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 unstable carbamate, urea, and thiourea pesticides and herbicides. Environmentally significant approximately 7-ng quantities of the analytes were separated with microbore liquid chromatography, and the exact mass measurements were made in real time with a benchtop time-of-flight mass spectrometer. The positive 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 sodium ion, or an ammonium ion. Fragment ions and the rich mass spectra typical of electron ionization (EI) are generally not produced in the soft electrospray 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. Identifications of these analytes in environmental or other samples are less certain than identifications of volatile pesticides determinated by gas chromatography and EI mass spectrometry. The benchtop time-of-flight mass spectrometer was equipped with an electrostatic mirror, and resolving powers of 3500-5000 were routinely obtained and used for these exact mass measurements. This type of mass spectrometer is significantly less costly and complex than other types of mass spectrometers with exact mass measurement capabilities. The mean errors from three replicate exact mass measurements of the 10 test analytes were in the range of 0-5.4 parts-per-million. Potential interferences from substances with similar exact masses were evaluated.