Development of a methodology utilizing gas chromatography ion-trap tandem mass spectrometry for the determination of low levels of caffeine in surface marine and freshwater samples

摘要

A methodology for monitoring low level of caffeine in aqueous samples via gas chromatography coupled with an ion-trap tandem mass spectrometry detection system (IT-MS/MS) was developed. Four ITMSMS operating parameters, including the collision-induced dissociation ( CID) voltage, the excitation time (ET), the isolation time ( IT) and the maximum ionization time (MIT) were optimized in order to maximize the sensitivity of the IT-MS/MS technique towards the analyte and its isotope-labeled standard. After optimization, a limit of detection of 500 fg mu l(-1) with S/N= 3 was achieved. Taking into account blank values and the matrix background, a method detection limit of 1.0-2.0 ng l(-1) was derived and applied to all of the samples analyzed in the study. Various mass spectrometric conditions have been applied to caffeine and its trimethyl-C-13-labeled standard to elucidate fragmentation pathways for new and commonly occurring product ions observed in the collision-induced dissociation (CID) spectra produced by the ion trap. Ion structures and fragmentation pathway mechanisms have been proposed and compared with previously published data. An isotope dilution method using C-13-labeled caffeine as a surrogate internal standard was employed to determine and correct the recovery of native caffeine in water samples. The developed methodology has been applied for the determination of caffeine in surface marine and freshwater samples collected on the west coast of Vancouver Island in British Columbia, Canada. The results obtained for the marine water samples indicated a wide variation in the level of caffeine, ranging from 4.5 to 149 ng l(-1), depending on the location of the sampling site within the inlet. The concentrations of caffeine in samples from lakes associated with various residential densities ranged from ND to 6.5, 1.8 to 10.4 and 6.1 to 21.7 ng l(-1) for low, moderate and high residential densities, respectively.