Background Subtraction with Library Search

Once a mass spectrum from an eluting component has been acquired, the next step is to try to identify the component either through the skill of the mass spectroscopist or by resorting to a library search. Most modem GC/MS systems with an attached data station include a large library of spectra from known compounds (e.g., the NIST library). There may be as many as 50,000 to 60,000 stored spectra covering most of the known simple volatile compounds likely to be met in analytical work. Using special search routines under the control of the computer, one can examine 

Column bleed gives a mass spectrum (a) that is mixed with an eluting component to give a complex spectrum (b). By subtracting (a) from (b), the true spectrum (c) of the eluting component is obtained. 

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These data for successive scans are then stored for subsequent manipulation. The reconstructed total ion current trace, equivalent to that obtained from a flame ionisation detector in gas chromatography, shows the variation of total ion current with time and allows spectra of interest to be identified. A typical example is shown in Fig. 6A. The background may be subtracted to give clean spectra, and their identification may be attempted using libraries of standard spectra. If a composite spectrum is obtained from two unresolved peaks, complex subtraction routines may be used to obtain a pure spectrum of each of the components. These may be separately submitted for library searching. The spectra may then be plotted or obtained as a mass versus intensity listing. 

The 2 examples presented above were examined before we had obtained our computer. In the following the merits of the latter component should be apparent. Fig. 4 shows a case with a composite multiple intoxication picture. The dots just under the curve line indicate repetitive scans taken automatically and collected by the computer. During the GC the apparently important scans were plotted out in a background-subtracted normalized form and exposed to interpretation including a library search. It is hard to see how non-MS-systerns could have been so informative in an hour s time. Possibly with non-MS methods one had become satisfied to indicate one or two of the drugs, disregarding the presence of the other ones, with potential consequences for the treatment and for the interpretation of the case. 

Figure 3.32 NIST library search of the difference spectrum of Figure 3.31 after careful background subtraction. The RSI match value 833 is high, SI value with 718 is low due to other fragments from background ions, the probability value of 74% is high due to the unique spectrum of the first hit. The difference spectrum shows signals of the coeluting compound of Figure 3.31.

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