Spectroscopy - May 2008 - (Page 10)

10 SPECTROSCOPY GLOBAL EDITION MAY 2008 tively able to reduce all polyatomic interferences simultaneously, and for this reason is the preferred technique for multielemental determinations in complex or unknown matrices. Nonspectroscopic interference is the catch-all term for any interference that is, well, not spectroscopic. These include primarily enhancement and suppression effects that alter the response of groups of analytes. In general, nonspectroscopic interferences do not create a response where there was none, but rather alter the response of a group of analytes in a sample compared with a reference sample. These are commonly termed matrix effects because they are caused by sample matrix components that are different from the reference or calibration samples. There are many different sources of matrix effects, but they fall into a few common categories. Sample transport and nebulization effects result from physical attributes (such as viscosity, volatility, or surface tension) of the sample that can alter the efficiency of sample transport and nebulization, resulting in either increased or decreased sample reaching the plasma. Ionization suppression is caused when high concentrations of easily ionized elements in the plasma preferentially suppress the ionization of elements with higher ionization potentials. Space-charge effects preferentially suppress low mass ions in the presence of high concentrations of high mass ions by repelling the low mass ions from the highly positive ion beam in the ion optic region of the mass spectrometer. A reliable test for measuring matrix tolerance in general is the rate of formation of metal oxides in the plasma. Cerium oxide is normally used. Figure 1 shows the signal suppression effects of undiluted seawater on a group of elements for cerium oxide levels ranging from 0.2% to 2%. Not only is sensitivity higher at lower CeO/Ce ratios, but internal standard correction is simpler and more reliable and accuracy is much improved. Other effects can enhance the response of groups of analytes specifically. For example, it is known that the presence of organic carbon in the sample will enhance the response for arsenic and selenium and possibly a few other similar elements. Within limits, matrix effects are controlled through the use of internal standards. Internal standards are elements (or isotopes) other than the analytes that behave similarly to the analytes in the presence of matrix effects. As a result, their behavior can be used to correct for presumably similar (though unknown) effects on the analytes. To be effective, the internal standards must respond to the variable matrix conditions in exactly the same way as the analyte elements. They also must not be present in the original sample. The perfect internal standard is another isotope of the analyte element spiked at an unnatural abundance

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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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