Low-Mass Fragment Ions

Methyl esters with a y-hydrogen Pyridines and alkyl pyrroles 

Aromatic hydrocarbons with side chains Benzyl compounds with a y-hydrogen 

Alkyl aromatics Benzoyl compounds Aromatic hydrocarbons 

Any ion in this table may, of course, be substituted in such a way as to make it nearly unrecognizable. Only with practice can one make the proper assignments. In addition, other spectral data can play a usefnl role in this respect. For example, if the NMR spectrum of an unknown contains a peak characteristic of a methoxy group on an aromatic ring, one would look for a tropylium ion at miz 121, not at 91. 

Tropylium ions lose an acetylene fragment to only a slight extent (Equation 

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Laser microprobe MS (LMMS) can be used for direct analysis of normal-phase HPTLC plates [802,837]. Kubis et al. [802] used polyamide TLC plates polyamide does not interfere with compound identification by the mass spectrum, owing to its low-mass fragment-ions (m/z < 150). LMMS is essentially a surface analysis technique, in which the sample is ablated using a Nd-YAG laser. The UV irradiation desorbs and ionises a microvolume of the sample the positive and negative ions can be analysed by using a ToF mass spectrometer. The main characteristics of TLC-LMMS are indicated in Table 7.84 [838],... 

Obviously, the first generation fragment ions should be more closely related to the initial stmcture of ABXY than those of the second or even third generation. Fortunately, such higher generation (and therefore low-mass) fragment ions can also reveal relevant information on the constitution of the analyte. In particular, they yield reliable information on the presence of functional groups (Chap. 6). 

Selective detection of posttranslational modifications is possible through collision-induced formation of low-mass fragment ions that serve as selective markers for the modifications of interest. Collision-induced fragmentation, which is turned on while scanning the lower mass range, is turned off during the remainder of the scan (e.g. m/z 400-2400). In this way peptide molecular weight information and modification-selective, low-mass marker ions are detected in the... 

Castells et al. (2004) evaluated the suitability of positive ion chemical ionization (PICI) for the detection of CPs [45]. Figure 11 shows a comparison of an El and PICI mass spectra of a typical CP. Like El, mass spectra of CPs in PICI are characterized by the absence of the molecular ion and presence of numerous low-mass fragment ions corresponding to losses of HCl and chloride radical. As such, PICI has not been embraced as a tool for detecting CPs in environmental samples. 

Look for characteristic low-mass fragment ions (Table 2.12). 

Figure 6.1 Product-ion ESI mass spectra of lithiated 16 0-18 1 phosphatidylcholine after CID at different collision energy. Product-ion ESI-MS analysis of hthiated 16 0-18 1 dPC at m/z 766.5 in the presence of lithium hydroxide in the infusion solution was performed on a Thermo Fisher TSQ Vantage mass spectrometer. Collision activation was carried out with collision energy of 10 (a), 20 (h), 30 (c), and 40 (d) eV, and gas pressure of 1 mTorr. These results demonstrate two key points. First, while product-ion mass spectra acquired at different collision energy are very different, the fragmentation pattern of dPC species is identical under different experimental conditions. The pattern includes the neutral losses of 59,183,189, sn- FA plus 59, and sn-2 FA plus 59. Second, the low mass fragment ions increase as collision energy.

Fig. 12.23. Triton X-405 analyzed by positive-ion nanoESI-MS. (a) Molecular weight distribution appears trifold as [M+NH4] ", [M-i-2NH4], and [M+3NH4] ions series (b) only the triply charged ions undergo significant dissociation upon NSD to yield low-mass fragment ions (c) QD spectrum of the fragment ion at m/z 233 for endgroup analysis. Reproduced from Ref. [78] with permission. Elsevier, 2009.

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