Quantification in the analysis of transesterified soybean oil by capillary gas chromatography

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if. Quantitation in the Analysis of Transesterified Soybean Oil by Capillary Gas Chromatography1
B. Freedman*, W.F. Kwolek 2 and E.H. Pryde 3
Northern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Peoria, IL

A rapid quantitative capillary gas chromatographic method has been developed for studying transesterification of soybean oil (SBO) tofatty esters. Standard solutions containing methyl linoleate, mono-, di- and trilinolein were analyzed with a 1.8 m X 0.32 mm SE-30 fused silica column. The effect of carrier gas flow on reproducibility was determined. Prior to analysis, mono- (MG) and diglycerides (DG) were silylated with N,O-bis(trimethylsilyl) trifluoroacetamide.Tridecanoin was used as an internal standard. From plots of areaand weight relationships, slopes and intercepts for all four compound classes were determined. Agreement between the measured and calculated compositions of the standard solutions was good; the overall standard deviation was 0.4. Slopes and intercepts also were determined for SBO and its methyl and butyl esters. Complete separation of ester, MG, DG and triglyceride was obtained in 12 min bytemperature programming from 160 to 350 C. This method of analysis gave excellent results when used in a kinetic study of SBO transesterification.

of soybean oil (SBO) (4). The analytical method used to support this kinetic study was capillary gas chromatography (CGC) described in this paper. This method was chosen because it provided the accuracy and precision required. Capillary columns offer anumber of advantages over packed columns: they are more efficient, provide greater accuracy and precision, increase resolution and reduce analysis time (5-8). Nohl (9) noted that short capillary columns, 2-4 m in length, give the same resolution as packed columns. D' Alonzo et al. (10) successfully used a 6 m glass capillary column to separate a mixture of fatty acids, monoglycerides (MG), diglycerides(DG) and triglycerides (TG). Their separation required only 10 min compared to 35 min for the same separation on a 1.5 ft packed column. For our kinetic study, we required quantitation of classes of compounds, Le. ester, MG, DG and TG, but not of compounds within a class. Short capillary columns provided short run times and offered adequate separation of these classes. Thus, CGC met our majorobjectives to quickly analyze diverse and numerous samples accurately and precisely.
EXPERIMENTAL Materials. Reference standards used in this study, methyl linoleate (MeL), monolinolein (ML), dilinolein (DL), trilinolein (TL) and tridecanoin (TD), were purchased from Nu-Chek-Prep, Inc. (Elysian, Minnesota) and were chromatographically pure (>99%). Refined

Recently there has been a stong interestin simple fatty esters as an alternative to #2 diesel oil as a fuel for farm tractors. These esters are obtained from vegetable oils by transesterification. Previously we have studied variables affecting ester yields (1,2) with analyses by thin layer chromatography (TLC) using flame ionization detection (FID) (3), as well as transesterification kinetics
TABLEl

Effects of Helium Flow Rate onAjA, Reproducibility of Standard Solutions Methyllinoleate Solution number 1 Flow rate (in mllminl 50 200 350 50 200 350 50 200 350 Monolinolein iAjA .12 .11 .11 .21 .18 .20 1.22 1.09 Ll5 RSD(%) 7.2 3.3 2.1 11.4 1.7 3.7 1.7 1.7 0.8 Dilinolein iAjA .18 .16 .16 .15 .12 .15 2.21 2.36 2.31 RSD(%) 4.7 0.9 2.2 17.3 17.3 14.3 1.2 2.9 1.5 Trilinolein iAjA, .07 .06 .06 8.54 9.29 8.28 2.70 2.94 2.72 RSD(%)1.0 4.2 5.7 5.5 3.8 1.6 2.5 2.3 2.8

na
2 4 4 4 4 3 3 5 3

iAjA 10.88 7.96 8.01 .12 .07 .10 4.31 3.29 3.61

RSD(%) 2.8 2.1 4.5 43.1 1.3 ILl 3.0 1.9 2.3

3

5

an, number of replicates. bi A,/A" mean of area of compound (AJ/area of internal standard (A,). cRSD (%), % relative standard deviation.

'Presented at the AGCS meeting in Philadelphia in May 1985. 'Retired biometrician, North...