Semi-volatile samples were analyzed by high performance liquid chromatography (HPLC) coupled with a corona charged aerosol detector (corona CAD). Nine phthalates with different side chains (carbon numbers of 1–8) were used as samples, and their concentrations were changed from 6–188 μg mL
−1. Acetonitrile, methanol, tetrahydrofuran (THF), acetone, 2-propanol and their binary mixtures were used as mobile phase. All experiments were carried out under isocratic conditions. Phthalates with higher vapor pressure show lower sensitivity for corona CAD. Dimethyl phthalate (DMP) showed no peak in the experiments. The signal intensities of phthalate observed by corona CAD were drastically changed with the compositions of the mobile phase. When mixtures of acetonitrile and methanol were used as the mobile phase, all of the phthalates, except for DMP showed the maximum intensities at the composition of acetonitrile/methanol = 35/65 (v/v). Acetonitrile/THF and methanol/THF also showed the maximum intensities at the compositions of acetonitrile/THF = 80/20 (v/v) and of methanol/THF = 70/30 (v/v). On the other hand, the intensities were changed monotonously using acetonitrile/acetone or acetonitrile/2-propanol as mobile phases. The estimated sensitivity (
Sm) of corona CAD, calculated from an empirical equation, agreed qualitatively with all of the results monitored for the binary mixtures of mobile phase.
Sm generally varies depending on the size of aerosol prepared by nebulization of an analyte solution in corona CAD. The size of the aerosol changes with the composition in the mobile phase. Therefore, the signal intensity of a sample component observed by corona CAD should be changed with the mixed composition of the mobile phase solvents.
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