Nuclear magnetic resonance spectroscopic analysis of ethyl ester yield in the transesterification of vegetable oil: an accurate method for a truly quantitative analysis Giovanni Pimenta Mambrini, a Caue Ribeiro a * and Luiz Alberto Colnago a Biodiesel has recently gained importance as an alternative to fossil diesel. However, the development of more efficient biodiesel formation processes still depends on the use of fast and accurate analytical techniques to evaluate the conversion degree of the transesterification reaction. Nuclear magnetic resonance (NMR) spectroscopy has been used for this purpose, but some experimental details still need to be addressed. Therefore, in this communication, the experimental conditions for a truly quantitative NMR analysis of biodiesel formation are presented. The longitudinal relaxation time (T 1 ), which is the determining factor for quantitative analysis, was measured using an inversion-recovery method, and a maximum value of 2.35 s was obtained for a biodiesel sample. A linear determination coefficient of r 2 = 0.99 was obtained when a time delay between pulses longer than 5T 1 =15 s was used, whereas strong deviations were observed when using shorter delays. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: 1 H-NMR; biodiesel; transesterification; quantification; relaxation time Introduction Recently, much research investigating new energy sources has been conducted. Agriculturally derived fuels (biofuels) have attracted significant attention because they are derived from renewable sources, are biodegradable, have low toxicity, and have a low-pollution potential relative to their fossil-based counterparts. [1] The conversion of vegetable oils and fats into biodiesel therefore, is one of the most studied options for diesel replacement. [2] Particularly in Brazil, the various sources of natural oils and previous success in sugarcane ethanol production [3] provide strong motivation for the further development of different alternatives. However, the processes for biodiesel production are still a challenge in terms of productivity and cost, and significant efforts are necessary to attain a self-sustainable system for biodiesel production. [4,5] These efforts include discovering new catalysts and consolidating production processes. Naturally, these tasks depend on a reliable, efficient, and fast method for quantitative measurement of the conversion to biodiesel. In the literature, methods such as gas [6] or size-exclusion chromatography [7] have been proposed, but they are slow analytical methods. To the best of our knowledge, NMR-based methods are one of the best choices for fast and reliable measurement. For example, Ghesti et al. [3] proposed an NMR method that is based on the measurement of the relative areas of the peaks associated with the glycerol and ethoxyl groups in a mixture of oil and biodiesel. Additionally, Jin et al. [8] studied methyl ester formation using NMR spectroscopy, demonstrating that this technique allows the detection of diglycerides and monoglycerides in the reaction medium in addition to the starting oil and the product. The literature also contains research about the quantification of the oil content in seeds based on this technique, which keeps the seed intact after analysis. [9,10] Finally, Nagy et al. [11] used 31 P NMR to verify the biodiesel preparation process. These examples show how NMR can be useful for biodiesel research. Experimental conditions can strongly interfere with the NMR signal, thus impeding a truly quantitative measurement. Usually, the extent of transesterification is measured by integrating the area under the peaks corresponding to the biodiesel and oil. For quantitative NMR analysis, it is necessary to know the values of the longitudinal relaxation time, T 1 , such that a time interval between pulses (T p ) longer than 5T 1 can be used to allow the spins to return to their fundamental state. [12] It must be emphasized that very few papers describe the NMR parameters used in biodiesel conversion, and the routine adopted is often not adequate because it is simply an adaptation of other quantification protocols that were not developed for this specific purpose. [13] Therefore, in this paper, we discuss the importance of various NMR variables in a quantitative measurement of biodiesel in oil mixtures, focusing on the importance of an adequate choice of T p . The results show that significant deviations from linearity can appear due to an erroneous choice in this single parameter. Therefore, using a correct procedure with an adequate T p value can provide results that agree well with expected values, demonstrating the potential of NMR techniques for biodiesel analysis. * Correspondence to: Caue Ribeiro, Embrapa Instrumentação, Rua XV de Novembro 1452, São Carlos, SP, Brazil, CEP 13560–970. E-mail: caue@cnpdia.embrapa.br a Embrapa Instrumentação, Rua XV de Novembro 1452, São Carlos, SP, Brazil, CEP 13560-970 Magn. Reson. Chem. 2012, 50,1–4 Copyright © 2012 John Wiley & Sons, Ltd. Rapid Communication Received: 8 February 2011 Revised: 2 September 2011 Accepted: 8 September 2011 Published online in Wiley Online Library: 5 February 2012 (wileyonlinelibrary.com) DOI 10.1002/mrc.2831 1