Low Mass Ion Series

Try to identify any low-mass ion series before postulating specific structures for higher mass peaks. At lower masses there are fewer possible ion assignments for a particular mass value (and these are usually EE ) a sizable m/z 29 peak is usually C2Hs or CHO, whereas m/z 129 has literally hundreds of isobaric possibilities. Thus the information from low-mass ion series can limit the possible assignments for high-mass ions. 

The spectra of Unknowns 5.9 and 5.10 are very similar, with m/z 224 shown to be CigH32 in both. However, one is unique in containing an important ion series (note that these are best identified by starting at the low-mass end of the spectrum). What possible kinds of molecules are these  

Ion series separated by CH and C. Compounds with a hydrogen-to-carbon ratio much less than two cannot show a significant series of ions spaced at CH2 intervals (see Unknown 5.1, naphthalene, CioHg). The low-mass ions of such compounds can still show characteristic series. A very important series is that shown by aromatic hydrocarbons at m/z 38-9, 50-2, 63-5, and 75-8 (C Ho,5 to C H ). Heterocyclic compounds containing oxygen and nitrogen atoms and 

Recognizing ion series. Chemists quickly become able to identify carbonyl peaks in an infrared spectrum or CH3 peaks in a proton NMR spectrum. Similarly, you will feel much more at home with mass spectra if you can easily recognize the possible significance of mass values. Use the series 15, 

In Unknowns 5.11 and 5.12 notice in particular the general appearance of the spectra and their ion series. 

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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. 

These same fractions were then analysed by MS/MS using a Q-TOF in the negative ion mode. As displayed in Figure 8.71 for one of these unknowns, two series of peaks are observed. The first series, which dominates the spectra, corresponds to low-mass ions (m/z, 80, 107 and 124) suggestive of the presence of taurine conjugate. The second series corresponds to lower intensity ions distant from each other by 14 u that are characteristic of compounds containing hydrocarbon chains. 

Sulfur produces a series of intense peaks in both ionization modes. Peaks are more prolific in the negative mode, where signals are observed for the entire Sf to Si5 series. In contrast, in the positive mode, fewer peaks are observed from sulfur. Higher laser fiuences are required to observe peaks for very low mass ions such as mJz 32 and 64. Unfortunately, higher laser fluence results in the deterioration of peak resolution, shape, and intensity of the peaks for more abundant ions. However, with optimum adjustment of delayed extraction time and voltage, resolution of all isotopolog peaks can be achieved. Sulfur exists in nature as a mixture of four isotopes [94.93%], [0.76%],... 

SIMS Secondary Ion mass spectroscopy A beam of low-energy Ions Impinges on a surface, penetrates the sample and loses energy In a series of Inelastic collisions with the target atoms leading to emission of secondary Ions. Surface composition, reaction mechanism, depth profiles... 

Smith [83] classified large sets of hydrocarbon oil infrared spectral data by computer into correlation sets for individual classes of compounds. The correlation sets were then used to determine the class to which an unknown compound belongs from its mass spectral parameters. A correlation set is constructed by use of an ion-source summation, in which a low resolution mass spectrum is expressed as a set of numbers representing the contribution to the total ionisation of each of 14 ion series. The technique is particularly valuable in the examination of results from coupled gas chromatography-mass spectrometry of complex organic mixtures. 

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. 

Another source of metal carbide ions is the ruthenium compound Ru C(C0)i7. The mass spectrum of this compound exhibits an ion series Ru6C(CO) (n = 0 thru 16)30>. A low abundance of RusC+is also found30). The mass spectra of the Rub(CO) 14 (arene) derivatives have also been investigated. A series of ions Ru6C(CO)i4- (arene) + is observed 30>. 

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