Dynamic Headspace Gas Chromatography/Mass Spectrometry Technique for Determining Volatiles in Ambient Stored Vegetable Oils

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Abstract
A dynamic purge and trap technique is developed to analyze volatiles formed in ambient stored soybean oil. A Unacon 810B headspace concentrator interfaced to an HP 5992B benchtop mass spectrometer and equipped with capillary column and flame detector allows simultaneous quantitative and qualitative analysis. Previous techniques required excessive heating to purge volatiles from the oil and did not quantitatively measure the free volatiles because volatiles were generated from peroxides in the oil. The new technique offers ppm quantitation of individual volatiles rather than an arbitrary measurement of total volatiles, measured as total peak area. For this technique, one-half gram of vegetable oil is purged with nitrogen under mild heat. The volatiles are quantitatively trapped by a Tenax/Ambersorb/graphite trap, which can be stored for subsequent analysis or analyzed immediately by thermally desorbing and backflushing the volatiles onto the Unacon. Using nonane as an internal standard, volatiles of freshly deodorized soybean oil, which was stored at room temperature and in light for 67 days, are monitored. Analysis indicates an increase of total volatiles from 1.8 to 8.2 ppm during this time. Peroxide values range from 0.0 to 18.8 during the same interval. Specific volatiles identified by mass spectrometry are pentane, hexanal, and 2-heptenal, and response factors for the internal standardization technique vary ± 5%. Results indicate that tert-butyl hydroquinone (TBHQ) can offer some protection to the oil as determined by both total volatiles and peroxide buildup.

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