Determination of trace elements in biodiesel and vegetable oil by inductively coupled plasma optical emission spectrometry following alcohol dilution

摘要

A method for the simultaneous determination of Ca, Cu, Fe, K, Mg, Na, P, S and Zn in biodiesels and vegetable oils by inductively coupled plasma optical emission spectrometry (ICP-OES) has been developed. The method -based on the use of an ICP-OES instrument outfitted with a spectrometer in Paschen-Runge mount, equipped with linear charge coupled device detectors monitoring the entire spectrum from 130 to 770 nm - offers a high sample throughput as sample preparation is limited to dilution with alcohol, while all elements of interest are determined simultaneously. Ethanol is only suitable in the context of biodiesel analysis, whereas dilution with 1-propanol also allows application of the method, without any additional modification, to analysis of vegetable oils. As a result, the dilution with 1-propanol is preferable. Sample introduction was carried out with pneumatic nebulization and spectral interferences from carbon-containing compounds were reduced by cooling the cyclonic spray chamber to -5 °C. The remaining spectral interferences in the low-UV region were efficiently corrected for by the background correction system offered in the software of the ICP-OES instrument used. Calibration was carried out against inorganic standards diluted in ethanol or 1-propanol, while Y was used as an internal standard, correcting for non-spectral interference and sensitivity drift. The accuracy of the method was verified through the analysis of the NIST SRMs 2772 and 2773 biodiesel reference materials. Additionally, as for most of the target elements only indicative concentration values are available for these reference materials, recovery tests have been performed using inorganic and organic standards. The results obtained were in good agreement with the values found on the certificate for both ethanol and 1-propanol sample dilution, while the recoveries were between 87 and 116% for biodiesel and between 95 and 106% for vegetable oils. The measurement precision expressed as relative standard deviation (n = 3) was lower than 5% and limits of detection were at the low μg g~(-1) level. Subsequently, the method developed has been applied to the analysis of biodiesel and vegetable oil samples from different origins. Given the simple sample preparation, the limits of detection realized, the sufficient accuracy and precision obtained as well as the high sample throughput, this method is very well suited for routine analysis of biodiesel and vegetable oils.