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ICP-MS: A POWERFUL NEW ANALYTICAL TOOL FOR TRACE ANALYSIS OF ALL STABLE ELEMENTS AND INDIVIDUAL NUCLIDES
Barry N. Noller
Environment Division,
Department of Mines and Energy,
GPO Box 2901 Darwin NT 0801
ICP-MS (inductively coupled plasma-mass spectrometry) is a hybrid analytical technique utilising a plasma atomisation source and a quadrupole mass spectrometer as detector. The specific feature of ICP-MS which makes it a unique technique is its capability to detect the presence of each nuclide when sufficient mass is present. A scan of the spectrum shows the presence of all nuclides.
The first ICP-MS instruments were installed in 1983. After more than ten years of development ICP-MS is now being used routinely throughout the world for trace element analyses in a wide variety of applications(1). There are currently in excess of 600 installations throughout the world and more than 20 instruments in Australia. This wide availability now gives easy access to an ICP-MS instrument.
Apart from the capability to analyse for all stable elements at the trace and ultra-trace level, it is also possible to examine the isotopic distributions of elements from ICP-MS spectra. This gives a specific possibility concerning the nature of the elements for teaching purposes.
THE INSTRUMENT AND DATA SYSTEM
Details of the basic ICP-MS instrument are shown in Figure 1. Spectral overlapping by compounds, particularly oxides, with various isotopes cause interferences in the ICP-MS determination of elements. Mathematical processes are available for analysing the spectrum taken by an ICP-MS instrument, to correct for spectral overlap interferences (2). In ICP-MS, by comparison with ICP emission, the spectrum is relatively simple. Computing systems in current ICP-MS instruments incorporate the natural abundances of all isotopes to help determine contributions to a specific isotope. A file of 500-1000 possible interferences will cover nearly all cases(2).
APPLICATIONS
ICP-MS offers an analytical technique which has high sensitivity for most elements in the periodic table. The ICP-MS technique can be applied to analyse any kind of sample, provided it can be introduced into the plasma. The most common type of sample introduction is via direct atomisation via solution using a nebuliser. The alternative sample introduction systems for ICP-MS are as follows (1):
- flow injection analysis;
- laser sampling;
- ultrasonic nebulisation;
- electrothermal vaporisation;
- liquid chromatography;
- direct injection nebulisation;
- direct solid sample insertion;
- spark ablation;
- vapor generation; and
- slurry sampling.
The analytical capabilities provided by ICP-MS for the analysis of aqueous samples are as follows:
- the ability to measure the concentration of up to 70 elements directly at the trace to ultra-trace level (sub-ppb level);
- the possibility of screening water samples and digest solutions semi-quantitatively using scan mode, giving precision to ±20% or better and accuracy within a factor of 2 or better; and
- the identification of elements in processed waters at elevated concentrations, but not present in background surface of groundwaters, giving access to unique indicator elements.
Mining offers a useful area to explore the capability of ICP-MS to identify elements which are likely to be significant contaminants or tracers of waste waters. Various applications of the ICP-MS technique to mining activities in the Northern Territory have been described including uranium (4), (5), bauxite (6), manganese (7), gold (8), (5) and base metals (5).
SAMPLE COLLECTION, PREPARATION AND ANALYSIS FOR WATER SAMPLESWater samples are collected in 250 mL polyethylene bottles previously cleaned by filling with 5% nitric acid solution and rinsing with high purity water before use.
Water samples are processed as follows:
- direct acidification of sample with ultra-pure grade nitric acid to give a final concentration of 1% following collection of upon receipt by the laboratory undertaking the ICP-MS analysis (this gives elemental concentrations of acid-leachable and soluble forms);
- digestion of sample with nitric acid prior to analysis to give total concentration of elements as defined by the US EPA; and
- filtration through a 0.45μm membrane filter and acidification to give 1% final concentration with ultra-pure grade nitric acid.
