Ultra‐high performance separation of basic compounds on reversed‐phase columns packed with fully/superficially porous silica and hybrid particles by using ultraviolet transparent hydrophobic cationic additives

The use of the tetrabutylammonium additive was investigated in the ultra‐high performance reversed‐phase liquid chromatographic elution of basic molecules of pharmaceutical interest. When added to the mobile phase at low pH, the hydrophobic tetrabutylammonium cation interacts with the octadecyl chains and with the residual silanols, thus imparting a positive charge to the stationary phase, modulating retention and improving peak shape of protonated basic solutes. Two sources of additive were tested: a mixture of tetrabutylammonium hydroxide/trifluoroacetic acid and tetrabutylammonium hydrogen sulfate. Retention and peak shape of 11 basic pharmaceutical compounds were evaluated on commercially available ultra‐fast columns packed with octadecyl stationary phases (Ascentis Express C18 2.0 µm, Acquity BEH C18 1.7 µm, Titan C18 1.9 µm). All columns benefit from the use of additive, especially tetrabutylammonium hydrogen sulfate, providing very symmetric peaks with reasonable retention times. Focusing on the probe compounds amitriptyline and sertraline, efficiency and asymmetry values were investigated at increasing retention factor. The trend is very different to that obtained in reversed‐phase conditions and the effect lies in the complex molecular interaction mechanisms based on hydrophobic and ion exchange interactions as well as electrostatic repulsion.     

UHPLC Determination of Enantiomeric Purity of Sertraline in the Presence of its Production Impurities

A fast and low reagents consuming ultra-high performance liquid chromatography method was developed for the determination of the enantiomeric purity of sertraline in presence of its stereoisomers and four other process-related chiral impurities. The optimum chromatographic conditions were achieved using a commercially available C8 column with sub-2-μm particle size, and a mobile phase of 10 mM tetrabutylammonium hydrogen sulfate buffer pH = 3-acetonitrile (87:13, v/v) and 12 mM 2-hydroxypropyl-β-cyclodextrin as chiral additive, with ultraviolet detection at 220 nm. The method was validated in terms of ruggedness, specificity, linearity, accuracy, precision, and limits of detection and quantification. Different variables affecting the enantioresolution and retention time were optimized, such as pH, buffer concentration and type, chiral additive concentration and type, flow rate, stationary phase, mobile phase, and column temperature. Total analysis time was reduced by 50 % compared with conventional HPLC; also 94 % of acetonitrile and 90 % of 2-hydroxypropyl-β-cyclodextrin were saved.     

Liquid Chromatographic Analysis of Cephalexin in Human Plasma by Fluorescence Detection of the 9-Fluorenylmethyl Chloroformate Derivative

Abstract

A simple and sensitive precolumn derivatization method for the determination of cephalexin in human plasma has been developed. Cephalexin was derived with 9-fluorenylmethyl chloroformate (FMOC-Cl) in borate buffer (5 mM, pH 8.5) for 15 min at 25°C. Optimal conditions for the derivatization were described. The derivative was chromatographed on an XDB-C₁₈ column with water–acetonitrile (10:90, v/v) as mobile phase at a flow rate of 1.0 mL/min. The fluorescence excitation and emission wavelengths were 268 nm and 314 nm, respectively. The standard curve in spiked plasma was linear over the range of 0.0234–58.5 µg/mL; the detection limit (signal-to-noise ratio = 3; injection volume, 10 µL) was about 0.014 µg/mL. The performance of analysis was studied, and the validated method showed excellent performance in terms of selectivity, sensitivity, precision, and accuracy.     

