Interference standard and oxide ion detection as strategies to determine phosphorus and sulfur in fuel samples by inductively coupled plasma quadrupole mass spectrometry

Abstract

Due to their high ionization energies and severe spectral interferences, P and S are challenging elements in inductively coupled plasma quadrupole mass spectrometry (ICP-QMS). An alternative to improve sensitivity is to monitor P and S oxide and hydroxide species which are easily formed under cool plasma conditions. To improve accuracy, an interesting approach is the interference standard (IFS) method. Similar to internal standardization, the IFS method is based on the idea that interfering polyatomic ions and some Ar species (IFS) present similar behaviors in the plasma. Thus, by applying a conventional external calibration method and using the analytical–IFS signal ratio, one can minimize the contribution of the interfering ion to the analytical signal and improve accuracy. In this work, plasma operating conditions are adjusted to maximize the formation of ³¹P¹⁶O⁺, ³²S¹⁶O⁺ and ³²S¹⁶OH⁺. Under the optimal conditions and using the ³⁶ArH⁺ as an IFS probe, limits of detection as low as 5 and 600 μg l⁻¹ are obtained for P and S determined at m/z 47 and 49, respectively. The efficiency of the IFS method in reducing interferences, e.g.³⁰Si¹⁶OH⁺ on ³¹P¹⁶O⁺, and ³¹P¹⁶OH⁺ on ³²S¹⁶O⁺, is also evaluated. Accuracy is significantly improved in lubricating oil, diesel and biodiesel analyses. No significant differences are observed between the reference values and the ones obtained with the ³⁶ArH⁺ IFS probe at a 95% confidence level. The ³⁶Ar⁺ and ³⁸Ar⁺ IFS probes are also efficient in most determinations.