Fragmentation behaviour

From the FIA—MS overview spectrum, speculation that there can be more than just one structurally defined molecule type behind an observable signal i.e. the presence of isobaric compounds, cannot be excluded whenever one signal defined by the m/z-ratio is examined in FIA-MS spectra. Consequently, the information obtained by FIA-MS is quite limited whenever we deal with complex mixtures of environmental pollutants rather than the analysis of pure products or formulations with a known range of ingredients. LC separation is inevitable when mixtures of isomeric compounds should be identified with MS-MS. Therefore, in FIA-MS-MS special attention has to be paid to avoid the generation of mixed product ion spectra from isomeric parent compounds. This would block identification by library search and may lead to misinterpretations of product ion spectra because of the fragmentation behaviour observed. 

Fig. 2.6.10. APCI-FIA-MS-MS(+) (CID) daughter ion mass spectrum of selected [M + NH4]+ parent ion (mjz 340) of potential carboxylated non-ionic surfactant metabolite of precursor NPEO prepared by chemical synthesis structure of short-chain NPEC CgHi9-C6H4-0-(CH2-CH2-0)-CH2-C00H fragmentation behaviour under CID presented in the inset [28],... 

Fig. 2.6.13. Fragmentation behaviour of the acidic NPEO metabolites (CNPEC) (A) ESI— LC-MS(+) of underivatised compound and (M) di-methyl ester of HOOC(CH2)6-C6H4-0-(CH2CH20)C00H Source CID conditions, see Ref. [21]. Reprinted with permission from Ref. [21] 1998 by American Chemical Society.

The homologues of the methylated non-ionic EO/PO surfactant blend were ionised as [M + NH4]+ ions. A mixture of these isomeric compounds, which could not be defined by their structure because separation was impossible, was ionised with its [M + NH4]+ ion at m/z 568. The mixture of different ions hidden behind this defined m/z ratio was submitted to fragmentation by the application of APCI—FIA—MS— MS(+). The product ion spectrum of the selected isomer as shown with its structure in Fig. 2.9.23 is presented together with the interpretation of the fragmentation behaviour of the isomer. One of the main difficulties that complicated the determination of the structure was that one EO unit in the ethoxylate chain in combination with an additional methylene group in the alkyl chain is equivalent to one PO unit in the ethoxylate chain (cf. table of structural combinations). The overview spectrum of the blend was complex because of this variation in homologues and isomers. The product ion spectrum was also complex, because product ions obtained by FIA from isomers with different EO/PO sequences could be observed complicating the spectrum. The statistical variations of the EO and PO units in the ethoxylate chain of the parent ions of isomers with m/z 568 under CID... 

Fig. 2.9.26. ESI-FIA-MS-MS(-I-) (CID) product ion mass spectrum of [M + NH4]+ parent ion of fatty acid polyglycolester surfactant blend (C H2 +i-C(0)-0-(CH2-CH2-0)m-H) selected ion under CID m/z 406 fragmentation behaviour in the inset [24],...

When AESs of an industrial blend were examined by ESI—MS— MS(—), all AES homologues under standard CID conditions (collision energy +20-50 eV collision gasArgon pressure 1.5 mTorr) resulted in a very simple product ion spectrum that contained the parent ion and only one product ion mlz 97 ([HS04]-) (cf. Fig. 2.11.10). Vice versa, the parent ion scan mode applying mlz 97 helped to recognise AES in blends, formulations and environmental samples [49,60] however, interferences with sulfate compounds other than AES cannot be excluded. The same fragmentation behaviour was found if APCI-FIA-MS-MS(-) was applied however, the sensitivity was reduced compared with ESI-FIA-MS-MS(—). 

