Furnace Autosamplers

Electrothermal atomic absorption analysis has developed significantly, complementary to flame atomic absorption, and present apparatus is easier to use and has greater performance than that available even three years ago. Reliable temperature-controlled heating of the furnace and furnace autosampling accessories will certainly assist.the technique quickly to assume a similar degree of instrumental maturity and enable electrothermal methods to attain their deserved place in modem trace and ultra-trace metal analysis. 

The whole atomizer may be water cooled to improve precision and increase the speed of analysis. The tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre, often using an autosampler but occasionally using a micro-pipette with a disposable, dart-like tip. Solid samples may also be introduced in some designs, this may be achieved using special graphite boats. The sample introduction step is usually the main source of imprecision and may also be a source of contamination. The precision is improved if an autosampler is used. These samplers have been of two types automatic injectors and a type in which the sample was nebulized into the furnace prior to atomization. This latter type was far less common. 

Autosamplers in chromatography and graphite furnace atomic spectroscopy, for example, have improved injection precision by a factor of 3-10 compared with that attained by humans. 

Procedure Use the following furnace program with a suitable graphite furnace atomic absorption spectrophotometer set at 283.3 nm and equipped with an autosampler ... 

Apparatus Use a suitable spectrophotometer (Perkin-El-mer Model 6000, or equivalent), a graphite furnace containing a L vov platform (Perkin-Elmer Model HGA-500, or equivalent), and an autosampler (Perkin-Elmer Model AS-40, or equivalent). Use a lead hollow-cathode lamp (lamp current of 10 mA), a slit width of 0.7 mm (set low), the wavelength set at 283.3 nm, and a deuterium arc lamp for background correction. 

All work was carried out in a specially designed portable clean laboratory that also housed a Perkin-Elmer 5000 atomic absorption spectrophotometer with accompanying HGA-500 graphite furnace assembly and AS-40 autosampler (22). 

Tungsten-coil atomizers have to some extent been used in connection with atomic techniques in general and ETA-AAS in particular, and their popularity continues to rise. Their high simplicity and low cost make them attractive alternatives to graphite furnaces for many applications. However, they are much more interference-prone than their graphite counterparts. The interferences experienced by W-coil atomizers have been overcome in various ways, however. Thus, Barbosa et al. [25] avoid the interference from a salt matrix and implement a preconcentration step by electrochemically reducing Pb onto the coil surface. They use a flow injection system to deliver the sample through an anode inserted in the tip of the autosampler and the W-coil itself as cathode. The use of a W-coil as a platform inside a Massmann-type furnace provides no appreciable improvement over wall atomization [26]. 

To cope with matrix effects, and to achieve some enrichment, suitable separation methods can be applied, e.g. solvent extraction, coprecipitation, or volatilization directly from the inorganic solid sample (Heinrichs, 1979 Han et al., 1982 Sager, 1984). As the volatility of organic chelates is low, and the volatility of halogenides from organic solvents remains the same, organic phases obtained from solvent extractions have been put directly into the furnace. In this case, however, carry-over in the autosampler has to be prevented by an organic soivent, at ieast by an isopropanol/water mixture. 

Graphite furnace with autosampler and pyroiyticaliy coated graphite tubes... 

Ultrasonic vibration is utilized to promote acid extraction (e.g. with 5% HNO3 at 70°C) as well as for suspension of fly ash as slurry directly onto the furnace of an autosampler (Cresser et ah, 1992). 

Graphite furnace AAS generally offers highly sensitive detection for small sample amounts, however the sequential nature of the drying, ashing, and atomisation steps make it difficult to interface it to a continuous separation technique like HPLC. As a simple solution, the autosampler of a commercial GF-AAS instrament may be modified in such a way that the effluent is passed through a PTFE flowthrough cell from which the autosampler periodically injects a small aliquot (10—50 pL) into the furnace. Of course, this does not provide a continuous on-... 

Because the Curie-point filament is heated inductively, no connections are made to the wire. This facilitates autosampling and permits loading the wires into glass tubes for sampling and insertion into the coil zone. Unlike the isothermal furnace, which is on continuously, the Curie-point wire is heated only briefly and is cold the rest of the time. This necessitates heating the pyrolysis chamber separately to prevent immediate condensation of the fragments made during pyrolysis. Therefore, Curie-... 

In the interest of efficiency, autosamplers have become increasingly important in analytical laboratories. Autosampling systems are now available for all three types of pyrolysis equipment furnaces. Curie point, and resistively heated filament... 

Most of the autosampling pyrolysis systems available use a sample magazine or carousel to introduce prepared samples into a conunon heater. In the furnace type, samples are placed into deactivated steel cups that are dropped sequentially into the furnace for pyrolysis, then ejected pneumatically after the analysis. Tanperatures up to 800°C are achievable, and the carousel has space for up to 48 sample cups. 

Pyrolyzers have been adapted to provide automatic, imattended control of Py-GC. An early system used precoated pyrolysis wires held in quartz tubes on a turntable. These were sequentially loaded, accurately positioned in the induction coil, pyrolyzed, analyzed by capillary GC, and ejected. An alternative has used an automatic solids injector for samples enclosed in iron foil, and a furnace system has enabled sampling of the Martian surface. Autosampling systems based upon conventional pyrolyzers are now commercially available for resistively heated filaments, microfurnaces, and Curie-point pyrolyzers. One such system... 

Figure 6.30 The SS600 solids autosampler for direct solids analysis by graphite furnace AAS. Analytlk Jena, www.analytik-jena.de. Used with permission.)...

Atomic absoiption spectrometer with background correction Graphite furnace with autosampler and pyrolytically coated graphite tubes Monoelement aluminum hollow cathode lamp Aluminum standard, 1 g/Iiter (Al metal in 0.3 M HCl)... 

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