Kenneth W. Jackson

Mechanism of modification by palladium in graphite furnace atomic absorption spectrometry

Abstract The effects of palladium and mixtures containing palladium on the absorbance characteristics of lead, thallium, cadmium, selenium, manganese and cobalt are described. These data, together with results of scanning electron microscopy showing the distribution of palladium on the graphite surface, indicate that palladium has a physical mechanism of analyte modification. During furnace heating, the analyte dissolves in molten palladium and may combine with it chemically. However, the rate limiting step leading to atomization appears to be diffusion of the analyte from palladium. The addition of magnesium, molybdenum or powdered carbon increases the speed of diffusion by causing palladium to form smaller droplets, and hence produces sharper absorbance peaks. Palladium becomes less effective as the atomization temperature increases, because the rate of diffusion is higher. This accounts for palladium having only a small stabilizing effect on less volatile elements such as manganese and cobalt. The addition of ascorbic acid to palladium has no significant effect on its modifying properties in a dilute nitric acid matrix. Results of kinetic studies on the atomization of gold are consistent with analyte diffusion out of palladium as the rate-limiting step leading to atomization.

Modification by palladium in the analysis of slurries by graphite furnace atomic absorption spectrometry: a physical mechanism

Abstract The atomization of lead from sediment particles after introduction to the graphite furnace as a slurry is studied. Absorbance characteristics with and without the addition of a palladium/magnesium modifier mixture are compared, and scanning electron micrographs are presented to show the distribution of palladium and slurry particles in the graphite furnace during the heating cycle. The results are consistent with a physical mechanism of modification by palladium. When palladium is premixed with the slurry, prior to pipetting it into the furnace, palladium becomes absorbed on the slurry particles. The rate-limiting step leading to atomization is then diffusion of lead through the palladium layer. When palladium is pipetted into the furnace before the slurry, atomization proceeds via the release of lead from the slurry particle, followed by its collision with a palladium droplet, and subsequent rate-limiting diffusion out of the droplet.

Simultaneous Determination of Trace Wear Metals in Used Lubricating Oils by Atomic Absorption Spectrometry Using a Silicon-Target Vidicon Detector:

A multichannel atomic absorption spectrometer with a silicon-target vidicon detector was used for the simultaneous determination of trace metals in aqueous solution and in used lubricating oils. In most cases sensitivities were similar to those obtained by conventional single channel atomic absorption. Iron, magnesium, copper, and silver were simultaneously determined in lubricating oils, and, although precision was somewhat poorer than that normally attained by atomic absorption, it was adequate for this application. Good correlation was obtained between the vidicon method and a single channel method using a commercial atomic absorption spectrometer.

A 500-Channel Silicon-Target Vidicon Tube as a Photodetector for Atomic Absorption Spectrometry

Abstract The suitability of a vidicon TV tube as a detector for atomic absorption spectrometry (AA) was investigated. The spectral response was seen to be adequate throughout the range 217.0-313.8 nm. The instrument was then used as a detector in determining several elements by flame AA, and the results compared favorably with those obtained by a conventional detection system. The system should thus be feasible for simultaneous multielement analysis.

Wall-to-platform migration in electrothermal atomic absorption spectrometry Part 1. Investigation of the mechanism of chloride interference on thallium

Abstract In wall-to-platform migration, the sample aliquot is pipetted onto the graphite furnace wall. Heating of the furnace then causes migration of the analyte from the wall to the platform. This technique permitted the atomic absorption signal for 1 ng of thallium to be separated in time from the background absorption signal from 10 μg of a sodium chloride matrix. This precluded interference through a gas-phase reaction or through expulsion loss of analyte, since they would require analyte and matrix to be present in the gas phase at the same time. Hence, it was concluded that interference by chloride occurred in the condensed phase. A refinement of wall-to-platform migration, called the “two-platform method”, permitted an examination of the temperature dependence of analyte migration from the wall to the platform, and hence the stage of furnace heating when interference occurred. This showed that interference occurred between condensed-phase species, at temperatures from 500 to 700°C, during the pyrolysis stage of the furnace heating cycle.

Mechanism of the action of palladium in reducing chloride interference in electrothermal atomic absorption spectrometry

Abstract Interference by sodium chloride on the determination of thallium occurs through the condensed phase formation of thallium chloride during the pyrolysis stage of the electrothermal atomization cycle. This leads to loss of volatile thallium chloride during the pyrolysis and atomization stages. Palladium stabilizes the analyte and reduces this interference, but stabilization is only partially successful when palladium is premixed with the analyte and sodium chloride. Stabilization is much more effective if, prior to the atomization stage, palladium is first pre-pyrolyzed on the furnace platform at 900°C, the analyte in the sodium chloride matrix is introduced, and this is pyrolyzed at 1100°C. In this way, 1 ng of thallium can be determined in the presence of 200 μg of sodium chloride without interference. Stabilization occurs through adsorption of the analyte by underpotential deposition on the pre-pyrolyzed palladium deposit. The analyte remains adsorbed on palladium until the sodium chloride has volatilized during the 1100°C pyrolysis stage. On further heating, thallium then becomes embedded in molten palladium and diffuses out during the atomization stage. The addition of magnesium to the palladium improves the recovery of thallium.

Physical behaviour of nickel and copper modifiers used in the determination of selenium by electrothermal atomic absorption spectrometry

Scanning electron microscopy and visual examination were used to study the physical changes occurring with temperature when Ni and Cu were used as modifiers for Se. During the pyrolysis stage of the electrothermal atomizer heating cycle, the Ni or Cu matrix was observed to shrink as the metal salt decomposed to the oxide and then the metal. This may have resulted in physical entrapment of Se, which coprecipitated with the excess of modifier salt during the drying stage, and this could contribute significantly to the stabilizing effect of Ni and Cu. Pyrolysis curves for Se in the presence of Ni or Cu show a characteristic dip, or decrease in thermal stability, at a pre-treatment temperature of about 800 °C. However, replacement of the Ar purge gas by H2 during the atomizer drying stage eliminated this dip, and this may be due to chemical or physical effects. During the atomization heating stage, the modifiers formed molten droplets, and analyte atomization was governed by its kinetically controlled release from these droplets. This is considered to be the major cause of analyte stabilization during atomization.