Table II.
Data on Solvents for Near-lnfared Spectrophotometry Microns 1.0
1.2
1.1
1.6
1.8
2.0
2.2
Carbon t e t r a c h l o r i d e
Carbon d . s d f i d e Chloroform (ethanol s t a b i l i z e d ) hIeLh)ier.e c h l o r i d e
Dioxane
-
Di(n-butyl) ether T r i e t h y l e n e glycol dimethyl ether
Heptane
Benzene Acetonitrile
Dimethylformamide D i m ethyl
8 ulfoxide
prepared are representative but not complete. Compounds of unusual structure or with substituents which cause unusual interactions may shift a given vibration outside the limits shown on the correlation chart. Therefore, caution should be exercised in the use of any chart of this type, As more data are published the authors hope to publish revised and improved charts. The data in Table I have been obtained primarily from spectra run in
carbon tetrachloride solution. They apply quslitatively to most other solvents. I n conjunction with the use of the correlation chart, some knowledge of the spectral data on solvents is extremely desirable. Table I1 gives the spectral regions in which some solvents are useful and it is also an indication of the maximum path lengths which can be tolerated in qualitative work without causing the slits to be too wide for close
adherence to Beer's law. (The criterion for this chart was that the nominal or spectral slit width should not be more than three times the slit obtained with carbon tetrachloride in the reference beam of a double beam spectrophotometer.) Most of the solvents have been successfully used in various analgtical applications in the regions noted in the chart Many other solvents can be used such as chlorinated aromatic compounds in the 1.4-micron region. Those in the table are mentioned primarily as guides in solvent selection. Furthermore, the solvents in the chart may often be used in longer cells than are indicated over very narrow spectral regions. In addition, cells thinner than 1 cm. may occasionlly be used to advantage. Elevated temperatures are also useful for solving difficult solubility problems. However, one must be cautious of the effect of temperature on molar absorptivity ( I ) . The only region in which a number of transparent solvents is not available is that between 2.7 land 3.1 microns, where the fundamental hydroxyl and K-H (amines, amides, imines. hydrazines) stretching bands are found. LITERATURE CITED
(1) Hughes, R . H., Martin, R. J.. Coggeshall, N. D., J . ('hem. Phys. 24, 489
(1956). ( 2 ) KFye, V., Spertrochrnz -4cta 6 , 25; (1934).
Determination of Impurities in Helium b y Gas Chromatography Arthur W. Mosen and George Buzzelli, John Jay Hopkins Laboratory for Pure and Applied Science, General Atomic Division, General Dynamics Corp., Son Diego, Calif.
for determinA ing impuritiesmethod in parts per million N ANALYTICAL
range was needed for studies on helium as a reactor coolant. The Bureau of Mines ( I ) uses a liquid helium cold trap to concentrate impurities from helium prior to analysis on a mass spectrometer. Because the impurity level of interest was below the limit of sensitivity of the available instrument, a means of concentrating the impurities was required. Madison ( 3 ) retained fixed gases on a liquid nitrogen-cooled charcoal column. Kyryacos and Boord (2) have shown that Molecular Sieve columns are better for the resolution of the fixed gases than charcoal columns. Timms, Konrath, and Chirnside (4) used a Type 5A Molecular Sieve for determination of impurities in carbon dioxide. -4gas chromatographic method has been developed for the analysis of
helium. A liquid nitrogen-cooled charcoal trap is used to concentrate the impurities, and a Type 5A Molecular Sieve column is used for separation of hydrogen, oxygen, nitrogen, methane, and carbon monoxide. Carbon dioxide is determined on a separate sample using a silica gel column. EXPERIMENTAL
Apparatus. A Burrell Model K-2 Kromo-Tog was used. The adsorption columns are standard 100-cm. Kromo-Tog columns. One is packed with a 28- to 48-mesh Type 5A Linde Molecular Sieve; the other, with Burrell silica gel. Helium is the carrier gas. The cold trap is made from borosilicate glass tubing 2 mm. in inside diameter. This can be connected directly to the Kromo-Tog gas sampling manifold. The trap is packed with
0.75 cc. of 50- to 200-mesh coconut charcoal, Fisher S o . 5-690. The trap is initially conditioned by heating to 150" to 200' C. under vacuuni for 1 hour. Helium carrier gas is then passed through for a half hour. S f t w conditioning, the stopcocks are turned to isolate the U-tube. In use, the trap is attached to the system to be sampled and helium is swept through the three-way stopcocks and the rubber tubing to remove air from this part of the sampling system. The trap is cooled in liquid nitrogen for 5 minutes prior to collecting the sample, kept immersed in liquid nitrogen during sample collection, and stored a t this temperature until helium is analyzed. \Then the air has been removed from the connecting tubes, the stopcocks are turned so that helium will flon through the charcoal trap. The volume of gas flowing through the trap is measured with a wet-test meter. When the sample has been collected, the stopcocks VOL. 32, NO. 1, J A N U A R Y 1960
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