Photochemical vapor generation for the determination of chloride in fuel ethanol by ICP-MS/MS

The global trend of increasing ethanol demand requires implementation of rigorous quality control protocols, and the presence of chloride has been a matter of concern for regulatory agencies. A photochemical vapor generation (PVG) analyte introduction method was developed for the determination of Cl^- in fuel ethanol by triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS) to overcome the spectral and matrix interferences that hinder the determination of Cl^-. The use of ICP-MS/MS with a mass-shift approach using O₂ as the reaction gas (³⁵Cl⁺ → ³⁵Cl¹⁶O⁺) significantly reduced the background compared to conventional single quadrupole ICP-MS. The presence of only ethanol was insufficient to photogenerate volatile chlorine species. On the other hand, an evaluation of auxiliary agents for PVG of Cl^- (especially, but not limited to, Cu) revealed that the addition of copper acetate (CuAc₂) to the ethanolic medium enhanced CH₃Cl generation under UV irradiation. A 3-fold improvement in response compared to conventional pneumatic nebulization (PN) was achieved using 2.5% w w^-1 ethanol and 400 mg kg^-1 CuAc₂, with a 2 mL min^-1 sample flow rate (about 23 s of sample irradiation time). This optimized PVG ICP-MS/MS system yielded a blank-limited limit of detection (LOD) of 0.007 mg kg^-1, 6-fold better than that obtained using PN ICP-MS/MS. Accounting for a needed 40-fold dilution of the sample, a practical LOD of 0.26 mg kg^-1 was achieved for fuel ethanol samples. The proposed PVG ICP-MS/MS method represents a sensitive and accurate alternative for routine monitoring of inorganic Cl^- in fuel ethanol with sufficient LOD to comply with levels required by regulatory agencies.