Anal. Chem. 1982, 5 4 , 1589-1594
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Determination of Alternating Current Polarographic Behavior of Some Steroids in Aprotic Organic Solvents Jerome C. Schaar' and Donald E. Smith" Bpartment of Chemistry, Northwestern lJnlversi& Evanston, Illinois 6020 1
The work presented represents a continuation of our efforts to Illustrate the superiority of aprotic organic solvent-electrolyte systems for the rinalysis of many organic drugs. Measurement of the peak In-phase component magnitude of the ac polarographic or faradaic admittance (FAM) response provides the assay observaible. Investigations of five steroids show that three give esseilrtlally Ideal responses, characterized by a facile one-electron heterogeneous charge transfer step and a stable radical anion product In aprotic solvent systems. Two show clear evidence of a second order folbwlng chemkal reaction. Composite and single tablet assays are reported for two of the steroids.
We present here ac polarographic and faradaic admittance data for the steroids testosterone, methyltestosterone, progesterone, prednisolone, arid hydrocortisone. The structures of these compounds may be found in the literature (1). CAS Registry Nos. 58-22-0,58-18-4,57-83-0,50-23-7, and 50-24-8, respectively, should be consulted for proper chemical nomenclature. The fundamental harmonic ac response of these compounds in both aprotbc and protic solvent systems is illustrated. Peak height sensitivity comparisons are made by using the solvent systems acetonitrile, dimethylformamide, aqueous base, and aqueous acid. Tetraalkylammonium salts (0.010 M)provided the electrolyte, except for the aqueous acid. Composite and single tablet assay results are given for methyltestosterone and prednisolone, using both analog (2) and digital FFT-FAM (3)measurement systems. Comparisons are given of these results, as well as those obtained using United States Pharmacopeia (USF') methods (4).FFT-FAM results from drug standards and tablet extracts also are compared. Prednisolone and hydrocortisone behave nonideally, yielding nonlinear calibration curves in dry acetonitrile and dimethylformamide. Reasons for this are discussed and a chemical strategy for obtaining a linear calibration curve is presented for prednisolone. The data and accompanying discussion provide support for previously stated (5-7) advantages of aprotic organic solvent-electrolyte systems in the analysis of organic pharmaceuticals.
EXPERIMENTAL SECTION Reagents. Prednisolone, methyltestosterone, progesterone, testosterone, and hydrocortisonestandards were purchased from ICN Pharmaceuticals, Inc., Plainview, NY. Prednisolonetablets, USP, were obtained from Phillips Roxane Laboratories, Inc., Columbus, O H 5 mg tablets, lot no. 770659. Methyltestosterone tableta, USP, were obtained from Eli Lilly and Co., Indianapolis, IN: 25 mg tablets, lot no. 1WuIA; 10 mg tablets, lot no. 1GM92A. Alumina F-20was purchased from Alumina Company of America, Bauxite, AZ, and was activated at 350 "C for 12 h and stored in a desiccator prior to use. Spectroscopicgrade acetonitrile (ACN) and dimethylformamide (DIVXF) were purchased from Burdick and Jackson Laboratories, Inc., Muskegon, MI. Six hundred 'Present address: Abbott Laboratories,Chemical Division, Analytical Services and Methods Ilevelopment, 1400 Sheridan Road, North Chicago, IL 60064. 0003-2700/82/0354-1589$01.25/0
milliliters of these solventswas passed over a 2 cm X 30 cm column of activated alumina to remove water. For ACN the first 100 mL eluted from the column was discarded. Methanol, ethanol, hydrochloric acid, and perchloric acid were AR grade and were purchased from Mallinckrodt,Inc., St. Louis, MO, and were used as received. Tetrabutylammonium hydroxide (TBAOH) was purchased from Eastman Kodak Co., Rochester, NY, and used as received. Tetrabutylammonium iodide (TBAI), tetraethylammonium perfluoroborate (TEAPFB), tetrabutylammonium were perchlorate (TEMP),and tetraethylammoniumiodide (TEAI) purchased from SouthwesternAnalytical Chemicals,Inc., Austin, TX, and were used without purification. Water was purified with a Millipore Corp. "Milli-Q" water purification system. Prepurified argon Matheson Gas Products, Joliet, IL, was used as received to degas the polarographic solutions. It was saturated with electrolyte vapor or solvent vapor using a standard gas bubbler. (Saturating the argon with a 0.010 M electrolyte in the solvent was compared to saturating with only the solvent. No temperature or condensation problems were observed and solvent only was used to saturate the argon for the 0.010 M electrolyte solutions.) Pardilm "M" was purchased from American Can Co., Greenwich, CT. Minimal purification of reagents and low supportingelectrolyte concentrations characterize this work. This is done to minimize cost and complexity of the assay. A more detailed rationale has been given (7). Apparatus. An analog phase-sensitivefundamental-harmonic ac polarograph of design described by Smith ( 2 , 8 , 9 )was used with a three-electrode potentiostat equipped with positive feedback iR compensation (8,10,II). The computerized instrument used was equipped with the synchronous data generation and sampling (SYDAGES)system, which has been described elsewhere (3,12). The electrode system employed was a dropping mercury working electrode (DME), a platinum auxiliary electrode, and a homemade reference electrode. An Ag/AgI, saturated TEAI in ACN reference electrode with a Vycor frit solution junction was used for ACN solutions; an Ag/AgI, 0.10 M TEAI in DMF reference electrode was used for DMF solutions; an Ag/AgCl, saturated KCl reference electrode was used for the basic aqueous solutions; and a SCE reference electrode was used for aqueous acid solutions. Recommended Procedure for 5-mg Prednisolone Tablets. Supporting Electrolyte Solution (SES): 2.1 g of TEAPFB plus 30 mL of water in sufficient acetonitrile to make 1L. Stock Standard Preparation: 125mg of prednisolone/100 mL of SES. Standard Preparations: Dilute 3 , 4 , 5 , and 6 mL of the stock standard preparation to 25 mL with SES. These solutions correspond to 60, 80, 100, and 120% label claim, respectively. Sample Preparation: Weigh each tablet accurately and transfer to an 8-dram vial, grind to a fine powder, add 20 mL of SES, cap, and place in an ultrasonic bath for 3 min. Transfer the solution to a centrifuge tube, seal and centrifuge for about 10 min. Procedure: Transfer samples and standards to electrochemical cells, degas, and run the in-phase ac polarogram (30 mV peakto-peak, 100 Hz),scanning the dc potentials -1.0 V to -1.6 V vs. Ag/AgI, saturated TEAI in ACN. Recommended Procedure for 25-mg Methyltestosterone Tablets. Supporting Electrolyte Solution ( S E S ) : 2.1 g of TEAPFB/l L of acetonitrile. Stock Standard Preparation: 125mg of methyltestosterone/ 100 mL of SES. Standard Preparations: Dilute 10, 15,20, and 25 mL of the stock standard preparation to 25 mL with SES. These solutions 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54,NO. 9,AUGUST 1982 1
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