Aliphatic acids mass spectrum

Examination of the mass spectrum of n-decanoic acid (Figure 4.1) shows prominent ions at m/z 60 and 73. A m/z 60 ion (Section III) (see also Appendix 10) suggests the mass spectrum may represent an aliphatic carboxylic acid. This ion in combination with m/z 73... 

Figure 5.12 GALDI mass spectrum of shellac (from methanol solution). Peaks at m/z—570 are related to esters of aliphatic hydroxy acids with sesquiterpenoid carboxylic acids (see text and Table 5.4). Signals marked with crosses are contaminants in the spectrometer that accumulated over time (m/z 413, 469, and 507) peaks marked ( ) are contaminating graphite clusters from the matrix (m/z 264, 276, 288).

Figure 5 El (70 eV) mass spectrum of the TMS derivative of octadecanoic (stearic) acid. The four ions at m/z 117,129,132, and 145 are a characteristic feature of the TMS derivatives of fatty acids, but there are few ions that provide information on the structure of the aliphatic chain.

Mayers and Haines (1967) have isolated a sulfolipid, 1,14-docosyl disulfate, from the phytoflagellate, Ochromonas danica. Acid hydrolysis of the sulfatide yielded a diol. The structures of the sulfatide (lipoid sulfate ester) and the diol were determined by infrared, NMR, and mass spectroscopy. The infrared spectrum of the sulfatide diol had bands indicative of a long-chain diol 1060cm (primary alcohol CO), 1120 (secondary alcohol CO), and 718 cm (long-chain aliphatic CH2rocking). 

The MALDI spectrum of a pure polymer resembles a comb and the spacing between the comb s teeth equals the mass, Tr, of the repeat unit. This quantity is often diagnostic and it suggests an almost trivial use of MALDI, namely spectral identification of polymers. The reason is that if one computes LIr values for common polymers, most values are different, the number of superpositions being very low. For instance, the most common polyethers are poly (ethylene oxide), poly (propylene oxide), poly(tetrahydrofuran), and poly(2,5-dimethyl phenylene oxide), and some common aliphatic polyesters are poly(caprolactone),poly(lactic acid), poly (hydroxy-butyrate), and poly(hydroxyvalerate). No superposition occurs (Pr values are 44, 58, 72, 120, 114, 72, 86, and 100 Da, respectively). 

Esters are carboxylic acid derivatives, and the spectrum of ethyl acetate is shown in Figure 14.19D. The carbonyl absorption does not distinguish this compound from an aldehyde or a ketone, but there is the C-O absorption at about 1200 cm 1. Because this is in the fingerprint region, however, its position can be difficult to identify. This is clearly the case for methyl pentanoate, where the C-0 absorption can easily be missed or misidentified. Based only on the infrared, it may be difficult to distinguish an ester from an aliphatic aldehyde or ketone. If the formula is known, however (from mass spectrometry), the identification is easier because an ester has two oxygen atoms, whereas the aldehyde or ketone has only one. 

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