The analytical characteristics of a 27.12-and a 40.68-MHz inductively coupled plasma (ICP) are compared as ion sources for use in inductively coupled plasma mass spectrometry. The background spectra produced by the two sources are similar; however, there is evidence of greater entrainment of air in the higher frequency discharge. Calibration graphs are linear over 6-7 orders of magnitude with both operating frequencies, although the 40.68-MHz ICP suffers a factor of 2-5 loss in sensitivity. Accordingly, detection limits are also slightly worse at the higher operating frequency, as is the precision. Interference effects are independent of operating frequency, but the extent of doubly charged, oxide and hydroxide ion formation might be affected by a more significant orifice-linked discharge with the 40.68-MHz ICP.
A microwave-induced nitrogen discharge at atmospheric pressure (MINDAP) Is used as the Ion source for elemental mass spectrometry (MS) and compared to the use of the Inductively coupled plasma (ICP). Optimization studies are presented to Illustrate the dependence of signals on various Instrumental parameters. Detection limits determined for five elements range from 3 to 22 ng/mL, somewhat higher than those determined with an ICP and the same mass spectrometer system. The background mass spectrum from the MIN-DAP Is dominated by NO+; oxide and hydroxide Ion ratios are higher than for ICP-MS. The linear dynamic range is similar to that In ICP-MS, but Interferences caused by concomitant elements are much worse In MINDAP-MS.
Ionization temperatures in the inductively coupled plasma are determined with the use of the ionization fraction for iodine obtained by mass-spectral sampling. Temperatures profiled at different heights and different positions across the plasma are between 6900 and 7800 K at relatively low heights. Power increases raise the measured ionization temperature, while changes in nebulizer gas flow and the addition of an easily ionized element to the plasma have no measurable effect. Desolvation of the sample aerosol causes a decrease of about 500 K in the ionization temperature.
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