Determination of the elemental composition of trace analytes in complex matrices using exact masses of product ions and corresponding neutral losses

The emergence of time-of-flight (TOF) and hybrid quadrupole/time-of-flight (Q-TOF) mass spectrometers has offered new possibilities for determining the elemental composition of analytes present at trace levels. The mass accuracy provided by these instruments is currently in the range of 2-5 m m/z units, permitting the determination of the elemental composition of small molecules. The orthogonal information of relative isotopic abundances (RIAs) is used to reduce the number of elemental compositions that are possible, based on consideration of exact masses. Elimination of additional possible compositions has been reported when the analyte is fragmented and its resulting product ions and corresponding neutral losses are carefully analyzed. Published algorithms reduce the number of proposed precursor ions by deleting each precursor candidate which cannot be explained by summing any combination of postulated product ion and corresponding neutral loss elemental composition candidates. An extension of such algorithms is described in this paper. This approach compares not only the precursor ion with the different fragments, but tests the possible descent of any ion from all other recorded ions. This extended algorithm has been tested by processing published data. Algorithms analyzing product ion spectra can be used for real-life data. However, there is a risk that an ion which originates from the mobile phase or from a co-eluting matrix compound can be mathematically correlated to the investigated precursor ion. Such an incorrect correlation can lead to the deletion of a correct elemental composition. This is an important issue if TOF rather than Q-TOF instruments are used. Therefore, ultra-performance liquid chromatography (UPLC) and a peak deconvolution algorithm were used to generate and process TOF chromatograms in order to minimize the number of ions which are not related to the analyte precursor ion. The combined use of chromatographic deconvolution and product ion spectra has been tested and is critically discussed. Copyright (C) 2007 John Wiley & Sons, Ltd.