Exact mass measurement of product ions for the structural confirmation and identification of unknown compounds using a quadrupole time-of-flight spectrometer: a simplified approach using combined tandem mass spectrometric functions.

This article describes a simple method to perform lock mass corrected accurate mass measurements in tandem mass spectrometry (MS/MS) with a quadrupole time-of-flight (Q-TOF) mass spectrometer. The experimental approach consists of using the protonated molecule of a known compound, which is measured in a MS/MS function using low collision energy (no fragmentation), as mass calibrator. The unknown compound is acquired in MS/MS mode albeit using high collision energy. After the acquisition, the two MS/MS spectra of unknown and mass calibrator are combined, and the fragments of the unknown are lock mass corrected by using the protonated molecule of the mass calibrator. To prove this concept, 10 compounds were analyzed using this approach, the fragments interpreted and, where possible, related to structural data available in the literature. All the unequivocally assigned fragments were accurately mass measured with mass errors within appropriate limits, i.e. for m/z values <200 with a mass tolerance of 3 mDa while for m/z > 200 the mass tolerance is expressed as 10 ppm.