Spectroscopy - May 2008 - (Page 8)

8 SPECTROSCOPY GLOBAL EDITION MAY 2008 do this with every sample, though clearly, we need to manage these interferences — but how, and to what extent? The “how” part is fairly well understood, given the tools currently available, and it depends upon dividing the interferences roughly into two groups that we call spectroscopic interferences and nonspectroscopic interferences. Spectroscopic and nonspectroscopic interferences are caused by different things and are handled differently, but the net effect is the same — if they are not reduced sufficiently, they can impact the analytical result. The “to what extent” is the topic of this column. 0.2% CeO/Ce 1% CeO/Ce 2% CeO/Ce 120 100 Signal recovery (%) 80 60 40 20 0 Ba Zn Cr Fe Sc Cu Cd 111 137 63 115 45 52 56 66 Elements Figure 1: The effects of increasingly efficient matrix management of signal suppression. Undiluted NASS-5 seawater (TDS ~ 3%) spiked with eight elements and measured by ICP-MS with typical matrix tolerance (CeO/Ce = 2%), good matrix control (CeO/Ce = 1%), and excellent matrix control (CeO/Ce = 0.2%). Spectroscopic and Nonspectroscopic Interferences Spectroscopic interferences contribute directly to a specific analyte signal by possessing the same mass-to-charge ratio (m/z) as the analyte ion. There are three types, all of which must be managed differently. The first type is isobaric (an isotope of another element with the same mass as the analyte isotope), for example, 100Mo and 100Ru. In nearly all cases, these can be avoided through the selection of an alternate analyte isotope that does not suffer an isobaric overlap. The second type is doubly charged. Because the mass spectrometer measures mass to charge, rather than purely mass, if the charge is other than 1, the mass to charge will be different from the mass. A number of common elements possess second ionization potentials sufficiently low to allow formation of doubly charged ions in an argon plasma. These ions will interfere with analytes at half their actual mass. For example, 136Ba2+ commonly interferes with 68Zn+. Again, isotope selection is the most common way to avoid doubly charged interferences. The third and most problematic type of spectroscopic interference is polyatomic. In this case, the presence of a molecular ion, either from the plasma or from interactions in the interface region or collision cell, possesses the same m/z as the analyte ion. These are extremely common and frequently result in multiple polyatomic interferences on a single analyte isotope. Table I shows some typical examples. Management strategies for poly- In

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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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