J.P. Matousek

Effect of anions on atomization temperatures in furnace atomic-absorption.

Atomization temperatures have been measured for silver, cadmium, chromium, copper, nickel, lead, tin and zinc. The effect of various anions on the atomization temperatures of these metals has been examined. Of the anions investigated which were added as acids, only phosphate affected the atomization temperatures. For elements which atomized at a lower temperature than tin, phosphate addition resulted in an increased atomization temperature but those which atomized at a higher temperature than tin were not affected. These observations suggested that there are two mechanisms of atom formation in the graphite furnace. The first involves reduction of the metal oxide by carbon and is applicable only to compounds which can form oxides at temperatures lower than those required for the reduction process to occur. The second mechanism is direct decomposition of the metal compound to give metal atoms and is applicable to compounds of higher thermal stability which decompose at temperatures higher than those required for the reduction process.

Interferences due to easily ionised elements in a microwave-induced plasma system with graphite-furnace sample introduction

Abstract Several aspects of both enhancement and suppression of the analyte emission intensity caused by an easily ionised element (EIE) have been studied in an atmospheric pressure He microwave-induced plasma (MIP). A sequence of experiments, designed to elucidate possible mechanisms of this EIE effect, examines the following aspects: the concentration dependence of the effect for various EIEs; spatially separated vaporisation of EIE and analyte into the plasma; the effect of operating parameters upon the EIE-induced enhancement; the influence of the EIE on the excitation temperature and on the efficiency of coupling of microwave energy to the cavity. The EIE-induced suppression of emission intensity is consistent with reduced power dissipation in the plasma, due to decoupling of the plasma from the microwave power source, whereas the EIE-induced enhancement of emission intensity is best explained by a radiative energy transfer mechanism.

Aerosol deposition in furnace atomization.

A novel approach to sample deposition in furnace atomization is suggested, which obviates the need for skilled application of microvolumes by syringe. The analyte in aerosol form is deposited under controlled conditions on the internal surface areas of graphite furnaces. Precision approaching that of flame atomization systems is achieved and at the same time, concentrational sensitivity may be increased simply by extending the deposition time. The amount of analyte deposited in the furnace is restricted only by the sample volume available and the matrix concentration. A single standard can be used to construct a calibration curve by simply varying the aerosol deposition time.

Spatial Emission Properties of a Microwave-Induced Plasma

The spatial dependence of emission from a microwave-induced plasma in argon has been studied. A graphite furnace atomizer was used as a means of sample introduction. Emission from metallic elements is localized to a few cm near the inlet of the discharge, but the exact position of the emission profile is element-dependent. For non-metals, such as I, a broad profile centered about the resonant cavity is found. This difference in behavior is shown to be due to the deposition of metals upon the walls of the discharge tube used to confine the plasma. The removal of analyte atoms is explained by a mechanism which involves ionization of the analyte and then radial acceleration of these ions under the influence of the inhomogeneous microwave field. This hypothesis accounts for the observed decrease in emission intensity as microwave power is increased.

Atomic absorption studies of CO2, laser-induced atomization of samples confined in a graphite furnace☆

Abstract A pulsed infrared CO 2 laser has been used to produce atomization for analysis by atomic absorption. The solid sample is contained in a tubular graphite furnace, enabling control of the initial temperature and confinement of the atomic plume within the monitoring light beam. Silver—copper alloys yield transient laser-induced silver atomic absorption signals of approximately 10 ms duration, from which quantitative analysis may be made. Interferences due to non-selective absorption are entirely absent.

