Capillary Electrochromatography Coupled to Atmospheric Pressure Photoionization Mass Spectrometry for Methylated Benzo[a]pyrene Isomers

Abstract

Benzo[a]pyrene, one of the most carcinogenic PAHs, has 12 monomethylated positional isomers (MBAPs). A strong correlation between the carcinogenicity of these isomers and methyl substitution has been reported. In this study, on-line coupling of capillary electrochromatography (CEC) and atmospheric pressure photoionization mass spectrometry (APPI-MS) provides a unique solution to highly selective separation and sensitive detection of MBAP isomers. The studies indicated that APPI provides significantly better sensitivity compared to electrospray ionization and atmospheric pressure chemical ionization modes of MS. A systematic investigation of APPI-MS detection parameters and CEC separation is established. First, several sheath liquid parameters (including type and concentration of volatile buffers, type and content of organic modifiers, use of dopants and inorganic/organic additives, and sheath liquid flow rate) and APPI-MS spray chamber parameters (capillary voltage, vaporizer temperature, nebulizer pressure) were found to have effects on detection sensitivity as well as the profile of mass spectrum. For example, when ammonium acetate was replaced with acetic acid in the sheath liquid, the MS signal was enhanced as much as 90% and the formation of ammonia adduct was effectively suppressed. Next, the separation of MBAP isomers was conducted on internal tapered columns packed with polymeric C18 stationary phase. With the use of a mobile phase consisting of slightly higher acetonitrile content (90%,v/v) and a small amount of tropylium ion, the analysis times were significantly shortened by 20 min without compromising the resolutions between the isomers. Finally, quantitative aspects of the CEC-APPI-MS method were demonstrated using 7-MBAP as the internal standard. The calibration curves of three of the most carcinogenic isomers, namely, 1-MBAP, 3-MBAP, and 11-MBAP, showed good linearity in the range of 2.5-50 microg/mL with a limit of detection at 400 ng/mL.