Primary amines undergo break of the β bond to the NH2 group [1, 3]: For instance, 2-phenylethylamine, found in chocolate, red wine, cheese and also involved in migraine, provides a molecular ion of relatively low abundance on electron impact, which undergoes fragmentation in the β position resulting in formation of two carbocations: Secondary amines have the same degradation pathway as esters (Figure 20). This mass is rendered by low-resolution spectrometers. Distinctive peaks by m/z 42 and 43 in the cyclohexanone spectrum arise from the following fragmentations: The molecular ion peak is predominant in aromatic ketones. The existence of the molecular ion in the spectrum allows accurate determination of molecular mass. , etc., peaks in the mass spectrum of a compound, given the natural abundance of the isotopes of carbon and the other elements present in the compound. A more advanced variation on this is the use of an internal standard which behaves very similarly to the analyte. Airport security screeners also use high-speed GC-MS instruments to look for residue from bomb-making chemicals on checked luggage. 4 x 12 = 48. All elements have several natural-state isotopes [46–49]. Finally, there is a detector, which detects and quantifies the separated ions. One fragment is an even-electron cation and another free radical with an unpaired electron [1, 3, 54]: In heterolytic cleavage, an electron pair moves together to the charged atom. In an EI source these molecules will behave very differently. This peak in 2-methylbutane is caused by: The ion formed is a secondary carbocation - it has two alkyl groups attached to the carbon with the positive charge. in the molecule [1, 3]. Primary alcohols mainly show a predominant peak due to the CH2=OH+ (m/z 31). This section will ignore the information you can get from the molecular ion (or ions). Compared to alcohols, ethers do not support fragmentation with water elimination. Mass spectra of pure compounds show difficult-to-interpret peaks. This is fairly clearly seen in the mass spectra of ketones like pentan-3-one. Similarly to toluene, β- and γ-picolines also render intense peaks by M–HCN, m/z 66, and l M−1, m/z 78. This is the one that has the largest, easily identifiable m/z ratio. Definitive Rules, 1979. Quite often, mass spectrometry is used in conjunction with a separation technique called gas chromatography (GC). The symbol M• (or M+•) does not refer to an added electron but to a post-ionisation unpaired electron. Compiled by A. D. McNaught and A.Wilkinson. In electrospray ionization (ESI), the protein sample, in solution, is sprayed into a tube and the molecules are induced by an electric field to pick up extra protons from the solvent. Identification of the molecular ion is the first stage for mass spectrum interpretation because the molecular ion is the source of information on molecular composition. Suffice it to say that in GC, a very small amount of a liquid sample is vaporized, injected into a long, coiled metal column, and pushed though the column by helium gas. Dominant fragmentation patterns depend on chain length as well as on the number and length of nitrogen-bound alkyl groups . This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The spectra produced by electron impact ionisation display weak or non-existent molecular peaks, because of amino acid capacity to easily lose their carboxyl group and of their esters to easily lose their carboalkoxyl group on electronic impact [76, 77]: Esters of amino acids basically have two fragmenting patterns: There is an average or high peak intensity of iminium ions, RCH=N+H2, as well as of the N+H2=CHCO2R′ ion. Mass spectrum is the balance of a series of competing and consecutive reactions [54–56]. When the molecule displays several isotopes such as those of bromine, of relatively close abundance, the dibromo-molecular ion, Br2, has three peaks (Figure 6): Molecular peaks of bromide compounds . The difference comes from one or several electrons. Pharmacokinetics needs the use of mass spectrometry because of the complex nature of the matrix (often blood or urine). A possible exam question. The M+1 peak (resulted on H⋅ collision with the molecular ion) is more visible. Schematics of an electrospray source where ions separate by the m/z ratio (z ≥ 1). Fatty ethers may be an exception. Isotopic percentage and nominal mass calculation are also explained along with fragmentation mechanism. The y-axis of a mass spectrum represents signal intensity of the ions. As such, it is relatively stable. In this way, the many proteins in a complex biological sample (such as blood plasma, urine, etc.) Following McLafferty rearrangement, straight-chain nitriles of more than seven carbon atoms render a characteristic peak by m/z 97: Simple fragmentation of the C–C bond, aside from the one next to nitrogen, renders a series of homologous peaks of even mass along the hydrocarbon chain by m/z 40, 54, 68 and 82, due to the (CH2)nC≡N+ ions, similar to hydrocarbons [1, 3]. Very volatile liquids are first vaporised and then introduced into the ionisation chamber. The molecular peak, the parent peak, corresponds to the M +• molecular ion in pure compounds. One instance of ortho-effect is that of o-methylbenzoic (o-toluic) acid [1, 3, 53]: Esters of aliphatic acids, even soaps, usually display one noticeable molecular peak. Solid, liquid or gaseous organic compounds can be analysed; however, inside the ionisation chamber, samples need to be in the gas phase. Analyte structures can sometimes only arise partly from the known structure, or isomers cannot be determined. Along the way, different compounds in the sample stick to the walls of the column to different extents, and thus travel at different speeds and emerge separately from the end of the column. The proposed molecular structure is correlated with mass spectrum data [82–87]. Hence, the mass spectrum of a sample is a pattern representing the distribution of ions by mass (more correctly: mass-to-charge ratio) in a sample. Source: PAC, 1991, 63, 1541 (Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). These compounds render easy-to-interpret spectra. There have been several suggestions (e.g. Aliphatic sulphides render an intense peak by M+2. Built by scientists, for scientists. The chapter presented here is an introduction to mass spectrometry, which, we think, helps the understanding of the mechanism of fragmentation corroborating spectral data and molecular structures. Fragmentation of the C15–C16 bond, with elimination of an olefine and formation of the m/z 232 ion of mass M–(C8H17+27).
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