Mass shift

A common feature of the reactions described in (1) is that the hydrogen migration can be traced directly from appropriate mass shifts in the spectra of suitably 2H labelled precursor ions. 

Figure 5.4 GALDI mass spectra of ursonic acid (7, m/z 477) fresh (a), photoaged with (b) and without (a) glass filter. Spectrum (b) depicts the normal ageing pattern (cf. Figure 5.3), but ageing without glass filter and therefore with much more UV light results in a mass shift of (M+2) in spectrum (c) relative to (b), which is explained by Norrish reactions...

The proteins in the mortars can be modified by gradual oxidation or other chemical processes. In mass spectra the peaks that can be interpreted as oxygen incorporation (the mass shift of +16 Da) or ammonia release (—15 Da) can be sometimes indicated. This observation is not surprising as several amino acids (Met, Trp, Tyr, etc.) can be oxidised under these conditions similarly, Gin and Asn can gradually release their ammonia by long-term hydrolysis in a wet inorganic matrix. 

Table 7.5 provides a complete list of common metabolites and their mass shifts relative to parent compounds.117 While the concept of metabolite profiling is not new,20 multiple advances in MS hardware and software allow researchers to look more easily for metabolites and include them in PK assays.118... 

Mass Shifts of Common Metabolites Relative to Initial (Parent) Compound Dosed... 

Here we encountered a typical situation in the de novo sequencing—there is a part of the sequence that is not covered by any ion series. Not all bonds between amino acids are of equal strength, and some of them might be particularly resistant to collisions, which in turn results in the missing mass-shifts (and missing residues). 

Short examination of the fragment ion spectrum from a singly charged precursor is not particularly helpful. Some previously found mass shifts can be verified but no peaks with m/z lower than 800 Th can be assigned. Detailed peak annotation is presented in Fig. 6.16. 

There are three basic approaches to identify the ion series. The most straightforward approach is based on a-ions linked to b-ions. If there is at least one or two mass shifts of 28 Da associated with any of the already identified peaks, there is a high probability that we found a b-ion series. However, this can only be considered speculation. 

In this example, we will use the third approach—as one ion series is already available, finding the reverse ion series should be easy. Indeed, starting from the peak at 300.1 Th (which comes from the cleavage of the same bond as the 1080.8 peak) we see the mass differences of 163.2 and 56.9 Th, corresponding to tyrosine and glycine. Using the same procedure, the entire previously assigned sequence can be proven correct, but also—extend the already revealed sequence by one more residue—the mass shift between 1206.6 and 1305.8 equal to 99.2 Th corresponds to valine. 

Similarly to 17 Th neutral loss, 18Th mass shifts are also very common, corresponding to the loss of water. Such shifts are (in most cases) caused by elimination of hydroxyl group from serine and threonine side chains. 

The N-terminal amino group can be modified by different reagents, including succinic anhydride, propionic anhydride, and /V-acetoxysuccinimide. Different approaches allow the use of different mass shifts between light and heavy populations and usually differ in fragmentation patterns. 

The peptide mixture on the MALDI target can be exposed to a chemical derivatization to confirm the identity of a peptide by the mass shift associated with the sequence-specific derivatization. A large number of possible derivatization reactions can be combined with the MALDI-TOF analysis. Their usefulness depends critically on the kinetics of the derivatization reaction, whether the reaction is complete with small amounts of peptides and whether only one product is generated. A visible MALDI signal can be generated from low atomole of peptide present under the laser beam (Vorm et al., 1994), but these amounts are often not sufficient... 

One approach is to use an enzyme that cleaves the targeted modification from the peptides. A comparison of the peptide map before and after cleavage can reveal modified peptides by identifying the specific mass shift in the spectrum. This approach has been used more systemati-... 

Figure 5A, B shows the isotopic distribution, of protonated bosentan (C27H30N5O6S, Mr 552.6) with a mass resolution of 0.5 and 0.1 at FWHM, respectively. It is worthwhile to observe the mass shift of the most abundant ion from m/z 552.2006 to m/z 552.1911. This value does not change with a mass resolving power of 15 000 (Fig. 1.5C) or even 500000 (Fig. 1.5D). Accurate mass measurements are essential to obtain the elemental composition of unknown compounds or for confirmatory analysis. An important aspect in the calculation of the exact mass of a charged ion is to count for the loss of the electron for the protonated molecule [M+H]+. The mass of the electron is about 2000 times lower than of the proton and corresponds to 9.10956 x 10 kg. The exact mass of protonated bosentan without counting the electron loss is 552.1917 units, while it is 552.1911 units with counting the loss of the electron. This represents an error of about 1 ppm.

Pound force x seo/lb mass shifting equilibrium pressure ratio 1000 14.7. 

---