Determination of P, S and Si in biodiesel, diesel and lubricating oil using ICP-MS/MS
The feasibility of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) to overcome polyatomic interferents in severely affected isotope determination is demonstrated here. This instrument arrangement presents an octopole reaction/collision system (ORS) in between two quadrupole mass analyzers. The first quadrupole operates as a mass filter which allows just the target mass-to-charge ratio (m/z) to enter the ORS and rejects all other species with m/z different from the target analyte's m/z. Once in the ORS, analytes react with the reaction gas introduced into the ORS, and the second quadrupole selects the product of the reaction's m/z (mass shift operation mode). We describe here a procedure for accurate determination of P, S and Si in biodiesel, diesel and lubricating oil by ICP-MS/MS. Fuel standard reference materials (SRMs) were microwave-assisted acid-digested using nitric acid and hydrogen peroxide. The oxygen gas flow rate in the ORS was optimized considering the limit of detection (LOD) values and accuracies reached for P, S and Si determinations in the lubricating oil SRM and the best oxygen gas flow rate was 0.75 mL min−1. Adopting optimized conditions, limits of detection were 1.2, 0.49, 0.31, 0.33 and 0.78 μg L−1 for 28Si16O+, 29Si16O+, 31P16O+, 32S16O+ and 34S16O+, respectively. Adequate precision, accuracy and sensitivity were obtained when using the mass shift mode and recoveries for biodiesel, diesel and lubricating oil digests ranged from 95.0 to 113%. No significant differences were observed between the certified values and the ones obtained using ICP-MS/MS at a 95% confidence level.