The use of salting out for the extraction of hydrophilic biologically active substances from the aqueous solutions for their further chromatographic determination

Using gas chromatography methods with the quadrupole mass spectrometric detector and flame ionization detector, as well as high-performance liquid chromatography with the diode-matrix detector, the distribution constants of codeine, phenobarbital, modafinil, adrafinil, pseudoephedrine for hexane-water, hexane-water systems of inorganic salt solutions (sodium chloride, dipotassium phosphate, potassium carbonate and ammonium sulfate), chloroform-water, chloroform-aqueous solutions of inorganic salts (sodium chloride, dipotassium phosphate, potassium carbonate and ammonium sulfate) were determined. The principal possibility of quantitative extraction of all the investigated substances from the aqueous matrices by salting out was shown. The patterns of salting out of the studied substances from the aqueous solutions were established. In particular, the salting out ability of salts increased sharply with the increasing charge of the salt anion. A significant effect on the salting out effect was exerted by the nature of the extracted compound. It has been shown that the following compounds could be quantitatively recovered in the hexane: codeine, modafinil, and afrafinil. Due to their high hydrophilicity, phenobarbital and pseudoephedrine were not quantitatively extracted with the hexane even at the salt concentration close to the saturation. For the extraction of phenobarbital and pseudoephedrine, the more active but less selective chloroform was used. Based on the obtained values of the distribution constants of the substances and their acid-base properties, the extraction methods for the sample preparation of the various objects were proposed with determining the studied substances in them using salting out. The methodology for the determination of pseudoephedrine and codeine in drugs was tested. It was based on the preliminary removal of the matrix components by chloroform from the aqueous acidic solution and subsequent alkalization, salting out, and extraction with chloroform. The characteristics of the methods were as follows: detection limit for pseudoephedrine - 0.09 g/dm 3 and for codeine - 0.07 g/dm 3 ; standard deviation did not exceed 4% for both substances, range of determined concentrations for all-ephedrine  was 0.09 – 3.00 g/dm 3 , and for codeine - 0.07 - 4.00 g/dm 3 respectively. Key words : salting out, chromatography, increment of methylene group, solution structure, Sechenov constant (Russian)  DOI: http://dx.doi.org/10.15826/analitika.2019.23.4.004 S.M. Leshchev 1 , O.N. Mikhniuk 2 , K.D. Kryzhny 2 , M.F. Zayats 1 1 Belarusian State University, ul. Leningradskaya, 14, Minsk, 220050,  Republic of Belarus 2 State institute of advanced training and retraining of customs authorities of the Republic of Belarus, ul. Mogilevskaya, 45/4, Minsk, 220007, Republic of Belarus