Some Observations on the Vaporization and Atomization of Samples with a Carbon Filament Atomizer D. J. Johnson,’ B. L. Sharp,2 and T. S. West213 Department of Chemistry, Imperial College of Science and Technology,London, SW7 2AY, U.K.
R. M. Dagnall Huntingdon Research Center, Huntingdon, U.K.
An equation governing the rate of evaporation of atoms from the surface of a filament Is developed, and used to calculate the temporal variation of the signal observed with this technique. Good agreement between the observed and calculated signal profiles is not observed in ail cases, but by experimentally measuring the variations of filament temperature and anaiyte signal with lime, it Is possible to relate the commencement of evaporation to the filament temperature, and here reasonable agreements with the theory are found.
One of the significant advances to occur in atomic spectrometry during recent years has been the development of non-flame methods of atomic vapor production. The devices described in the literature (1-3) can be divided into two basic groups, the filament and the tube furnaces. While papers continually appear revealing new applications and adaptions of these devices, little has been written on their theory of operation. This can be contrasted with the v01ume of theoretical discussion which has appeared on flames. Papers have appeared in the journals describing determinations of atomic vapor temperature in non-flame cells ( 4 ) and discussing the influence of heating rate on an analytical signal (5), but the most important work to date is that of L’vov (6),who attempted to explain in empirical terms atomic vapor production and the concept of temporal signal profile. An understanding of the processes of vaporization and atomization is important since it will ultimately enable prediction of optimum analytical conditions (as opposed to purely experimental determinations) and, perhaps more importantly, provide information about the mechanism of interference from concomitant sample species. A further importance must be attached to the nature of the signal profile because faithful recording of the signal event, and therefore correct design of equipment, is a prerequisite to obtaining accurate and precise analytical data. A proposed theory should, within the limits of ideal experimentation, be able to describe the observed facts and, better still, predict new ones. A theory thus proved can then be used in the reverse sense to elucidate fundamental processes from experimental observations. Before discussing our work, it is as well to realize that analysis of the signal profile of filament and tube furnaces represents two limiting cases. The signal duration of the tube furnace is principally determined by the rate of loss of atoms by diffusion from the analytical volume. The work of L’vov on the graphite cuvette can be described under these condi-
Present address, Department of Chemistry, University of Florida, Gainesville, FL 32611. Present address, Macaulay Research Institute, Craigiebuckler, Aberdeen AB92QJ., Scotland. 3 Author to whom reprint requests should be addressed. 1234
ANALYTICAL CHEMISTRY, VOL. 47. NO. 8, JULY 1975
tions and the agreement of observations with theory can be attributed to the rate determining nature of the diffusion process. The situation for filament atom reservoirs is somewhat different, since efficient and rapid flushing of the vaporization surface should ensure that the signal duration is principally determined by the rate of thermal evaporation from that surface. The net result, of course, is that shorter signal durations (typically
