Study of the atomization of boron in electrothermal atomic absorption spectrometry and hollow cathode furnace atomic non-thermal excitation spectrometry

Abstract

Coating a total pyrolytic graphite tube with tungsten carbide or lanthanum carbide increased the optimum pyrolysis temperature of B from 850 to > 2200 °C. Addition of a calcium–magnesium modifier to the boron solutions increased the pyrolysis temperature to 1200 °C, whereas a titanium-ascorbic acid modifier had no significant effect. The lowest characteristic mass of B, 0.8 ng, was obtained with the calcium–magnesium modifier. The low temperature loss of boron, without a modifier, was investigated using dynamic secondary-ion mass spectrometry, which confirmed that vaporization of boron species occurs above 900 °C. Boron atomic emission signals were obtained at <800 °C by hollow cathode furnace atomic non-thermal extraction spectrometry (HC-FANES) with a 30 Torr helium plasma. Molecular dissociation and boron atom excitation apparently occurred through a one-step collisional process. The detection limit of B by He HC-FANES, under non-optimum conditions, was 71 pg (no modifier). The study suggests that atomization of B in electrothermal atomic absorption spectrometry (ETAAS) is likely to occur through molecular dissociation rather than solely by sublimation of B. The modifiers probably prevent low temperature dissociative desorption of B₂O₃ occurring at active carbon sites and so increased the optimum pyrolysis temperature to >850 °C. The poor detection limit of B in ETAAS is due to the inefficient thermal dissociation of the B-containing species (probably oxides and carbides) produced by dissociative desorption of B₂O₃. Also, once formed, B atoms apparently undergo a series of condensation–vaporization steps, which cause a persistant plateau in the tail of the AAS signal, and result in severe memory effects.