Development and Validation of Stability-Indicating HPLC Methods for Quantitative Determination of Pravastatin,Fluvastatin, Atorvastatin,and Rosuvastatin in Pharmaceuticals

Abstract

High-performance liquid-chromatographic (HPLC) methods were validated for determination of pravastatin sodium (PS), fluvastatin sodium (FVS), atorvastatin calcium (ATC), and rosuvastatin calcium (RC) in pharmaceuticals. Two stability-indicating HPLC methods were developed with a small change (10%) in the composition of the organic modifier in the mobile phase. The HPLC method for each statin was validated using isocratic elution. An RP-18 column was used with mobile phases consisting of methanol–water (60:40, v/v, for PS and RC and 70:30, v/v, for FVS and ATC). The pH of each mobile phase was adjusted to 3.0 with orthophosphoric acid, and the flow rate was 1.0 mL/min. Calibration plots showed correlation coefficients (r) > 0.999, which were calculated by the least square method. The detection limit (DL) and quantitation limit (QL) were 1.22 and 3.08 µg/mL for PS, 2.02 and 6.12 µg/mL for FVS, 0.44 and 1.34 µg/mL for ATC, and 1.55 and 4.70 µg/mL for RC. Intraday and interday relative standard deviations (RSDs) were <2.0%. The methods were applied successfully for quantitative determination of statins in pharmaceuticals.     

A simple isocratic LC method for quantification of trace-level inorganic degradation impurities (ferricyanide,ferrocyanide, nitrite,and nitrate) in sodium nitroprusside injection and robustness by quality using design approach

This study developed and validated a trace-level quantification inorganic impurities method using reversed-phase HPLC and performed the robustness check using quality-by-design approach by varying the multiple factors simultaneously. This method is economical and simple and exhibits its stability-indicating nature [for the determination of ferrocyanide ([Fe(CN)₆]^4–), ferricyanide ([Fe(CN)₆]³⁻), nitrate (NO₃^–), and nitrite (NO₂^–)] in sodium nitroprusside (SNP) drug substance and liquid dosage form. Chromatographic separation was achieved using a USP L43 column (ACE PFP, 150 × 4.6 mm, 3 μm) with a simple isocratic elution. The buffer consists of potassium dihydrogen phosphate (50 mM), tetrabutylammonium hydrogen sulfate (9 mM), and tetrabutylammonium hydroxide (25 mM). The buffer pH was adjusted to 7.2 with tetrabutylammonium hydroxide. The mobile phase was mixed with the buffer and acetonitrile (68:32 v/v). The flow rate was 0.8 mL/min, column temperature was maintained at 30°C, and injection volume was 5.0 μL. The SNP impurities were monitored at 225 nm using a UV detector. Further, the method was validated per the International Council for Harmonisation (ICH) guidelines, and forced degradation studies were carried out under different stress conditions. The detector responses were plotted against concentrations, and correlation was linear (r > 0.999) over the range of 0.8–7.5 μg/mL for ferricyanide; 1.0–37.5 μg/mL for SNP; and 0.2–7.5 μg/mL for ferrocyanide, nitrite, and nitrate. The method repeatability was established for all the impurities with relative standard deviation (%), and the results were found to be less than 2.0.     

Multivariate optimization of a capillary zone electrophoresis assay method for simultaneous quantification of metformin and vildagliptin from a formulation

A rapid, accurate, precise, and optimized capillary zone electrophoresis assay was established and validated for the simultaneous quantification of metformin and vildagliptin in tablets. The electrophoretic separation was achieved on an untreated bonded silica capillary with a background electrolyte comprising 25 mM of borate buffer at pH 7.5 at 207 nm. The concentration of the buffer and the pH of BGE were optimized using the multivariate optimization method for determining the retention time and peak area. Furthermore, the sample injection time, capillary oven temperature, and applied voltage were optimized. The capillary zone electrophoresis technique was validated for all required parameters as per the International Conference on Harmonization recommendations. The linearity ranged in the concentrations of 5–500 µg/mL and 5–100 µg/mL with the limit of detections of 0.22 µg/mL and 0.40 µg/mL for metformin and vildagliptin, respectively. In addition, the percent relative standard error for repeatability and inter-day precision was within the acceptable range. The mean recoveries determined by the capillary zone electrophoresis method were 99.2% and 100.4% for metformin and vildagliptin, respectively. Finally, the capillary zone electrophoresis process was effectively used for the assays of metformin and vildagliptin in their solid dosage form, and statistical outcomes were in agreement with the outcomes of the previously validated RP-HPLC method.     

