Background-subtracted mass spectrum

Figure 3.17 Background-subtracted mass spectrum obtained from the component eluting after 4.65 min in the LC-MS analysis of a pesticide mixture. From applications literature published by Micromass UK Ltd, Manchester, UK, and reproduced with permission.

Fig. 2.11 An automatical generated drug screening ESI-MS report for the ESI background subtracted mass spectrum, illustrated in Fig. 2.1 OC, obtained from the GPC spin column eluate of the ten-component mixture incubated with RGS4 protein from well Al. Note that the highlighted components are the three compound hits that non-covalentl) bind to the RGS4 protein.

This isotope cluster technique [61] was used to facilitate the identification of omeprazole metabolites in rat urine [62]. Omeprazole is a selective inhibitor of gastric acid secretion and is used in the treatment of acid-related disorders. A 1 1 mixture of natural [ 5]-omeprazole and labelled [ S]-omeprazole was administered to a rat. More than 40 related substances were found in the LC-MS chromatogram of the rat urine sample. All these compounds were related to omeprazole, because they showed the expected isotopic cluster (two peaks of similar height two m/z units apart). The summed background-subtracted mass spectrum of the entire sample is shown in Figure 10.9. This spectrum represents a good overview of the metabohc pattern and the metabolic routes omeprazole is involved in. 

Another example of excellent spectral integrity under these d anding conditions is illustrated in Figure 15.41. The ion of m/z 200 for atrazine is extracted from the TIC and the background subtracted mass spectrum is a good match to the reference mass spectrum. 

Analyzing the background-subtracted mass spectrum once matrix ions have been identified, they can be subtracted from the spectrum. The resulting spectrum provides information on the molecular idaitity of the chlorinated unknown for instance, the molecular waght and the number of chlorine can be readily obtained fiom the quasi-molecular itms. The comparison of the background-subtracted spectram with the spectrum of the synthesized reference standard, if available, that matches in retention time with the chlorinated unknown will establish the idmtification of the unknown analyte. 

Fig. 2.10 Application of the background subtraction algorithm between two consecutivel) acquired ESI mass spectra from GPC spin column eluates of ten-component mixtures incubated with RGS4 protein. (A) Foreground ESI mass spectrum for the GPC spin column eluate of a ten-component mixture incubated with RGS4 protein from well Al. (B) Background ESI mass spectrum corresponding to the GPC spin column...

Fig. 8. Mass spectrum, with background subtracted, of pbo-toionized (Cfto) Rbv clusters containing both singly and doubly ionized species tbe solid line connects peaks belonging to groups of singly ionized clusters with a fixed value of n. Note tbe dominant peaks corresponding to (C (,Rb, ) Rb and (QoRb,.,) Rb2 (marked...

While m/z 229 and 251 were observed in the spectrum of the previously eluting component (see Figure 3.16), it is necessary to confirm that their presence in this spectrum is solely from that source and not from another component whose mass spectrum also contains these ions. If this can be done, the spectrum of the second component can be obtained by background subtraction. 

Further experimental work involving cone voltage studies may provide further confirmatory evidence but the most likely explanation is that the mass spectrum of the component with retention time 4.65 min is that shown in Figure 3.17, while the mass spectrum of the second component is that obtained by background subtraction, and is shown in Figure 3.22. 

Background-subtracted spectrum A mass spectrum from which ions arising from species other than the analyte have been removed by computer manipulation. 

FJciureJyT. (a) Mass spectrum at the top of the chromatographic peak (b) background spectrum (c) analyte spectrum after background subtraction (d) library spectrum of hexachlorobiphenyl (score 99%). 

UV Etching. A typical mass spectrum of the vaporized UV etching products is shown in Figure 4, together with a background spectrum obtained without UV irradiation. The comparison clearly shows that UV irradiation causes an increase in intensity for various mass peaks. For example, the intensity of the peaks of m/e=15, 31, 59 and m/e=41, 69 increased drastically by UV irradiation. The former three are due to side-chain scission caused by UV absorption at the C=0 unit, while the latter two are due to main-chain scission initiated by side-chain scission (11). The structure and mass numbers of typical vaporized species are shown in Table I. From here on, we use the spectral intensity after the background is subtracted. 

FIGURE 22 (a) HPLC/UV chromatogram, (b) ESI LC/MS total ion chromatogram, (c) extracted mass spectrum for RT = 25.35 min peak without background subtraction, (d) Extracted mass spectrum for the same peak with background subtraction. 

In this case the survey scan was set as a full scan and the dependent scan as a product ion scan. The problem with data dependent acquisition is to determine the selection criteria. In most cases the system picks up the most abundant ion in the full scan spectrum. An inclusion list with masses of potential metabolites or exclusion list of known interferences significantly improves the procedure. In the example shown in Fig. 1.39, a procedure called dynamic background subtraction (DBS) was applied. This procedure considers chromatographic peak shapes and monitors not the most abundant signal in the spectrum but the largest increase of an ion in a spectrum. The advantage is that once a signal of a peak has... 

Figure 11. (b) Mass spectrum (subtracted for background averaged over 10 scans) of mid-chain 2,5-dialkylthiophenes XV from the sample at 137m. 

In order for an identification to be made from mass fragmentograms sufficient ions (at least six) must be scanned. It is sometimes possible to obtain a full, interpretable mass spectrum in such cases by background subtraction. The scan (spectrum) in which the ions due to the compound of interest are at a maximum is determined from the mass fragnentograms. This spectrum is then "cleaned up" by subtraction of those ions contributed by the contaminant. It is also possible to compare the background-subtracted spectrum directly with authentic spectra stored in the instrument. 

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