Characterization of an Ultrasonic Nebulizer–Membrane Separation Interface with Inductively Coupled Plasma Mass Spectrometry for the Determination of Trace Elements by Solvent Extraction

Abstract

The problems of using an ultrasonic nebulizer–membrane desolvation separation interface (USN–MEMSEP) for the determination of trace elements by solvent extraction and ICP-MS were studied. The interference effects of chloroform and the behavior of trace metal chelates as a function of MEMSEP temperature and sweep gas were studied. In comparison with conventional nebulization, the use of the interface resulted in an approximately 10-fold increase in analyte signals. Although the interface removed much of the chloroform vapor from the aerosol stream by selective permeation and argon counter gas purging, residual solvent resulted in polyatomic ion interferences that affected the limits of detection. The addition of a small flow of oxygen to the auxiliary gas minimized these interferences and prevented carbon deposition on the torch tubes and sampler cones. The ⁴⁰ Ar ¹² C ⁺ signal was attenuated, but those of ⁴⁰ Ar ¹⁶ O ⁺ and CeO ⁺ /Ce ⁺ increased slightly. An increase in MEMSEP desolvation temperature resulted in a decrease in ³⁵ Cl ¹⁶ O ⁺ and ⁴⁰ Ar ¹² C ⁺ signals due to enhanced rejection of chloroform. Thermal desolvation of the metal organic compound vapors and aerosols resulted in a decrease in the ion counts of the chelated analytes with increasing temperature, probably due to their volatilization and rejection from the membrane. An internal standard could be used to compensate for the responses to changes in temperature. Signal responses of the metal dithiocarbamates to changes in MEMSEP desolvation temperature were significantly different to those in chloroform solutions of oil-based standards, and as a consequence the latter were unsuitable for calibration. The advantages of the technique include matrix elimination, marked reduction in polyatomic carbide ions, enhanced LODs and reduced plasma interferences.