Biodiesel: Testing for Glycerin/Glyceride Contamination

ASTM D6751 outlines specifications that must be met for B100 biodiesel to be considered an acceptable fuel source, and include a maximum limit for glycerin content. Specifications are set for free glycerin and also for total glycerin (bound in mono-, di-, and tri-glycerides). The need to verify that the levels of these contaminants are below established guidelines is because of their potential to clog fuel systems. The source of each contaminant can be traced back to a specific manufacturing step:
  • Triglycerides remain if not converted to FAMEs during the transesterification reaction step in the manufacturing process.
  • Mono- and di-glycerides remain due to the incomplete conversion of triglycerides to FAMEs during the transesterification reaction step in the manufacturing process.
  • Glycerin remains due to incomplete separation of FAMEs and glycerin into two fractions during a later step in the manufacturing process.

The actual procedures for testing B100 biodiesel for glycerin/glycerides can be found in ASTM D6584 and DIN EN 14105. Both methods provide for the quantitative determination by silylating the sample with N-methyl-N-(trimethylsilyl) trifluoroacetamide (MSTFA) followed by high temperature gas chromatography using cool-on-column (COC) injection beginning at 50 °C. Use of a heated injection port can lead to sample discrimination and is not suggested as a replacement for COC. The syringe needle used must have a diameter small enough to fit inside the 0.53 mm I.D. guard column. Automated injection is highly recommended for consistency.

Technical Article:
Determination of Free and Total Glycerin in B100 Biodiesel