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

Abstract

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.