4 or HF. Determining the formation and volatility of such compounds requires systematic studies under conditions associated with the decomposition of air particle samples. At present, such studies are not available.
Since its commercial introduction in the early 1980s, inductively coupled plasma mass spectrometry (ICP–MS) has aroused great attention as a sensitive instrument for measuring elemental concentrations in various kinds of samples 6, 7, 8, 9, 10, 11. However, interference problems due to the acid-derived background ions formed during the ion extraction process of ICP–MS may significantly degrade the sensitivities of many elemental determinations, especially that of 75As (100%) and 51V (99.7%) 12, 13. Previous literature 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 revealed that the chloride, introduced via reagents or samples, can give rise to the formation of 40Ar35Cl+, 37Cl14N+ and 35Cl16O+, which may seriously inhibit the detection of 75As+ and 51V+.
Many methods have been employed in the reduction of these interferences, e.g. through interface design [25], or the use of a mixed gas plasma [26]. However, polyatomic ions associated with primary plasma ions remain a problem. The best solution to this isobaric interference problem is to analyze the digested sample under high mass spectral resolution ICP–MS 27, 28, 29. At a resolution of m/Δm=3000, the signal for ClO+ is easily resolved from that for 51V+ even if only trace amount of V is to be determined. A much higher resolution (m/Δm = 10 000) is required for resolving 75As+ from ArCl+ interference.
The purpose of this investigation is to test and modify a previously developed acid mixture digestion method for determining As and V in airborne related reference materials using ICP–MS. Quadrupole ICP–MS and sector-field ICP–MS were employed and compared. The attributes of the two techniques were used to characterize the influence of polyatomic interferences as well as chemical loss from chlorinated acid agents or samples for As and V in the ICP–MS measurement. Acceptable precision and accuracy of the results were confirmed via recovery tests, replicate analyses and standard addition methods.
Section snippets
Apparatus
A pressure bomb digestion system consisting of a 25 ml PTFE vessel and thermostatically controlled heating block supplied by Berghof (Germany) was used for sample digestion. Details regarding the instruments used herein can be found elsewhere 2, 3. Two types of ICP–MS instruments were employed in this investigation: (a) the quadrupole instrument used was a Perkin–Elmer Sciex Elan Model 5000 quadrupole mass spectrometer (Thornhill, Ontario, Canada), and (b) the sector-field instrument used was a
Validation of optimized digestion condition
Reference materials were dissolved and the solution thus obtained was analyzed to validate the sample preparation techniques. As and V concentrations determined under different digestion conditions are summarized in Table 2. An erroneous, excessive recovery was observed for both As and V data involving the addition of HClO4 to the digestion procedure. Obviously, this can be attributed to the ArCl+ interference for As determination. Similar results for V can also be attributed to ClO+ or ClN+
Conclusions
This study examined sample digestion techniques and instrumental interference for determining As and V in airborne related reference materials using ICP–MS. It was found that the detection limits of quadrupole ICP–MS would seriously deteriorate if a chlorinated acid was employed during the digestion procedure. A modified HNO3+H2O2+HF mixed acid digestion with a low temperature evaporation procedure was proposed for ICP–MS to effectively determine the As concentration in various samples.
Acknowledgements
This work was supported by the National Science Council, Taiwan, under contract number NSC87-2113-M007-039. The authors would like to thank Mr. W.F. Chang, National Science Council's Regional Instrument Center at Hsinchu, for the ICP–MS measurement. Prof. P.C. Chiang, Institute of Environmental Engineering, National Taiwan University is appreciated for his long-term assistance and valuable comments.
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