RP-HPLC-UV validation method for levofloxacin hemihydrate estimation in the nano polymeric ocular preparation

The specific and accurate reversed-phase HPLC-UV method has been validated to determine levofloxacin hemihydrate (LEVH) level. The separation was conducted at C 18 analytical column by administering mobile phase acetonitrile, methanol, and phosphate buffer (pH 3) with the ratio of 17:3:80. The flow rate of the mobile phase was 1 mL/min with a UV detector at 295 nm wavelength. Analytical methods validation evaluated includes specificity, linearity, accuracy, precision, LOD, LOQ, and robustness. The implementation of the analytical method was employed to determine LEVH level in ocular polymeric nanoparticles preparations. The test was specific for LEVH with the retention time of 7.66 min. Linearity was obtained from the concentration range of 4.8 µg/mL to 29.04 µg/mL. All method validation criteria are within the acceptable range. The developed method can be applied for LEVH polymeric nano-formulation analysis.     

Determination of aromatic amines in environmental water samples using solid-phase extraction modified with sodium dodecyl sulphate and micellar liquid chromatography

Extraction and determination of seven aromatic amines in environmental water samples were performed with solid-phase extraction (SPE) and micellar liquid chromatography (MLC) using experimental design. Extraction of aromatic amines was carried out with a C₁₈ cartridge modified with sodium dodecyl sulphate (SDS). The washing solution and elution solvent for extraction of aromatic amines were aqueous solution containing 5% (v/v) acetonitrile and 5% (v/v) acetone and 3 mL methanol, respectively. The chemometrics approach was applied for the separation optimisation of these compounds using MLC. Different mobile phase compositions were used for modelling based on retention times to obtain the best separation using central composite design. The optimum mobile phase composition for separation and determination of analytes in water samples was 69 mM SDS, 9% v/v 1-propanol and pH = 6.4. Recoveries were between 84.8–93.5% with relative standard deviation (RSD) less than 5.8% (n = 5). Limits of detection and linear range were 1–4.5 and 3.1–125.0 µg/L, respectively. The proposed method was applied to determine the aromatic amines in real samples (river and well waters). Amount of 4-nitroaniline and 3-nitroaniline in river water sample were 2.15 and 1.91 µg/L, respectively.     

A Simple,Rapid, and Validated LC Method for the Estimation of Nimesulide in Human Serum and Its Application in Bioavailability Studies

Abstract

A new, simple, and rapid, sensitive reversed phase liquid chromatographic method was developed for the estimation of nimesulide in blood serum using a 60:40 mixture of acetonitrile and orthophosphoric acid (pH 3.0) as the mobile phase at 230 nm. The mobile phase and other chromatographic conditions were optimized to minimize interference from the serum matrix and at the same time provide sufficient sensitivity for the method to be adopted for in vivo studies of oral formulations of nimesulide. Acetonitrile was used to precipitate proteins from serum during sample preparation. Detector response was found to be linear in the region of 100–1000 ng/ml. The detection and quantitation limit, as per the error propagation theory, was found to be 50 ng/ml and 100 ng/ml, respectively. The linear equation obtained by the least square regression method was Area = 37.29 × Conc.(ng/ml)?1699.89 with a retention time of 3.97 ± 0.04 min. The results of the analysis were treated statistically, as per ICH guidelines for validation of analytical procedures, USP-2000. and by recovery studies. An internal standard was not employed in the method, as sample recoveries were in good agreement with their label claims. The results were found to be accurate, reproducible, and free from interference. The developed methods were further used for estimation of nimesulide for oral bioavailability of designed sustained release formulations of nimesulide.     

