CAD spectra

Figure 8. The m/z 137 ion was selected from the source and colli-sionally decomposed (N ) to give the CAD spectra. The top spectrum is the fragment arising from the isolated xenognosin and the bottom two spectra arise from the isomeric benzyl alcohols ((3).

Cadmium thiocyanates, 17 333-336 Cadmium trifluoroacetates, 17 25, 26 CAD spectra, see Collisionally activated dissociation spectra... 

Mechanism. The abundance of high mass ions combined with the expectation that the charge should reside at the carboxylate site, suggest that the losses of the elements CnH2n+2 preferentially occur from the end of the carbon chain remote from the carboxylate. This assumption was shown to be valid for the CH4 loss from palmitic acid by comparing the CAD spectra of the (M-H)- ion of unlabeled palmitic acid with the spectra of the (M-H)- ion of... 

The MS/MS approach to the analysis of surfactant mixtures seems to be applicable to most types of anionic surfactants (jj). While each class exhibits certain unique fragmentations, which depend on the functional groups present, the characteristic negative ion CAD spectra reveal structural information pertaining to alkyl chain length and branching for all cases studied thus far except sulfosucclnates. 

Mixtures of more complex nitrogen-containing compounds such as amine oxides and ethoxylated quaternary amines are also amenable to the type of Investigation described above ( ). The CAD spectra of these substances are dominated by a few informative ions which result from specific fragmentations characteristic of the class of compounds. The Crfi2n+2 1099 series also occurs but at a less abundant level for the ethoxylated compounds, and the series is of low abundance for the amine oxides. 

McLafferty et al. (37) studied the collisionally activated dissociation (CAD) of the ions resulting from the fast-atom bombardment ionization of cyano- and methylcobalamins, using a tandem mass spectrometer. They showed that mlz 1329 is produced only from the quasi-molecular ion. Similar CAD spectra are produced by both the quasi-molecular ions and the 1329 peak, indicating that essentially all fragmentation arises from this latter ion. CAD spectra produced for the 1329 ion derived from methylcobalamin or vitamin B12 coenzyme are also similar. This is consistent with the work of Taylor (10), who used linked-scan metastable ion techniques in a conventional double-focusing mass spectrometer to show the same thing. The sequential... 

The basic NR mass spectrum contains information on the fraction of undissociated (survivor) ions and also allows one to identify dissociation products that are formed by purely unimolecular reactions. NRMS thus provides information on the intrinsic properties of isolated transient molecules that are not affected by interactions with solvent, matrix, surfaces, trace impurities, radical quenchers, etc. However, because collisional ionization is accompanied by ion excitation and dissociation, the products of neutral and post-reionization dissociations overlap in the NR mass spectra. Several methods have been developed to distinguish neutral and ion dissociations and to characterize further short lived neutral intermediates in the fast beam. Moreover, collisionally activated dissociation (CAD) spectra have been used to characterize the ions produced by collisional reionization of transient neutral intermediates [51]. This NR-CAD analysis adds another dimension to the characterization of neutral intermediates, because it allows one to uncover isomerizations that do not result in a change of mass and thus are not apparent from NR mass spectra alone. 

MIKE/CAD spectra, in addition to their interest for structural determinations, are... 

Indeed, it is possible to obtain MIKE/CAD spectra of isobaric ions [154a,b]. For example, after elimination of 43 amu fragment, cyclohexanone leads within the... 

Figure 28. MIKE/CAD spectra of various protonated molecules of 1,4-cyclohexane ether (a), cyclohexanone (b) and cyclohexane oxide (c) produced under tC4,H9/CI [152].

The MIKE/CAD spectra of each ion are sufficiently different so that we may admit the separation of these ions, as shown by the non-existence of ions arising from the loss of 4,5 or 6 hydrogen atoms, as well as that of the m/z 39 ion in the MIKE/CAD spectrum of the isobaric ion CaHjO]". Inversely, the very low intensity of the m/z 41 peak in the MIKE/CAD spectrum of the C4H7] ion is proof of the good separation of these isobaric ions Fig. 29). 

Figure 29. MIKE/CAD spectra of C3H3O and QH, isobaric ions [m/z 55) produced from cyclohexanone under El conditions [154a].

Figure 34. MIKE/CAD spectra showing positively and negatively charged products both arising from the [M-H] anion of p-chlorobenzoic acid [164].

