Precursor mass

A good way to start sequencing is to begin near the precursor mass. Mass differences between precursor and first ions in C- and N-terminal series are specific to the ion series. Using this approach, we may be able to identify first N- and C-terminal amino acids, respectively. 

Figure 6.11. Product ion spectrum used in Example 1. The spectrum was obtained on the quadrupole ion trap mass spectrometer with ESI source and low energy fragmentation. Precursor mass is 574.3 Da.

Figure 6.12. MS3 of the peptide used in Example 1 from Fig. 6.11. Precursor mass in the first stage was 574.3 Th, and 278.1 Th in the second stage.

Figure 6.14. Example 2 fragmentation spectrum of singly charged peptide, precursor mass of 1098.6.

Fig, 9.2.4 Mean diameter of silver particle versus the cubic root of the precursor mass. (From Ref. 23.)... 

To take advantage of these new dissociation techniques, Joshua Coon and colleagues have recently introduced a third acquisition paradigm, namely, the decision tree acquisition (DTA) approach, which has proven to significantly improve peptide identification rates (24). DTA uses precursor mass, charge, and m/z to automatically determine, in real time, whether to employ CID or ETD during MS2. Later, this acquisition technique has been extended to also employ HCD (25). 

A second source of cross talk occurs for mixtures of precursor-product ions with similar masses. Figure 6b shows the interference occurring when product and precursor ions within 1 Da of each other are monitored simultaneously for an analyte with the structure shown in Fig. 6a. The actual analyte signal should be observed only for a channel monitoring the 416.9-to-244.9 transition. Similar, although less intense, peaks are observed, however, for a precursor mass of 415.9 and/or a product ion mass of 243.9. This cross talk is caused by insufficient mass selectivity in the first quadrupole that allows the M — 1 to be transmitted. 

From an economical point of view the overall conversion efficiency of the precursor is another important parameter for cost considerations. This efficiency is defined as the ratio of infiltrated mass to the precursor mass introduced into the reaction chamber. Investigations indicate that the conversion efficiency is dependent on the mass transport method, residual time of the precursor, pressure and temperature, etc. As shown in Figure 5.9 the conversion efficiency of propylene (C3H6) increases with an increase in residence time but decreases with the partial pressure of the precursor gas. In particular, it is worth noting that the conversion efficiency in the I-CVI process is rather low and generally is in the range of 0.78 to 2.14% [17],... 

C4H7+ ions were generated by collisionally activated dissociation (CAD) in the gas phase from various precursors. Mass spectrometric analysis showed that homoallyl chloride and cyclopropylmethyl chloride generated primarily cation 103, whereas cyclobutyl chloride gave a substantial amount of bicy-clobutonium ion 104. 

The rate of increase in foulant deposit thickness x is the difference between the precursor mass flux to the reaction zone and the foulant mass flux away fi-om the zone A o. i ... 

According to Nitta the introduction of chlorides or bromides into the catalyst salt mass during the formation of deposited catalyst, promotes an increase in ee by virtue of the inerease in D, but the overall rate of the reaetion decreases under these eonditions. Additions of NiCfy NaCl, FeCfy and NaBr or of HCl or HBr to the eatalyst precursor mass prepared by method A (see Table 4.8.) increases ee from 36.1% to 49.7-57.5%, and also increases D from 13 to 20-30 nm while narrowing the size distribution of the niekel crystallites... 

Figure 10.3 An example of peptide identification with MS/MS. For a peptide of interest at m/z 1 777.97, an MS/MS spectrum is acquired at the precursor mass using the biological sample with the highest intensity level for the peptide. The MS/MS spectrum is searched against a database of human protein sequences using the MASCOT search...

The search parameters include, among others, ions mass tolerances appropriate for the type of instmment used and expected peptide modifications. For example, 5-ppm precursor mass tolerances for a high-resolution mass spectrometer and 0.5-Da fragment tolerance for the ion trap fragmentation. The expected peptide modifications include (i) static (fixed) that apply to all amino acid residues in a sample, for example, cysteine modification due to the alkylation step, and (ii) dynamic (variable) which may or may not be present at each amino acid site. 

The data obtained allow us to conclude that different peptides are differently localized within the cc and ca, even with multiple precursor masses being closely spaced within a single mass unit. Mass accuracy, as calculated from over 10 peptides of known sequences, is well within or better than the 3 ppm instrument specification, accuracies rarely associated with MSI experiments, particularly for peptides. 

Table 3.24 Chemical composition, oxidation degree eind pheise concentration of catalyst precursor (mass fraction)...

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