Analysis of Insulin by High Performance Liquid Chromatographic Method with Precolumn Derivatization with 4‐Chloro‐7‐Nitrobenzo‐2‐Oxa‐1,3‐Diazole

Abstract

A high performance liquid chromatographic method (HPLC) with precolumn derivatization and fluorescence detection for insulin was developed and applied for the quantification of insulin in spiked serum. To covalence couple with insulin, 4‐chloro‐7‐nitrobenzo‐2‐oxa‐1,3‐diazole (NBD‐Cl) was selected as fluorescent reagent. The optimal derivatization conditions were as follows: temperature 50; time 2 h, in the dark; 0.1 M phosphate buffer (pH 9.0). Analytical separation was carried out on a C18 column and the mobile phase including acetonitrile‐water containing 0.1% trifluoroacetic acid (TFA) (v/v∶ 30/70). The excitation/emission wavelengths were 470/540 nm. Under the conditions, the retention time and capacity factor of the adduct of insulin‐NBD were 10.03 min (flow rate 1 mL/min) and 3, respectively. The recovery of insulin in serum was 95.06% and the detection limit was 90 nM. In the investigated concentration ranges (0.46 µM～16.10 µM), R² was 0.9934, which indicated the potential for the application of NBD‐Cl derivatization to the analysis of insulin in the biological matrices, although with the shortcoming of long analytical time.     

Liquid Chromatographic Determination of Glyoxal and Methylglyoxal from Serum of Diabetic Patients using Meso‐Stilbenediamine as Derivatizing Reagent

Abstract

Meso‐stilbenediamine has been used as derivatizing reagent for liquid chromatographic (LC) determination of glyoxal (Go), methylglyoxal (MGo), and dimethylglyoxal (DMGo) at pH 3. Liquid chromatographic elution and separation was carried out from the column Kromasil 100 C‐18, 5 µm (15×0.46 mm i.d.) with methanol: water:acetonitrile (59:40:1, v/v/v) with a flow rate of 1 mL/min and ultraviolet detection at 254 nm. The linear calibration curves were obtained for Go, MGo, and DMGo within 0.97–4.86 µg/mL, 1.52–7.6 µg/mL, and 1.41–7.08 µg/mL with detection limits of 48 ng/mL, 76 ng/mL, and 70.8 ng/mL, respectively. The method was applied for the determination of Go and MGo from serum of patients suffering from diabetes and ketosis. The amounts of Go and MGo found were 0.150–0.260 µg/mL and 0.160–0.270 µg/mL with coefficient of variation (C.V.) 2.6–4.7% and 2.5–4.6%, respectively. The results obtained were compared with normal subjects with Go and MGo contents of 0.025–0.065 µg/mL and 0.030–0.070 µg/mL with C.V 1.5–4.9% and 1.6–4.8% in the serum.     

Development of an ion-pair reversed-phase HPLC method with indirect UV detection for determination of phosphates and phosphites as impurities in sodium risedronate

A method based on RP-HPLC with indirect UV detection was developed for the determination of phosphates and phosphites as impurities in sodium risedronate. RP separation of the phosphates and phosphites was achieved by adding tetrabutylammonium hydroxide as an ion-pairing agent in the mobile phase. Potassium hydrogen phthalate was added to the mobile phase as an ionic chromophore in order to obtain high background absorption of the mobile phase. Separation was performed on a C18 column using a mixture of pH 8.2 buffer (containing 0.5 mM tetrabutylammonium hydroxide and 1 mM phthalate) and acetonitrile (95 + 5, v/v) as the mobile phase, with indirect UV detection at 248 nm. The validation of the method included determination of specificity/selectivity, linearity, LOD, LOQ, accuracy, precision, and robustness. The LOD was 0.86 microg/mL for phosphates and 0.76 microg/mL for phosphites. The LOQ was 2.60 microg/mL for phosphates and 2.29 microg/mL for phosphites. The developed method is suitable for quantitative determination of phosphates and phosphites as impurities in QC of sodium risedronate.     

