Spectroscopy - May 2008 - (Page 12)

12 SPECTROSCOPY GLOBAL EDITION MAY 2008 using a technique called isotope dilution. These unusual isotopes are expensive, when available, and for some elements are not available at all. Furthermore, while isotope dilution is excellent at eliminating matrix effects, it cannot eliminate spectroscopic interferences. As a result, because perfect internal standards generally are not the norm, matrix effects still must be minimized as much as possible. This is achieved in one of two ways. Matrix matching — that is, making up standards and blanks in the same matrix as the samples — effectively cancels the effects of the matrix and works well when the matrix is well characterized and the same for all samples. The method of standard addition is a special case of matrix matching, where the calibration standards are added to the sample itself, thus ensuring a perfect matrix match. This is time consuming because it involves the preparation and measurement of multiple spikes into every sample. When the samples in a group are not identical in terms of matrix, then matrix matching is not practical. In this case, the matrix must be reduced or eliminated. This can be achieved through various techniques of matrix elimination, or more commonly through sample dilution. Both methods have disadvantages, including analyte loss or reduction of analyte concentration, contamination, the possibility of error, and increased time and effort. The Role of Interferences in Practical Limits of Detection As ICP-MS instrument manufacturers and users, we tend to get caught up in the “How low can you go?” game of competitive detection limits. Because everyone knows that ICP-MS is very sensitive, the tendency is to see who can push that limit the furthest, and we speak routinely in terms of partsper-quadrillion (ppq) or even subpart-per-quadrillion levels as though it is something we can grasp easily. A concentration of 1 ppq is equal to 1 part in 10 15, or the first 0.15 mm on a trip from Earth to the sun. And while a simple calculation of blank counts per second (cps) divided by maximum response (cps/concentration) for a standard can in fact give us blank equivalent concentrations (BECs) in the parts-per-quadrillion range, most of us need to ask ourselves: “Does my application really need that?” and “What are the real-world limitations and costs associated with the quest for parts-per-quadrillion detection limits?” This leads me to my point, which is how we can achieve sufficient sensitivity (limits of detection or quantification) to answer our question or solve our problem at an acceptable cost in equipment, supplies, time, and frustration. Detection limits in ICP-MS are determined by measuring the standard deviation of the blank signal times some confidence factor and multiply-

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Spectroscopy - May 2008 Contents A Pragmatic Approach to Managing Interferences in ICP-MS Pittcon 2008: Back to New Orleans and Coffee and Beignets for Breakfast News Market Profile Calendar

Spectroscopy - May 2008

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