This program contains all the possibilities of rapid and sensitive analysis of metastable spectra, regardless of the mode employed, with averages of MIKE (or MIKE/CAD) spectra. 

In order to obtain more intense molecular peaks, Levsen and co-workers [197] chose the field ionization (FI) method, much softer than electron impact ionization, and utilized collision-induced decompositions rather than unimolecular decompositions. They were thus able to identify estrone (m/z 270) and progesterone (m/z 314) in an artificial mixture (Table 13). The FI/MIKE/CAD spectra of the molecular ions are... 

Analysis of steroid mixtures using MIKE/CAD spectra under FI conditions (FI/CAD spectra) [197]... 

As stated above, the analysis of certain of these peaks in MIKE/CAD spectra is very useful for determining the structure of molecules [209], especially for demonstrating the presence of unsaturations, of cyclopropenes on side chains, etc. (Table 18). 

In practice, the situation is not as simple as that, since MIKE/CAD spectra have a low resolution, leading to a few difficulties when isomers (or fragment) ions are present (or compounds with new structures) and when performing certain identifications. Nevertheless, the objective can be achieved, as shown by this review [209]. 

Cooks and co-workers [210] identified each individual protonated molecule MH from various steroids contained in biological matrices by studying their MIKE/CAD spectra. Collisions are necessary in this ionization mode in order to increase the number of characteristic daughter ions. In certain cases, however, spontaneous decompositions are sufficiently numerous, especially in the first FFR. 

The study of MIKE/CAD spectra of protonated molecules of various compounds characterized by a A -3-keto system (such as testosterone, corticosterone, norgesterel, etc.) leads to the demonstration of peaks with non-negligible intensities at m/z 147, 135, 123, 109 and 97, with ratios that are very close. Certain fragment ions, including the [MH-CH3OH] ion, produced from hydroxycortisone (with the same A -3-keto sequence) also generate the same characteristic ions, and thus a certain amount of caution is required. 

Protonated molecules from steroids with a phenolic A ring may be characterized similarly by the presence of intense peaks at m/z 157, 135 and 107, as well as [MH-H20] in their MIKE/CAD spectra. 

Protonated isomeric molecules, such as testosterone and dehydroepiandrosterone, are characterized in the MIKE/CAD spectra by peak widths, but also by intensities, which vary as a function of collision gas pressure. 

Thus, there is no difficulty in determining the sequence of the AcN-Gly-Ala-Leu-OCH3 peptide. In a more thorough report of this work [218], the authors presented all the aspects offered by the use of MIKE/CAD spectra, including the study of a mixture of oligopeptides with sufficient sensitivity the decreased possibility... 

The major disadvantage of MIKE/CAD spectra, however, is related to the widening of metastable peaks, which reduces energy resolution. 

All three include the C4H9 radical and can decompose spontaneously (or collision-ally) to eliminate either the primary (-CH2-CH(CH3)2) or secondary (-CH(CH3)-CH2-CH3) isobutyl radical. Consequently, several differences are observed in the MIKE/CAD spectra. However, the greatest differences appear in the decomposition spectra of m/z 103 ion (Table 22), which is consistently present when the polypeptide has an He or Leu residue in terminal position. 

MIKE and MIKE/CAD spectra of ion (CD3NH =CH(C4H,))m/z 103 produced from peptides where the Leu (or lieu) group is present in various positions [218]... 

Comparison of fragments lost from cyclopeptides a and b observed in the FD/B/fi/CAD spectra [223]... 

The elucidation of the structure of cyclopeptidic indoles in a mixture could be determined by analyzing the B/E/CAD spectra of molecular ions. The FD spectra of this mixture indicated the presence of high-molecular-weight ions, including m/z 593 and 573. It was impossible to separate this mixture, even with liquid chromatography, as a result of the similar structures of these molecules and the mixture had to be studied as it was (about 1 pg of sample). Collisions on these ions led to side-chain cleavage, which led to an interesting approach to molecular structure. Table 23 lists the principal ions observed in B/E/CAD spectra from the M molecular ions produced in FD conditions. 

It is to be borne in mind that it is not always necessary to perform MIKE/CAD spectra, since occasionally spontaneous decompositions are sufficient. 

These MIKE/CAD spectra are in general characterized by the presence of an m/z 18 ion, which may have originated from either or NH4. The presence of a non-... 

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