Evaluation of a low cell constant conductance detector for detection of charged species in high-performance liquid chromatography

Abstract

The present study evaluates high performance liquid chromatography (HPLC) detection based on a commercial conductance detector with a low cell constant of 0.005 cm⁻¹ and a volume of 2 μL and a photodiode array UV-vis detector typically used in HPLC. When detecting a static NaCl solution, the conductance detector yields a limit of detection (LOD, 3 × noise) for NaCl of 39 parts per trillion. When flowing methanol through both conductance and UV-vis detectors and injecting benzoic acid/methanol, the signal-to-noise (S/N) ratio of the conductance detector is 8-fold higher than that of the UV-vis at its optimal wavelength. Using an HPLC with a C-18 column, flowing a 75 : 25 water : methanol solution, and using an acetylsalicylic acid (ASA)/water solution, the conductance detector yielded an ∼18-fold higher S/N ratio. It was found that HPLC system noise reduces the S/N ratio of the conductance detectors. The conductance detector detected non-chromophoric species generated by atmospheric CO₂ as well as by decomposition of ASA. Conductance chromatograms yielded ASA peak heights and areas that varied linearly with the ASA concentration from 0.5 ppm to 10 ppm with linear correlation coefficients exceeding 0.999. In view of the sensitivity of conductance detection, its potential application as a sensitive tool for cleaning assessments for pharmaceutical equipment was confirmed by dispersing 50 μg of ASA on a 2′′ × 2′′ stainless steel sheet, swabbing the surface, dissolving the collected material in water and injecting the solution into the HPLC. The results in this study demonstrate that low cell constant conductance detection can be remarkably sensitive to ionized/charged species and thereby has potential to serve as an analytical tool for this important class of molecules.