NPEO-SO4 is one of the rare anionic surfactant compounds on which aerobic biodegradation monitoring has been performed, where metabolites could be observed by API-MS. Using FIA-MS, however, differentiation of precursor compounds and metabolites was impossible. Both compounds showed the same molar masses but could be recognised because of their quite different RTs in RP-LC [15]. MS CID performed by trap confirmed a fragmentation behaviour of the metabolite quite different from precursor NPEO-SO4 compounds, whose structure is not yet clear. 

Fig. 2.12.6. Identification of esterquat compounds FIA-APCI-MS-MS(+) (CID) product ion mass spectrum of selected [M — RCO]+ base peak ion of cationic surfactant blend of di-hydrogenated tallowethyl hydroxyethyl ammonium methane sulfate type (mlz 692 general formula (R(C0)0CH2CH2)2-N (CH3)-CH2CH2(0H)CH30S03) fragmentation behaviour under CID...

MS—MS was applied to characterise alkyl quats. The blend of cetyl-dimethyl-ammonium bromide was examined by FIA—APCI—MS— MS(+). Besides the nitrogen-containing fragment at m/z 46, mainly alkyl chain fragments from the C46 chain could be observed at m/z 43, 57, 71, and 85. The APCI(-I-) product ion spectrum is presented in Fig. 2.12.12. The inset contains the pattern of the CID fragmentation behaviour observed on a triple quadrupole instrument [39]. 

Fig. 2.12.15. FIA-APCI-MS-MS(+) (CID) product ion mass spectrum of cationic surfactant compound (m/z 538) fatty acid polyglycol amine type observed in the Saale river, Germany (general formula R—N H((CH2—CH2—OH)x)—(CH2—CH2—OH)y X fragmentation behaviour of [M]+ parent ion at m/z 538 under CID conditions is presented...

Limonene, one of the most prominent natural monoterpenes (cf Section VII), represents a particular derivative of 4-vinylcyclohexene since it has been studied with respect to the pronounced energy dependence of its fragmentation behaviour (Scheme 7). Counterintuitively, and in contrast to 4-vinylcyclohexene, the radical cations of limonene (27) do not undergo the retro-Diels-Alder reaction if the internal energy of the ions is low. As... 

All research workers would agree that our knowledge of ion structures is not as good as in normal chemistry. However, many seem to think that proposed structures are probably correct , probably the best representation , or at least plausible , whereas our conclusion is that knowledge of ion structures at present is very poor. The basis for plausibility is usually a comparison with ground-state closed-shell systems, but the existence of species such as Hs, CHj (Melton, 1963), and the unusual fragmentation behaviour of alkanes suggests that... 

Aromatic ethers display a somewhat stronger M peak, and show a fragmentation behaviour similar to that of aliphatic ethers. 

Losses of small nentrals snch as HjO and CO are observed in the negative-ion mode as well. Again, the loss of CHj can be observed from methoxylated flavonoids, as was stndied in detail by Justesen [22-23], [M-H] of monomethyl-flavonoids show the loss of CHj and subseqnent losses of CO or HCO, while polymethyl-flavonoids show losses of CHj and snbseqnent losses of CHj, or CHj and CO. It is not clear whether the loss of two CH3 radicals was checked by accurate-mass determination. Alternatively, the loss of 30 Da conld be explained by the loss of H2C=0, which is frequently observed in the fragmentation of polymethoxylated compounds. Isomeric methoxylated flavonoids showed different fragmentation behaviour, but standards were still needed for determination of the methoxy-position. The product-ion MS-MS spectra of rhanmetin and isorhamnetin are shown in Figure 15.3. No loss of the prenyl substitnent is observed for prenyl flavonoids in negative-ion mode [14],... 

The fact that sometimes 1.3- or 1.4-disubstituted arenes exhibit a fragmentation behaviour which is characteristic for the ortho isomer may be caused by different effects. Among these the most familiar ones are the migration of substituents (e.g., via valence isomerization) prior to elimination reactions. Alternatively the fragment (M—HZ)+ might be generated via consecutive radical eliminations14. ... 

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