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An Improved Method for Determining Medium- and Long-Chain FAMEs Using Gas Chromatography

Abstract

The existing protocols for analyzing fatty acid methyl esters (FAMEs) using a one-step acetyl chloride (AC) catalyzed transesterification and extraction procedure cannot accurately determine the medium- and long-chain fatty acids simultaneously in clinical (enteral, parenteral) formulations. For example: (1) addition of AC at room temperature generates an exothermic reaction that often results in loss of sample and possible injury to the analyst; (2) certain polyunsaturated fatty acids (PUFAs) are less stable at elevated temperatures during the transesterification and contribute to the over-estimation of the C16:0 and C18:1 fatty acids; and (3) the flame-ionization detector (FID) response varies depending on the carbon chain length of the fatty acids, that consequently impacts the underestimation of medium-chain fatty acid (C6–C10) recoveries. To overcome these deficiencies and accurately determine FAMEs, we have developed an improved one-step transesterification method that employs the addition of AC in tubes kept on a dry ice bath, the transesterification at room temperature, and the data analysis using relative response factors. Using this modified protocol, we determined the fatty acid composition of lipid emulsions (Omegaven® and Lipidem®) on a Shimadzu GC2010 gas chromatography (GC) system using a capillary GC column (Zebron ZB-WAX plus, 30 m, 0.25 mm ID, 0.25 μm). Our data suggest that the improved method can be easily used to accurately determine fatty acids (C6–C24) in functional foods and lipid emulsions.

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Abbreviations

AC:

Acetyl chloride

AOAC:

Association of Analytical Chemists

Calc.:

Calculations

Exp:

Experimental

FAME:

Fatty acid methyl esters

FAs:

Fatty acids

FID:

Flame ionization detector

GC:

Gas chromatography

IS:

Internal standard

PUFAs:

Polyunsaturated fatty acids

REF:

Relative response factor

Theo:

Theoretical

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Acknowledgments

The work presented in this manuscript is supported by a grant from Baxter Healthcare Corporation. Deerfield, IL 60015, USA.

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Correspondence to Rafat Siddiqui.

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Xu, Z., Harvey, K., Pavlina, T. et al. An Improved Method for Determining Medium- and Long-Chain FAMEs Using Gas Chromatography. Lipids 45, 199–208 (2010). https://doi.org/10.1007/s11745-009-3382-7

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Keywords

  • Medium-chain fatty acids
  • Long-chain fatty acids
  • Relative response factor
  • Lipid emulsion
  • Transesterification