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
Technology (AIST), is providing chemical and physical metrology service to the world, including the development and providing of CRMs for the analysis of fuels based on a managing system in accordance to the ISO Guide 34:2009. 2 Up to date, the NMIJ has issued RMs and CRMs of sulfur calibrating solutions (NMIJ CRMs 4215-a, 4217-a, 4221-a, NMIJ RM 4216-a) and bioethanol (NMIJ CRM 8301-a) for the analysis of fuels. 3 In accordance to the development plan of CRMs for fuel analysis, a CRM for the
... asurements of water, methanol, density, kinematic viscosity, and regulated elements (Na, Mg, K, Ca, P, and S) was developed by the present authors' group following ISO Guide 35:2006, 4 which is an international technical guide for the development of RMs and CRMs covering general and statistical principles for certification, e.g. the production of candidate RMs, assessments of the homogeneity, stability study, and characterization. Homogeneity is one of the basic requirements for candidate materials of CRM, and the uncertainty of the homogeneity should be studied and combined to obtain the uncertainty of a certified value. 4 According to the ISO Guide 35:2006, 4 "The minimum number of bottles selected at random is between 10 and 30, but should generally not be smaller than 10". Furthermore, at least 3 sub-samples were taken for each bottle to check the reproducibility of the sample pretreatment. Therefore, measurements of over 30 sub-samples are required for a homogeneity assessment of a candidate RM. High throughput and reliable sample pretreatment methods are preferred for such kind of chemical analysis to lower the cost and to improve the working efficiency. The concentrations of Na, Mg, K, Ca, P, and S in biodiesel fuel (BDF) are regulated so as to ensure the engine performance and for environmental conservation. 5 As a result, measurements of these elements are required for quality monitoring of the BDF. In responding to this requirement, certifications of the elements' contents were covered by the present candidate RM. In our previous work on the development of CRMs, Na, Mg, K, Ca, and P, were often measured by an inductively coupled plasma optical emission spectrometer (ICP-OES) and an atomic emission spectrometer (AES), 6-8 while S was measured using a total sulfur analyzer attached with a trap and release unit. 9-11 In these methods, the measurement was often carried out at specific 2017 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: O2 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, O2 flow rate (reaction cell gas) to remove any spectral interference with the S, H2 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.