Decomposition in the mass spectrometer

The types of ion fragments produced by the decomposition of organic halogen compounds in the mass spectrometer have been summarised by McLafferty - . Brief reviews have also been given by Beynon and by Budzikiewicz et al . These authors have also summarised the results of other workers, such as the early studies on the monohalides by Stevenson and Hippie and by Dibeler and Reese (ref. 146) and on the polyhalides by Bernstein et by McDowell et al and by Dibeler et Electron impact studies on aromatic halogen compounds have been reported by Majer and Patrick . The interpretations of the mass spectra of halogen compounds have been provided in some detail by McLaffierty , and the main features only of these spectra are discussed very briefly here. 

Two competing processes are of importance for monochlorides and, to a lesser extent, for monobromides. These are (1) cleavage at the carbon-halogen bond and (2) loss of hydrogen halide from the parent molecule, viz. 

The behaviour of these carbonium ions and the energetics of their formation in the mass spectrometer have been discussed by Maccoll et and analogies 

In the mass spectrometer, processes (1) and (2) are comparable for ethyl chloride, but ion formation by process (2) occurs more readily for higher n-alkyl chlorides. Process (1) is of increasing importance for secondary chlorides and most important for tertiary chlorides. It is also of major significance in the case of methyl and aromatic chlorides. 

With polychlorides, polybromides and polyiodides fragmentation by carbon-halogen rupture becomes increasingly important, and the mass spectra are correspondingly more complicated. 

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Table II. Relative mass intensities evolved from the 1 4 chloride complex at three temperatures during thermal decomposition in the mass spectrometer. Nominal temperature values are given.

Fig. 32. The variation of the peak at M/E 60 (resulting from peracetic acid decomposition in the mass spectrometer) during the cool-flame oxidation of acetaldehyde [47]. ...

Figure 7.9. Thermochemical energy relationships for unimolecular ion decompositions in the mass spectrometer.

Examination of the mass spectrum of P2VPY taken during the maximum decomposition rate reveals the major decomposition products as methylpyridine (93 a.m.u.), protonated vinyl pyridine (106 a.m.u.), and protonated dimer (211 a.m.u.) with ion ratios 74 100 59 respectively. Trimeric and tetrameric protonated species (316 and 421 a.m.u.) are also observed but in relatively small amounts. Protonated ions, rather than the simple monomers and dimers observed for the decomposition of poly(styrene) by MS11, may be created by a mechanism similar to that reported for the decomposition of 2-(4-heptyl)pyridine12 in the mass spectrometer. 

The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

They find 222> that in the mass spectrometer, octaborane(12) is produced by the first order decomposition of nonaborane(15) and in turn decomposes by a first order process to hexaborane(lO). 

In the vapor phase at 690°, regardless of its environment, tetraphenyl-phthalic anhydride exhibits much of the same behavior reflected in its decomposition pattern under electron impact in the mass spectrometer. 

Polyesterurethanes, polycarbonate and silicone rubbers have been studied by TG-Tenax-FTIR/MS. The degradation of polyesterurethanes yields C02, water, tetrahydrofurans, cyclopentanone, dicarbonic acid, and aliphatic diols and esters. The thermal decomposition of silicone rubbers leads to the formation of polychlorinated biphenyls which are produced in small amounts and can be observed in the mass spectrometer [86]. 

A number of papers concerned with the decomposition behaviour of organic halogen compounds in the mass spectrometer and related systems is to be found in the journal Organic Mass Spectrometry. For example, in the case of fluoro-compounds, McCarthy has discussed mass spectral correlations for fluorinated alkanes studies by other workers have included those on aromatic fluoro-com-pounds , fluorine-containing dimethyl esters , and tetrafluoroethanes . In the same journal, studies on other halogen compounds include dichlorocyclo-propanes , norbornyl chlorides and bromides , j8-phenylethyl bromides , and chloro-substituted benzynes . 

The walls of the expansion chamber as well as those of the RF region must be inert (glass or gold-coated), and the expansion chamber should be heated (at moderate temperatures 150-200° C) to reduce condensation. Common problems for this type of pyrolyser are condensations on the cool portion of the system. On the other hand, heating the walls of the sample region (with resistors) may generate decomposition of the sample before pyrolysis. Also, the expansion chamber extends the time for the sample to be introduced in the mass spectrometer ion source and therefore the time... 

The simplest compound of the type RN=NR, di-imide, has been identified in the mass spectrometer among the solid products of decomposition of N2H4 by an electrodeless electrical discharge at 85°K. Both isomers have been identified (i.r.) in inert matrices at low temperatures (as also has imidogen, NH ), and the structure (a) has been assigned to the cis (planar) isomer... 

Various forms of radiation have been used to produce ions in sufficient quantitites to yield neutral products for subsequent analysis. In principle, it should be possible to use intense beams of UV below ionization threshold for this purpose. To date, however, efforts to collect neutrals from resonant multiphoton ionization (REMPI) have not succeeded. In one experiment, 1 mbar of gaseous -propyl phenyl ether was irradiated at room temperature with a 0.1 W beam of 266 nm ultraviolet (from an 800 Hz laser that gives 8 n pulses) concurrent with a 0.5 W beam at 532 nm. The beams were intense enough not only to ionize the ether in the mass spectrometer, but also to excite it so that it expels propene. After several hours of irradiation < 10% of the starting material remained. Production of carbon monoxide and acetylene (decomposition products of the phenoxy group) could be detected by infrared absorption spectroscopy, but the yield of neutral propene (as measured by NMR spectroscopy) was infinitesimal. 

The authors studied the thermal dissociation of thorium iodide mass-spectrometrically, and suggested that substantial decomposition to lower iodides and iodine vapour occurs when both the solid and liquid are heated. Coarse ciystals of Thl4(cr), prepared from the elements, were used to minimise the oxygen and water contents of the samples used in the (unspecified) Knudsen cell. The variation of the intensities of the TI1I4, TI1I3, TI1I2,12 and I ions observed in the mass-spectrometer was studied from ca. 625 to 800 K. 

Thermal degradation of components can 1 a difficulty in GC/MS. Not only can the CiC injection port and GC column cause degradation but also the heated metal surfaces in the mass spectrometer ion source may cause problems. L.owering the temperature can minimize degradation. Often, however, the m iss spectrometer can be used to identify decomposition products, which can lead to chromatographic modifications lhat solve the degradation problem. 

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