766 Subsurface Geologic Methods 



plants such as higher alcohols, aldehydes, and ketones. Similarly, benzene 

 and related aromatic solvents can be used in certain instances. Some of 

 the aliphatic solvents have a relatively high solvent action toward the 

 mineral waxes and a low desolving ability toward the vegetable waxes. 

 In most instances it is necessary to prepare a synthetic solvent com- 

 pounded from a mixture of polar and nonpolar solvents so designed as 

 to have a high selective solvation for a specific compound or group of 

 compounds. 



The degasing of the samples consists of placing the sample in a flask 

 and reducing the pressure to abput 60 mm. of mercury. The sample is 

 treated with phosphoric acid to decompose the carbonates, and the carbon 

 dioxide resulting therefrom is removed. The flask is heated at 100° C. 

 and the evolving gas is carefully scrubbed and introduced into the analy- 

 tical apparatus at 10'^ mm. of mercury. The analysis consists of con- 

 densing the gases in a liquid nitrogen trap at —196° C. The substances 

 which are still gaseous are drawn off, and then the temperature is raised 

 to —145° C. and the resulting volatile constituents removed. A third frac- 

 tion is made up of the constituents which are liquid at —145° C. but 

 which are gaseous at atmospheric conditions. 



The first group consists mainly of air, methane, and hydrogen. The 

 second group contains any ethane, propane, and butane which might be 

 present; and the third fraction is made up of pentane and heavier hydro- 

 carbons. Analysis for the methane consists of burning the gas over a 

 glowing platinum wire, and measuring the volume of the resulting carbon 

 dioxide. The quantities of the other fractions are determined by measur- 

 ing the volumes with a McLeod gauge before and after combustion. The 

 weights of the different fractions are calculated from the gas laws and the 

 final results expressed in parts per billion. 



Several other methods have been used for ultra-sensitive detection and 

 quantitative determination of the hydrocarbon constituents. Important 

 among these are infra-red absorption spectroscopy, fluorophotometry, 

 polarography, and mass spectroscopy. 



The fluorescence of certain of the hydrocarbons when activated by 

 ultraviolet light has offered an extremely sensitive method for detecting 

 vanishingly small amounts of these substances. Also, the presence of 

 some of the inorganic, secondary minerals is quickly and readily deter- 

 mined by fluorescence. The details of the fluorescence technique differ 

 considerably with different users. In some instances the sample of soil is 

 exposed to ultraviolet light and the fluorescence determined either by ob- 

 servation with the eye, with a photo-cell, or in the extreme cases by long 

 exposure to a photographic plate. Increasing fluorescence is related to 

 increasing concentration of the sought-after constituent in the sample. 

 Another variation of this technique is concerned with the ultraviolet ex- 

 amination of the solvent extract from the samples. In still other instances, 

 an effort is made to enhance or depress the fluorescence of certain con- 



