Transversally-heated furnace

The transversely heated furnace (THGA, Fig. 8. IB) avoids many of the problems associated with the longitudinally heated furnace. The graphite tube of a transversely heated... 

Vanadium Is a refractory element determined from the wall of the furnace but otherwise using STPF conditions (Manning and Slavin, 1985). Urine diluted in 2% HNO3 and Triton X-100 was analyzed for V by Paschal and Bailey (1990) using STPF methods with a detection limit of 2//g/L in the urine. Shuttler (1992) used the transversely heated furnace (the Perkin-Elmer 4100ZL) and, with only dilution of the urine, was able to obtain a detection limit better than 0.5//g/L. 

Figure 6.12 A graphite tube for a transversely heated furnace. [Courtesy of PerkinEhner Inc., Shelton, CT (www.perkinehner.com).]...

A cool down step before the atomization step is used for longitudinally heated furnaces. This has been shown to improve sensitivity and reduce peak tailing for some refractory elements. This improves the accuracy of the measurement for these elements. The cool down step is not used in transversely heated furnaces. 

Normal graphite furnaces have a temperature profile and thus differences in the spreading of the analyte over the graphite surface may lead to changes in the volatilization behavior from one sample to another. This effect can be avoided by using a transversally-heated furnace, where the temperature is constant over the whole tube length (Fig. 84). The latter furnace, which has been proposed by Freeh et al. 

M. El Ati-Hellal, F. Hellal, M. Dachraoui and A. Hedhili, Optimisation of Sn determination in macroalgae by microwaves digestion and transversely heated furnace atomic absorptions spectrometry analysis. Can. J. Anal. Sci. Spectrosc., 2008, 53(6), 262-274. 

A special coil configuration is used to heat thin strips of metal that caimot be heated efficiently with a coil that encircles the load, as the strip thickness is small compared to the depth of penetration. The transverse flux induction coil is positioned on either side of a strip to produce a uniformly heated strip with good efficiency in a much smaller space than conventional radiant or convective strip heating furnaces (6). 

Nowka R, Muller H (1997) Direct analysis of solid samples by graphite furnace atomic absorption spectrometry with a transversely heated graphite atomizer and D2-background correction system (SS GF-AAS). Fresenius J Anal Chem 359 132-137. 

Temperature distribution in (a) longitudinally and (b) transversely heated graphite furnace. 

The stabilized temperature platform furnace (STPF) concept was first devised by Slavin et al. It is a collection of recommendations to be followed to enable determinations to be as free from interferences as possible. These recommendations include (i) isothermal operation (ii) the use of a matrix modifier (iii) an integrated absorbance signal rather than peak height measurements (iv) a rapid heating rate during atomization (v) fast electronic circuits to follow the transient signal and (vi) the use of a powerful background correction system such as the Zeeman effect. Most or all of these recommendations are incorporated into virtually all analytical protocols nowadays and this, in conjunction with the transversely heated tubes, has decreased the interference effects observed considerably. 

Figure 21-8 (a) Transversely heated graphite furnace maintains nearly constant temperature over its whole length, thereby reducing memory effect from previous runs. The i vov platform is uniformly heated by radiation from the outer wall, not by conduction. The platform is attached to the wall by one small connection that is hidden from view. [Courtesy Perkin-Bmer Corp., Norwalk, Cl] (to) Heating profiles comparing analyte evaporation from wall and from platform. [From W. Slavin, Atomic Absorption Spectroscopy, Anal. Chem. 1982,54,685A.]... 

Gawalko and coworkers elucidated the characteristics of closed-vessel and focused open-vessel MW sample preparation techniques for the subsequent determination of Cd, Cu, and Pb, and Se in wheat, wheat products, corn bran, and rice flour by ET-AAS with transverse-heated graphite furnace [23]. Both techniques were evaluated using a total of 15 CRMs for the three food types. In the case of rice, these were the NIES 10a, 10b, and 10c Rice Flour and the NRC Rice Flour. The two techniques were equivalent in terms of agreement with the certified figures. Up to 12 samples could be processed with the closed system with minimal amounts of acid and reduced risk of contamination. Larger masses of samples could be treated by the open-vessel system. 

E. J. Gawalko, T. W. Nowicki, J. Babb, R. Tkachuk, Comparison of closed-vessel and focused open-vessel microwave dissolution for determination of cadmium, copper, lead, and selenium in wheat, wheat products, corn bran, and rice flour by transverse-heated graphite furnace atom, Int. J. Assoc. Off. Anal. Chem., 80 (1997), 379-387. 

Fig. 8.1. Typical graphite furnaces (A) longitudinally heated model, (B) transversely heated model. For details, see text. (Reproduced with permission of Perkin-Elmer.)...

Recently, the use of transverse heated filter furnace for the analysis of trace elements in automotive fuels was proposed [14]. This instrumental technique showed interesting results for volatile elements, such as Cd and Pb. The filter furnace approach was the subject of several papers and of continuous developments in order to improve the design [15], to extend the range to low volatile elements [16], to employ the furnace in transverse heated atomizers [17], and to determine trace elements in aqueous and organic solutions [18,19]. 

Tittarelli et al. SSC, Milan) employed a transverse heated filter atomizer with atomic absorption spectrometry to determine a number of trace elements in automotive and jet fuels. Sub-ppm detection limits were obtained. The use of filter furnace reduces the risk of elemental loss during drying and pyrolysis steps, and decreases the interferences due to molecular absorption and light scattering. 

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