Mass scanning spectrum

FIGURE 15.39 Extracted ion profile taken from the TIC shown in Figure 15.34 at m/z 314 representing the compound chlorpyriphos. The full mass scan spectrum of chropyriphos is shown in the bottom part of the figure. 

The mass scan mode is used to survey the entire mass spectrum within a certain volume of the specimen. 

Another advantage of mass spectrometry is its sensitivity - a full-scan spectrum, and potentially an identification, can be obtained from picogram (pg) amounts of analyte. In addition, it may be used to provide quantitative information, usually to low levels, with high accuracy and precision. 

The limited resolution and mass accuracy of the early MALDI-TOF instruments made the combination of MALDI with magnetic sector instruments (Chap. 4.3) desirable, [148,149] but this set-up suffered from low shot-to-shot reproducibility and poor sensitivity getting a full scan spectrum required thousands of laser shots while scanning the magnet. Even though eutectic matrix mixtures were introduced to circumvent such problems, [90,91] the MALDI-magnetic sector combination never became established. 

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... 

The existence of a metastable ion and its relationship to m1 and m2 thus confirm that the ion m2 was in fact formed directly from mx. There are in theory many possible solutions to the equation, but the actual solution is normally obtained by inspection of the spectrum using major peaks, usually of similar intensity, as possible values for mx and m2. For spectrometers which have an exponential mass scan this is a simple operation since the distances between m and m2, and m2 and m1 will be identical. 

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. 

Fig. 1 XPS wide-scan spectrum a of copper particles coated with PVFA-co-PVAm (molar mass of the PVFA pre-polymer = 340000 g mol degree of hydrolysis >90%, adsorption at pH = 8). The insets show the C Is (b), N Is (c) and Cu 2p (d) element spectra...

The instrument is a 90° sector, 5 cm radius analyser and can resolve adjacent masses up to about 140. It can detect partial pressures of 10" torr and when used with a suitable electron multiplier, the mass scan can be displayed on an oscilloscope. The spectrum can be scanned in a few milliseconds thus enabling rapid reactions to be followed. 

Full scan spectrum Scanning analyzer N/A Not required Qualitative analysis Nonse-lective detection. Mass spectrum obtained. Single- or triple-quadruole... 

This analytical mode has a resolution sufficient for the fragment ions. For instance, the B/E linked scan spectrum of the unimolecular decomposition of the MH ion (m/z 259) of biotin methyl ester, produced by Cl ammonia, has a good resolution (< 1 u). The m/z 221, 241 and 243 ions are due to the elimination of neutral fragments (CH3OH, HjO and CH4), which characterize Cl mass spectra (Fig. 21). 

Figure 24. Cl mass spectrum and BjE — E/Eo) linked scan spectrum of neutral loss of ketene (42 amu) for a mixture of various acetylated phenols [149].

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