Simultaneous Direct Determinations of Na, Mg, K, Ca, P, and S in Biodiesel Fuel by ICP-QMS/QMS after Xylene Dilution: Development and Application of a High-throughput Method for a Homogeneity Assessment of a Candidate Reference Material

Inductively coupled plasma tandem quadrupole mass spectrometry (ICP-QMS/QMS) measurements after xylene dilution were investigated as a method for determining the elements (Na, Mg, K, Ca, P, and S) in a biodiesel fuel (BDF) candidate reference material (RM). Optimizations were respectively carried out for the following parameters to obtain the best performance for measurements: O-2 flow rate (additional gas to the carrier gas) to ensure complete combustion of the xylene solvent in the plasma, plasma power to obtain lower background signal intensities for Na and K, O-2 flow rate (reaction cell gas) to remove any spectral interference with the S, H-2 flow rate so as to remove spectral interference with Ca. After optimization, the lower detection limits of Na, Mg, K, Ca, P, and S were 0.0004, 0.00004, 0.0003, 0.00012, 0.00005, and 0.002 mg kg(-1), respectively. Typical relative standard deviations were 2.1, 2.0, 1.7, 1.1, 2.5, and 2.5% for Na, Mg, K, Ca, P, and S, respectively, where the elemental concentrations in the BDF sample were, respectively, ca. 1 mg kg-1 each for Na, Mg, K and Ca, ca. 2 mg kg(-1) for P, and ca. 6 mg kg(-1) for S. The established method was applied to the homogeneity assessment of a candidate RM of BDF made from palm oil. The relative uncertainties of the homogeneity were 0.3, 0.4, 0.6, 0.3, 1.6, and 0.6% for Na, Mg, K, Ca, P, and S, respectively.