A new simple spectrophotometric method was developed for the simultaneous determination of drugs with interfering spectra in binary mixtures without previous separation. The new method is based on a simple modification for the ratio subtraction method. This modification enabled wider range of application. The proposed ratio difference method was applied for the determination of brimonidine and timolol in laboratory prepared mixtures with mean percentage recoveries 100.40±2.29 and 101.23±1.30 respectively, and in their pharmaceutical formulation with mean percentage recoveries 101.08±0.44 and 100.66±0.52 respectively. The suggested ratio difference method was validated according to USP guidelines and can be applied for routine quality control testing.

An innovative spectrophotometric method was developed for simultaneous determination of compounds with interfering spectra in binary mixtures without previous separation, showing significant advantages over the conventional methods regarding minimal data manipulation and applicability. The proposed method was applied for the determination of retinoic acid and hydroquinone in laboratory-prepared mixtures with mean percentage recoveries 100.13 ± 0.31 and 99.99 ± 0.04, respectively, and in their pharmaceutical formulation with mean percentage recoveries 100.13 ± 0.86 and 100.07 ± 0.58, respectively. The method was validated according to USP guidelines and can be applied for routine quality control testing.

A novel, simple and highly sensitive sensor for the simultaneous determination of isopropamide iodide (ISP) and trifluoperazine hydrochloride (TFP) was developed. A carbon paste electrode was modified with m-cresol purple (MCP) and carboxylated multiwalled carbon nanotubes (MWCNTs). Different techniques were used to characterize the nanostructure and performance of the sensor. Under the optimized experimental conditions, ISP and TFP showed linear responses over the ranges of 5.50 × 10⁻⁶–1.70 × 10⁻⁴ and 5.00 × 10⁻⁷–1.34 × 10⁻³ mol L⁻¹, respectively. The detection limits for ISP and TFP were found to be 9.91 × 10⁻⁷ mol L⁻¹ and 1.10 × 10⁻⁷ mol L⁻¹, respectively. Satisfactory recoveries of analytes from these samples were demonstrated, indicating that the suggested sensor is highly suitable for clinical analysis, quality control and routine determination of ISP and TFP in pharmaceutical formulations and human fluids.

Two chromatographic methods were validated for the determination of the widely prescribed analgesic and antipyretic drug combination of paracetamol (PC) (recently integrated into the supportive treatment of COVID-19), propyphenazone (PZ) and caffeine (CF) in the presence of two PC impurities, namely 4-aminophenol and 4-nitrophenol. A “dual-mode” gradient high-performance liquid chromatography method was developed, where the separation was achieved via “dual-mode” gradient by changing both the ternary mobile phase composition (acetonitrile: methanol: water) and the flow rate. This enables a good resolution within a relatively shorter analysis time. The analysis was realized using Zorbax Eclipse XDB column C18, 5 μm (250 × 4.6 mm) and the UV detector was set at 220 nm. The other method is a thin-layer chromatography densitometry method, where the separation was achieved using a mobile phase composed of chloroform: toluene: ethyl acetate: methanol: acetic acid (6: 6: 1: 2: 0.1, by volume). Densitometric detection was performed at 220 nm on silica gel 60 F254 plates. The developed methods were fully validated as per the ICH guidelines and proved to be accurate, robust, specific and suitable for application as purity indicating methods for routine analysis of PC in pure form or in pharmaceuticals with PZ and CF in quality control laboratories.

A new voltammetric method was developed for simultaneous determination of lidocaine hydrochloride (LH) and its toxic metabolite (2, 6-dimethylaniline (DMA)) based on their oxidation and adsorption on C18 silica modified carbon paste electrode (SMCPE). Several experimental factors including the effect of pH, scan rate, percentage of silica, accumulation potential and time were studied and the optimum conditions were found to be: pH 11 ± 0.10 using Britton Robinson buffer, 100 mV/s scanning rate, −0.6V accumulation potential, 150 s accumulation time and 5% C18 silica. Under the optimized conditions, applying square wave voltammetry (SWV) displayed two signals for DMA and LH at 0.56 and 0.87 V, respectively. The method revealed satisfactory results in terms of linearity (7.94 × 10⁻⁶ to 1.07 × 10⁻⁴ M) for LH and (1.20 × 10⁻⁶ to 1.07 × 10⁻⁵ M) for DMA, accuracy, and precision. The limit of detection was found to be 2.19 × 10⁻⁶ and 2.15 × 10⁻⁷ M for LH and DMA in pure forms, respectively. The developed SWV method was further applied for the assay of LH in its pharmaceutical dosage form and DMA in spiked milk samples.