Comparison of pneumatic nebulization and hydride generation inductively coupled plasma mass spectrometry for isotopic...

A comparative investigation between pneumatic nebulization and continuous hydride generation as sample introduction methods for inductively coupled plasma mass spectrometry was carried out for isotopic analysis of selenium in biological samples of interest to human metabolic studies. Experimental parameters known to affect the analytical performance of the system were evaluated: instrument operating parameters, analyte solution/NaBH4 flow rate, and NaBH4 concentration. Signal-to-background ratio was examined for the three stable isotopes 74Se, 77Se, and 82Se. While background count rates for the hydride system were 3-5 times larger than those for the nebulization method, the signal-to-background ratios, normalized for Se concentration, were 30-50 times greater for the hydride system. Absolute detection limits (3 sigma) for the two systems were 20-60 (nebulization) and 0.6-1.8 (hydride) ng of Se. Overall memory of the hydride system was evaluated. Measurable effects were observed within 400 s from switching to analyte solution with differing isotopic composition, only if the sequence of analysis was from high to low ratio (1-4% bias). However, if the sequence was from low to high ratio, precise and linear calibration plots could be obtained over the isotope ratio range of an order of magnitude or higher. While further improvements might lead to potential enhancement of sensitivity and precision of as much as an order of magnitude, the present performance of the hydride system was satisfactory in relation to the requirements of isotopic analysis for metabolic investigations employing 74Se as the in vivo stable isotope tracer.