24 February / March 2018
Figure 5: Chromatographic profiles of extracted ions in one dimensional chromatography (left) and GCxGC, right.
accuracy increases to 92% upon reviewing the data after the study was unblinded. This accuracy would certainly increase after injecting the individual reference standards (future work), particularly those compounds which are missing from NIST and using them for confirmation. Or alternatively, the limitation of being constrained to those available only from NIST would be relaxed and the accuracy of identification and total percentage of identified spiked components, will only increase.
The use of a large curated database is a very significant advantage for this methodology. The latest GC/MS (EI, 70eV NIST 17 mainlib and replib) databases contain spectra for over 260,000 different chemical compounds and has been in use and validated for identification for many years [4-8]. Contrasting the GC/MS database against the ever-growing MS/MS database (for LC/ MS) with spectra for greater than 15,000
Table 3: Analyte information for both 1D and GCxGC chromatography examples (Figure 3). Note the * indicates the accuracy for Chemical Ionisation in this case.
compounds, truly sets the identification power of GC/MS for potential identifications over and above that for LC/MS. GCxGC HRAM TOF-MS does a superior job of identifying known unknowns at on average of 84% to the full compound list available from NIST library, as demonstrated above for these exposome standard sample mixtures.
The advantages of this approach and method are readily apparent from the percent accuracy of identification; nearly 84% correct assignments while blind is an outstanding number. Using this method practically 4 out of every 5 unknowns will be identified if they are in the NIST database. The consistency in identification across
Figure 6: Accuracy of identification of the experimental method of blind and unblinded results. Accuracy is defined as the correct identifications versus the total number of expected identifications or Accuracy = (correct/total) x 100% of the Identifiable NIST compounds.
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