Signal correction using molecular species to improve biodiesel analysis by microwave-induced plasma optical emission spectrometry

• Plasma naturally occurring species are used to correct analytical signals.
• N2+ and OH are used as molecular probes to improve accuracy in biodiesel analysis.
• Ca, K, Mg and Na are determined by MIP OES with LODs of 70, 7, 9 and 8 μg/L.
• Direct analysis of 1-propanol-diluted biodiesel by injecting air into the plasma
• Little to no sample preparation and recoveries in the 82–104% range

Microwave-induced plasma optical emission spectrometry (MIP OES) and a simple dilute-and-shoot sample preparation procedure with 1-propanol are used to determine Ca, K, Mg and Na in biodiesel fuel. MIP naturally occurring molecular species are used to probe plasma condition variations during sample introduction, and then correct analytical signals to improve accuracies. The electronic transitions for N2+ (0–0, B 2 ∑+u → X 2 ∑+g) and OH (0–0, A 2 ∑+ → X 2πi) are used as molecular probes. A biodiesel standard reference sample is analyzed and the results for the molecular probe signal correction method are compared with values obtained with a traditional external standard calibration (EC). Improved accuracies were observed for Ca using the N2+ probe, and for K, Mg and Na using OH. Recoveries were in the 82–104% range for all analytes (60–130% for EC). The limits of detection (LODs) for Ca, K, Mg and Na are 70, 7, 9 and 8 μg/L, respectively (2 mg/kg for Ca, and 0.2 mg/kg for all the other analytes). The method was applied to three biodiesel fuel samples and the concentrations ranges for Ca, K and Na were 9.0–16.0, 10.6–142.1, and 7.8–52.7 mg/kg, respectively. Magnesium values were lower than the LOD in all cases. Analytical signal correction using plasma naturally occurring molecular species has the potential to significantly improve accuracy and sample throughput. It allows for effective determinations without the need for time-consuming calibration methods such as standard additions.