Organotin speciation using fast flow glow discharge mass spectrometry

Abstract

A low power (≤5 W) dc fast flow glow discharge (FFGD) ion source, coupled to gas chromatography, was evaluated for the speciation of tetramethyl-, tetraethyl-, triethyl- and tributyltin compounds. This source differs from other GC-GD techniques in that the sample is introduced into a flowing afterglow and not the active discharge region. The rapid flow of Ar gas carried the analyte towards the sampling cone, reducing diffusional losses and source fouling. The Ar discharge gas flow rate, source pressure, discharge voltage and ion extraction voltage (applied to the sampling cone) were optimised with respect to the Sn⁺ and SnH⁺ signals. Absolute detection limits of 0.2–0.4 pg were determined. The technique demonstrated excellent linearity (response indices 0.98–1.01) with a dynamic range of 3–4 orders of magnitude. The fundamental chemistry behind the trends observed is considered. The relative ion intensities were shown to vary with the source operating conditions. The ion extraction voltage, in particular, could be varied to obtain controllable fragmentation without affecting the primary discharge. From measurements of the relative ion kinetic energies, the observed fragmentation has been attributed to dissociation by electron bombardment in the ion extraction field. Structural information was discernible at the 0.03–10 ng level. The FFGD technique combines sensitive and selective detection with low cost and simplicity (relative to other MS methods). As such, FFGD-MS has potential applications in routine monitoring and identification of unknown organotin compounds in the environment.