The stabilized temperature platform furnace

This chapter is dedicated to Professor John Ottaway, who had expected to write it. 

The furnace can be thought of as a cell within which the sample vapor is partially confined. If the sample is put on the wall of the furnace, illustrated in Fig. 2, the various materials in the sample will vaporize as the wall heats up. But the temperature and the rate at which the analyte will vaporize depend upon the compounds in which the analyte is present. The L vov theory requires that the gas phase temperature while the analyte is an atomic vapor be the same for standards and samples. So we cannot tolerate the thermal ambiguity that occurs when we put the sample on the wall of the furnace. Therefore a small platform, Fig. 3. is added within the furnace. The platform is heated by radiation from 

Integration of the absorbance signal was part of the original L vov concept. There are many uncontrolled factors that alter the peak shape, therefore the peak absorbance. By now, the literature has numerous examples of the advantage of using integrated absorbance and there is nothing that is more a feature of the STPF technique. In this chapter, the 

When Cr is determined in a Ca matrix, illustrated in Fig. 4 from Slavin et al. (1983), the peak shape is greatly different compared to simple standards. But note that the integrated absorbance signal is the same for both valence states of Cr and independent of the presence of Ca. 

Early designs of the furnace used just the power to achieve the desired final temperature. However, if more power is applied to achieve the final temperature rapidly, the peaks are higher and narrower. This is especially important for the refractory elements. The maximum power available is used to achieve final temperature quickly, using a photodiode to determine when the finai temperature has been reached and triggering a power reduction at that temperature. This has been shown to provide much better performance for metals like V, Ti and Mo. 

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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. 

Slavin W, Carnrick G and Manning D (1982) Graphite-tube effects on perchloric acid interferences on aluminum and thallium in the stabilized-temperature platform furnace. Anal Chim Acta 138 103-110. 

Slavin, W., Carnrick, G.R., Manning, D.C. and Pruszkowska, E. (1983). Recent experience with the stabilized temperature platform furnace and Zeeman background correction. Atom. Spectrosc. 4. 69-86. 

Interferences, such as background absorption, are reduced to a minimum in the AAS-HGA analytical technique by taking full advantage of the Stabilized Temperature Platform Furnace (STPF) concept. STPF includes all of the following parameters (5.2.) ... 

In LS GF AAS, according to the Stabilized Temperature Platform Furnace (STPF) concept [133], signal evaluation should be done exclusively by means of time-integrated absorbance, the so-called peak area value. Applying this principle to HR-CS GF AAS, a spectrum is obtained which is summed over time, whereby the integrated absorbance at individual pixels A is calculated by ... 

In order to determine Bi by GF AAS under stabilized temperature platform furnace (STPF) conditions using the Pd-Mg modifier, a pyrolysis temperature of 1200 °C must be applied (Hiltenkamp and Werth 1988). The optimum atomization temperature under these conditions is 1900 °C the characteristic mass with Zeeman effect background correction (BC) is 28 pg, while in a non-Zeeman instrument it is about 20 pg. 

Weighed tissue samples (kidney, liver, meat, bone and wool) of approximately 1 and 0.5 g CRM 185 bovine liver were placed in PTFE vessels, 5 ml of concentrated HNO3 and 5 ml of distilled H2O were added. A microwave oven program with temperature and pressure settings of 150°C and 130 p.s.i., respectively, was performed for all analyzed matrices. The digestion solutions were diluted to a final volume of 50 ml all dilution, excluding blood samples, were performed with 1% HNO3. Zinc analysis was performed on a Flame AAS 2100 (Perkin-Elmer), with an air-acetylene flame. Lead and cadmium analysis was performed on a GF-AAS 5100 ZL (Perkin-Elmer). Analysis on GF-AAS were carried out under stabilized temperature platform furnace (STPF) conditions. 

Stabilized Temperature Platform Furnace. The conditions in a Massmann furnace are by no means optimum during atomization. The sample is dispensed onto a cold wall of the tube which is then heated rapidly for atomization. After vaporization the sample is in an environment that is not in equilibrium with respect to volume or time. This can cause chemical interferences. Fewer interferences would occur in a continuously heated, isothermal furnace. It has been shown that about 60% of the atoms formed diffuse to the cooler cuvette ends and condense. In addition, at 2800 K a temperature gradient of 1000 K between the middle and the ends of a... 

Perkin Elmer introduced, in 1984, a Stabilized Temperature Platform Furnace (STPF) (Figure 49). There are two inert gas flows (internal and external). The heating rate of the furnace is high (about 2000 Ks ), which allows the use of lower atomization temperatures. After the thermal pretreatment a small temperature step (<1600 K) is used in order to prevent the atomization of the sample before the equilibrium has been reached. 

Gas phase interferences due to compound formation of the analyte element with a concomitant should not be very significant in ETAAS because a much longer time is available for dissociation compared to FAAS. It was shown by high-temperature equilibrium calculations that gas phase interferences at the temperatures used in ETAAS should actually be rather insignificant [18], The reason why the literature is nevertheless full of reports on such interferences is largely due to an improper use of this technique. Slavin et al. [19], based on the systematic work of L vov [20], introduced a concept which they called stabilized temperature platform furnace (STPF). It is in essence a package of measures which eliminates most nonspectral interferences in ETAAS by atomization under local thermal equilibrium conditions. 

Graphite furnace atomic absorption spectrometry (GFAAS) is an excellent method to provide sub-ng/mL minimum detection limits [110]. Continuing advancements such as Zeeman correction, and stabilized temperature platform furnaces, have made GFAAS an effective analytical method for magnesium determination. Depending on the sample matrix, pretreatment can vary from direct analysis of fluids, to wet mineralization, dry ash, acid extraction, and by using PPRs (e.g., Triton X-100). 

The application of XRF to the determination of Se in very small blood [105] and blood serum samples [106] has been reported. Spectrofluorimetry and Zeeman-corrected, stabilized-temperature, platform-furnace AAS have been compared [107] for the determination of Se in milk. 

Leung and Henderson (1982) have not been able to confirm these findings of grossly different curve slopes for different types of serum and standard samples. Bettinelli et al. (1985) made a thorough evaluation of the direct measurement of aluminium in serum using GF-AAS with the stabilized temperature furnace with the L vov platform. Their recommendation was to use pyrolytically coated graphite tubes and to keep the method as simple as possible with minimal sample pre-treatment. Slavin (Personal Communication) also has been unable to observe major slope change problems between aqueous and... 

For urine analysis sample aliquots are diluted 1 1 with distilled water before application to the AAS stabilized temperature platform (Leung and Henderson, 1982). Fecal analysis require considerably more complicated preparation steps than serum or urine. The procedure developed in the author s laboratory (Brown et al., manuscript in preparation) is summarized as follows Frozen specimens are thawed and distilled water is added (1 mL per 2 g feces) and the sample is homogenized in a sealed container on a paint shaker. A 10 mL aliquot is ashed at 550°C in a muffle furnace, dissolved in dilute HNO3 snd analyzed by GF-AAS. 

If the concentration of cadmium in a sample solution is too low for quantitation by this flame AAS analytical technique, and the sample is to be averaged with other samples for TWA calculations, aliquots of the sample and a matrix modifier are later injected onto a Lvov platform in a pyrolytically-coated graphite tube of a Zeeman atomic absorption spectrophotometer/graphite furnace assembly for analysis of elemental cadmium. The matrix modifier is added to stabilize the cadmium metal and minimize sodium chloride as an interference during the high temperature charring step of the analysis (5.1., 5.2.). 

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