Organic salts, liquid at room temperature, as mobile phases in liquid chromatography

Abstract

The synthesis of six alkylammonium salts of the general formula R n—a NH + a X −, where R = ethyl, propyl, butyl or sec.−butyl, X − = nitrate or thiocyanate, n < 4 and a = 0, 1,2 or 3 is described. Four of these salts, which are liquid at room temperature, were previously unknown. Structural characterization was performed by NMR ( 1H, 13C, 15N), IR and fast-atom bombardment mass spectrometry. The above nitrate and thiocyanate salts and three other known liquid salts, triethylhexylammonium chloride, were characterized by standard physical measurement techniques to determine their suitability as chromatographic solvents. The physical properties measured include melting point, refractive index, UV spectra, miscibility with common organic solvents, aqueous solution pH, density and viscosity. It was concluded that the alkylammonium nitrate the thiocyanate salts could be used as mobile phases in liquid chromatography when mixed with a second solvent of low viscosity. Suitable equations are given to predict the viscosity of mixtures of alkylammonium nitrate and thiocyanate salts in all volume proportions with water, methanol, acetonitrile, tetrahydrofuran and dichloromethane. The liquid organic salts form immiscible solvent pairs with non-polar organic solvents and should also be suitable for use in liquid—liquid chromatography and liquid—liquid extraction.