In order to address the problem of compounds coeluted within the TIC, the next step is the deconvolution of background-free peak spectra. The speed of data acquisition by the TOF-MS system allows the summation of many single scans to scan sets. The scan sets provide high S/N values and spectra that are unaffected by the phenomenon of spectral skew, which in turn enables effective deconvolution.
Deconvolution is followed by chemometric data analysis of the distinct spectra.When a target compound is detected, the software derives a match coefficient (0–1).Ultimately, a report is generated listing all positive compound identifications.
The background-compensated TIC of the urine sample within the deconvolution software can be seen in Figure 3. The software has interrogated the TIC for compounds listed in the drug library created. Detected target compounds are represented by red bars and the heights of the bars represent their peak sum. A section of the report of targets found can be seen in Table 1.
A number of compounds that would have been difficult or even impossible to identify from quadrupole data or using traditional compound identification software have been identified in the sample. In particular, it has been difficult historically to detect benzodiazepines using GC-MS due to the polarity of the non derivatized analytes. The high sensitivity and non skewed classical spectra provided by the TOF-MS system in combination with the advanced deconvolution software used has resulted in the confident identification of this class of compounds in their non-derivatized form.
Figure 4 illustrates the capabilities of the system, showing a saturated amisulpride peak that is overlapping a small concentration of alprazolam completely. By limiting the mass range between 100 and 400amu, alprazolamis positively identified, showing good spectral similarity to the library spectrum (Figure 5) and, as a result, a high match coefficient (0.88). The deconvolved spectrum versus the library spectrum (green) is shown in Figure 5. The quality of spectra is excellent despite the large matrix interference. Using the advanced software, it is also possible to library search unknown peaks in the NIST database for confident and comprehensive profiling of the sample.