Homologous ions

The alkane series is present in mass spectra of any compound containing an alkyl group. In case of isobaric series (e.g., alkanes and ketones) one should pay attention to the intensities of the isotopic peaks. Thus, for the isobaric ions of m/z 43 (CH3CO and C3H7) the abundance of the isotopic peak (m/z 44 ion) will be 2.2% and 3.3%, respectively. The situation is very simple with A + 2 elements. In this case there are two homologous ion series due to A and A + 2 ions. It is worth emphasizing that... 

TABLE 5.8. Homologous Ion Series of the Main Classes of Organic Compounds... 

The correct analysis of the homologous ion series has certain limitations. Low abundances of peaks in some series require the attention and experience of a researcher. Usually alkane series are dominated in the mass spectra of the most various compounds. Fragmentation initiated by one functional group may completely suppress or notably camouflage other reactions of polyfunctional substances. In the latter case it is useful to consider IR-spectroscopy data in mass spectral interpretation. 

Upon extrapolation to larger ketones one can expect to observe larger acylium and alkyl fragments. The occurrence of series of homologous ions is a feature that can be very helpful to deduce structural information from mass spectra. Ions such as the acylium ion series and the carbenium ion series are also known as characteristic ions. Learning the nominal masses of the first members of each series by heart is useful (Tables 6.2 and 6.3). 

Low volatile bituminous Pocahontas 3 coal shows different early mobile phase components in terms of alkylsubstituted benzenes (m/z 92, 106, 120) and further shows similar components (several homologous ion series) compared to medium volatile bituminous Upper Freeport coal (e.g. at m/z 192,206,220,234 m/z 216, 230, 244, 258 and m/z 266, 280,294). From the above observations on seven coals, it is clear that there is a chemically and/or physically distinct mobile phase, at... 

Trihydroxycyclopropenium ion and homologous ions Hogeveen dication [C6(CH3)6] 256 11 Oxygen-stabilized carbodications 257 References 258... 

It is well known that two silicate minerals, stable at the same time, will have a specific distribution of homologous ions between them, depending upon the temperature and pressure conditions prevalent. This is... 

MicroChannel plate detectors are particularly useful in time-of-flight mass spectrometry, as they are flat, minimizing time spread and subsequent mass resolution of homologous ion packets. In addition, they have reasonable gain (104—107 per plate) and fast response time (100-psec time resolution). The major limitation of multichannel plate detectors is the recovery time needed for the detector to rechaige. When a channel is discharged, a recovery time on the order of 10 nsec is typical. This becomes problematic if an ion follows another into a particular... 

The ambident 4-pyridylmethide ion can be trapped on the nitrogen with ethyl chloroformate (135). The homolog ion forms spiro derivatives on treatment with hard organohalogen compounds. However, it is not known whether C-alkylation would occur with the soft alkyl halides. 

There have been a few attempts to correlate AfH(M ) in a simple additive maimer. Franklin devised a group equivalents method, with application to gaseous free radicals and carbonium ions [63]. His results are not conclusive as to the validity of this method. Holmes et al. [64] developed an additivity scheme for estimating the heats of formation of gas-phase organic ions, based on data for unbranched homologous ions but additivity methods are not generally successful for ions, mainly because ionization energies are not simply proportional to the number of added sub-units. 

If the hydrogen atom affinity (HA) of homologous ions is a constant, then... 

Another characteristic homologous ion series of phenylalkanes can be seen in Figure 3.7 at masses corresponding to CgH5(CH2) (m/z 77, 91, 105, 119,...) the abundances of the individual ions are much more dependent on the structure of the molecule than those of the low-mass ion series. However, ions stabilized by the aromatic nucleus can be formed by a variety of fragmentation pathways, so that the presence of a large peak, such as m/z 91, signifles only that the molecule contains one or more of several possible structural features. 

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