MIKES spectra

Note In MIKES Ei represents the full electric field necessary to transmit the precursor ion through the ESA, and Ei is often denoted as starting value Eq. Then, the abscissa of MIKE spectra is divided in units of EIEq = m2/mi. 

Fig. 7. Mass-analyzed ion kinetic energy (MIKE) spectra of C6FsSi(CH3)3. [Reproduced with permission from J. M. Miller, J. Ross, J. Rustenberg, and G-. L. Wilson, Anal. Chem. 45, 627 (1973) copyright by the American Chemical Society.]...

Mass spectrometry is suitable for the identification of pyrrolo-benzodiaze-pines and the differentiation of their isomers (1996M1653). A structural distinction can be made easily from their mass spectra or the metastable mass-analyzed ion kinetic energy (MIKE) spectra, produced by their molecular or most intense fragment ions. Established fragmentation pathways have been supported by MNDO and AMI semiempiiical calculations. 

For instance, HF is lost from 114 only when R3 = F and CH2CO is lost only when R2 = F. The MIKE spectra of [M + NH4]+ adducts formed under Cl (NH3) conditions show exclusively consecutive losses of NH3 and CH3OH for all four compounds, to yield, at least formally, ions 114. However, the CID spectra of the [M + NH4]+ ions do not display the characteristic secondary fragmentations attributed to 114 and observed in the CID/EI experiments221. 

The presence of longer alkyl chains or aromatic rings as substituents may provide the opportunity for rearrangements to take place. They are dominant at low energy and generally appear as metastable transitions in standard or MIKE spectra. 

Certain quinazolines, e,g. l-methylquinazolin-4(l//)-one, fragment by a retro-Diels-Alder process. The MIKE (mass analyzed ion kinetic energy) spectrum of the molecular dication of quinazoline is very similar to that of quinoxaline. MIKE spectra of quinazolinethiones show the loss of NCS radicals from the molecular ionA ... 

Kemp, D. L., Cooks, R. G. and Beynon, J. H. Simulated mass-analyzed ion kinetic energy (MIKE) spectra from a conventional double focusing mass spectrometer. Int.. Mass Spectrom. Ion Phys. 21 93-101, 1976. 

The measurement of MIKE spectra to elucidate ion structures and to study fragmentation mechanisms remains the method of choice, providing they lead to numerous fragmentations, which is not always the case when soft methods, such as Cl, FD, etc., are utilized. It is necessary to use an ionization which transmits a higher energy to the molecular ions. A larger number of fragments can be observed for the ions which decompose in the second FFR. 

In the case of bicyclic ketones, such as ethyl hydrindanones (a and b) [125], such a partial molecular isomerization does not prevent their identification (Fig. 18), since their MIKE spectra are quite different (Fig. 19). 

These results show the caution that must be exercised in special cases, in terms of the conclusions to be drawn from identical MIKE spectra. 

Using this methodology, Beynon and co-workers [126,127] showed how the use of MIKE spectra provided the possibility of obtaining family trees for the ions. 

Figure 19. Comparison of MIKE spectra of [M-C2H4] fragment ion from isomeric ethyl hydrindanone, methyl decalone and the molecular ions, M , formed from isomeric hydrindanones [125],...

As an example, it is difficult to distinguish the structures of the protonated molecules MH produced from isomeric compounds with the elemental formula CgHioO (cyclohexanone, cyclohexane oxide and 1,4-cyclohexane ether) [152] on the basis of their respective unimolecular decomposition spectra. Indeed, only the loss of water was observed in the MIKE spectra of these products. Collision-induced decomposition spectra (MIKE/CAD), on the other hand, lead to an easy structural distinction (Fig. 28). 

The non-negligible disadvantage of this mode in analytical applications should not be forgotten the considerable widening of some peaks which are already wide in spontaneous decomposition MIKE spectra. 

The sensitivity can be improved considerably if - rather than recording peak intensity during electric field scanning (MIKE spectra after setting on the main beam) - the electric field is set on the transition, e.g., 304 -+182" and peak area is recorded as a function of time. 

Figure 42. Comparison of MIKE spectra of ions m/z 270, 111 and 288 produced in El from mixture of estrone (a), estradiol (b) and estriol (c) (as reflected spectra) with the same respective ions formed for authentic samples of these compounds [196].

Figure 43. MIKE spectra of isomeric molecular ions m/z 286 for 16-keto estradiol (d) and 16-hydroxy-estrone (e) [196],...

The interest of choosing the [R] and [R + 42] ions is related to additional data which may be obtained from the MIKE spectra of these ions, as well as confirmation of structures. These must be compatible among themselves, considering that the only difference is due to the presence of C-15, 16 and 17 (forming the D ring) in the [R + 42] fragment ion. 

Ronje and Grutzmacher [216] studied systematically various polypeptide sequences using MIKE spectra from derivative (labeled or not) peptides and mixtures under El and Cl conditions. 

Figure 65. (a) MIKE spectra of corresponding fragment ions formed in the source for (a) AcN-Val-Ala-Leu-COOMe and (b) AcN-Leu-Ala-Val-COOMe isomeric peptides. 

In particular, when the abundance of the precursor is low, artifact signals may appear which are difficult to attribute. Certain of them, however, may be interpreted, especially those which may be due to unimolecular decompositions produced in the first FFR, for signals obtained in MIKE spectra in the second FFR when these artifacts are present in BjE linked scan spectra. 

These results are sufficiently encouraging for studying mixtures of non-derivatized peptides with the following advantages normal increase of the abundance of the deprotonated molecule [M-H] which affects sensitivity directly no useless increase of the molecular weights of peptides, which had an unfavorable effect on the resolution of MIKE spectra. 

Figure 94. MIKE spectra of m/z 166 obtained from (a) a crude D. longimamma extract and (b) purum ubine hydrochloride [241].

Spontaneous decompositions of M ions produced by El (or during charge transfer) lead to characteristic MIKE spectra. The characteristic decomposition chosen is that which gives the retro Diels-Alder, which is particularly sensitive to the cis or trans ring junction of these alkaloids, as shown by Tamas et al. [242]. 

The interpretation of the MIKE spectra of heterocyclic compounds may be ambiguous. The study of the decomposition spectra of natural isotopic ions may simplify this situation. Occolowitz et al. [248] studied the structure of the C7H12NO2S ions (m/z 174) produced during the fragmentations of M molecular ions. 

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