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.