Influence of Organic Modifier Additives to Separate Steroids by Micellar Electrokinetic Chromatography: Determination of Solute-Micelle Association Constants at Different Acetonitrile Concentrations

In this report, the determination of testosterone, progesterone, estrone, 17-β-estradiol, and ethynilestradiol by micellar electrokinetic chromatography (MEKC) is described. Several organic modifiers were investigated using sodium dodecyl sulfate as the surfactant agent in the background electrolyte. The effect of the acetonitrile concentration on the migration time of the steroids and on the selectivity was also studied by using different background electrolytes. Under the optimized conditions that included a sodium tetraborate (pH 9.3; 25 mM) buffer with 10 mM sodium dodecyl sulfate, 20% (v/v) of acetonitrile, 27 kV running voltage, and injection with a plug of background electrolyte (7 mbar × 1 s), the analytical performance of the method was evaluated. Good linearity (correlation coefficients, R ² ≥ 0.99) and adequate precision were achieved, with limits of detection of 1.27, 2.17, 0.6, 1.13, and 1.7 µg/mL for testosterone, progesterone, estrone, 17-β-estradiol, and ethynilestradiol, respectively. To study the effect of the acetonitrile concentration on the solute-micelle interaction, the retention factor and association constants were determined. In all cases, the association constants decreased by increasing the acetonitrile concentration from 10% to 30%, suggesting that the presence of large amounts of organic modifier decreased the steroid-micelle interactions.     

Development of an Ion-Pair HPLC Method for Determination of Acebutolol in Pharmaceuticals

A simple and accurate ion-pair reversed-phase high-performance liquid chromatographic (RP-HPLC) method has been developed and validated for determination of acebutolol (ACE) in tablet dosage forms. Both ACE and ambroxol (internal standard) were well separated using a reversed phase column and a mobile phase consisting of a mixture of methanol-0.05 M acetic acid (containing 8 mM sodium 1-heptanesulfanate) (65:35 v/v) with pH adjusted to 3.2 with triethylamine. The mobile phase was pumped at 0.80 mL min^?1 flow rate and ACE was detected by diode-array detection at 240 nm. The retention times for ACE and internal standard (IS) were 4.574 and 8.236 min, respectively. A linear response (r = 0.9998) was observed in the range of 0.27–2.93 μg mL^?1 in mobile phase. The limit of detection and limit of quantification were found as 0.07 and 0.23 μg mL^?1 in the mobile phase. Validation parameters such as precision, accuracy, selectivity, reproducibility, and system suitability tests were also determined. The excipients did not interfere with the assay of ACE in tablet dosage forms. It is suggested that the proposed method can be used for routine quality control and dosage-form assay of ACE.     

Simple Determination of 1-Deoxynojirimycin in a New Dietary Supplement by Liquid Chromatography

A simple and fast chromatographic method using ultraviolet diode-array detector (UV-DAD) was developed for the high performance liquid chromatography determination of the content of 1-deoxynojirimycin (DNJ) in a new dietary supplement in the form of granules for oral solution preparation. The derivatization reaction was carried out at room temperature for 15 min at pH 7. The reaction reached completeness at a reagent to analyte molar ratio of about 60. The chromatographic separations were performed on a C18 Phenomenex Synergi Fusion stainless steel column (250 mm?×?4.6 mm; 4 µm) with detection at λ?=?254 nm. The mobile phase consisted of triethylammonium (TEA) phosphate buffer (0.05 M; pH 3) and acetonitrile under gradient conditions at a flow-rate ramping from 1 to 1.2 mL/min. The validation parameters (linearity, sensitivity, accuracy, precision, specificity and stability) were satisfactory. Intra-day precision (relative standard deviation, RSD) was ≤?2.23% for peak area and retention time without significant differences between intra- and inter-day data. Recovery studies gave good results (93.59%; n?=?15) with a RSD of 2.64%. The developed method is suitable for the quality control of DNJ in raw material and industrial products. The method can be applied in any analytical laboratory and does not require sophisticated instrumentation.     

