Structurally diagnostic fragment ions 218

The ion of m/z 77 (C6H5+) usually points to the presence of a benzene ring. The ion of m/z 91 (C6H5CH2+ formed by a-cleavage) is the signature of phenyl alkanes [reaction (6.20)]. If m/z 92 (C7Hg+ ) is also present along with m/z 91, it provides additional information that the substituent hydrocarbon chain is at 

Ethyl or longer-chain phenyl ethers, in addition, would produce an ion of m/z 94 (C6H60+ ) via McLafferty-type rearrangement [reaction (6.29)]. The ion of m/z 93 (C6H7N+) is also prodnced from ionized 2-alkyl and 4-alkylpyridines via the McLafferty rearrangement. The m/z 105 ion (CeHsCO ), which is produced via a-cleavage, is evidence for the presence of phenyl ketones and esters and amides of benzoic acids. Esters of benzoic acids will also produce a characteristic ion of m/z 122 as a result of the McLafferty rearrangement  

Benzyl esters can be recognized on the basis of an ion of m/z 108 that is formed by the elimination of a ketene molecule [reaction (6.32)]. Ions at m/z 127 and 128 are indications of iodo compounds. A strong signal at m/z 149 (9) is always present in the spectra of esters of phthalic acids and in samples that might be contaminated with plasticizers. 

Solution From the high-mass cluster it is obvious that the unknown contains a bromine atom. The difference in mass between m/z 122 and 43 supports this observation. The elemental composition of m/z 43 is C3H7+, which could be an isopropyl or propyl cation. The unknown is either 1- or 2-bromopropane, both of which give nearly identical mass spectra. 

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It provides unsurpassed molecular specificity because of its unique ability to measure accurate molecular mass and to provide information on structurally diagnostic fragment ions of an analyte. 

Since both ESP and ISP produce quasimolecular ions, more sophisticated techniques, such as LC-MS-MS are required to obtain diagnostic fragment ions and, thus, analyte structure elucidation (117, 118). Identification can often be achieved by using daughter ion MS-MS scans and collisionally induced dissociation (CID), most commonly on a triple quadrupole MS in this way, dissociation of the quasimolecular ion occurs and diagnostic structural information can be obtained (119). 

Tandem mass spectrometry allows more structural information to be obtained on a particular ionic species, because the used ionization method yields relatively few structurally diagnostic fragments, or because its fragmentation is obscured by the presence of other compounds in the mixture introduced in the source, or because it is obscured by other ions generated from the matrix in the course of ionization. 

The partial positive ion FAB mass spectrum (obtained by Xe bombardment) is shown in Fig. 2 and incorporates an abundant protonated molecular ion at m/z 1040. Three clearly defined series of diagnostic fragment ions, corresponding to the general structures (1) (N-terminal), (2) (N-terminal) and (3) (C-terminal) identify the sequence as PhCO-Ala-Phe-Val-Ile-Asp-Asp-Glu-Gln (or /.yo-Gln). The negative ion FAB spectrum showed an abundant [M —H] ion at m/z 1038 and the series of ions corresponding to structure (4) was able to confirm the sequence of the six C-terminal amino acids. 

Figure 12. Structure of the SKPl gjycopeptide deduced from the Q-TOF data showing some of the diagnostic fragment ions used to assign the position of attachment of the sugar.

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