Helium fractionation

The paper [106] reports data on the effect of dilution with helium on the selectivity of methanol formation and complete oxygen conversion temperature for the DMTM reaction in a small d = l mm) fused silica capillary at P = 100 atm and = 10 s. At a constant total pressure and reaction time, the increase of the helium fraction (decrease in the concentrations of the reactants) in the mixture was accompanied by the monotonic growth of the complete oxygen conversion temperature and the reduction of the selectivity of methanol formation to zero at [He] = 90% (Fig. 4.10). 

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. 

The most familiar gas laser is the helium—neon laser (23,24). Sales of commercial helium—neon lasers exceed 400,000 units per year. The helium—neon laser is a compact package that produces a continuous beam of orange-red light. The inside diameter of the tube is commonly around 1.5 mm. The output of helium—neon lasers available commercially ranges from a fraction of a milliwatt to more than 35 mW. They have many appHcations in the areas of alignment, supermarket scanning, educational demonstrations, and holography. 

Should be distd under reduced pressure under nitrogen and stored in the dark. Purified via the nitrosochloride [Waterman et al. Reel Trav Chim Pays-Bas 48 1191 7929]. For purification of optically active forms see Lynn [J Am Chem Sac 91 361 1919]. Small quantities (0.5mL) have been purified by GLC using helium as carrier gas and a column at 90 packed with 20 wt% of polypropylene sebacate on a Chromosorb support. Larger quantities were fractionally distd under reduced pressure in a column packed with stainless steel gauze spirals. Material could be dried with CaH2 or sodium, and stored in a refrigerator CaS04 and silica gel were not satisfactory because they induced spontaneous isomerisation. [Bates, Best and Williams 7 C/iem Soc 1521 7962.]... 

Fractions were analyzed by vapor-phase chromatography (column 0.3 X 120 cm., 20% SE-52 on Chromosorb P 60/80, 130°, helium flow rate of 60 ml./min.). Retention times of 1.9 minutes for dicyclopentadiene and 4.6 minutes for the 7,7 dichlorobicyclo[3.2.0]hept-2-en-6-one were found. 

Capture efficiency is the fraction of generated contaminant that is directly captured by the hood. Measurement of capture efficiency involves measuring concentration of process-generated contaminant or a tracer material. Using process-generated contaminant requires use of instruments suited to each specific contaminant and its conditions (temperature, pressure, concentration, form, etc.). In order to facilitate these measurements, a tracer is often substituted for the process-generated contaminant. The tracer is usually a gas (sulfur hexafluoride, nitrous oxide, helium, or similar), but an aerosol (particles) can also be used (potassium iodide, polystyrene particles, microbiological particles, etc.). The chosen tracer should be as similar to the real contaminant as possible, but at the same time should... 

Figure 12.9 Typical pyrolysis chromatogram of fraction from a styrene-acTylonitiile copolymer sample obtained from a miciocolumn SEC system 1, acrylonitrile 2, styrene. Conditions 5 % Phenylmetliylsilicone (0.33 p.m df) column (50 m X 0.2 mm i.d.) oven temperature, 50 to 240 °C at 10 °C/min carrier, gas, helium at 60 cm/s flame-ionization detection at 320 °C make-up gas, nitrogen at a rate of 20 mL/min. P indicates tlie point at which pyrolysis was made. Reprinted from Analytical Chemistry, 61, H. J. Cortes et ai, Multidimensional cliromatography using on-line microcolumn liquid cliromatography and pyrolysis gas cliromatography for polymer characterization , pp. 961-965, copyright 1989, with permission from tlie American Chemical Society.

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.

After a preliminary study by Mortenson and Leighton S the thorough study by Edwards, Day and Overman s is notable. They analysed solutions of spblCHj) in benzene, octane and CCI4 for non-volatile forms of °Bi. Similar analyses were made on gaseous Pb(CH3)4 at 10 mm pressure, both pure and diluted with He, Ne, At, Kr and Xe. In solution at concentrations over 5 mole percent, about 50% of the Bi remained in a volatile form on dilution to mole fraction 0.05, the retention fell to 18% and rose again to over 90% in very dilute solutions. The retention values in the gas phase were then practically a continuation of those in dilute solution—between 80% and 90% for the pure gas at 10 mm pressure. With helium as diluent, the retention reached its maximum of 97% and the values decreased slowly to about 90% with xenon. 

The recoil-free fraction /a of transition metal complexes or proteins in frozen solution can be as small as 0.1-0.3, when measured just below the melting point, but the /-factor increases strongly when the temperature is lowered to fiquid nitrogen temperatures (77 K), and at fiquid helium temperatures (4.2 K) it may reach values of 0.7-0.9 [35]. This makes a substantial difference to the acquisition time of the spectra because of the square dependency on the signal (3.1). 

Gunther D, Heimich CA (1999) Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier. J Anal At Spectrom 14 1363-1368 Habfast K (1998) Fractionation correction and multiple collectors in thermal ionization isotope ratio mass spectrometry. Inti J Mass Spectrom 176 133-148... 

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