Simultaneous determination of Ca, P, Mg, K and Na in biodiesel by axial view inductively coupled plasma optical emission spectrometry with internal standardization after multivariate optimization

A simple and rapid method for the simultaneous determination of Ca, P, Mg, K and Na in biodiesel by axial view inductively coupled plasma optical emission spectrometry by simply diluting the sample in ethanol and using Y as internal standard is proposed. The sample (≈2.5 g) is diluted by making up the volume to 25 mL with ethanol, after the addition of 2 μg mL−1 Y(III). The plasma radiofrequency power, oxygen flow rate, the auxiliary gas flow and nebulizer pressure were optimized by factorial analysis. An oxygen flow was used to decrease the background. The obtained detection limits, in μg g−1, considering 2.5 g of sample in a final volume of 25 mL, were: 0.03 for Ca, 0.5 for P, 0.005 for Mg, 0.3 for K and 0.1 for Na. Samples of biodiesel produced from different raw materials, such as soybean oil, castor oil, cotton and animal fat, were analyzed and spiked with 5 μg g−1 of the analytes. Calibration was carried out with standard solutions in ethanol, allowing the use of stock aqueous standard solutions. All recoveries were in the range from 82 to 114% for all analytes, demonstrating the accuracy of the proposed procedure. Most of the obtained concentrations of the analytes, with RSD lower than 9%, were in the μg g−1 level.