Spectroscopy - May 2008 - (Page 13)

MAY 2008 SPECTROSCOPY GLOBAL EDITION 13 ing that by the response factor (concentration/cps) for the particular analyte isotope. Any sample dilution required before analysis also must be factored in. The response factor is fairly easy to determine with good accuracy and precision, and the relative impact of interferences is small. It is the blank that is problematic. Lots of things can play havoc with the blank, including of course, the counting statistics. Mainly, these are contamination (the blank contains the analyte or analytes of interest) and interferences (the blank contains something that is not the analyte of interest but is indistinguishable to the instrument). The possible sources of both of these are numerous and become myriad as we attempt to measure at lower and lower levels. For all practical purposes, reporting limits in almost all cases are blank-limited; that is, you can’t report a concentration that is not different statistically from the method blank or blanks. If we use a very expensive blank, treat it well, and measure for a sufficient length of time in a very expensive clean room, we can achieve instrument detection limits for most elements in the low to sub-part-pertrillion range or even lower. For some applications, particularly in the semiconductor industry, this is necessary and worth the cost and effort. In many other applications, this type of sensitivity and precision are not necessary or even achievable within reasonable limits of cost and effort. For example, what if the “blank” isn’t very reproducible at all? Remember, the most representative “blank” is the actual sample with the analytes removed. A real blank should include all possible sources of error that might affect a sample, including contamination, human error, matrix effects, and longterm measurement imprecision. Typically, this is accounted for through the use of a preparation blank that is subjected to the same sampling and preparation steps as the sample. While this certainly accounts for potential contamination introduced via sample preparation, it does not account for the imprecision in preparation of the replicate preparation blanks or for matrix effects, which can vary significantly from sample type to sample type. Summary At the end of the day, the first question we need to ask is “How low do we really need to measure reliably?” A good example would be drinking water measurement — a fairly simple sample with important public health ramifications. Most regulated elements in drinking water have maximum contamination levels in the single-digit parts-per-billion range, about a million times higher than the parts-perquadrillion limits that are possible under perfect conditions with ultraclean samples. In this case, we don’t

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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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