Mass spectrometry branched-chain fatty acids

Egge et al. (1972) found at least 50 branched chain fatty acids in human milk fat by identification with GLC-mass spectrometry follow-... 

J. A. ZirroUi and R. C. Murphy, Low-enragy tandem mass spectrometry of the molecnlar ion derived from fatty acid methyl estes a novel method for analysis of branched-chain fatty acids, J. Am. Soc. Mass Spectrom. 4, 223-229 (1993). 

Within a given carbon number group, some resolution is achieved for combinations of fatty acids of different chain lengths. In Figure 8.5, a separation of a hydrogenated butter fat is illustrated [288]. The C46 fraction, for example, may contain MPP, MMS, LaPS, CSS and many more species, where M = 14 0, P = 16 0, La = 12 0 and C = 10 0. Intermediate fractions containing odd-chain and branched-chain fatty acids are also well resolved. It is not easy to identify the components within particular peaks without access to mass spectrometry (see below). Similar separations of butter fat and vegetable oils [384,912] have been reported with stationary phases... 

Marine lipids with their diversity of unsaturated and branched chain acid moieties are a difficult class of materials to analyze. Ruminants (sheep, goats, cows, etc.) have a bacterial "factory" in the rumen which is able to produce branched-chain partially-hydrogenated lipids from ingested plant lipids. These lipids are incorporated into the milk and meat of the animals and eventually into animals which feed upon the ruminants. As a rule animal lipids are highly complex in comparison to plant materials. Although the branched chain materials are usually present in low concentration when compared to the common fatty acid moieties, complete description of these fats requires more sophisticated GC and thus long open tubular columns in tandem with mass spectrometry and computer analysis of the data has become an important approach. Even with a 100-m column, subcutaneous lipids of barley-fed lambs were so complex that prior fractionation with urea adducts was necessary (17). 

Because fatty acids derived from natural sources are present in a mixture, an ideal analysis method for these molecules should be applicable to mixtures without requiring a prior separation or derivatization. Mass spectrometry is an excellent tool for determining the structure of fatty acids present in a mixture. It is possible to determine not only the molecular weight and thus the elemental composition but also, in most cases, the nature and position of the branching and the other substituents on the carbon chain. [268,269] Furthermore, such an analysis requires low quantities ranging from 10 pg to 100 ng of total lipid, depending upon the analysed sample, the ionization method used and the configuration of the spectrometer. [270,271]... 

Butterfat is the most complex natural fat on the basis of its fatty acid (Table 3.153 Jensen etal, 1962) and triglyceride (Table 3.157) composition. There are approximately 15 major acids but over 500 acids have been characterized. The composition of butter-fat is further complicated because it contains numerous isomers of the monoenoic, dienoic and branched chain acids (Table 3.154) (Jensen et ai, 1962 Hay and Morrison, 1970 Deman and Deman, 1983 Van der Wen and de Jong, 1967). Such complex mixtures are probably best characterized by combinations of HPLC, capillary GLC and mass spectrometry. The higher-molecular-weight fatty acids have been characterized in some detail (Table 3.154) (Iverson et ai, 1965). 

When combined with mass spectrometry, GLC can confirm the identity of most of the closely related components of complex mixtures found in wax, cutin or suberin analysis. Examples of particular uses of this technique include the identification of branched fatty acids (Tulloch, 1976 Jackson and Blomquist, 1976), location of methyl branches in alkanes (Fig. 6.13), location of functional groups such as carbonyl and hydroxyl moieties on aliphatic chains following a-cleavage (Tulloch, 1976), and identification of wax esters (Kolattukudy, 1980). 

Combined glc and mass spectrometry provide the capability to deal with the complex mixtures of closely related compounds often found in plant cuticles. Even though identification of new compounds solely by their mass spectra cannot be considered reliable, mass spectrometry has become an invaluable tool in identifying known types of compounds in cuticular lipids. For example, methyl branches in alkanes can be located by cleavage on both sides of the substituted carbon (Fig. 5). Mass spectrometry is also the most suitable technique for identifying branched fatty acids (Tulloch, 1976 Jack-son and Blomquist, 1976 Nicolaides and Apon, 1977). Functional groups such as carbonyl groups and hydroxyl groups in the aliphatic chain can be... 

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