Coupled in situ electrodeposition-electrothermal atomic absorption spectrometry: a new approach in quantitative matrix free analysis☆

Abstract A new approach for in situ matrix elimination in electrothermal atomic absorption spectroscopy (ETAAS) is described. In an initial electrodeposition step (possible by use of a Pt/Ir delivery tube on the autosampler) the furnace is coated with about 0.25 μg Pd. Quantitative deposition of metallic analytes onto this renewable substrate is achieved from 25–40 μl samples by electrolysis for 60 s at 3.5–5.0 V (35–45 mA). Reprogramming of the autosampler to remove spent electrolyte after the electrolyses and to provide a rinse cycle facilitates removal of > 99.5% of a 0.5 M NaCl matrix prior to atomization. It is proposed that the analyte is bound onto the metallic modifier, rather than encapsulated within it. Binding of the analyte with Pd significantly increases the appearance temperature for Cd and Pb. The ashing loss for these analytes deposited onto Pd from a Cl − matrix is observed above 900°C and 1300°C, respectively. This stabilization facilitates separation of the residual NaCl matrix before atomization. It has been established for Cd that sensitivity of the determination remains constant for matrices as diverse as 1% HNO 3 , 0.5 M NaCl and sea water.

Halogen assisted volatilization in electrothermal atomic absorption spectroscopy: reduction of memory effects from refractory carbides

Abstract Memory (carry-over) effects associated with electrothermal atomic absorption spectroscopic (ETAAS) analysis of metals which form refractory carbides can be overcome by a halogen (Cl 2 ) purging process. Residual carbides are removed from the graphite or pyrolytic graphite-coated furnaces after each analysis, or group of analyses, by injection of Cl 2 into the heated furnace. Alternatively, Cl 2 may be injected during the atomization step immediately after maximum absorption is observed, thus avoiding a separate heating/purging cycle. The application of this method to the determination of V and Cr is reported. Optimum temperatures were established for volatilization of residual refractory carbides prepared in situ . The results are supported by thermodynamic calculations.

Determination of gold by atomic fluorescence spectroscopy in premixed flames

Abstract The atomic fluorescence of gold in premixed air-hydrogen, separated air-acetylene and oxy-hydrogen-argon flames, excited with a high-intensity hollow-cathode lamp was studied. Fluorescence of the gold resonance lines at 2428.0 and 2676.0 A was observed and also direct-line fluorescence at 3122.8 A. The emission at 3029.2 A appears to arise from thermally assisted direct-line fluorescence. The most intense line at 2428.0 A permits atomic fluorescence determination to be made with a detection limit of 0.005 p.p.m. in the oxy-hydrogen flame diluted with argon; the fluorescence signal in this flame is ca . 2.5-fold stronger than that in the air-hydrogen flame. The detection limit by atomic absorption (with the same instrument and source but 10-cm air-acetylene flame) is 10 times greater. With an extraction procedure, detection limits of 0.0002 and 0.00007 p.p.m. were obtained in the nitrogen-sheathed air-hydrogen and hydrogen-oxygen-argon flames, respectively. The method was used for the determination of gold in mine waters.

Mechanistic studies on the trapping and desorption of volatile hydrides and mercury for their determination by electrothermal vaporization-inductively-coupled plasma mass spectrometry

Abstract Metallic coatings in the pyrolytic graphite furnace have been used for the pre-concentration of Hg (using electrochemically reduced Au) and AsH 3 (using both thermally and electrochemically reduced Ir and Pd/Ir) prior to electrothermal volatilization. The analyte trapping efficiency during accumulation and the analyte and modifier release processes during volatilization were monitored in real time. This was achieved by using the fast response capability of inductively-coupled plasma-mass spectrometry to determine simultaneously both the analyte and modifier elements as a function of time. The temperature dependence of the analyte trapping process points to contrasting mechanisms for Hg adsorption on Au (reversible, physical adsorption/amalgamation) and for AsH 3 adsorption on Pd, Ir or Pd/Ir (physical adsorption followed by an irreversible hydrogen abstraction reaction at active sites). Results also indicate that only minor volatilization of modifier occurs at the temperature required for volatilization of analyte from the furnace.

Atomic-fluorescence spectroscopy of lead

The fluorescence spectrum of lead excited with a high-intensity hollow-cathode lamp has been investigated and the probable mechanism of fluorescence transitions is suggested. It is confirmed experimentally that the most intense fluorescence line at 405.78 nm is mostly due to direct-line fluorescence. The premixed air-hydrogen flame, the separated air-acetylene flame, and the oxy-hydrogen flame diluted with argon have been used, the last mentioned giving a detection limit of 0.02 ppm with the line at 405.78 nm.