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The application of trace metals speciation analyses has increased exponentially in the past decade due to the dissemination of information regarding the direct applicability of the data for risk assessment and focused treatment identification. Quantification of trace metals species has evolved over the past thirty years with the advent of new technology which, when operated in the hands of an experienced analysts, can provide more absolute results. The evolution of trace metals speciation analyses has evolved from reactionary based chemistry to chromatographic separation finally resulting with speciated isotope dilution mass spectrometry (SIDMS).

Speciated isotope dilution mass spectrometry diverges from the traditional analytical methods in the sense that calibration curves, matrix duplicates, and matrix spikes become unnecessary. Each sample is spike with isotopically enriched speciation standards (e.g. ⁵⁰Cr(III) and ⁵³Cr(VI)) at known concentrations and is analyzed in triplicate or quadruplicate (for statistical purposes). The trace metals species of interest in the sample are assumed to conform to the relative natural abundances documented and accepted by the scientific community (e.g. 4.345% ⁵⁰Cr, 83.789% ⁵²Cr, 9.501% ⁵³Cr, and 2.365% ⁵⁴Cr). The different isotopes are then monitored during the speciation analysis (ion chromatography inductively coupled plasma mass spectrometry) and isotope ratios are generated for each trace metals species (e.g. ⁵⁰Cr/⁵²Cr and ⁵³Cr/⁵²Cr for each chromium species). Monitoring the isotope rations for each target species allows for identification of species conversion during sample preparation and analysis.

A Representation of the Deconvolution Mathematics

 

If a sample was spiked with ⁵⁰Cr(III) and the sample matrix was oxidative in nature, the ratio of ⁵⁰Cr/⁵²Cr for Cr(VI) would be elevated. The same can occur if the sample is reducive in nature (monitoring ⁵³Cr/⁵²Cr for Cr(III)). By monitoring all isotopes for the target analyte during speciation analysis and applying statistic from replicate analyses quality control is essentially being performed on each sample.

 

 

 

Notice how the ratios of ⁵²Cr and ⁵³Cr are different between chromatograms?

 

While application of SIDMS is very straightforward for aqueous matrices, solid and sludge samples require one or more extractions which are not always 100% efficient at extracting and stabilizing the target species. The isotopically enriched speciation standards must be applied prior to the extraction procedures to monitor conversion during the extraction process. If species conversion did occur during the extraction procedure, the isotopic ratios will identify the issue and can mathematically correct for it.

Although SIDMS can quantitate, and account for species conversion, for the different species of interest with considerable precision it provides limited information regarding the extraction efficiency for insoluble species. To identify the extraction efficiency total trace metals analysis must be performed on the native material for any sample analyzed by SIDMS. If the extraction procedure does not account for 100% of the total trace element in the sample the end user of the data must consider the following: (1) more than two species can exist in almost any sample matrix, (2) certain matrix components may have high adsorption capacities for the target trace metals species, or (3) not all trace metals species have been documented. Before a more rigorous extraction procedure is applied to increase the extraction efficiency (as dictated by the end user of the data) the propensity of species conversion must be identified; otherwise, science is merely being manipulated to fit the inflexible requirements of project data quality objectives.

It should be noted the trace metals of interest must qualify before SIDMS can be applied. The trace metal analyte of interest (regardless of species) must meet the following requirements: (1) the element must have at least three different isotopes, (2) the target isotopes must be stable (not radioactive), and (3) the target isotopes cannot be a product of radioactive decay. Many trace metals fit the requirements including: Cr, Se, Hg, Sn, and Fe.

As with any mass spectrometry based analytical method the quantitation of each isotope must be free from isobaric interferences (same m/z). Interfering compounds can be monitored with a standard ICP-MS and the final isotope measurements can be adjusted accordingly but this can significantly increase the variability of the method. Reduction of these interferences can be accomplished via application of an ICP-MS with dynamic reaction cell (DRC) technology. The benefit of a DRC versus a collision cell based instrument is the flexibility of using the specific reactionary gases and ability to optimize the reactionary gas parameters to provide an almost interference free signal to the mass spectrometer. It is highly recommended that any client requesting SIDMS analysis inquire about the laboratory’s methods for mitigating isotopic interferences.

As with any analytical method one of the most important variables associated with the analysis is the experience of the analyst. The theory of SIDMS couples three different chemical concepts: ion chromatography, ICP-MS analysis, and nuclear chemistry. Although a basic understanding of each of the concepts alone would suffice for pursuit of that given field, very few laboratories and chemists have the experience of coupling all three together. It is highly recommended that prior to submitting samples to any laboratory touting to provide SIDMS (EPA Method 6800) analyses please inquire about the following:

 

1. What quality control parameters are associated with the analysis?
2. Does the laboratory have any protocols in place for failing QC?
3. What are the analyst’s credentials?
4. Does the Project Manager (or customer service representative) understand the basic concepts of SIDMS to have an educated discussion? If not, who handles questions regarding the data?
5. Does the laboratory recommend field spikes? If so, they probably do not understand the basics of SIDMS since the isotopically enriched standards must be spiked at a specific ratios of the target concentration of the elemental species to provide representative results.
6. How does the laboratory account for isobaric interferences with any of the target isotopes?
7. What instrument platform is the laboratory currently using?
8. Has the laboratory performed any interference studies for any of the target isotopes? If so, please request a copy for review.

The technology associated with Method 6800 is patented and subject to licensing by Applied Isotope Technologies, Inc. Applied Speciation and Consulting is a licensed company with an AIT-certified analyst. Any parties interested in attaining a license for commercial applications of Method 6800 should contact Applied Isotope Technologies, Inc.