Potential of electrospray ionization mass spectrometry (ESI-MS), using direct infusion, to quantify fatty acid methyl esters (FAMEs) in biodiesel

Biodiesel is a very important biofuel obtained from vegetable oil or animal fat by a chemical reaction known as transesterification. It consists mainly of fatty acid methyl esters (FAMEs). In biodiesel quality control, the limits and methodologies for each parameter are well established, and one of them is related to the total amount of FAMEs, which is measured by gas chromatography. However, the development of other analytical methods for such measurements can serve as alternative techniques. In this work, the use of a mass spectrometer with direct infusion into an electrospray ionization source (ESI-MS) is shown as a promising technique to quantify the FAMEs present in biodiesel. Due to the chemical difference and the behaviour of the methyl esters, the ionizations occur at different scales. Thus, to evaluate this behaviour, the concept of relative ionization efficiency (RIE) in the ESI source was used for quantification. Uncertainty estimations of intensity measurement by ESI-MS were also proposed. Four biodiesel samples (canola, coconut, crambe and soy) were synthesized, and their concentrations were determined by the reference methods, GC-MS and GC-FID, and by the proposed method, ESI-MS. Then the efficiency of the method was evaluated by comparing the methods with two approaches, one using the uncertainties and another considering a statistical analysis. In both situations, the methods could be considered as equivalent.