Stability-indicating method for the determination of assay and quantification of impurities in amlodipine–atorvastatin combination dosage form by RP-HPLC

A simple stability-indicating RP-HPLC method was developed and validated for quantification of amlodipine, atorvastatin, and its impurities on Waters HPLC using Unisol C₁₈ 5?µm, 250?×?4.6?mm column in their combined tablet dosage as per ICH guidelines. The gradient (T/%B) at 0/42, 18/42, 22/75, 30/75, 32/42, and 35/42 of 40?mM 4.7 pH ammonium acetate as mobile phase A and acetonitrile as mobile phase B of flow rate 1.5?mL/min and 240?nm wavelength. Peak purity compiled for amlodipine and atorvastatin in all stressed conditions. For impurities: Precision was found in between 1.5 and 3.6%. The limit of detection and quantification for amlodipine, amlodipine impurity A, and atorvastatin was found to be 0.06 and 0.18?µg/mL, for atorvastatin Impurity A, B, C, and H was determined as 0.04 and 0.11?µg/mL, for Atorvastatin Impurity D was measured as 0.11 and 0.28?µg/mL, respectively. The linear regression achieved >0.9999 from 0.22 to 7.5?µg/mL. Recovery was observed in between 97 and 101%. For assay: Precision was determined in between 0.1 and 0.2%. The linear regression achieved >0.9999 for amlodipine and atorvastatin. Recovery ranged from 100 to 101%. The validated method was found to be accurate, precise, reliable, and robust to determine the assay as well as impurities in amlodipine–atorvastatin combination dosage formulation.     

Comparison of liquid chromatography with fluorescence detection to liquid chromatography-mass spectrometry for amino acid analysis with derivatisation by 6-aminoquinolyl-N-hydroxysuccinimidyl-carbamate: Applications for analysis of amino acids in skin

The analysis of nineteen amino acids found in collagen was optimised using 6-aminoquinolyl-N-hydroxysuccinimidyl-carbamate (AQC) as a derivatisation reagent. The analysis and detection of nineteen AQC-amino acids using fluorescence and mass spectrometry were compared at different mobile phase pH’s and column temperatures. The pH range of the mobile phase was set between 2.7 and 6.0 and column temperatures, 15–60 °C. The majority of amino acids produced a mono-derivatised product with AQC, except cystine, lysine and hydroxylysine which were di-derivatised. Hydroxylysine’s retention time was affected most by changes in the pH, whilst hydroxyproline’s retention time was more affected by column temperature. Hydroxylysine was detected as two diastereomers which were completely resolved. The relative standard deviation of the retention times of AQC-amino acids was less than 1% and the limit of detection (LOD) and limit of quantitation (LOQ) were ranged from (0.05–0.23) µM and (0.07–0.76) µM on fluorescence and (0.02–0.10) µM and (0.06–0.33) µM on mass spectrometry respectively. This method was successfully applied for the quantitation of amino acids in different animal skins.     

Analysis of streptokinase by validated liquid chromatography methods and correlation with an in vitro bioassay

Reversed‐phase and size‐exclusion liquid chromatography methods were validated for the assessment of streptokinase. The reversed‐phase method was carried out on a Jupiter C₄ column (250 mm × 4.6 mm id) maintained at 25°C. The mobile phase consisted of 50 mM sodium sulfate solution pH 7.0 and methanol (90:10, v/v), run isocratically at a flow rate of 0.8 mL/min. The size‐exclusion method was carried out on a Protein KW 802.5 column (300 mm × 8.0 mm id), at 25°C. The mobile phase consisted of 40 mM sodium acetate solution pH 7.0, run isocratically at a flow rate of 1.0 mL/min. Retention times were 19.3 min, and 14.1 min, and calibration curves were linear over the concentration range of 0.25–250 μg/mL (25.75–25 750 IU/mL) (r ² = 0.9997) and 5–80 μg/mL (515–8240 IU/mL) (r ² = 0.9996), respectively, for reversed‐phase and size exclusion, with detection at 220 and 204 nm. Chromatographic methods were employed in conjunction with the in vitro bioassay for the content/potency assessment of Streptokinase, contributing to improve the quality control and ensure the efficacy of the biotherapeutic.     

