Tracing the S-Curve: Molecular Applications of ICP-MS

Jan 01, 2008
Volume 23, Issue 1

Kenneth L. Busch
"Mass Spectrometry Forum" columns focus on the basics of mass spectrometry (MS), and therefore often delve into the tools of the trade: MS instruments and hardware. The columns often begin with a restatement of elementary facts and assumptions so that we can see how those tools fit with analytical needs. We do so again here, by reminding ourselves that MS is one analytical tool, among many tools, that can provide information that we use within "an overall measurement perspective" to solve problems. An exclusive focus on hardware associated with a particular choice within MS — be it a choice of ionization method, of mass analyzer, of a combination known as a hyphenated method, or even of a particular data processing approach — is sometimes an overly artificial construct that focuses on the tool rather than on the creation and completion of an analytical approach that provides data that solve the problem. A properly constructed analytical approach begins with the careful definition of the problem, configures a sampling methodology, collects the samples, prepares the sample, acquires data, and culminates with the integration of the MS data with other data, acquired with other analytical tools. The overall protocol confronts the discrepancies and inconsistencies, and, assessing the data in perspective, finally reaches a reasoned conclusion. The acquisition of MS data is only a part of the measurement suite, and only part of an overall analytical approach that must first determine if MS analysis is even possible; and if so, whether the data will be pertinent. One joy of teaching introductory MS courses is bearing witness to the expansion of MS into new areas of application, and a growing recognition of the synthetic skills that underlie the creation of an insightful analytical approach. The ultimate value of an approach is based upon its unique utility — the rightness of the method for the job — and not on hardware or tools, no matter what commercial acronym describes the instrument, and no matter how much it costs. When MS data is the linchpin, as it often is, mass spectrometrists earn their rightful commendation as analysts. It requires only funds to acquire MS hardware; it takes an insightful analyst to prepare samples, design and validate a method, obtain results of both accuracy and precision, and place the results in proper perspective.

The subject of this column is molecular applications of inductively coupled plasma (ICP) linked with MS, and how those applications have developed. If we were to pull our analytical chemistry textbook from the shelf, we might read that ICP-MS is used to provide qualitative and quantitative information about elemental (mostly metal and metalloid) species, with samples reduced to atomic ions within the plasma ionization source, the mass analyzer separating the ions by mass (with higher mass resolution used to avoid isobaric interferences), and optimized detectors providing highly accurate information even at very low limits of quantitation. We would read in the textbook that ICP-MS competes with other spectroscopic methods for elemental analysis but has become a method of choice because of unmatched specificity and sensitivity that routinely achieves parts-per-billion levels. Early reviews and books (1–3) in the field describe tools and instrumentation, and the relevant applications almost entirely within fields of elemental analysis (inorganic MS), including geochemical and environmental analyses. We would read, perhaps, that isotopic dilution methodology has been used extensively in ICP-MS analyses (4,5), and that careful sampling is an intrinsic part of a high-sensitivity analytical method. Therefore, as an example of insightful sampling as part of an overall analytical scheme, we could consider spatially resolved sampling and high-sensitivity ICP-MS applied to the analysis of tree-ring samples to assess environmental exposure to heavy metal toxicants (6). However, the concept that ICP-MS might have molecular applications is usually missing from the textbooks. If our imaginations stopped at the atomic level, satisfied with these extraordinary accomplishments now become ordinary, we would be shortchanging ourselves in our role as analysts.

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