Filament temperature

Figure 7.9 shows a schematic representation of this effect, in which the ratio of the two isotopes changes with time. To obtain an accurate estimate of the ratio of ion abundances, it is better if the relative ion yields decrease linearly (Figure 7.9) which can be achieved by adjusting the filament temperature continuously to obtain the desired linear response. An almost constant response for the isotope ratio can be obtained by slow evaporation of the sample, viz., by keeping the filament temperature as low as is consistent with sufficient sensitivity of detection (Figure 7.9). 

Thermal ionization has three distinct advantages the ability to produce mass spectra free from background interference, the ability to regulate the flow of ions by altering the filament temperature, and the possibility of changing the filament material to obtain a work function matching ionization energies. This flexibility makes thermal ionization a useful technique for the precise measurement of isotope ratios in a variety of substrates. 

Arkel refining a sample of tire impure metal, for example zirconium, is heated to a temperature around 550 K in contact with low pressure iodine gas in a sealed system which has a heated mngsten filament in the centre. The filament temperature is normally about 1700K. At the source the iodides of zirconium and some of the impurities are formed and drese diffuse across the intervening space, where tire total pressure is maintained at 10 auiios, and are decomposed on the filament. The iodine then remrns to form fresh iodide at the source, and the transport continues. 

In case of excessive carbonization the rate of boiling should be increased or the filament temperature slightly lowered. In general the vapor velocity should be as high as possible without exceeding the capacity of the copper condenser. 

Thermal Conductivity Detector In the thermal conductivity detector (TCD), the temperature of a hot filament changes when the analyte dilutes the carrier gas. With a constant flow of helium carrier gas, the filament temperature will remain constant, but as compounds with different thermal conductivities elute, the different gas compositions cause heat to be conducted away from the filament at different rates, which in turn causes a change in the filament temperature and electrical resistance. The TCD is truly a universal detector and can detect water, air, hydrogen, carbon monoxide, nitrogen, sulfur dioxide, and many other compounds. For most organic molecules, the sensitivity of the TCD detector is low compared to that of the FID, but for the compounds for which the FID produces little or no signal, the TCD detector is a good alternative. 

The high filament temperature used causes additional radiative heating of the substrate [530, 531]. Feenstra et al. [531, 548] have developed a heat transport model of their setup (Figure 68). All heat exchange is assumed to occur via ra-... 

With the setup described, a series of depositions was carried out [531, 548], in which the substrate temperature was varied between 125 and 650°C. the pressure between 0.007 and 0.052 mbar. the gas flow rate between 15 and 120 seem, and the dilution of the silane gas with hydrogen ([SiHaj/flSiHa] + [Hi])) between 0.1 and 1. Under these conditions the deposition rate varied between 1 and 2.5 nm/s [531 ]. Molenbroek et al. [530] reported a variation of the deposition rate between 1 and 9 nm/s for similar process conditions, at a filament temperature of 2000°C. 

Fig. 3.4. ZnO film conductivity as a function of amount of adsorbed hydrogen atoms. 1 - Film temperature -196 C filament temperature 1,000 C 2 — Film temperature -1% C filament temperature 1,100 C...

Fig. 3.3. Relationship between the number of adsorbed H-atoms on the film and the number of electrons caused an increase in its conductivity. The calculation has been performed on the basis of experiments with the following film temperatures 1 - -33 C 2 - -78 C 3 -196 C. Filament temperature was fix at 1,100 C.

The results on pyrolysis of acetone displayed in Fig. 4.5 are consistent with formula (4.8). Thus, variation of the concentration of free radicals near the sensor surface and, consequently, variation of the value idv/dt)tMi = o as functions of the filament temperature are governed by relation (4.8). As the acetone pressure increases, this relation fails because of fast interaction of CH3 radicals with acetone molecules. 

We use commercial Ti02 crystals (Pi-Kent) cut and polished to within 0.3° of the (110) face and we prepare them further with cycles of Ar + bombardment and U H V annealing to approximately 950-1100 K, typically 5-10 min for each cycle. The samples are mounted onto tantalum back-plates via strips of tantalum spot-welded to the back-plate. Annealing is performed by high-energy electron bombardment of the back-plate from a hot filament. Temperatures are measured from optical pyrometers (Minolta) focused on the back-plate. The temperatures are not measured directly from the samples because they are translucent and get darker with each sputter/anneal cycle. 

In these gages, a wire inside the gas whose pressure is to be measured, is electrically heated by a constant power (see Fig. 1.29). As the gas density decreases, the heat loss from the filament to the envelope walls decreases and hence the filament temperature increases (not linearly). The temperature (200-300°C) is read by a thermocouple in thermal contact with the wire. 

Data on the critical temperature of deposition of silver on glass are unsatisfactory. Knudsen (47) gives the critical temperature as above 575°. Cockcroft, however, reports critical temperatures ranging from —90 to —15°, depending on the filament temperature, for deposition of silver on mica. 

When the gas composition is altered due to adsorption or desorption, the value of k changes by Ak which in turn alters the filament temperature by Aff. Under the new conditions equation (15.2) can be rewritten as... 

Fig. 49a-f. Formation of the torus from a filament Temperature is 46 °C a thicker filament formed after the fusion of thinner filaments b if the temperature is still held constant, an initial loop forms c-f as time elapses, the rest of the filament is drawn into the loop to form a torus (f). Scale bar, 10 pm [363]... 

The apparatus and procedures for studying the ionization related to catalytic combustion have been described (20). A potential of 300 volts d.c. was maintained between the cylindrical collector and the heated filament, with the collector being negative. After the filaments had been conditioned at 1000°C. for a few minutes, the background current was less than 0.01 X 10"9 amp., even at the highest filament temperatures used (900°C.). 

Lead Analysis by TEMS. Lead analyses are made using the gel technique described by Cameron et al. (10). Lead ions are thermally produced at rhenium filament temperatures between 1100-1300°C, depending on the sample size. Ten ng of sample allows an analysis of 10 runs with 200 sweeps each across the masses of interest. 

Filament temperature Filament resistance Voltage across filament Recorder response Detector response factor... 

The carrier should be chosen with a TC either larger than most of the samples to be run, nitrogen with organic samples, for instance, or smaller, such as argon with some fixed gas analyses. Temperature dependence and linearity of response should be investigated carefully for all compounds to be analyzed. It should be remembered that a much lower current is necessary to reach the same filament temperature in low-conductivity gases. Mixtures of carrier gas, particularly where one of the constituents is also a sample compound, lead to further complexities. 

The large value of the filament temperature sensitivity shows how important temperature control of the detector is. Problems in this area appear most frequently with temperature controllers which oscillate about their setpoint, with a period of a few minutes. If the cells of the detector respond to this change with different delay times, then the oscillation will appear in the baseline. 

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