Appearance of the mass spectrum

The number of abundant ions in the mass spectrum and their distribution is indicative of the type of molecule. As discussed above, the mass and the relative abundance of the molecular ion gives an indication of the size and general stability of the molecule. An abundant molecular ion is expected, for example, from aromatic and saturated polycyclic molecules, provided that no easily cleaved group is present. A spectrum consisting of a few prominent ions suggests there are only a few favoured decomposition pathways indicating a small number of labile bonds or stable products. 

Characteristic ion series are also produced by aromatic compounds, the exact positions being dependent on the nature of the substituent (Table 3.5). 

Certain types of compounds give characteristic ions in the mass spectrum which are often readily picked out and are useful indicators of possible struc- 

The following scheme is suggested as a general approach to the interpretation of the mass spectrum. Each spectrum presents its own challenge and therefore too rigid adherence to any scheme is unwise. Reference should be made to the appropriate paragraph of this section for fuller details of each step. 

Determine the elemental composition and the index of hydrogen deficiency, i.e. the number of double bonds and rings. 

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The appearance of the mass spectrum is closely similar to that provided by a magnetic-sector instrument. 

When chlorine, CU, is examined in a mass spectrograph, Cl/, Cl+, and Cl+I ions are formed. Remembering that there are two isotopes in chlorine, 35 (75%) and 37 (25%), describe qualitatively the appearance of the mass spectrum. Which ion will produce lines at the largest radius Which at the smallest radius How many lines will each ion produce ... 

The flow rate of liquid in the HPLC-electrospray system is paramount in determining performance both from chromatographic and mass spectrometric perspectives. The flow rate affects both the size and size distribution of the droplets formed during the electrospray process (not all droplets are the same size) and, consequently, the number of charges on each droplet. This, as we will see later, has an effect on the appearance of the mass spectrum which is generated. It should also be noted that the smaller the diameter of the spraying capillary, then... 

At this point, there is no need to worry about why bromoform behaves like this upon electron ionization. Instead it is sufficient to accept the occurrence of such fragments and to focus on the consequences in the appearance of the mass spectrum (Chap. 6.1.4). 

The introduction of a charge-localizing heteroatom into a molecule is accompanied by obvious changes in the appearance of the mass spectrum. Withdrawal of an electron upon El must not necessarily affect a o-bond, because the electron can be supplied from one of the free electron pairs of the heteroatom. 

Check which ionization method was used and examine the general appearance of the mass spectrum. Is the molecular ion peak intensive (as with aromatic, heterocyclic, polycyclic compounds) or weak (as with aliphatic and multifunctional compounds) Are there typical impurities (solvent, grease, plasticizers) or background signals (residual air, column bleed in GC-MS) ... 

Next to these two processes, there are at least two other gas-phase processes, which partly determine the analyte ionization and to a large extent determine the appearance of the mass spectrum of the analyte observed. 

The general appearance of the mass spectrum depends on the type of compound analyzed. Seeing the patterns that distinguish, say, a normal alkane from an aromatic hydrocarbon requires practice and lots of it. The following examples include simple gas molecules and simple compounds representative of a variety of organic chemicals. 

In ESI-MS, and also in APCI-MS, the appearance of the mass spectrum depends on the mobile-phase composition. In addition, the appearance of mass spectra may change by applying instruments from different manufacturers and/or with different ion source geometries. Some results with the cholesterol reducing agent simvastatin may serve as an example. In acetonitrile/water or acetonitrile/aqueous ammonium acetate (1 nunol/L, adjusted to pH 4.0), simvastatin shows [M+Na] (Sci-ex ionspray interface and API-Ill) [113]. Under similar mobile-phase conditions but on a different instrument (Sciex turboionspray and API-365), [M-i-H]+ and were most abundant. With 2 nunol/L ammonium acetate in the mobile phase, the [M+NH ]" actually is the most abundant ion, next to [M-i-H]+, [M-l-Na] and [M+K]+being present [114, 115]. Attempts to force the ionization toward the generation of [M -i-Na]+ by the addition of sodium acetate in the case of simvastatin compromised the performance of the method. 

The main spectrum (in blue) is accompanied by a subspectrum (in red), and in each spectrum, the spacings between consecutive lines is mjz 74. Rationalize the appearance of the mass spectrum. 

Try to predict the appearance of the mass spectrum of 3-methyl-3-heptanol. 

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