No abstract

A new method for the determlnatlon of lead In aqueous samples Is described. The method Involves the extraction of Pb2+ Into chloroform as the dlthlocarbamate complex followed by solvent evaporation and methylatlon of the resfdue with methylllthlum to form tetramethyllead. The analyte vapor Is then trapped on a short column of Porapak Q from which It Is eluted Into a quartz furnace atomlc absorptlon detector. A relative standard devlatlon of 6.8% was obtained at the 50 ng/mL leuel. The working range extends from the detection llmlt of 5 ng to an upper llmit of approxlmately 200 ng. The results of the analysis of lead-contalnlng Envlronmental Protectlon Agency Water Pollution Quality Control Samples are reported.Metal volatilization techniques provide a number of significant benefits in atomic absorption spectrometry. First, matrix effects can be circumvented. Generally, it is possible to selectively derivatize the analyte while leaving behind the interferences. Second, separation of the analyte from the solvent eliminates the expenditure of thermal energy upon solvent evaporation in contrast to direct aqueous aspiration. Third, due to the thermal instability (1) of many volatile metal-containing compounds (compared to their inorganic counterparts), decomposition to the free atoms is facile at relatively low flame or furnace temperatures. Because of the increase in free atom population, the enhancement of atomic emission, absorption, and fluorescence signals is expected. Finally, sensitivity improvement through preconcentration can be obtained when a trapping system is employed in connection with the volatilization scheme. In this manner it is feasible to measure very low analyte concentrations because the volatilized metal compound can be collected from a large solution volume.A vapor generation technique that has not received significant consideration for many metals is the alkylation of the analyte to form an organometallic species. The alkylated derivatives of most post-transition metals and metalloids are both volatile and stable enough to be trapped and subsequently eluted into a suitable detection system (2). Because hydride generation techniques (3) cannot be applied to all metals, alkylation may be a possible alternative in many cases, despite the fact that most alkylating agents cannot be used directly with aqueous solutions. The metal whose alkylation has been investigated most thoroughly as an analytical approach is mercury. Mono-and dialkylmercury compounds have been formed from inorganic species and separated by using gas chromatography (4, 5).The phenylation of dithiocarbamate complexes has been used as a volatilization technique in the determination of As, Sb, T1, Se, Te, Hg, Bi, and Sn by Schwedt et al. (6). Their work utilized metal extraction followed by reaction with phenylmagnesium bromide to form the phenylated metal species. Gas chromatographic separation with flame ionization detection was used.The work to be described here involves the extraction of lead into chloroform, the evaporation of ...

A stable, low gas-flow torch has been developed for use with a helium microwave induced plasma (MIP). A toroidal plasma with central analyte introduction is obtained by the addition of a tantalum coupling probe injector tube. This injector penetrates through 100% of the total cavity depth and aids in the efficiency of power transfer to the cavity, in plasma initiation, and in circumventing the effects of a lack of homogeneity in the microwave field on analyte distribution in the plasma. The tangential helium flow was 4 1/min and the microwave power was 60 W.

A standard pyrolytlc graphite L'vov platform was compared to one treated by soaking In a 10% thorlum nitrate solution.The half-wldhs d fwnace atomk absorptbn peak profiles for Ai were reduced by as much as a factor of 4 by the modlfled surlace, and the maxlmum pennWble drarrlng temperature was extended by more than 800 O C . The peak area preckh (1.5% relative standard devlatbn for 200 pg of AI) and sen-sltivHy were comparable to those obtained with untreated platforms. Thls behavlor was stlli observed afler 500 flrlngs of a treated platform at 2450 O C .Improvements in trace-metal analyses using graphite furnace atomic absorption spectrometry (GFAAS) have been hindered by the lack of fundamental knowledge concerning the high-temperature chemistry taking place in the furnace itself. Due to this uncertainty many analysts have resorted to an empirical approach in their attempts to modify the behavior of analyte in the furnace. Such research necessarily has a trial-and-error appearance-various reagents and conditions examined until success is reached. This has been particularly true in the area of graphite atomizer surface modification.The treatment of graphite with various metals or solutions of their salts has produced new surfaces exhibiting properties markedly different than those of the original graphite. The chief pattern that has emerged is the fact that most metals used to modify graphite are known to form metal carbides.It is not known whether the carbide-forming metals not reported in the literature as surface modifiers have yet to be tried or have been found ineffective.The most commonly reported advantage of a metal-treated surface is an increase in analyte sensitivity (1-8). It should be pointed out that in some instances various authors explicitly mentioned that peak heights were used in preference to integrated peak areas or did not mention which mode of quantification was used. It is possible in some of these cases that the reported sensitivity improvement for peak heights was in fact a peak sharpening, leaving integrated area un-changed. Theory (9) predicts that integrated peak area is the better indicator of the extent of free atom formaton. It should also be pointed out that most of the reported sensitivity enhancements were in comparison to nonpyrolytic graphite tubes, which have now been largely supplanted by pyrolytically coated graphite. In some instances the advantages indicated might not have been realized over the newer graphite.The benefits of metal-modified graphite surfaces have extended beyond signal enhancement. It was reported by Norval et al. (10) that a tungsten-or tantalum-treated surface resulting from sputtering of the metal led to an improved resistance of the graphite surface toward oxidizing acids. A similar result was found by Sotera et al. (11) for a borontreated surface. Vickrey et al. found that a zirconium carbide surface allowed organolead (12) and organotin (13) compounds to be determined by using aqueous standards in the furnace while this could not be accomplished on ...

The feasibility of using metal oxide cloths as atomization surfaces In furnace atomic absorption spectrometry was studied. Y203 cloths were Introduced Into a preheated graphite furnace on a probe attached to the autosampler arm. This permitted evaluation of the new surfaces with little system modification. Data on temperature dependence and sensitivity for Ag, Se, Cd, and Pb were obtained. With Pb as the analyte, studies on precision and interferences were performed and compared to data obtained by using a conventional furnace program and graphite platform atomization.