The homoconjugated triene (‘linolenic acid unsaturation’) and tetraene (‘arachidonic acid unsaturation’) pattern can be readily recognized and localized in aliphatic compounds by chemical ionization (NO) mass spectrometry. This method may be used for the screening of biological material, especially in food analysis.

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A series of polyunsaturated vinyl-hydrocarbons (n-C15 to n-C17) (10, 11, 12, 15, 16 and 17) including the as yet unknown all-cis-1,7,10,13-hexadecatetraene (15) as well as some rare sesquiterpenes such as isocomene (8) or modhephene (7) have been isolated from the root oil of Senecio isatideus. Unambigous positioning of the double bonds of the polyolefines was easily established by 13C-NMR spectroscopy [16]. A new mass spectroscopical method utilizing NO as a reactant gas for chemical ionization [1] made possible the reliable identification of minor and trace constituents without any chemical pretreatment or further derivatisation.

It is shown that the position of one double bond in long-chain unsaturated fatty acids and their esters can be determined by chemical ionization (NO) mass spectra. Homoconjugated dienoic acids (as linoleic acid) do not give a characteristic fragmentation pattern, in contrast to those which contain three or more homoconjugated double bonds.In continuation of our investigations on the fragmentation behaviour of compounds with double and triple bonds under chemical ionization (CI) conditions using NO as the reagent gas we now wish to report on n-alkenoic acids and their methyl and isopropyl esters. ' With the exception of compounds with a double bond in the 2-or w-position (which will be discussed below) all acids and methyl esters investigated show a typical pattern in the upper mass region, viz. Fig. 3). Methyl esters are therefore best suited for analysis, as they also show a less pronounced formation of unspecific (hydrocarbon) ions than free acids (probably due to the lower temperature required for evaporation). The fragmentation processes specific for the various double-bond positions in the hydrocarbon chain are, however, not influenced by the nature of the functional group. 2-n-Alkenoic acids (esters) (Clo, C12, c 1 6 ) § show the typical behaviour of saturated acids (esters) (which has also been observed for 2-alkynoic acids/esters2), viz.[M+NO]+ and loss of HONO (CH30N0 etc.) therefrom (base peak) (see Fig. 1 NO 3-n-Akenoic acids (C9, CIJ (see Table 1) show a fragment at m / z 104 (Fig. 2) which is shifted to mlz 118 for methyl esters. It is formed directly from [M+NO]+ (m*) and has an elemental composition of C3H6NO3 (both determined for 2-3-dodecenoic acid). This implies a genesis of this ion by cleavage of the double bond and transfer of 2 H atoms as indicated schematically:4-n-Akenoic acids (esters) (C,,, CI2) show only fragmentation of the acid (ester) function as described above but no ions initiated by an attack of NO+ at the C=C double bond (Fig. 3). n-Alkenoic acids (esters) with double-bond positions from As to form two ions which are characteristic for the position of the double bond. They belong to the series [C,H2,-4N02]t, a, and [ C,H2n-6NO]+, b (the elemental composition C7H8N0 has been determined for E-6-dodecenoic acid isopropyl ester and 2-6-octadecenoic acid methyl ester). These ions have their counterparts in the series [ cnH2n-6N02]+ and [C,H2,-8NO]' observed for alkynoic acid esters: and a similar mechanism for their formation may be invoked (the alternative fragment formed by addition of NO' at the sp2-C close to the carbalkoxyl group is not of any significance).

n-Alkyn-1-01s show fragmentation patterns which are influenced by the length of the chain and by the relative positions of the hydroxyl group and triple bond, and which allow a localization of the site of unsahuation.