| A gas chromatographic (GC) protocol for analysis of trans fatty acids (TFAs) was established, and then applied to survey TFAs distribution in commonly consumed oils and oil-involved foods.The effects of column temperature and carrier gas flow rate were evaluated on the GC separation of fatty acids (FAs) in a hydrogenated soybean oil on a highly polar capillary column (CP - Si188, 100m×0.25mm×0.2μm). At constant column temperatures maintained at 170℃, 175℃, and 180℃, respectively, the critical FAs pair (13/14t & 9c) was better separated at higher temperatures; while the individual positional isomers of trans 18:1 FAs were so at lower temperatures; and the individual positional isomers of trans 18:2 FAs generally reached baseline resolution regardless of the temperatures tested. Under programmed temperatures, however, the resolution and peak profile of 13/14t & 9c was significantly improved, and initial column temperature, temperature gradient, and carrier gas velocity exhibited significant effects on individual isomers of t-18:1 in terms of resolution peak profile. The optimum conditions for TFA resolution was obtained by a series of factorial experiments: carrier gas pressure 220kPa, initial temperature 120℃, remain 3min , 8℃/min to 175℃, remain 28min, 3℃/min to final temperature 215℃, remain 20min, split ratio 1:50,sample load 0.5μL, sample concentration 5mg/mL.The protocol established was shown to have good specificity and high sensitivity, accuracy and precision with a detection limit≤5.5μg/mL. The linear range of saturated FAs (18:0, 20:0) was 8.0×10-3 ~ 5.4×10-1mg/mL and unsaturated FAs (9t-18:1,9t12t-18:2,9c-18:1,9c12c-18:2) was 1.66×10-2 ~ 1.06mg/mL(R2>0.993). T-tests proved no significant difference between the determined value and the true value (α=0.05) in the reference. And the relative standard deviation (RSD) of oil sample was 0.18%-9.89%. Most of the TFAs in the refined oils were trans polyunsaturated FAs (PUFAs) including t-18:2 and t-18:3. The contents of the TFAs were related to the refining process and the oil seed cultivars. The contents of the TFAs in the refined oils tested were as follows: soybean oil 0.89%-3.48%, rapeseed oil 1.97%-4.76%, maize oil 2.14%-6.24%, sunflower seed oil 1.48%-2.36%,peanut oil 0.10%-0.47% and palm oil 0.12%-0.28%. The TFAs average mean of all vegetable oils analysed was 2.03%.All of the special oils analysed, except for shortening, were mainly composed of partially hydrogenated vegetable oils with t-18:1 being the major TFAs. The difference in the functions of special oils requires different degree of hydrogenation, and is, therefore, related to different contents of TFAs. The contents of the TFAs in several special oils tested were: cocoa butter replacer (CBR) and powdered oil 40%-50%, cocoa butter substitute (CBS) and torrefy and filling oils 3%-5%. Shortening sample analysed was mainly composed of palm oils with a TFAs contents as low as 2% consisting of trans PUFAs (t-18:2). There may be some shortenings which consist of hydrogenated vegetable oils were not included in this article. The TFAs average mean of all special oils analysed was 17.30%.The contents of TFAs in oil-containing foods were remarkably different: potato chips and instant noodles (~1%); sandwich biscuits (3%~18.30% depending on degree of hydrogenation of the oils); wafer and cookies (~3%); and french fries, fried chicken, deep-fried twisted dough sticks, and fried dough twists (1%~13% depending on oil categories and total frying time).The dietary intake levers of TFAs was 1.44g/d, city 1.82g/d, rural 1.30g/d and the percent of total energy was 0.58%, 0.77% and 0.51% respectively.
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