Low mass region

Examination of the low-mass region showed a m/z-42 peak that is characteristic of (CH3)2N—. The proposed structure was (CH3)2N—C6H4— C02C2H5. 

Low Mass Region. All spectra shown in the examples were acquired using the quadmpole ion trap mass spectrometer. As noted previously, this widely used and relatively cheap mass analyzer suffers the low-mass cut-off phenomena. In addition to techniques used in the examples shown above, other mass analyzers applied for tandem mass spectrometers may cover the low mass region of the fragmentation spectmm that can be information rich. 

In the spectra of the untreated silica sample, no specific peaks in the low mass region up to 150 amu (atomic mass units) such as from 3+, 10+, and no cluster peaks in the higher mass region are found. The spectra of the acetylene-treated sample do show these specific plasma-polymerized acetylene peaks in the... 

As mentioned in Sect. 3.1.1, the as-prepared (CH)X film surface is usually oxidized to some extent. Figure 23(a) and Fig. 23(b) show, respectively, the negative static SIMS spectra of a high density cis-(CH)x film before and after sputtering by Ar+ beam [193]. The negative ion spectrum for the as-received film is dominated in the low mass region by signals due to C2H (25 amu), C2... 

There are many reasons why mass spectrometry misses the high masses. One of these is that the detectors measure in a linear mass mode which soon loses small numbers of molecules in the background noise, contrast this with SEC which collects logarithmically with mass. The detectors are often mass sensitive and this can be corrected to some extent by applying a data manipulation function. Other factors which need to be taken into consideration are loss of low mass regions due to either volatility or ionization problems. This is particularly apparent when looking at condensation polymers or acrylics from catalytic chain transfer polymerization. There are also effects on the mass distribution due to the laser power used thus the minimum laser power is often required but not always applied. 

The CID spectrum (Fig. 5) of fraction 9 (Fig. 1) confirmed the sequence of the unmodified p(133-144) peptide as Val-Val-Ala-Gly-Val-Ala-Asn-Ala-Leu-Ala-His-Lys. Significant ions arising from fragmentation of the peptide backbone are indicated in standard nomenclature (28). For example, the mass difference 99 between the ions at m/z 1118 (w12) and 1019 (w ) shows the presence of valine. Similarly, the mass difference 71 between m/z 355 (y3> and 284 (y2) indicates the presence of alanine at that position. Peaks in the low mass region of CID spectra represent the immonium ions H2N+CHR (where R is a side-chain group) arising from individual amino acid residues. The major ion m/z 110 in the immonium ion region of the spectrum for fraction 9 arises from histidine. 

The cresols and other lower alkylphenols have been studied by multiphoton ionization mass spectrometry (MPI) with the aim of distinguishing positional isomers. Only slight differences were found in some cases, mainly concerning the low-mass region. Remarkably, the MPI mass spectrum of ortho-cresol was again distinct becanse water loss, i.e. a primary fragmentation reaction, from ions o-7 + was fonnd to be significantly more frequent than with the other isomers . ... 

Mass Spectra.— The mass spectral behaviour of the vinblastine alkaloids is similar to that of voacamine and its relatives. Transmethylation reactions are observed, e.g. for vinblastine (169) (M = 810) peaks occur at mje 824 and 838 this being one of the major reasons for the utilisation of hydrazide derivatives. The nature of the two structurally different components can be recognised from characteristic fragment ions in the low-mass region. ° ° ... 

The constitution of pleiomutine (230), apart from the decision in favour of C(15) or C(17) as a linkage position, can be established from the mass spectrum of the alkaloid. In the low-mass region fragments mje 278,249, and 208, characteristic of eburnamenine,are apparent, as well as the previously mentioned fragments V and w and the — 28 peak, all typical of pleiocarpinine (232). 

Figure 4, Comparison of the mass spectrometric determination of the photolytic products of 1 1 DjO-HtO with that of conventional electrolysis. Under comparable instrumental settings, the lines at masses 3 and 4 are not observed in the blank. Argon gas was used to purge the cell prior to the Tight reaction. Note changes in scale in the low-mass region.

Inspect the low-mass region for immonium ions. The first step in the interpretation is to inspect the low-mass region of the spectrum, observing the presence of any immonium ions (H2N=CHR" ) and the amino acid composition that they indicate. 

Inspect the low-mass region for the ji-ion. To assign the C-terminal amino acid, the low-mass region of the spectrum is inspected to identify the yi-ion at either m/z 147, for C-terminal lysine peptides, or m/z 175 for C-terminal arginine peptides. The m/z of the yi-ion is then used to calculate the m/z of the b i-ion, and the high-mass region of the product ion spectrum is inspected to identify that ion. 

---