Development of a tungsten filament electrothermal vaporizer for inductively coupled plasma time-of-flight mass spectrometry and its possibilities for the analysis of human whole blood and serum

Abstract

A methodology was developed using a tungsten filament electrothermal vaporizer (WETV) as a sample introduction system, coupled to an inductively coupled plasma time-of-flight mass spectrometer (ICP-TOFMS), for rapid and simultaneous determination of Cr, Cu, Li and Pb in human whole blood and serum samples. The design of the vaporizer is based on a W-filament from Osram, which is positioned in a low volume (7.3 mL) quartz cell that is especially suited for the evaporation of minute amounts of samples and is coupled to the plasma torch via an 8 cm (3 mm id) glass tube. The tungsten coil was connected to the secondary side of a conventional ac power supply, the primary side of which was interfaced with a newly designed LabView^™-based controllable four-channel switch box. Each channel was attached to a separate variable 0–220 V transformer, allowing indirect adjustment of the voltage on the secondary side of the power supply between 0 and 13.8 V. The voltage/current heating programs (drying, pyrolysis, vaporization and filament cleaning) for sample evaporation as well as the ICP-TOFMS operational parameters (carrier gas flow rate, modulation pulse width) for the simultaneous determination of 17 isotopes were optimized using sequential and “Modified Simplex” algorithm-based optimization techniques. The compromise operation parameters of the WETV system for multielemental determinations were found to be different for aqueous standard solutions, human whole blood and serum samples. An argon carrier gas flow rate of 1.16 L min⁻¹ provided optimum results for the detection of transient signals. By using 10 µL sample volumes of aqueous multielemental standard solutions absolute detection limits (3σ) in the range of 0.007–0.1 pg for Ba, Cd, Co, Cr, Cu, Ga, In, Mn, Ni and Sr, 0.11–0.19 pg for Ag, As, Bi, Li, Pb and Pd and 11 pg for Fe, and a precision of 2–6% for 10 replicates of 5 ng mL⁻¹ (20 ng mL⁻¹ of Fe) multielemental sample solutions were achieved. The method accuracy was verified by the analysis of Reference Materials SRM 909 (Human Serum) from NIST and 404108 (Human Whole Blood) from Nycomed Pharma regarding their Cr, Cu, Li and Pb concentrations. Except the addition of a phosphate-based matrix modifier, followed by appropriate dilution, no additional sample pretreatment was necessary. Internal standardization using multielemental solutions was used for calibration. Good agreement with the certified values was observed and the filament had a lifetime of more than 350 firings.