Helium analysis

Helium analysis is most conveniently carried out by mass spectrometry and even very simple instruments are capable of sensitivities better than 0.01 ppm at concentrations of 5-6 ppm He, i.e., those commonly found in soil and overburden gases and waters. 

Dyck, W. and Pelchat, J.C., 1977. A semi-portable helium analysis facility. Geol. Survey Canada, Paper 77-lC,pp. 85-87. 

Hinkle, M.E. and Kilbum, J.E., 1979. The use of vacutainer tubes for collection of soil samples for helium analysis. US Geol. Survey, Open File Report, 79-1441,23 pp. 

Rice, R.S., and Reimer, G.M., 1977. Helium analysis of subsurface waters as a uranium exploration tool. Symp. on Hydrogcochcmical and Stream Sediment Reconnaissance for Uranium in the USA. US Dept. Energy, Oct. 1977. 

Both helium and radon are readily lost to the atmosphere from water, and both will pass through plasticcontainers. These ga.ses need to be collected with minimum sample agitation and with no air bubble under the cap, as they are much more soluble in air than water. Radon may be collected in steel-capped glass jars, but samples for helium analysis are most safely collected in metal containers. Glass containers may be used if analysis is within a few weeks, but a correction should be made for lo.ss of helium by diffusion through the container. The measurement of helium and radon in surface water is unlikely to yield success, with the exception of water from the bottom of lakes. 

Liquid helium s use in magnetic resonance imaging (MRI) continues to increase as the medical profession accepts and develops new uses for the equipment. This equipment has eliminated some need for exploratory surgery by accurately diagnosing patients. Another medical application uses MRE to determine (by blood analysis) whether a patient has any form of cancer. 

Elemental chemical analysis provides information regarding the formulation and coloring oxides of glazes and glasses. Energy-dispersive x-ray fluorescence spectrometry is very convenient. However, using this technique the analysis for elements of low atomic numbers is quite difficult, even when vacuum or helium paths are used. The electron-beam microprobe has proven to be an extremely useful tool for this purpose (106). Emission spectroscopy and activation analysis have also been appHed successfully in these studies (101). 

Cathodoluminescence microscopy and spectroscopy techniques are powerful tools for analyzing the spatial uniformity of stresses in mismatched heterostructures, such as GaAs/Si and GaAs/InP. The stresses in such systems are due to the difference in thermal expansion coefficients between the epitaxial layer and the substrate. The presence of stress in the epitaxial layer leads to the modification of the band structure, and thus affects its electronic properties it also can cause the migration of dislocations, which may lead to the degradation of optoelectronic devices based on such mismatched heterostructures. This application employs low-temperature (preferably liquid-helium) CL microscopy and spectroscopy in conjunction with the known behavior of the optical transitions in the presence of stress to analyze the spatial uniformity of stress in GaAs epitaxial layers. This analysis can reveal,... 

External-beam PDCE refers to measurements with the specimen removed from a vacuum environment. This mode permits the analysis of large or volatile specimens and consists of allowing the panicle beam to exit, through a thin window, the vacuum of the beam line and impinge on the specimen held at atmospheric pressure of air or other gases (e.g., helium). 

Within the last 5—10 years PIXE, using protons and helium ions, has matured into a well-developed analysis technique with a variety of modes of operation. PIXE can provide quantitative, nondestructive, and fast analysis of essentially all elements. It is an ideal complement to other techniques (e.g., Rutherford backscattering) that are based on the spectroscopy of particles emitted during the interaction of MeV ion beams with the surface regions of materials, because... 

The atom probe field-ion microscope (APFIM) and its subsequent developments, the position-sensitive atom probe (POSAP) and the pulsed laser atom probe (PLAP), have the ultimate sensitivity in compositional analysis (i.e. single atoms). FIM is purely an imaging technique in which the specimen in the form of a needle with a very fine point (radius 10-100 nm) is at low temperature (liquid nitrogen or helium) and surrounded by a noble gas (He, Ne, or Ar) at 10 -10 Pa. A fluorescent screen or a... 

For intermediate temperatures from 400-1000°C (Fig. 11), the volatilization of carbon atoms by energetic plasma ions becomes important. As seen in the upper curve of Fig. 11, helium does not have a chemical erosion component of its sputter yield. In currently operating machines the two major contributors to chemical erosion are the ions of hydrogen and oxygen. The typical chemical species which evolve from the surface, as measured by residual gas analysis [37] and optical emission [38], are hydrocarbons, carbon monoxide, and carbon dioxide. 

In addition, solute foeusing is possible by maintaining a low initial temperature (e.g. 40 °C) for a long period of time (8-12 min ) to allow the mixture of deeom-pressed earbon dioxide, helium gas and the solutes to foeus on the GC eolumn. The optimization of the GC inlet temperature ean also lead to inereased solute foeusing. After supereritieal fluid analysis, the SF fluid effluent is deeompressed through a heated eapillary restrietor from a paeked eolumn (4.6 mm i.d.) direetly into a hot GC split vaporization injeetor. 

Figure 12.22 SFC-GC analysis of aromatic fraction of a gasoline fuel, (a) SFC trace (b) GC ttace of the aromatic cut. SFC conditions four columns (4.6 mm i.d.) in series (silica, silver-loaded silica, cation-exchange silica, amino-silica) 50 °C 2850 psi CO2 mobile phase at 2.5 niL/min FID detection. GC conditions methyl silicone column (50 m X 0.2 mm i.d.) injector split ratio, 80 1 injector temperature, 250 °C earner gas helium temperature programmed, — 50 °C (8 min) to 320 °C at a rate of 5 °C/min FID detection. Reprinted from Journal of Liquid Chromatography, 5, P. A. Peaden and M. L. Lee, Supercritical fluid chromatography methods and principles , pp. 179-221, 1987, by courtesy of Marcel Dekker Inc.

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