Quality control in pharmaceutical analysis by diode-array thin-layer chromatography: quantification of trimethoprim and sulfamethoxazole as a case study

The combination of trimethoprim and sulfamethoxazole as a fixed-dose combination in the ratio 1:5 is known as cotrimoxazole. It is used as antibiotic to treat a variety of bacterial infections. Cotrimoxazole is part of the World Health Organization’s list of essential medicines. Cotrimoxazole is an example of a drug that was partially unavailable in Germany during the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic and the Ukraine war. The dependency on foreign sources of medicines is well known in Europe and resulted in the Pharmaceutical Strategy for Europe 2020, a strategy concept “will support the competitiveness and innovative capacity of the EU’s pharmaceutical industry”. High-performance thin-layer chromatography (HPTLC) is a cost-effective method for quantifying pharmaceutically active compounds. Diode-array detection (DAD) in conjunction with HPTLC can simultaneously detect ultraviolet‒visible (UV‒VIS) and fluorescence spectra directly from the plate. Visualization as a contour plot helps to identify the optimal wavelengths for compound quantification and reduce uncertainty in the determination. The quantification of trimethoprim and sulfamethoxazole is presented in a case study that highlights the key aspects for HPTLC quantification of pharmaceutical fixed-dose combinations with minimal uncertainty. HPTLC‒DAD allows quantification of trimethoprim and sulfamethoxazole with a required relative standard deviation of less than 2.5%.