Optimal procedure for combined high-performance thin-layer chromatography/high-sensitivity liquid secondary ion mass spectrometry

Abstract

An optimized procedure is reported for the analysis of non-volatile compounds from the surface of silica gel high-performance thin-layer chromatography plates. Following chromatography individual compound bands, excised from the plate with their aluminium support and attached to the normal probe target with an electro-conducting adhesive, were ionized by liquid secondary ion mass spectrometry (LSIMS). Several compound types were analysed to assess sensitivity and specificity of detection. Urinary porphyrins provided molecular weight information from subnanomolar amounts. The detection sensitivities of free, glycine-conjugated and taurine-conjugated bile acids were approximately 125 pmol, 100 pmol and 20 pmol, respectively. A 21-component mixture containing oligosaccharide derivatives was used to evaluate the retention of spatial resolution and specificity achievable by the method. Prevention of diffusion with optimal definition of sample bands, avoidance of electrical insulation between matrix and target probe (particularly in negative ion mode) and matrix conditions were important determinants of sensitivity for in situ LSIMS analysis. Irreversible adsorption levels on the silica gel plate were shown to be greater using solvent extraction of compounds from silica removed from the plate than from in situ LSIMS detection.