# Versatile High-Performance Liquid Chromatography and Ultraviolet Detection-Based Method for the Determination of Thioproline in Pharmaceutical and Cosmetic Products

## TL;DR

This paper introduces a new HPLC-UV method to accurately measure thioproline in pharmaceutical and cosmetic products.

## Contribution

The first HPLC-UV method for quantifying thioproline in cosmetics and pharmaceuticals is developed and validated.

## Key findings

- The method achieved linearity in the range of 0.5–125 μmol/L with a quantification limit of 0.5 μmol/L.
- The method demonstrated high accuracy (93.22%–104.31%) and acceptable precision for intra- and inter-assay measurements.
- The method was successfully applied to Polish-market pharmaceutical and cosmetic products.

## Abstract

The article presents the first method based on high-performance liquid chromatography and ultraviolet detection (HPLC-UV) for the determination of timonacic (thioproline, 1,3-thiazolidine-4-carboxylic acid, tPro) in pharmaceutical tablets and face care products (creams, sera, foundations, suncreams). Sample preparation primarily involves solid-liquid extraction (SLE) of tPro with 0.2 mol/L phosphate buffer pH 6, derivatization with 0.25 mol/L 2-chloro-1-methylquinolinium tetrafluoroborate (CMQT), followed by polytetrafluoroethylene (PTFE) membrane filtration. The chromatographic separation of the stable UV-absorbing 2-S-quinolinium derivative is achieved within 14 min at 25 °C on a Zorbax SB-C18 (150 × 4.6 mm, 5 µm) column using gradient elution. The eluent consists of 0.1 mol/L trichloroacetic acid (TCA), pH 1.7, in a mixture with acetonitrile (ACN) delivered at a flow rate of 1 mL/min. The analyte is quantified by monitoring at 348 nm. The assay linearity was observed within 0.5–125 μmol/L. The limit of quantification (LOQ) was found to be 0.5 μmol/L. The accuracy ranged from 93.22% to 104.31% and 97.38% to 103.48%, while precision varied from 0.30% to 11.23% and 1.13% to 9.64% for intra- and inter-assay measurements, respectively. The method was successfully applied to commercially available on the Polish market pharmaceutical and cosmetic products.