Fragment Ions Under Mass

To summarize, mass spectrometry has successfully been used for the identification of compounds containing a Zn—C bond, which have a large diversity of structures and complexity. These complexes have been subjected to different ionization methods (such as El, Cl, EAB and ESI) and in many cases they generated numerous Zn-containing fragment ions. Under soft (Cl or FAB) experimental conditions, some of these compounds produced protonated molecules [M-t-H]" " or even protonated dimerlike species [M2H — R]+. Electron ionization was successful for the characterization of many volatile Zn-containing compounds. Peaks of molecular ions M+ were frequently observed, but the majority of the mass spectra were dominated by Zn—C bond dissociation products. 

The majority of LC/MS systems use either quadruple or ion-trap mass analyzers. Because of the general paucity of fragment ions under routine operating conditions, instruments with true MS/MS capability, which allow CID of selected ions, have a clear advantage for identification and in providing low limits of detection. 

Physical Chemical Characterization. Thiamine, its derivatives, and its degradation products have been fully characterized by spectroscopic methods (9,10). The ultraviolet spectmm of thiamine shows pH-dependent maxima (11). H, and nuclear magnetic resonance spectra show protonation occurs at the 1-nitrogen, and not the 4-amino position (12—14). The H spectmm in D2O shows no resonance for the thiazole 2-hydrogen, as this is acidic and readily exchanged via formation of the thiazole yUd (13) an important intermediate in the biochemical functions of thiamine. Recent work has revised the piC values for the two ionization reactions to 4.8 and 18 respectively (9,10,15). The mass spectmm of thiamine hydrochloride shows no molecular ion under standard electron impact ionization conditions, but fast atom bombardment and chemical ionization allow observation of both an intense peak for the patent cation and its major fragmentation ion, the pyrimidinylmethyl cation (16). 

The first step in the application of mass spectra is obviously to obtain a list of fragment ions formed by electron bombardment of the molecule under study and their relative amounts by appropriate techniques. The goal from this point will necessarily be to relate the positive ions to the molecular structure whether it be an unknown structure to be identified, or a known structure of which a knowledge of fragmentation is desired. The fragment ions observed indicate the pieces of which the molecule is composed... 

Compilations of Reference Spectra There are several compilations of reference mass spectra available of which the oldest is the American Petroleum Institute (Ref 82) collection of spectra obtained mostiy on the older type instruments. Recent collections index spectra variously, eg, under reference number (Ref 19). molecular weight (Refs 12 19), molecular formula (Ref 19), fragment ion values (Ref 19), and base peak (Refs 12 19). A quarterly journal, Archives of Mass Spectral Data ... 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

Figure IQ Postulated structure for the major fragment ion of xenognosin B under electron impact mass spectrometry.

Applications Early MS work on the analysis of polymer additives has focused on the use of El, Cl, and GC-MS. The major drawback to these methods is that they are limited to thermally stable and relatively volatile compounds and therefore are not suitable for many high-MW polymer additives. This problem has largely been overcome by the development of soft ionisation techniques, such as FAB, FD, LD, etc. and secondary-ion mass spectrometry. These techniques all have shown their potential in the analysis of additives from solvent extract and/or from bulk polymeric material. Although FAB has a reputation of being the most often used soft ionisation method, Johlman el al. [83] have shown that LD is superior to FAB in the analysis of polymer additives, mainly because polymer additives fragment extensively under FAB conditions. 

If during the ionization the amount of energy deposited on the molecule is low, as occurs in soft techniques, i.e. Cl, ESI, DESI and MALDI, the mass spectrum is very simple. It is characterized by protonated/deprotonated molecules, and eventually few adduct ions but very few or no fragment ions. This implies that it is easy to obtain the molecular weight of the analyte under investigation, but structural information is missing. As an example, the ESI mass spectrum of a small molecule is reported in Figure 2.20. There are two main ions one at m/z 556 and another at m/z 578. As the mass spectrum has been obtained in positive... 

Fig. 2.6.7. APCI-FIA-MS-MS(+) daughter ion mass spectrum of [M + NH4]+ ion of octylcyclohexylethoxylate homologue at m/z 582 from reduced octylphenolethoxylate mixture (CgHi7-C6Hio-0-(CH2-CH2-0)m-H) (inset) fragmentation scheme under...

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