Loss of H2O from Alkanols

Aliphatic alcohols show a strong tendency to thermally eliminate a water molecule. This is of special relevance if volatile alkanols are introduced via the reference inlet system or by means of a gas chromatograph. Then, the mass spectra correspond to the respective alkenes rather than to the alkanols that were intended to be analyzed. The water is often not detected, simply because mass spectra are frequently acquired starting from m/z 40 to omit background from residual air. 

Example The El mass spectra of 1-hexanol, Mj = 102, and 1-hexene, Mr = 84, show close similarity because the molecular ion peak is absent in the mass spectrum of hexanol (Fig. 6.40). However, a more careful examination of the hexanol spectrum reveals peaks at m/z 18, 19, 31, and 45 that are absent in the hexene spectrum. These are due to H20, HsO and to oxonium ions (H2C=OH and H3CCH=0H in this case) which are reliable indicators of aliphatic alcohols and ethers (Table 6.8). 

In addition to the observation of oxonium ions, alkanols may occasionally be identified from the occurrence of a seemingly [M-3] peak while the molecular ion is absent (Fig. 6.41). The unusual difference of 3 u results from neighboring [M-CHs] and [M-H20] peaks. In these cases, a [M-33] peak indicates consecutive losses of CH3 and H2O in either order, i.e., [M-HzO-CHs] and [M-CHs-HzO]. (Chap. 6.6.4). Alternatively, the sequence of [M-H20] and [M-H20-C2H4] may occur. 

Ammonium, phosphonium, oxonium salts and the like cannot be evaporated without substantial or complete decomposition, and thus it is not useful to employ El- 

MS for their characterization. Nevertheless, if a sample that happens to be an organic salt has incidentally been analyzed by EI-MS, it is helpful to know about their recognition. The El mass spectra usually appear to result from the corresponding amines, phosphines, or ethers, but fortunately, the anion cannot hide perfectly from us. 

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