Silica gel âSilasorb 600â with specific surface area of about 600 m*/g and average particle size of u 10 pm was used as the sorbent. Phenol and the o-, m-, ...
Talanta, Vol. 36, No. 5, pp. 513-519, 1989 Printed in Great Britain. All rights reserved
0039-9140/89$3.00+ 0.00 Copyright 0 1989 Pergamon Press plc
NORMAL-PHASE HIGH-PERFORMANCE LIQUID CHROMATOGRAPHIC DETERMINATION OF PHENOLS S. N. LANIN and Yu. S. NIKITIN Department of Chemistry, Moscow State University, 119899 Moscow, USSR (Received 27 May 1987. Revised 23 September 1987. Accepted 17 November 1988)
Summary-Normal-phase high-performance liquid chromatography has been used for separation of phenol and its monoderivatives. Multi-component mixtures of hexane (non-polar component) with butan-l-01, chloroform, butyl bromide, butyl chloride or diethyl ether (polar additives) were used as selective eluents. Silica gel “Silasorb 600” with specific surface area of about 600 m*/g and average particle size of u 10 pm was used as the sorbent. Phenol and the o-, m-, p-isomers of cresol were concentrated by extraction with n-butyl acetate from aqueous solutions. A method for determination of microamounts of phenols in aqueous solutions in the presence of 160-fold amounts of aromatic hydrocarbons has been developed.
Phenol and its derivatives are among the most toxic and widely spread pollutants in industrial effluents and natural waters. Current methods (mostly spectrophotometrk-‘) usually fail to determine phenols without a preliminary separation. Unfortunately, the preliminary separation considerably complicates the analysis and does not always ensure accurate results, especially for isomers. Existing techniques determine only the total content of phenols in effluents and natural waters, and in many cases this approach is hardly satisfactory. It is often extremely important to identify individual pollutants and to determine individual concentrations, as the permissible concentration limits for various phenols and their isomers significantly differ, sometimes by factors of tens and even hundreds.4 High-performance liquid chromatography (HPLC) is especially advantageous for separation of organic mixtures, e.g., of phenols by either the normal-phase (polar sorbent, non-polar eluent) or reversed-phase (non-polar sorbent, polar eluent) technique.r-” In the present investigation normal-phase HPLC was applied. It is characterized by higher selectivity in separating O-, I?I- and p-isomers of organic compounds’4*‘s and better reproducibility of the surface adsorption properties for silica gels than for “bonded” phases (and the former are considerably cheaper than the latter). As the content of phenol derivatives in effluents and especially in natural waters is often rather low, preconcentration by adsorption or extraction is recommended. The latter has the advantage of transferring the phenols from the aqueous to the organic phase, as required for normal-phase HPLC.
EXPERIMENTAL Apparatus
A “Tswett-306” liquid chromatograph equipped with a spectrophotometric detector (2 200-700 nm) and a piston
pump giving eluent flow-rates in the range OS-5 (kO.1) ml/min was used. Stainless-steel columns (200 x 6 mm and 300 x 6 mm) were slurry-packed, using the suspension procedure, with silica gel Silasorb 600 (“Lachema”, Czechoslovakia), (specific surface area about 600 m’/g and average particle size -10 pm). The column efficiency in terms of p-nitrophenol, was ~5000 theoretical plates. A “Milichrom” microcolumn liquid chromatograph equipped with a spectrophotometric detector (1190-360 nm) and piston pump (2500 pl capacity) for eluent flow-rates in the range 2-600 pl/min was also used. Stainless-steel columns (120 x 2 mm) were slurry-packed with Silasorb 600 (-600 m*/g specific surface area, average particle size -6 pm). The column efficiency in terms of p-cresol was 9000-10000 theoretical plates.
Reagents
Samples were introduced into the chromatograph with an MS-10 microsyringe. Two-component mixtures (from 99:l to 30 : 70 v/v) of hexane or heptane (non-polar component) with butan-l-01, chloroform, butyl bromide, butyl chloride or diethyl ether (polar additives) were used as eluents. Phenol, and its o-, m- and p-substituted methyl, chloro, bromo, iodo, nitro, amino and hydroxyl derivatives were used as test materials. Phenol and m-cresol (pure) were distilled under reduced pressure (79-W/18 mmHg and 92-93”/12 mmHg, respectively). The purity of the substances was verified chromatographically and by refractometry. Absorption spectra of the phenol solutions in the eluents and in butyl acetate were recorded with a Hitachi-124 spectrophotometer. Procedure
A stable solution of phenol (205 pg/ml) was preparedI by dissolving 0.0517 g of distilled phenol and 0.001 g of maleic acid in 96% ethanol and making up to volumes with the same solvent in a 250-ml standard flask. Phenol solutions in distilled water and in n-butyl acetate were prepared for chromatographic analysis by dissolving weighed amounts of phenol in the appropriate volume of solvent. The solutions were further diluted as required, for preparation of the calibration graph. The phenols were extracted into n-butyl acetate by a salting-out procedure in which 45 g of anhydrous sodium sulphate and 3 ml of n-butyl acetate were added to 250 ml of aqueous phenol solution and the mixture was shaken for 15-20 min, until equilibrium was attained. 573
S. N. LANZNand Yu. S. NIKE~N
574
stance (CC&) (ml) and VRsp,,the retention volume of phenol (ml). It follows from Tables 1 and 2 that the retention of phenols si~fi~ntly depends on both the nature and the position of the substituent. The retention of halogen- and methyl-substituted phenols is the weakest; it conside~bly increases with change of substituent, in the series Cl~CH,
