Recognition of the Molecular Ion Peak

The molecular ion peak directly provides valuable information on the analyte. Provided the peak being of sufficient intensity, in addition to mere molecular mass, the accurate mass can reveal the molecular formula of the analyte, and the isotopic pattern may be used to derive limits of elemental composition (Chaps. 3.2 and 3.3). Unfortunately, the peak of highest m/z in a mass spectrum must not necessarily represent the molecular ion of the analyte. This is often the case with El spectra either as a result of rapidly fragmenting molecular ions or due to thermal decomposition of the sample (Chaps. 6.9 and 6.10.3) 

In general, the stability of the molecular ion increases if n-bonding electrons for the delocalization of the charge are available and it decreases in the presence of preferred sites for bond cleavage, e.g., by a-cleavage. 

Rule of thumb The stability of molecular ions roughly decreases in the following order aromatic compounds conjugated alkenes alkenes alicyclic compounds carbonyl compounds linear alkanes ethers esters amines carboxylic acids alcohols branched alkanes. [81] 

In El mass spectrometry, the molecular ion peak can be increased to a certain degree by application of reduced electron energy and lower ion source temperature (Chap. 5.1.5). However, there are compounds that thermally decompose prior to evaporation or where a stable molecular ion does not exist. The use of soft ionization methods is often the best way to cope with these problems. 

Note Even for the softest ionization method, there is no guarantee that the highest m/z ion must correspond to molecular mass. 

Quite often, under electron impact (El), recognition of the molecular ion peak (M) poses a problem. The peak may be very weak or it may not appear at all how can we be sure that it is the molecular ion peak and not a fragment peak or an impurity Often the best solution is to obtain a chemical ionization spectrum (see below in this section). The usual result is an intense peak at M + 1 and little fragmentation. 

Many peaks can be ruled out as possible molecular ions simply on grounds of reasonable structure requirements. The nitrogen rule is often helpful. It states that a molecule of even-numbered molecular weight must contain either no nitrogen or an even number of nitrogen atoms an odd-numbered molecular weight requires 

A useful corollary states that fragmentation at a single bond gives an odd-numbered ion fragment from an even-numbered molecular ion, and an even-numbered ion fragment from an odd-numbered molecular ion. For this corollary to hold, the ion fragment must contain all of the nitrogen (if any) of the molecular ion. 

Consideration of the breakdown pattern coupled with other information will also assist in identifying molecular ions. It should be kept in mind that Appendix A contains fragment formulas as well as molecular formulas. Some of the formulas may be discarded as trivial in attempts to solve a particular problem. 

The presence of an M - 15 peak (loss of CH3), or an M - 18 peak (loss of H20), or an M — 31 peak (loss of OCH3 from methyl esters), and so on, is taken as confirmation of a molecular ion peak. An M — 1 peak is common, and occasionally an M - 2 peak (loss of H2 by either fragmentation or thermolysis), or even a rare M - 3 peak (from alcohols) is reasonable. Peaks in the range of M - 3 to M — 14, however, indicate that contaminants may be present or that the presumed molecular ion peak is actually a fragment ion peak. Losses of fragments of masses 19-25 are also unlikely (except for loss of F = 19 or HF = 20 from fluorinated compounds). Loss of 16 (O), 17 (OH), or 18 (H20) are likely only if an oxygen atom is in the molecule. 

The intensity of the molecular ion peak depends on the stability of the molecular ion. The most stable molecular ions are those of purely aromatic systems. If substituents that have favorable modes of cleavage are present, the molecular ion peak will be less intense, and the fragment peaks relatively more intense. In general, 

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Development of the molecular formula starts with recognition of the molecular ion peak (Section 1.5). We assume the usual situation high-resolution MS instrumentation is not readily available. Let us also assume for now that the peak of highest m/z (except for its isotope peaks) is the molecular ion peak and is intense enough so that the isotope peak intensities can be determined accurately and the presence and number of S, Br, and Cl atoms can be ascertained. Look also at the fragmentation pattern of the mass spectrum for recognizable fragments. If the molecular ion peak is an odd number, an odd number of N atoms is present. 

Crown ethers have been used for improving the detection of fullerenes by electrospray mass spectrometry [13]. Sawada et al. have developed a FABMS methodology for the determination of chiral recognition of amino acid esters by crown ethers [14]. This method requires that the racemic mixture of the guests contains one enantiomer in its isotoplcally labeled form. Mass spectral analysis of the molecular ion peaks for H + and H + G(5) allows a direct comparison of their relative abundances, where H and G are host and guest, respectively. 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

Regrettably, alkyne-substituted cluster complexes seem particularly prone to fragmentation and very few accurate mass spectroscopic studies have been reported. A recent exception has been the field-desorption and electron-impact mass spectral investigation of mono-and oligo-nuclear ferracyclic ring systems of the from Fei(CO)J,(C2R2)2 (x = 1, 2, 3 y = 6, 8) (384). These species show intense molecular ion peaks, which enable ready recognition of the molecular composition. 

An aid to identification is the ability to determine the relative molecular mass (RMM) with a high degree of accuracy and to establish the empirical molecular formula. The former depends on recognition of the parent or molecular ion peak M produced when an electron is ejected from the molecule... 

Note One alkali adduct ion almost never occurs exclusively, i.e., [Mh-H]", [MH-Na]" and [Mh-K]" (Mh-1, M+23 and Mh-39) are observed with varying relative intensities at 22 u and 16 u distance, respectively. This makes the recognition of those peaks straightforward and effectively assists the assignment of the molecular weight. 

The addition of a mass-signature element that contains a 1 1 ratio of one or more stable isotopes ( C, N. or H) splits the molecular ion into two equal peaks (similar to bromine). All compounds that contain the peak-splitting component are easily recognized in the spectrum as goalpost-like peaks separated by the mass difference between the natural and isotopically labeled components. The peak-splitting element provides a valuable means for recognition of the compounds synthesized on the solid phase. 

The TMS spectra of the polyhydroxy 3-, 4-, 5- and 6-carbon atom diacids (aldaric acids) have been comprehensively reported by Petersson (1972b). In general the molecular ions are of very low intensity, but the M-15 peaks are adequate for recognition and are more intense than the corresponding organic acids (see Fig. 5.16). Aldaric acid TMS-ethers-TMS esters are dominated by TMS rearrangement processes, with less prominent ions from chain cleavages than the aldonic acids. 

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