It is essential to have simple rapid methods for the determination of fatty acid structures. Traditionally, fatty acids are analysed by gas chromatography using their methyl ester derivatives (FAME). However, their corresponding mass spectra exhibit molecular ions but are usually devoid of ions indicative of structural features and, notably, the position of double bounds on the aliphatic chains [1]. In the most useful approach to structure determination, the carboxyl group is derivatised with a reagent containing a nitrogen atom. Recently, a convenient method for preparing picolinyl esters from intact lipids has been published [2]. However, some problems occurred in our laboratory when this method was used, leading to some modifications and optimisation. Thus, hexane and water have been added while sodium bicarbonate has been removed in order to lower contamination. Temperature and length of the reaction have then been optimised in order to get 100% derivatisation for different kinds of lipids (45 °C and 45 min for FAME). Finally, a comparison of the response factors has confirmed the better sensitivity of the picolinyl derivative against FAME (five times more).
ABSTRACT:The elastic modulus E l of the crystalline regions in the direction parallel to the chain axis was measured by X-ray diffraction for polyimide derived from polyamic acid (biphthalic dianhydride and p-phenylene diamine). These specimens were cured by two different routes: curing at 200°C, and at 400°C for 1 h, respectively (2STEPS), and curing from 80°C to 400°C stepwise (nine steps) for 1 h at each step (STEPWISE). The E l values of 54 -169 GPa were obtained for the STEPWISE specimen and 80 -178 GPa for the 2STEPS specimen, depending on the meridional reflection employed for measurement of the E l value. A linear relationship between the E l value and the fiber identity period was obtained from each meridional diffraction, such that the E l value increased with an increase in the fiber identity period. The E l value of the fully extended structure was estimated to be 210 GPa. These are considered to be due to the coexistence of polymorphs with different skeletal structures. The crystalline regions of the 2STEPS specimen seems to consist of a more extended skeleton than those of the STEPWISE specimen.
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