° 100 ° 



thermal conductivity^ that are applicable to all kinds of volatile sub-- 

 stances. The principal interactions in systems employed with gas chro- 

 matography are shown by Figure IVjSo 



Variation of Solvent 



Liquids employed as solvents for solution chromatography range 

 from the nonpolar hydrocarbons to various polar substances. They in- 

 clude inorganic liquids, various aqueous solutions^ and even fused 

 salts „ Some of the various kinds of solvents, arranged in approximate 

 order of increasing polarity^s5^ are reported in Table lYjIVo Polarity 

 is however, a relative property i hence the sequence of the solvents in 

 the series varies with the mixture and the sorbent as may be inferred 

 from Table IV, III, 



With the development of gas chromatography, the solvents for 

 chromatographic separations were extended to include gases as well as 

 liquids. These aeriform fluids most useful in gas chromatography in- 

 clude nitrogen, hydrogen, and the rare gasse3--helium and argon-'s » 



Variation of Sorbent 



Along with the variation of mixtures and solvents^, there has 

 been a corresponding variation of sorbents", (See Table IV^IV,) Many 

 different kinds of powdered solids have been employed as the sorptive 

 agents. Some of these have been activated in various ways. Not only 

 sorption at gas'-solid and liquid-solid interfaces have been utilized 

 but also sorption at liquid-liquid interfaces^* 9 , 



Natural and synthetic zeolites were employed for some of the first 

 separations of various inorganic ions. And with the development of 

 cation and anion exchange resins, ion exchange has become an importajit 



