Considerations in the gas flow design of a graphite furnace vaporization interface: effects of a halocarbon atmosphere and sample matrix. Invited lecture

Abstract

A commercially available graphite tube furnace was coupled with an air–acetylene flame using two different designs of interface. The transverse sample introduction hole of the horizontal graphite tube was used as the exit for the evolved substances in one of the designs (‘upward streaming’), while in the other version the vaporized product passed through one of the ends of the graphite tube (‘end-on streaming’), the latter design being more generally applicable to plasma excitation–ionization sources (e.g., the inductively coupled plasma). Cadmium was selected as the test element as it is known that there are major changes in the transport efficiency of this element, in the presence of different matrices. Using fast heating (analytical conditions), the end-on streaming resulted in a 3-fold increase in integrated absorbance values for 20 ng of Cd, volatilized in an Ar atmosphere compared with upward streaming, and this factor decreased to 1.2 when using an atmosphere of Ar + CCl₄ in the furnace. The signal enhancement factor due to the introduction of a halocarbon was 3.9 and 1.6 for upward and end-on streaming, respectively. The enhancement effect of 20 µg of Se (as Na₂SeO₄) without halogenation was 260 and 40%, and with halogenation it decreased to 21 and 12% for upward and end-on streaming, respectively (measured as the integrated absorbance for 20 ng of Cd). This means that the analytical performance was much better with end-on streaming in the absence of a halocarbon, but the difference in this respect decreased if CCl₄ vapour was introduced. With the use of end-on streaming, the absorbance–time profile for Cd deteriorated with slow heating, owing to the re-vaporization of the partially condensed material from the end of the graphite tube. Therefore, the upward streaming system is recommended for use in volatilization studies (thermal evolution analyses).