Simultaneous determination of four hormonal compounds in oral contraceptive tablet formulations by high performance liquid chromatography

A rapid, accurate, and precise HPLC method has been developed for simultaneous determination of four contraceptive hormonal compounds namely ethinylestradiol (EE), drospirenone (DR), gestodene (GS), and levonorgestrel (LV) in oral contraceptive tablet dosage form. The chromatographic separation was achieved on a C18 (150 × 4.6 mm, 5μ) column; the mobile phase consists of acetonitrile: water (50:50, v/v) pumped at a flow rate of 1.0 mL/min; and UV detection was set at 200 nm. The limit of detection was 0.0086 µg/mL for (EE), 0.0397 µg/mL for (GS), 2.80 µg/mL for (DR), and 0.229 µg/mL for (LV), whereas the limit of quantitation (LOQ) was 0.028 µg/mL for (EE), 0.132 µg/mL for (GS), 9.500 µg/mL for (DR), and 0763 µg/mL for (LV), respectively. The correlation coefficient (r) values of the four compounds ranged from 0.99995 to 0.99999. The method was validated as per ICH guidelines and USP 34 for estimation of (EE), (DR), (GS), and (LV) in commercially available tablet dosage form. The validation results were found satisfactory. The proposed method can be useful in quality control of bulk manufacturing and pharmaceutical dosage forms.     

Comparison of Ion-Pairing and Reversed Phase Liquid Chromatography in Determination of Sulfamethoxazole and Trimethoprim

Abstract

Two simple, rapid, and sensitive HPLC methods have been developed for the simultaneous determination of sulfamethoxazole and trimethoprim in their pure and dosage forms, one utilizing reversed phase HPLC and the other ion-pair HPLC. In the reversed phase HPLC method (A) the mobile phase consists of 0.05% aqueous solution of formic acid with pH adjusted to 4.5±0.2 with triethylamine : acetonitrile:tetrahydrofuran 50 : 49 : 1 (v/v), and the mobile phase pumped at flow rate of 1.0 ml min^?1. An Appolo LC18 column (5.0 µm), 250 mm length × 4.6 mm diameter, was utilized as the stationary phase. Detection was affected spectrophotometrically at 254 nm. In the ion-pair HPLC method (B) the mobile phase consisted of methanol : buffer 35 : 65 (v/v) with the buffer composed of potassium dihydrogen phosphate 0.3 M and sodium heptan sulfonic acid 5.0 mM. To 500 ml of buffer was added 2.0 ml triethylamine, and then the pH was adjusted to 5.0 with phosphoric acid, and the mobile phase was pumped at a flow rate of 1.2 ml min^?1. A Hypersil C₁₈ column (5.0 µm), 150 mm length × 4.6 mm diameter, was utilized as the stationary phase. Detection was affected spectrophotometrically at 254 nm. Linearity ranges for sulfamethoxazole and trimethoprim were 1.0–110 and 1.5–98 µg ml^?1, respectively, with method A and 0.5–100 and 1.0–125 µg ml^?1, respectively, with method (B). Minimum detection limits obtained were 0.1969 and 0.3451 µg ml^?1 for sulfamethoxazole and trimethoprim, respectively, with method A, and 0.1377 and 0.2454 µg ml^?1 with method (B). The proposed methods were further applied to the analysis of tablets containing the two drugs, and the results were satisfied.