THERMOSPRAY NEBULIZATION AS SAMPLE INTRODUCTION FOR INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

A sample introduction system, consisting of a thermospray nebulizer and a desolvating unit, was coupled to an inductively coupled plasma mass spectrometer. Several parameters were optimized including the sample uptake rate, the power delivered to the capillary tube, the temperature of the aerosol and of the cooling-water, the carrier gas flow rate and the r.f. power. Under optimized conditions, the sensitivity obtained with the thermospray system is a factor of 10 higher than with a pneumatic nebulizer combined with a spray chamber. The increase was observed for the following elements over the whole mass range: Be, Al, Sc, Co, In, Gd, Tl, Th and U. Since similar background levels, 10-40 counts s-1, were observed, better detection limits could be achieved (from 0.19 ng l-1 for U to 1.3 ng l-1 for Be). The relative standard deviation (RSD) on analyte signals for a short-term stability test (10 min) is between 3 and 8%. These values can be improved to less than 4% by the use of internal standardization. No drift in analyte signal during several hours was observed. The levels of oxide (MO+:M+) and doubly charged (M2+:M+) ions obtained with the thermospray system are about a factor of 2.5 lower than those obtained with the pneumatic system. Consequently, the oxide ion levels for elements with the highest MO bond strength and the doubly charged ion levels for elements with the lowest second ionization energy are not above 1%.