Atomization and vaporization of lead from a blood matrix using rhodium-coated tungsten filaments with pseudo-simultaneous electrothermal atomic absorption and inductively coupled plasma mass spectrometric measurements

Abstract

In an effort to understand the pyrolysis and atomization behavior of Pb in a dilute blood matrix when determined using a tungsten filament electrothermal atomic absorption spectrometer (W-filament ETAAS), we directly coupled a W-filament ETAAS instrument (J. Anal. At. Spectrom., 2001, 16, 82) to an inductively coupled plasma mass spectrometer (ICP-MS). The W-filament instrument was operated both as an electrothermal atomizer (ETA) and as an electrothermal vaporizer (ETV). The experimental arrangement was used to monitor simultaneously the analyte (²⁰⁸Pb), the permanent modifier (¹⁰³Rh) coating the W surface, a surrogate for the organic components of the blood matrix (⁴⁰Ar¹²C), and the metal atomizer (¹⁸⁰W, ¹⁸³W and ¹⁸⁴W¹⁶O), while also measuring Pb by AAS pseudo-simultaneously. Electrothermal parameters used in the ETV/ETA experiments were precisely the same as those described previously (Spectrochim. Acta, Part B, 2002, 57, 727) for ETAAS work. This was necessary in order that the ICP-MS data could be used to observe the vaporization of analyte, matrix, modifier, and metallic atomizer throughout the electrothermal cycle. The purge gas used to transport vaporized material from the atomization cell to the ICP was Ar containing 6% H₂. Pseudo-simultaneous ETAAS and ETV-ICP-MS measurements of Pb in blood were achieved using this experimental arrangement, with the MS signals appearing roughly 2 s after those by AAS, consistent with the delay caused by the 15 cm long Teflon transfer line connecting the two instruments. During pyrolysis, a strong signal observed at mass 52 is shown to be due to ⁴⁰Ar¹²C; this signal increases as filament power for pyrolysis increases. When plotted, the data show the effective removal of much of the carbon-based matrix during pyrolysis, but without significant loss of analyte. In contrast, the pre-coated permanent modifier, Rh, is only lost during the final cleaning stage of the program in which maximum power is necessary to remove the carbonaceous matrix. However, when Rh is added to the diluent, it is lost even during pyrolysis, and at a much lower temperature. This explains why addition of Rh directly to the diluent yields no modification benefits, and why periodic re-coating of the filament surface for the analysis is necessary.