Nurullah Şanli

Determination of pKa values of some sulfonamides by LC and LC-PDA methods in acetonitrile-water binary mixtures

The dissociation constants of seven sulfonamide antibiotics, sulfadiazine, sulfathiazole, sulfamerazine, sulfamethazine, sulfamonomethoxine, sulfadoxine and sulfamethoxazole, have been determined in different acetonitrile-water binary mixtures (15, 30, 40, 50% v/v) by means of reversed-phase liquid chromatographic data. Also, a method based on the absorbance spectra at the maximum of chromatographic peaks previously obtained has been applied to determine the pKa values. This method can be applied to data obtained from LC-UV (photodiode array detection (PDA)) instruments and retains all the advantages of LC and spectrophotometric methods. Linear relationships were observed when the calculated pKa values of sulfonamides in different solvent mixtures were plotted against the acetonitrile molar fraction.

Determination of pKa values of some antihypertensive drugs by liquid chromatography and simultaneous assay of lercanidipine and enalapril in their binary mixtures

In this study, pK(a) values were determined using the dependence of the retention factor on the pH of the mobile phase for three ionizable substances, namely, enalapril, lercanidipine and ramipril (IS). The effect of the mobile phase composition on the ionization constant was studied by measuring the pK(a) at different methanol-water mixtures, ranging between 50 and 65% (v/v), using LC-DAD method. Two simple, accurate, precise and fully validated analytical methods for the simultaneous determination of enalapril and lercanidipine in combined dosage forms have been developed. Separation was performed on an X-Terra RP-18 column (250 mm x 4.60mm ID x 5 microm) at 40 degrees C with the mobile phase of methanol-water 55:45 (v/v) adjusted to pH 2.7 with 15 mM orthophosphoric acid. Isocratic elution was performed in less than 12 min with a flow rate of 1.2 mL min(-1). Good sensitivity for the analytes was observed with DAD detection. The LC method allowed quantitation over the 0.50-20.00 microg mL(-1) range for enalapril and lercanidipine. The second method depends on first derivative of the ratio-spectra by measurements of the amplitudes at 219.7 nm for enalapril and 233.0 nm for lercanidipine. Calibration graphs were established for 1-20 microg mL(-1) for enalapril and 1-16 microg mL(-1) lercanidipine, using first derivative of the ratio spectrophotometric method. Both methods have been extensively validated. These methods allow a number of cost and time saving benefits. The described methods can be readily utilized for analysis of pharmaceutical formulations. The methods have been applied, without any interference from excipients, for the simultaneous determination of these compounds in tablets. There was no significant difference between the performance of the proposed methods regarding the mean values and standard deviations.

Determination of protonation constants of some tetracycline antibiotics by potentiometry and lc methods in water and acetonitrile-water binary mixtures

An accurate estimation of dissociation constants of tetracycline antibiotics in acetonitrile-water binary mixtures is very important for several separation techniques such as liquid chromatography and capillary electrophoresis that use these solvent mixtures. In this study, the pKa values of five tetracyclines (tetracycline, oxytetracycline, chlortetracycline, doxycycline and metacycline), have been determined in water and acetonitrile-water binary mixtures (10%, 20% and 30% (v/v)) by potentiometry and liquid chromatography (LC). The obtained results show a similar trend for the pKa1 values of five tetracycline antibiotics as they increase with increasing concentration of organic modifier, which allows a linear relationship between pKa1 values and mole fraction of acetonitrile to be obtained. The pKa values obtained in aqueous medium have been compared with the values predicted by the SPARC on-line pKa calculator.

Analysis of amino acid neurotransmitters from rat and mouse spinal cords by liquid chromatography with fluorescence detection.

A RP-LC-FL detection method has been developed to identify and quantitate four amino acid neurotransmitters including glutamic acid, glycine, taurine and γ-aminobutyric acid in rat and mouse spinal cord tissue. 3-(4-carboxybenzoyl)-2-quinolinecarboxaldehyde (CBQCA) was employed for the derivatization of these neurotransmitters prior to RP-LC-FL analysis. Different parameters which influenced separation and derivatization were optimized. Under optimum conditions, linearity was achieved within the concentration ranges of 0.50-50.00 μM for all analytes with correlation coefficients from 0.9912 to 0.9997. The LODs ranged from 0.03 μM to 0.06 μM. The proposed method has been successfully applied to the determination of amino acid neurotransmitters in biological samples such as rat and mouse spinal cord with satisfactory recoveries.

Optimization of the experimental conditions for macrolide antibiotics in high performance liquid chromatography by using response surface methodology and determination of tylosin in milk samples

The simple, rapid and sensitive liquid chromatographic separation of five macrolides (tilmicosin, erythromycin, tylosin, roxithromycin and josamycin) widely used in food producing animals was developed. Response surface methodology was used as an optimization method of mobile phase, column temperature and pH to provide the best resolution of these analytes. The separation was performed by using an end-capped X-Terra RP-18 column (250 × 4.6 mm I.D × 5 m) with an isocratic system of 15 mM hydrochloric acid (pH 2.5)-acetonitrile as the mobile phase at a temperature of 30°C and flow-rate of 1.0 mL/min. The suitability of the method for multi-residue determination of the macrolides is demonstrated by the analysis of milk samples spiked with tylosin. Roxithromycin was used as internal standard. The recovery of tylosin was quite good as 90.8%. The limit of quantification and detection limit were 0.024 and 0.007 μg/mL, respectively. The method was successfully applied to determination of macrolides at levels below the maximum concentration legally allowed in milk samples.

Spectrophotometric Determination of Acidity Constants of Some Novel Methotrexate-Like Compounds in Water and Acetonitrile-Water Binary Mixtures

The acidity constants of four novel methotrexate like compounds (NPALA ; (N-[4-[[2-hydroxy -l-naphtyl)methylene]amino]benzoyl-ß-alanin) TPALA; (N-[4-(2-thiophene-aldiminobenzoyl)-ß-alanin), SAL-ALA; N-[4-[[2hydroxyphenyl)methylene]amino]benzoyl-ß-alanin], MeO-SAL-ALA; (N-[4[[(2-hydroxy-4-methoxyphenyl)metylene]amino]benzoyl-ß-alanin)) have been determined in water and acetonitrile-water binary mixtures (10%, and 20% (v/v)) by spectrophotometric method. The effect of the acetonitrile concentration on the ionization constant was also studied.