Mass spectra characteristics

The structure (IX) of 1,2-dehydroaspidospermidine rests on comparison of its UV- (Table III) and mass spectra (characteristic peaks at m/e 280, 251, and 210) with that of 1,2-dehydroaspidospermine (VII, Section II, B). Lithium aluminum hydride reduction also gave aspido-spermidine (XXXI, 28, 18, 51a, b). 

Aldononitriles give sharp, single peaks on g.l.c., and their methylated derivatives give mass spectra characteristic of the pattern of substitution. Their use instead of alditol acetates has been recommended, as the possible ambiguity introduced on reduction of the sugar is avoided. 

The component in peak A gave a mass spectrum identical with that of the alditol acetate of a 3,6-dideoxy-2,4-di-0-methyIhexose. As tyvelose is the only 3,6-dideoxy-hexose present, the component of the first peak must be the corresponding alditol acetate, methylated at 0-2 and 0-4. The component in peak B gave a mass spectrum characteristic of a mixture of the alditol acetates from a 6-deoxy-2,3-di-0-methylhexose (27) and a 2,3,4,6-tetra-O-methylhexose (28). 

On analysis of T6 by TLC/EASI-MS, it was revealed that no signal corresponded to MDMA, caffeine, or ketamine (Figure 16.9b). However, GC-MS revealed the presence of caffeine, as was indicated in the TLC. The retention time and respective mass spectrum characteristic of caffeine compound corresponded to those obtained for the caffeine standard solution (data not shown) [23]. 

In GC-MS effluent from the column is introduced directly into the mass spectrometer s ionization chamber in a manner that eliminates the majority of the carrier gas. In the ionization chamber all molecules (remaining carrier gas, solvent, and solutes) are ionized, and the ions are separated by their mass-to-charge ratio. Because each solute undergoes a characteristic fragmentation into smaller ions, its mass spectrum of ion intensity as a function of mass-to-charge ratio provides qualitative information that can be used to identify the solute. 

This fragmentation is characteristic for a given substance, similar to a fingerprint, and is referred to as a mass spectrum. 

The mass spectrum is characteristic for different substances and can be used like a fingerprint to identify a substance, either by comparison with an already known spectrum or through skilled interpretation of the spectrum itself (Figure 3.2). 

A single instrument — a hybrid of a quadrupole and a TOF analyzer — can measure a full mass spectrum of ions produced in an ion source. If these are molecular ions, their relative molecular mass is obtained. Alternatively, precursor ions can be selected for MS/MS to give a fragment-ion spectrum characteristic of the precursor ions chosen, which gives structural information about the original molecule. 

There are other characteristics of quadrupoles that make them cheaper for attainment of certain objectives. For example, quadrupoles can easily scan a mass spectrum extremely quickly and are useful for following fast reactions. Moreover, the quadrupole does not operate at the high voltages used for magnetic sector instruments, so coupling to atmospheric-pressure inlet systems becomes that much easier because electrical arcing is much less of a problem. 

A second use of arrays arises in the detection of trace components of material introduced into a mass spectrometer. For such very small quantities, it may well be that, by the time a scan has been carried out by a mass spectrometer with a point ion collector, the tiny amount of substance may have disappeared before the scan has been completed. An array collector overcomes this problem. Often, the problem of detecting trace amounts of a substance using a point ion collector is overcome by measuring not the whole mass spectrum but only one characteristic m/z value (single ion monitoring or single ion detection). However, unlike array detection, this single-ion detection method does not provide the whole spectrum, and an identification based on only one m/z value may well be open to misinterpretation and error. 

An alternative approach to peptide sequencing uses a dry method in which the whole sequence is obtained from a mass spectrum, thereby obviating the need for multiple reactions. Mass spec-trometrically, a chain of amino acids breaks down predominantly through cleavage of the amide bonds, similar to the result of chemical hydrolysis. From the mass spectrum, identification of the molecular ion, which gives the total molecular mass, followed by examination of the spectrum for characteristic fragment ions representing successive amino acid residues allows the sequence to be read off in the most favorable cases. 

In a mass spectrum, the ratios of isotopes give a pattern of isotopic peaks that is characteristic of a given element. For example, the mass spectrum of any corn ound containin carbon, hydrogen, nitrogen, and oxygen will show patterns of peaks due to the, 7C, 7N, gO, gO, and... 

The use of chemical mapping is demonstrated in the following example involving the delamination of a silicone primer and polytetrafluoroethylene (PTFE) material. The positive mass spectrum acquired from the delaminated interface contains peaks known to be uniquely characteristic of PTFE (CF3 at mass 69) and the silicone primer (Si(CH3)3 at mass 73). Figures 6 and 7 are secondary ion im es of the CF3 and (Si(CH3)3 fragments taken from a 1-mm area of the delaminated interface. These maps clearly indicate that the PTFE and the silicone primer exist in well-defined and complementary areas. 

The molecular ion is the most abundant ion. Characteristic fragment ions in the mass spectrum occur at in/z 89. 90, and 151. 

The NMR spectrum for TsHs displays a single peak at 4.20 ppm ( /n-si = 341 Hz) characteristic of Si-H protons, while the solid state and solution Si NMR spectra each show a single peak at —83.86 and —84.73 ppm, respectively, showing that any difference in the environment of the silicon atoms (particularly in the solid state) is not resolvable. The mass spectrum of TsHs has also been reported. ... 

The power of mass spectrometry lies in the fact that the mass spectra of many compounds are sufficiently specific to allow their identification with a high degree of confidence, if not with complete certainty. If the analyte of interest is encountered as part of a mixture, however, the mass spectrum obtained will contain ions from all of the compounds present and, particularly if the analyte of interest is a minor component of that mixture, identification with any degree of certainty is made much more difficult, if not impossible. The combination of the separation capability of chromatography to allow pure compounds to be introduced into the mass spectrometer with the identification capability of the mass spectrometer is clearly therefore advantageous, particularly as many compounds with similar or identical retention characteristics have quite different mass spectra and can therefore be differentiated. This extra specificity allows quantitation to be carried out which, with chromatography alone, would not be possible. 

The great advantage of the mass spectrometer is its abihty to use mass, more accurately the mass-to-charge ratio, as a discriminating feature. In contrast to, for example, the UV detector, which gives rise to broad signals with little selectivity, the ions in the mass spectrum of a particular analyte are often characteristic of that analyte. Under these conditions, discrete signals, which may be measured accurately and precisely, may be obtained from each analyte when they are only partially resolved or even completely umesolved from the other compounds present. 

McLafferty rearrangement A molecular rearrangement that occurs under certain ionization conditions which results in the production of characteristic ions in the mass spectrum of the analyte from which it has been generated. 

A mass spectrometric study was carried out to establish tbe structure of compoimd 69. Its mass spectrum contains tbe molecular ion peak m/z 252 (16.98%) and a base peak (100%) at m/z 210, corresponding to 2-(2-hydroxypbenyl)benzimidazole (70). A tendency towards decreasing the heterocycle size is characteristic of the mass spectrometric behavior of 1,5-benzodiazepin-2-ones [61] and consequently the mass spectra of these compounds contains intense peaks of the corresponding benzimidazoles. It is also known that the mass spectrometric fragmentation of 1,5-benzodiazepines is similar to their thermal or acid decomposition. In fact, refluxing compound 69 in concentrated sulfuric acid yields benzimidazole 70 as the main product. 

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