Determination of selenium in serum in the presence of gadolinium with ICP-QQQ-MS

Gadolinium (Gd)-based magnetic resonance imaging (MRI) contrasting agents interfere with the determination of selenium (Se) when analysed by single quadrupole inductively coupled plasma-mass spectrometry (ICP-MS). This paper demonstrates that an ICP-triple quadrupole-MS (ICP-QQQ-MS) with oxygen mass shift overcomes Gd⁺⁺ interference on Se⁺ and mitigates typically encountered matrix and spectral based interferences. Normal human serum was diluted in a solution containing isopropanol, EDTA, NH₄OH and Triton X-100. Samples were unspiked (control) serum; serum spiked with 0.127 μmol L^-1 Se or 127 μmol L^-1 Gd; and serum spiked with both 0.127 μmol L^-1 Se and 127 μmol L^-1 Gd. Consideration of collision/reaction gases and conditions for interference mitigation included helium (He); a 'low' and 'high' hydrogen (H₂) flow, and oxygen (O₂). The instrument tune for O₂ was optimised for effective elimination of interferences via a mass shift reaction of Se⁺ to SeO⁺. The ICP-QQQ-MS was capable of detecting trace (>9.34 nmol L^-1) levels of Se in serum in the presence of Gd in our simulated post-MRI serum sample. The multi-tune capabilities of the ICP-QQQ-MS may be adapted to eliminate other specific isobaric interferences that cause false positive results in other analyses where the analyte is confounded by doubly charged and/or polyatomic species.