Tantalum sampling system

As illustrated in Figure 12, the reaction mixture contains mono-, di-, and tri-brominated glycols, hydrobromic acid, and water. The mixture is extremely corrosive, and the reactor is operated at a temperature just above the freezing point of the product. The key to successfully sampling this mixture was the use of a corrosion-resistant tantalum sampling system. In addition, the sample line was continuously flushed with reactor solvent except during sampling. 

The tantalum boat atomization technique provides very low detection limits coupled with small sample size. The reproducibility of the sample size is apparently the limiting factor on precision of results. Figure 10-25 shows results obtained on replications of zinc samples. Relative standard deviations of between 2.8 and 3.4% were obtained for zinc concentrations between 1 X 10 and 5 x 10 g. Some typical absolute detection limits with the tantalum boat systems are given in Table 10-4. They range from 10 to... 

This paper deals with the control of weld depth penetration for cylinders in gold-nickel alloy and tantalum. After introducing the experimental set-up and the samples description, the study and the optimization of the testing are presented for single-sided measurements either in a pulse-echo configuration or when the pump and the probe laser beams are shifted (influence of a thermal phenomenon), and for different kind of laser impact (a line or a circular spot). First, the ultrasonic system is used to detect and to size a flat bottom hole in an aluminium plate. Indeed, when the width of the hole is reduced, its shape is nearly similar to the one of a slot. Then, the optimization is accomplished for... 

In the direct insertion technique, the sample (liquid or powder) is inserted into the plasma in a graphite, tantalum, or tungsten probe. If the sample is a liquid, the probe is raised to a location just below the bottom of the plasma, until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent, due to selective volatilization of the sample. The intensity-time behavior depends on the sample, probe material, and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently, no commercial instruments using direct sample insertion are available, although both manual and h ly automated systems have been described. ... 

In contrast, the combustion temperatures recorded for the tantalum-nitrogen and niobium-nitrogen systems were much lower than the melting points of the respective metals. Photographs of the cross-sections of undiluted samples do not indicate any macroscopic effect of melting, and neither do SEM photographs of the products. 

Fig. 9.6. Schematic diagram of the experimental system for laser ablation-assisted radio-frequency atomization excitation. (I) Sample holder, (2) tantalum lid, (3) graphite cup, (4) graphite disc, (5) rf power source, (6) different types of sample holder for sintered ceramics, (7) NdiYAG laser, (8) laser beam-focusing lens, (9) spectrometer, (10) quartz window for laser irradiation, (11) central electrode, (12) discharge chamber, (13) quartz window for optical observation. (Reproduced with permission of the Royal Society of Chemistry.)...

The accelerator is used as radiation source in the development of new dosimetry systems (physical and chemical) and in the characterization of existing dosimeters in different conditions (a water cooled tantalum converter is used for the generation of X-ray and there is the possibility of controlling the temperature of small samples under irradiation). In Table I the technical specifications of the accelerator are reported. 

In one ETV device the electrothermal vaporization chamber is connected directly to the ICP torch assembly. In this design the distance that the sample must travel from the surface of the vaporizer to the plasma is kept minimal (about 30 cm) and the vaporization chamber is small (about 50 cm ) in order to prevent diffusion of the analyte peak and to minimize loss of the sample to the walls of the delivery system. The sample is inserted into a tantalum boat, mounted on brass supports through which the power is applied. 

Plasma sources, dc excited or excited with radiofrequency energy, provide temperatures from 6000 to 10,000°K and are very stable when compared with the dc arc. They are usually used with liquid samples, which are aspirated into the source continually during the excitation period. The problem of fractional distillation thus is avoided and the source is very stable. Dickinson and FasseF devised a system to introduce a solid sample into a plasma by vaporizing the sample from an electrically heated tantalum boat. 

The function of the sample cell is to convert the sample into ground state atoms in the optical path of the atomic absorption system. The ground state atoms must be obtained from the sample in a reproducible manner if quantitative data are to be obtained. The sample is usually in liquid form, although recent developments with carbon rods and tantalum boats have made possible the use of solid samples. 

Another technique to produce an atomic vapor from a solid sample is the use of a tantalum boat. A tantalum boat is electrically heated, in a manner similar to the carbon rod system, to produce an atomic vapor. The high melting point of tantalum (2996°C) and its ability to be shaped into a boat make it useful for this purpose. Figure 10-24 shows a tantalum boat and a sample cell that can be used for this purpose. The boat is shaped with a small... 

---