Furnace calibration

Today s calorimeters have a furnace calibration routine that is separate from the temperature and enthalpy calibration routines. Follow the manu-... 

After completing the determination of the Curie temperatures, then proceed with furnace calibration. All instrument manufacturers include this step in their calibration protocols. 

One of the advantages of electrothermal atomization is that some materials can be atomized directly, thus avoiding the solution step. For example, liquid samples such as blood, petroleum products, and organic solvents can be pipetted directly into the furnace for ashing and alomization. Solid samples, such as plant leaves, animal tissues, and some inorganic substances. can be weighed directly into cup-type atomizers or into tantalum boats for introduction into tube-type furnaces. Calibration is. however, usually difRcult and requires standards that approximate the sample in composition. 

The furnace and thermostatic mortar. For heating the tube packing, a small electric furnace N has been found to be more satisfactory than a row of gas burners. The type used consists of a silica tube (I s cm. in diameter and 25 cm. long) wound with nichrome wire and contained in an asbestos cylinder, the annular space being lagged the ends of the asbestos cylinder being closed by asbestos semi-circles built round the porcelain furnace tube. The furnace is controlled by a Simmerstat that has been calibrated at 680 against a bimetal pyrometer, and the furnace temperature is checked by this method from time to time. The furnace is equipped with a small steel bar attached to the asbestos and is thus mounted on an ordinary laboratory stand the Simmerstat may then be placed immediately underneath it on the baseplate of this stand, or alternatively the furnace may be built on to the top of the Simmerstat box. 

The furnace. For heating the tube packing, a small electric furnace E is used, similar to that described in the carbon and hydrogen determination. It is 22 cm. in length and 1 5 cm. in diameter. The furnace is maintained at 680 C., as before, by a calibrated Simmerstat and its temperature is checked from time to time with a bimetal pyrometer. 

Muffle furnaces. An electrically heated furnace of muffle form should be available in every well-equipped laboratory. The maximum temperature should be about 1200 °C. If possible, a thermocouple and indicating pyrometer should be provided otherwise the ammeter in the circuit should be calibrated, and a chart constructed showing ammeter and corresponding temperature readings. Gas-heated muffle furnaces are marketed these may give temperatures up to about 1200 °C. 

The use of an integral video screen in instruments presents very great advantages, both in the ease of operation and in the ability to develop and understand analytical methods. Complete analytical records can be stored in the instrument and a visual display of good calibration curves can be stored in memory and recalled at will. It is most useful to have a graphical display of atomisation peaks when using a furnace where a distinction can be made of the total absorbance peak and that due to the analyte absorbance. 

With regard to operational considerations, the recirculation pump pressure gauges (which measure pressure differentials and consequently indicate possible blockage in the coiled tube) require frequent calibration. Regular cleaning and good housekeeping are also required, especially within the furnace area. 

For isothermal measurements, it is advisable to use a furnace of low thermal capacity unless suitable arrangements can be made to transport the sample into a preheated zone. The Curie point method [132] of temperature calibration is ideally suited for microbalance studies with a small furnace. A unijunction transistor relaxation oscillator, with a thermistor as the resistive part with completion of the circuit through the balance suspension, has been suggested for temperature measurements within the limited range 298—433 K [133]. 

An important prerequisite for the use of CRMs as calibrants, at least for optical methods and particularly all AAS modes, is that they should match the matrix and level of analyte contents of the materials to be analyzed as closely as possible, so that potential matrix effects will be compensated if calibrant and sample material are affected by the applied method, e.g. the temperature program for furnace techniques, in the same way. Further it is very important for all methods that the CRMs used should not show a nugget effecf, i.e. particles with extremely high analyte content that can lead to a high analyte heterogeneity (Kurfiirst 1991 Kurfiirst et al. 

Application to solid polymer/additive formulations is restricted, for obvious reasons. SS-ETV-ICP-MS (cup-in-tube) has been used for the simultaneous determination of four elements (Co, Mn, P and Ti) with very different furnace characteristics in mg-size PET samples [413]. The results were compared to ICP-AES (after sample dissolution) and XRF. Table 8.66 shows the very good agreement between the various analytical approaches. The advantage of directly introducing the solid sample in an ETV device is also clearly shown by the fact that the detection limit is even better than that reported for ICP-HRMS. The technique also enables speciation of Sb in PET, and the determination of various sulfur species in aramide fibres. ETV offers some advantages over the well-established specific sulfur analysers very low sample consumption the possibility of using an aqueous standard for calibration and the flexibility to carry out the determination of other analytes. The method cannot be considered as very economic. 

