Metastable decompositions

Tandem quadrupole and magnetic-sector mass spectrometers as well as FT-ICR and ion trap instruments have been employed in MS/MS experiments involving precursor/product/neutral relationships. Fragmentation can be the result of a metastable decomposition or collision-induced dissociation (CID). The purpose of this type of instrumentation is to identify, qualitatively or quantitatively, specific compounds contained in complex mixtures. This method provides high sensitivity and high specificity. The instrumentation commonly applied in GC/MS is discussed under the MS/MS Instrumentation heading, which appears earlier in this chapter. 

In order to find further support to this mechanism49 (equation 38) the ratio of the intensities of two metastable decompositions of the [M — CH20] + ion of 113, namely... 

Tabet, J. C. Jablonski, M. Cotter, R. J. Hunt, J. E. Time-resolved laser desorption The metastable decomposition of Chlorpophyll-A and some derivatives. Int. J. Mass Spectrom. Ion Phys. 1985, 65,105-117. 

In all three mixed cluster ions systems, the intensity distributions show94 that there is a maximum at n + m = 5. Results of metastable decomposition studies95 of the mixed cluster ions were determined to be ... 

Two other molecules considered in the same context, but with proton affinities larger than ammonia, were pyridine (C5H5N) and trimethylamine (TMA) (CH3)3N whose proton affinities are 220.8 cal mol-1 and 225.1 cal mol-1, respectively.190 Metastable decomposition studies of NH3(C5H5N)mH+ (m = 1-5) yield the following results ... 

While metastable decompositions have been extensively studied for many decades, nowadays the use of tandem MS is almost exclusively limited to the study of decompositions caused by collision. Different kinds of experiments can be carried out by tandem MS the most common is the product ion scan. This is carried out by selecting ions with a given m/z value among all those formed in the source and detecting selectively their product ions formed due to CIDs. It follows that at least two analyzers (the tandem) are required. In... 

The kinetic method [102,104-106] compares the relative rates of the competitive dissociations of a proton-bound adduct [A-H-B] formed by admitting a mixture of A and B to a Cl ion source. [104,105] There, the proton-bound adduct [A-H-B] is generated amongst other products such as [AH] and [BH]. Using standard tandem MS techniques, e.g., MIKES, the cluster ion [A-H-Bref] is selected and allowed to undergo metastable decomposition ... 

The kinetic energy releases in corresponding metastable decompositions may be very small. 

In addition to those ions formed during or soon after primary ion impact, as in the processes just described, other ions arise through subsequent events. Unimolecular reactions of ions, akin to metastable decompositions in magnetic sector mass spectrometry, occur in the free vacuum. The resulting fragment ions have intensities which contradict the notion that SIMS is a "soft" ionization technique, although some fraction of the ion production events can be so characterized. 

While the spectrum shown in Figure 1 may be very useful in many applications, it does not reveal structural information. In many cases, such information may be obtained by studying the metastable decomposition of major ions. The Instrument used in this laboratory may be operated in a manner in which an ion such as the (M-H) ion of the fatty acid is selected, even at high resolution if necessary, by using MS-I and its decompositions in the field free region between MS-I and MS-II followed by scanning the second ESA (MS-II). However, the metastable decompositions of the (M-H) ions of the... 

This reaction was observed to occur as a metastable decomposition process for n = 4-9. For clusters with n > 9, retention of water was observed as a prompt reaction ... 

Recently, Foltin et al. (199 la,b) have reported an unusual metastable decomposition channel in singly charged argon cluster ions involving the loss of a number of argon atoms. The number of argon atoms lost increases from two for Ar to ten... 

Mass spectrometric investigation of gas phase clusters has come to play a significant role in cluster research because it is presently the only technique that allows size-dependent analysis of cluster beams. We anticipate that, in the near future, CMS studies utilizing tandem mass spectrometric techniques will be invaluable in confirming the importance of metastable decompositions and in determining the factors responsible for the enhanced intensities of various ions in the CMS. Such an instrument is currently under construction in our own labs, and we should soon have the ability to perform collision-induced dissociation studies on large vdW clusters. 

Metastable loss of H2 is only observed for the chloro and bromo compounds. For X = F, the exothermicity of the protonation-dissociation sequence (7 kcalmol-1) is probably not sufficient to exceed the energy barrier for H2 loss, which for the 1,2-H2 elimination is predicted by theoretical calculations to be 18 kcalmol1 (thermochemical data for a 1,1 -H2 elimination are not available)106. For X = I, the protonation-dissociation reaction is exothermic by 41 kcal mol-1 and the absence of a metastable peak for this process is probably due to a very high activation barrier. The existence of an energy barrier for the H2 (HD) elimination from protonated chloro- and bromohalomethane, and their deuterated isotopomers was indeed revealed by the large KER values (140 eV) associated with their metastable decomposition. An intensity ratio of about 50 was measured for metastable peaks due to H2 and HX losses in accordance with the known thermochemical data which establish that H2 loss is less endothermic than HX loss by 10 kcalmol-1 and 28 kcalmol-1 for X = Cl and Br, respectively109. 

With the advent of very sensitive ionization techniques such as matrix assisted laser desorption (MALDI) coupled with time-of-flight (TOF) mass analysis, measurement of the intact mass of peptides at sub-pmol levels has become a reality (2) of which we have taken advantage for the systematic screening of HPLC fractions. Partial sequence information can be obtained by carrying out enzymatic hydrolysis with exoproteinases (e.g. carboxypeptidases and aminopeptidases) (3, 4). More recently, MALDI has been used to measure metastable decomposition occurring in the first field free region of a reflectron TOF instrument (referred to as post source decay (PSD)) with only marginally more sample (5-7). 

Fragmentations of aromatic isothiazoles and benzisothiazoles are characterized by strong molecular ions and typical pathways are given in CHEC(1984) <1984CHEC(6)131>. Two important primary fragmentation processes are the losses of HCN and CS, both also yielded by metastable decompositions. 

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