Fatty Acid Methyl Esters

Introduction

The fatty acid composition of a sample can reveal the origins of said sample. Different combinations of fatty acids are associated with different types of oils. This information can be especially useful in the field of archaeology. Dried clay holds lipid residue fairly well, so if a clay pot is crushed up and prepared for GC-MS analysis, the fatty acid composition of the oils in the clay can be discovered. This process can also be carried out with modern cooking oils to determine the different fatty acid composition of different oils or even of different brands of oil. This project is ideal for someone who is just beginning to conduct experiments that require GC-MS analysis, as it allows the person to gain experience in developing methods for data collection as well as allows them to gain experience analyzing mass spectra.

Fatty Acid Methyl Ester Structures

Significance of Fatty Acid Methyl Esters

Transesterification

Transesterification is the process of converting a carboxylic acid to an ester.

Image . This reaction is known as transesterification. The methyl from the methanol group is added to the carboxylic acid of the fatty acid to create a fatty acid methyl ester.

Methods and Materials

Sample Preparation/Trasesterifcation

• Combined 200 ul of the sample with 40 ul of anhydrous methanol into a microcentrifuge tube
• Added 4.3 ul of 5 M KOH to the top methanol layer (catalyst)
• Sonicated the tubes in an ultrasonic bath for 30 minutes
• Added 200 ul of 0.1 M acetic acid (neutralize the base catalyst)
• Centrifuged the solution at 6,000 rpm for 5 minutes
• Transferred 40 ul of the top layer to a GC vial and diluted with pentane

Results

GC-MS Analysis

GC-MS Method for the Analysis of Fatty Acid Methyl Esters:

Grapeseed Oil Data

Grapeseed oil was treated as described above. The transesterifcation products were palmitic acid, linoleic acid, and oleic acid. The oleic acid peak was much smaller than expected. While we have no definitive answer as to why this peak was so small, one theory is that the grapeseed oil was old and had undergone oxidation reactions. | || |}

Corn Oil Data

Sunflower Oil Data

References

Lipid Residue Analysis of Archaeological Pottery: An Introductory Laboratory Experiment in Archaeological Chemistry Clare S. Harper, Faith V. Macdonald, and Kevin L. Braun Journal of Chemical Education 2017 94 (9), 1309-1313 DOI: 10.1021/acs.jchemed.7b00225