898 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



Sokolov*^ also developed an apparatus for the separate determination of 

 methane and heavier hydrocarbons by low-temperature fractionation, com- 

 bustion with air, and volumetric determination of the carbon dioxide thus 

 formed. This procedure is very similar to that applied in soil analj'^sis 

 and will be discussed in section d. 



An adaptation of the mass spectrograph to gas and soil analysis was 

 recently proposed bj^ H. Hoover, Jr., and H. Washburn. The mass 

 spectrograph is an electron gun for determining the mass of positive ions 

 (canal rays) by deflection in a combined electrical and magnetic field. It 

 consists of an ionization chamber in which gas molecules are bombarded by 

 electrons. The positive ions thus formed pass through a small slit, are 

 accelerated to a high velocity by a potential of several hundred to a thou- 

 sand volts, and then pass through another slit into a semicircular evacuated 

 tube placed in a magnetic field. At the end of the tube is a slit through 

 which only ions of a predetermined radius of path curvature can pass. 

 Behind the slit is a collector for measuring the number of arriving ions. 

 The ion current is stepped up by an amplifier and recorded as a propor- 

 tionate plate current V)y an oscillograph galvanometer. If the magnetic 

 field H is held constant, the; only quantity determining the radius of 

 curvature r of the ion path is the accelerating voltage E, since for a given 

 ion 7n/e = r^H^/2J^\ A mixture of gases can therefore ))e analyzed by 

 slowly varying the accelerating voltage so that ions with various masses 

 are admitted successively by the exit slit, where their number is recorded. 

 It is claimed that an accuracy o£ 0.2 parts per billion b>^ weight is readily 

 obtainable and that an analysis for the first members of the paraffin series 

 in a gas mixture takes only a few minutes. 



Other methods of gas analysis which have been proposed include: 

 (1) infrared absorption spectroscop}^, (2) Ramann effect spectrograph}', 

 and (3) low-voltage excitation and observation of resonance radiation. 

 Although some of these methods may also be applicable to laboratory pro- 

 cedure, none of them has found, as yet, practicable application in gas and 

 soil analysis work. 



D. Soil Analysis 



As the name indicates, soil analysis differs from gas detection in that 

 soil samples and not gas samples are analyzed. By breaking down the 

 mineral grains mechanically and, if necessary, chemically, it is possible to 

 obtain access to the entrained and occluded hydrocarbon constituents in- 

 stead of depending solely on the interstitial air. Otherwise, in some phases 



46 JjQQ^ Q-il 



^^ A.I.M.E. Tech. PubL No. 1205, May, 1940. 