Bishop [75] determined barium in seawater by direct injection Zeeman-modulated graphite furnace atomic absorption spectrometry. The V203/Si modifier added to undiluted seawater samples promotes injection, sample drying, graphite tube life, and the elimination of most seawater components in a slow char at 1150-1200 °C. Atomisation is at 2600 °C. Detection is at 553.6 nm and calibration is by peak area. Sensitivity is 0.8 absorbance s/ng (Mo = 5.6 pg 0.0044 absorbance s) at an internal argon flow of 60 ml/min. The detection limit is 2.5 pg barium in a 25 ml sample or 0.5 pg using a 135 ml sample. Precision is 1.2% and accuracy is 23% for natural seawater (5.6-28 xg/l). The method works well in organic-rich seawater matrices and sediment porewaters. 

Pruszkowska et al. [135] described a simple and direct method for the determination of cadmium in coastal water utilizing a platform graphite furnace and Zeeman background correction. The furnace conditions are summarised in Table 5.1. These workers obtained a detection limit of 0.013 pg/1 in 12 pi samples, or about 0.16 pg cadmium in the coastal seawater sample. The characteristic integrated amount was 0.35 pg cadmium per 0.0044 A s. A matrix modifier containing di-ammonium hydrogen phosphate and nitric acid was used. Concentrations of cadmium in coastal seawater were calculated directly from a calibration curve. Standards contained sodium chloride and the same matrix modifier as the samples. No interference from the matrix was observed. 

In this method, inorganic lead in seawater samples are converted to tetra-ethylead using sodium tetraethylboron (NaB(C2H5)4) which is then trapped in a graphite furnace at 400 °C. Quantitation is achieved by using a simple calibration graph prepared from aqueous standards. An absolute detection limit of (3relative standard deviation. 

Graphite furnace atomic absorption spectrophotometry has been used for the determination down to 5 ng/1 inorganic and organic mercury in seawater [61]. The method used a preliminary preconcentration of mercury using the ammonium pyrrolidine dithiocarbamate-chloroform system. A recovery of 85 - 86% of mercury was reproducibly obtained in the first chloroform extract and consequently it was possible to calibrate the method on this basis. A standard deviation of 2.6% was obtained on a seawater sample containing 529 ng/1 mercury. 

Calibration Test. Before the wall tests were carried out, a calibration test was conducted to evaluate burner performance and to check for agreement between fuel gas and heat release calculations. For this test, the furnace was closed with a masonry wall lined with a layer of ceramic blanket material. The ASTM E-119 time-temperature curve was followed for 60 min. 

An alternative approach has been to use Curie-point pyrolysers. The use of the Curie point in accurately reproducing a temperature has already been discussed for the calibration of TG furnaces (p. 481). In a slightly different way the Curie point can be used for accurately reproducing pyrolysis conditions with the added advantage that the rise time is only about 0.4 s. The... 

Aliquots (0.3-0.5g) of dry sediment (passing 100 mesh) were weighed and then transferred into 10cm3 Coors alumina crucibles. The uncovered crucibles, contained in a suitable tray, were placed into a warm muffle furnace and ignited at 550°C. The samples were maintained at 550°C for 1.5h, then removed, allowed to cool, and transferred into 100ml calibrated flasks 50ml of 1.0M hydrochloric acid were then added to the flasks. The mixtures were next shaken for 14-18h at about 22°C. 

One important experimental parameter is also the furnace which is used in the thermogravimetric apparatus. There always exist pronounced radial and vertical temperature gradients which can be found by calibration runs. As an example, the determination of the vertical temperature distribution by Wiedemann41 may be referred to. Also the amount and shape of the sample can contribute to a temperature gradient. This fact is of special importance for kinetic studies. Considerable temperature differences - up to several °C - can exist at different locations of the sample holder. 

After having calibrated the instrument as directed above, introduce each solution into the generator three times and record the steady reading. If the generator is a flame, wash the apparatus thoroughly with water if a furnace is used fire it after each introduction. 

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