Monoglycerides represent a reaction residue in renewable biodiesel fuel. Among the possible glyceridic forms they are poorly soluble and have high melting point. For this reason during the in field use of biodiesel neat or in blend with diesel, monoglycerides can cause problems as regards to the cold stability of the fuel and the plugging of filters. The problem of quantitatively evaluating the monoglycerides content in biodiesel has been addressed and solved during the last 20 years, generally by means of GC. Recently the need for a strong reduction of total and saturated monoglycerides content appeared. In parallel with the technology changes new analytical methods have to be developed, in order to fulfill the requirement of accuracy and precision at low concentration levels. The technique presented here allows the determination of total monoglycerides content and the separate quantification of glycerol monopalmitate and stearate. The proposed method is based on acetylation of hydroxy groups of the sample, followed by micro column solid phase separation, isolation of polar fraction containing all acetylated glycerides, and GC quantification with internal standard. By using this technique a low noise GC path is obtained thus allowing the identification and the quantification of peaks. Our results are discussed in comparison to those obtained with the existing standards and demonstrated a good repeatability even at low concentration levels.
Practical applications: The results of the analytical setup of a new method for total and saturated monoglycerides in biodiesel allow to fulfill a request coming from the automotive world for the reduction of these partial glycerides in biodiesel to be used pure or in blend with diesel fuel. The currently available methods are not sufficiently precise at concentrations below 0.3% and do not allow separate evaluation of the saturated monoglycerides content. This analytical proposal might allow to overcome these problems and it is ready for a future standardization activity.