Exact mass measurements for confirmation of pesticides and herbicides determined by liquid chromatography/time-of-flight mass spectrometry

The accuracy and precision of exact mass measurements are determined using positive ions formed in the electrospray of 10 nonvolatile or thermally uns table carbamate, urea, and thiourea pesticides and herbicides. Environmenta lly significant similar to 7-ng quantities of the analytes were separated w ith microbore liquid chromatography, and the exact mass measurements were m ade in real time with a benchtop time-of-flight mass spectrometer. The posi tive ion electrospray mass spectra of the analytes generally consist of one or a few ions which are usually adducts of the molecule with a proton, a s odium ion, or an ammonium ion. Fragment ions and the rich mass spectra typi cal of electron ionization (EI) are generally not produced in the soft elec trospray ionization process. Confirmation of the identity of a nonvolatile pesticide or herbicide depends largely on the masses of the few ions formed and the retention time, which can vary with chromatography conditions. Ide ntifications of these analytes in environmental or other samples are less c ertain than identifications of volatile pesticides determinated by gas chro matography and El mass spectrometry. The benchtop time-of-flight mass spect rometer was equipped with an electrostatic mirror, and resolving powers of 3500-5000 were routinely obtained and used for these exact mass measurement s. This type of mass spectrometer is significantly less costly and complex than other types of mass spectrometers with exact mass measurement capabili ties. The mean errors from three replicate exact mass measurements of the 1 0 test analytes were in the range of 0-5.4 parts-per-million. Potential int erferences from substances with similar exact masses were evaluated.