Use of Large Graphite Beakers in Double-Arc Analysis [antikvár]

Use of Large Graphite Beakers in Double-Arc Analysis E. Schroll and D. Sauer Basic Research Institute of il^e Federal Testing and Research Institution, Arsenal, Vienna, Austria * Large graphite beakers have been developed for analysis using tlie double-arc technique. A two-chamber system prohibits the spraying of sample powder. The new beaker can be filled with more than 10 g of sample. The larger sample gives the advantage of better detection limits (to parts per billion). A new e.xcitation and heating source, called "mixed source," consisting of an exciting-spark or ac source and a separately controlled heating de arc, is described. INTRODUCTION The process of excitation in emission speetrography must be separated into two different events: the heating and evaporation of the sample, and the excitation of the vapors. The conventional double arc is a good approximation of an ideal light source because both events can be controlled separately.' Shaw, Joensuu, and Ahrens- described a vertical double-furnaee electrode that took the form of a microbeaker. The volume of the crater is 0.8 ce, thus a maximum sample may be 1-6 g, depending upon, the density of the sample ('-'0.5-1 g of rock sample). This form of the electrode is generally used in double-are analysis. Preuss ^ developed a horizontal carbon-tube furnace for direct-distillation analysis. The sample was loaded into a boat and heated in the carbon tube by a 10-kW current at 12-25 V. The gases were excited by de arc. The weight of sample was 3 g of powdered rock. The double-furnace electrode of Wedepohl^ continued this development, hut the form of the electrode was simplified for routine analyses. With the volume of this crater being 0.33 co, the sample weight eould amount to only 1 g of powdered rock. It should be borne in mind that a special vertical-burner furnace was constructed by Edwards and Urey® for the analysis of alkali metals in meteorites. The graphite erueible containing the sample was induction-heated in a water-cooled jacket, and the vapors were excited by flame. All these electrode forms restrict the weight of samples of powdered roek to 1 g. However, the determination of traces in geological materials and technical high-purity substances requires detection limits in ranges of parts per billion. One way to obtain better sensitivity is to use larger samples, which would also provide for better sampling. I. EXPERIMENTAL Large graphite beakers have been developed and tested in the vertical arrangement that does not re- quire a complicated arc stand. For the production of the furnace electrodes, graphite rods of reactor quality (Siemens-Plania, Germany) of 3 em diam were used. Four types of such electrodes were manufactured— 1. a shape that is similar to the graphite beakers of Shaw, Joensuu, and Ahrens but has a flat bottom and a volume of 6 ce; 2. the same beaker with a chimney like cap ; 3. a tall, thin beaker with flat bottom and a volume of 4 ce ; 4. a graphite beaker of normal shape (like Type 1) that contains an inset perforated with 50 holes. The volume, in this case, amounts to 1.5 cc. The graphite beaker of Type 4 is represented with its measurements in Fig. 1. Figure 2 shows the comparison of the size against the usual graphite beaker of Shaw, Joensuu, and Ahrens. The beakers of Types 1 and 2 can he filled with more than 10 g if the specific gravity of the sample is greater than 2. The testing of the electrodes was combined with studies about a separation of heating for evaporation from the electrical excitation. SOholes i Fig. 1. Two-chamber graphite beaker with perforated inset. (Dimensions are in mm.) 404 Volume 20, Number 6, 1966