Design and characterization of a thermochemical high-performance liquid chromatography flame photometric detector interface for the speciation of sulfur

A total consumption high-performance liquid chromatography flame photometric detector (HPLC-FPD) interface compatible with mobile phases delivered at conventional HPLC flow rates (0.6–1.0 ml min–1) and containing up to 100% methanol is described. The all-silica apparatus enclosed four consecutive thermal processes: (a) thermovaporization of the HPLC methanolic effluent; (b) pyrolysis of the organic matrix (including S or P containing species) in a kinetic methanol–oxygen flame; (c) volatilization of the analytes in a reducing post-combustion stage fuelled by H2; and (d) transport of the volatilized analytes to the hydrogen radical rich surroundings of an inverted H2–O2 diffusion flame. Molecular emissions induced in the last step were integrated as a narrow beam in a light-guide positioned remotely from the analytical flame and oriented towards a photomultiplier detection unit. Radioisotopic assays demonstrated that sulfur (as H235SO4) was transferred quantitatively to the analytical flame, whereas the phosphorus (as H332PO4) transport efficiency was semi-quantitative (25–43%). Indirect evidence suggested that sulfur and phosphorus were hydrogenated in the post-combustion step via the same thermochemical hydride generation process previously found to mediate the formation of H2Se and H3As under similar conditions. The linearity of calibration graphs (0.9955 < r < 0.9983, where r is the correlation coefficient) and unprecedented HPLC-FPD limits of detection for sulfur species (1.5 ng s–1 for 2-methylthiophene, 2.25 ng s–1 for CS2, and 4.5 ng s–1 for ethanesulfonic sulfonate) allowed the speciation of sulfur in garlic extracts.