NONPOLAR-SOLVENTS FOR NORMAL-PHASE LIQUID-CHROMATOGRAPHY AND POSTCOLUMN EXTRACTION IN THERMOSPRAY LIQUID-CHROMATOGRAPHY MASS-SPECTROMETRY

The use of n-hexane, cyclohexane and dichloromethane as liquid chromatographic eluents has been explored in thermospray liquid chromatography/mass spectrometry (TSP LC/MS). By use of positive and negative ion modes (PI and NI, respectively), and the filament-on mode, three different groups of model compounds have been studied: Organophosphorus pesticides, chlorophenols, and chlorinated phenoxy acid herbicides. In the PI mode, [M + H]+ was the base peak for the organophosphorus pesticides in all solvents, and [M].+, [M - H]+, or [M - Cl]+ were the base peaks for the other compounds depending on the solvent used. In the NI mode, the functional group fragment [FG]−, [M - R]− (R being methyl or ethyl), [M + Cl]−, and [M - HCI]− were the base peaks for the organophosphorus pesticides. Depending on the compound and the solvent used, [M - HCI]−, [M - H]−, [M + H]−, and [M + Cl]− were the base peaks for the chlorophenols and the chlorinated phenoxy acids. The formation of a chloride attachment ion, [M + Cl]−, was more important when cyclohexane or dichloromethane were used as eluents. By use of PI and NI modes under full scan conditions, detection limits in the low nanogram range were obtained. The sensitivity In the PI mode improved 1 order of magnitude compared with previous studies using reversed-phase eluents In TSP LC/MS. Two applications are reported: (1) the analysis of organophosphorus pesticides, using n-hexane-dichloromethane (50:50) as LC eluent, and (2) the use of a postcolumn extraction system with cyclohexane-dichloromethane-1-butanol (45:45:10) as extraction solvent after the ion-suppressed LC separation of chlorinated phenoxy acids using acetonitrile-water containing a nonvolatile buffer. © 1990, American Chemical Society. All rights reserved.