Direct determination of V and Ni tetrapyrroles by HPLC-ICPMS using gradient elution

Complete text of publication follows. A method was developed for the determination of V and Ni as tetrapyrroles in organic solutions by HPLC-ICPMS using gradient elution. The standards used were octaethylporphines of Ni(II) and V(IV) oxide (Ni-OEP and V-OEP) and an etioporphyrin of Ni(II) (Ni-EtioP; all: Aldrich, MI, USA). Two HPLC columns containing the same C18 reversed phase of 5 μm pore diameter were compared. Column A (Lichrosphere RP18, 150 mm x 4.6 mm, Supelco, USA) was connected to an external 10 mm 4.6 mm pre-column. Column B (Lichrosphere RP18, 125 mm x 4 mm, Merck, Germany) had an internal 5 mm 5 mm pre-column. Optimization of the elution gradient was performed at 400 nm with a 2 mg L-1 solution of the tetrapyrroles in methanol. The gradient changed from pure methanol up to 20% of CHCl3 in methanol. Column B showed better performance for the most strongly retained Ni-OEP compound, resulting in lower Minimum Detectable Concentration, MDC, (8 ng mL-1) and corrected retention time (t'R = 11.6 min). The ICP-MS sample introduction system consisted of a concentric nebulizer coupled to an IsomistTM system operated at -5 deg C (both: Glass Expansion, USA), and an eluting flow rate of 1.00 mL min-1 was used. Best gas flow rates for Ar (0.53 L min-1) and O2 (0.05 L min-1) were obtained by factorial design with a 10 ng mL-1 solution of Rh, Ba, Ce, Mg, Pb in methanol, at 1200 W RF-power. The V-OEP standard was eluted already with pure methanol, thus avoiding ClO+ interference. A FIAS connected between the HPLC and the ICPMS was used for external quantification (Laborda et al., Spectrochim. Acta B, 59 (2004) 857-870). Recovery tests were performed with 2 mg L-1 solutions of V-OEP (V: 0.17 mg L-1), Ni-OEP (Ni: 0.20 mg L-1) and Ni-EtioP (Ni: 0.22 mg L-1) and showed to be quantitative. Sensitivity for Ni was about 3-times lower in 20% CHCl3-methanol when compared to pure methanol; however, much faster elution was obtained with gradient elution than under isocratic condition. Acknowledgements: We thank Petrobras and CNPq for financial support.