A number of ICP-MS instruments are available commercially; most of the data referred to here have been obtained from a VG Plasma Quad instrument located at the South Australia State Chemistry Laboratories, Adelaide. Semi-quantitative scans of 65 or more elemental concentrations are achieved to within a factor of 2 but ±10% or better for elements that can be corrected against Standard Reference Water values, run simultaneously (eg using NIST 1643b Trace Elements in Water). Precision of all elemental concentrations is ±20%. Quantitative arsenic and selenium determinations are made when the concentration of either element exceeds 2μg/L since the ICP-MS scan results for these two elements may be unreliable.
Both soluble and insoluble forms of an element may be present. The physicochemical nature of such forms may be described operationally as particulate, colloidal and soluble concentration (compared with total) in fraction prepared from the sample by filtration through various membrane filters and subsequently analysed (see Noller, this volume).
ISOTOPE ABUNDANCESThe measurement of stable isotope abundances has been a specialist activity of very sophisticated magnetic focussing mass spectrometers (9).
Apart from displaying patterns of nuclides (see figure 2 and 3), the existence of minor isotopes of various elements can be easily demonstrated such as Hg-196 (0.146% natural abundance), the least abundant of mercury's 7 stable nuclides; U-235 (0.72% natural abundance) can be compared with U-238 (99.2746% natural abundance). In contrast U-234 (0.0054% natural abundance) and also the radioactive nuclide Pb-210 are not seen because their masses are too low to be detected - radionuclides, however, may be detected at lower mass levels through their properties of radioactivity. Gold has only one stable nuclide, Au-197.
Stable isotope abundances of metals and nonmetals are traditionally derived from textbooks or monographs (eg. CRC Handbook of Chemistry and Physics) and more recently from computer files (eg "Elementary Data", Clare P. Marshall and Brian D. Marshall 1989) are rarely seen as data derived from a mass spectrometer. In contrast, quadrupole mass spectrometers provide a faster means of scanning the mass range but are limited to the introduction of solid samples.
The inclusion of the quadrupole mass spectrometer in the ICP-MS instrument presents two interesting features regarding the measurement of nuclides in solution:
- the ability to scan the entire mass range and, with highly sophisticated software, to give a semi-quantitative analysis of all elements by using an internal standard, usually indium or bismuth (3); and
- displaying the distribution of stable nuclides, thus offering a relatively easy method to demonstrate the distribution of elements in samples and respective isotopic abundance. ICP-MS therefore expands the possibility of accessing the periodic table as a teaching aid.
EXAMPLES OF NATURAL ISOTOPE VARIATIONS
An example of natural isotopic variation is easily demonstrated for lead. Naturally occurring lead has 4 stable nuclides with the following abundances: Pb-204 (1.42%), Pb-206 (24.1%),Pb-207 (22.1%) and Pb-208 (52.4%) - data from CRC Handbook for Chemistry and Physics. Of the four lead isotopes, three are the stable end products of radioactive decay of uranium and thorium: Pb-208 from Th-232, Pb-207 from U-235 and Pb-206 from U-238 (10). The fourth isotope, Pb-204, has no known parent and its abundance has remained constant since the formation of the Earth. A comparison of the relative intensities of lead nuclides, given above, with the spectrum of lead nuclides in a sample or runoff water from an ore stockpile at the Ranger uranium mine, Northern Australia (see figure 4) shows the relative enhancement of Pb-206 (approximately 77%), compared with the relative intensities of Pb-207 and Pb-208, due to its derivation from U-238.
SPECIFIC APPLICATIONS OF STABLE NUCLIDES
Stable nuclides are much easier to handle than radionuclides. Stable nuclides and their compounds, not necessarily enriched in nature, may be used to follow metabolic processes (eg. Cu-63 and Cu-65, Zn-68 and Zn-70 and Fe-54 and Fe-58) (11) or other distribution processes such as potassium in soil using K-41. Such stable nuclides may be purchased at varying cost dependent on the rarity of the particular nuclide (eg. ISOTEC Inc., Miamisburgh OH 45342 USA).
CONCLUSION
The applications described show the ease with which ICP-MS scans and data may be use to demonstrate features of isotope distributions and elements of the periodic table.
ACKNOWLEDGEMENT
Special thanks go to Con Parouchais and Lyn palmer, Trace Element Group, SA State Chemistry Laboratories, Adelaide, South Australia for providing ICP-MS spectra and data from their VG plasmaquad instrument.REFERENCES
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