Helium sampling

The Cr203 content of each catalyst was determined by atomic absorption spectroscopy (Varian/Spectr AA-20 plus) on acid-digested samples. Total surface areas were determined by a single point BET method (nitrogen adsorption-desorption at 77.5 K) using a mixture of 29.7% N2 in helium. Samples were wet-loaded into the flow tube and dried at 423 K in a hydrogen flow for 15 minutes and then for another 30 minutes at 513 K before cooling in helium. 

Figure 3. TGA, DTA, and DTG curves for titanium copper hydride. The heating rate was 25°C/min in purified helium. Sample 32.1 mg, contained in an A/203 crucible.

The results listed in Table 6.1 were disappointing. It was clear that the heating capacity of the helium sample purge via the MS heated transfer line proved insufficient to compensate for the heat losses in the non-heated end-part of the MS capillary. 

DC Arc Plasma Jet Gas Stabilized Arc DC arc discharge formed between nonsample electrodes in flowing gas streams of argon or helium. Sample introduced separately. liquids, powders 1500-3500... 

Since the bubble chamber will be used with both hydrogen and helium, samples of aluminum epoxied to Plexiglas have been cycled to liquid-helium temperature several times with no visible degradation of the epoxy joint. 

Consider a sample of neon gas weighing 10.09 g. How many atoms of neon are present in the sample What mass of helium gas would you need for the helium sample to contain the same number of atoms as the neon sample ... 

The equation derived for t ie calculation of the total impurity concentration in the original helium sample is as follows ... 

Percentages of each contaminant in the concentrated impurity sample is multiplied by the total parts per million to give the ppm concentration of each component in the original helium sample. These data, for the above example, are given in Table II. 

The column is swept continuously by a carrier gas such as helium, hydrogen, nitrogen or argon. The sample is injected into the head of the column where it is vaporized and picked up by the carrier gas. In packed columns, the injected volume is on the order of a microliter, whereas in a capillary column a flow divider (split) is installed at the head of the column and only a tiny fraction of the volume injected, about one per cent, is carried into the column. The different components migrate through the length of the column by a continuous succession of equilibria between the stationary and mobile phases. The components are held up by their attraction for the stationary phase and their vaporization temperatures. 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

Rutherford backscattering spectrometry is the measurement of the energies of ions scattered back from the surface and the outer microns (1 micron = 1 pm) of a sample. Typically, helium ions with energies around 2 MeV are used and the sample is a metal coated silicon wafer that has been ion implanted with about a... 

Figure Bl.24.10. Random and aligned (chaimelled) backscattering spectrum from a single crystal sample of silicon. The aligned spectrum has a peak at the high energy end of the Si signal. This peak represents helium...

Figure Bl.24.17. An example of scanning transmission ion microscopy (STIM) measurements of a human oral cancer cell. The different images indicate different windows in the energy of transmitted helium ions as indicated in the figure. White indicate areas of high counts. The teclmique offers a thickness scan through the sample, and, in this case, the cell walls of one specific cell can be seen in the areas dominated by thicker structures (data from C A Pineda, National Accelerator Centre, Fame, South Africa).

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