Performance of a modular thermospray interface for signal enhancement in flame atomic absorption spectrometry coupled on-line to flow injection or liquid chromatography

Abstract

A modular thermospray interface for improving sample transport efficiency in conventional flame atomic absorption spectrometry (AAS) burners has been developed and optimized for flow injection (Fl)-AAS and high-performance liquid chromatography (HPLC)–AAS applications. The installation of this interface required no modification of the conventional flame AAS burner. The thermospray unit consisted of two separate thermoelectric elements which heated a 50 µm silica capillary carrying the mobile phase and the nebulizer gas (air), respectively. The capillary and the nebulizer gas were guided through their respective heating elements using inexpensive quartz tubes–Swagelok fittings assemblies. The capillary tube was inserted into the sample aspiration needle of the heated nebulizer nozzle and positioned in the mixing chamber where the overheated mobile phase expanded as a fine thermospray. Under optimum conditions, the thermospray unit completely nebulized 1 ml min l^–1 of the analyte solution. The unit was chemically compatible with water as the flow injection (Fl) carrier and 0.1 mol l^–1 ammonium carbonate, 0.1 mol l^–1 pyridinium ion or 10–100 mmol^–1 TRIS-HCl [TRIS =tris(hydroxymethyl)methylamine] buffer as HPLC mobile phases. In the Fl-thermospray-AAS mode the signal-to-noise ratios were improved 6–8 times over conventional Fl sample introduction, providing signal enhancements that were comparable to figures obtained with more complex and costly systems. An additional contribution to signal enhancement by the heated nebulizer gas (air) was attributed to desolvation of the thermospray aerosol in the mixing chamber. The absolute limits of detection were 1.9 ng of cadmium, 8.5 ng of copper and 27 ng of lead. The interface was also applied for the speciation of cadmium–metallothionein isoforms I and II in the HPLC–AAS mode.