Development and Characterization of Microscale Samplers Coupled to HPLC for Near Real-Time Reaction Monitoring

Abstract

The ability to sample a chemical reaction mixture on-line and on a continuous basis can lead to a better understanding of the chemical process, resulting in improved yield and quality of product. This is particularly important for the development of active pharmaceutical ingredients (APIs). Two primary areas of improvement needed for near real-time sampling in the pharmaceutical industry are sampling from small-scale reactions (several milliliters), such as may be used during research and development, and sampling from heterogeneous reactions that contain solid particles. For small-scale reactions, a sampling device that removes microliters of the reaction mixture is required. A “push-pull” sampler was constructed from coaxial fused silica capillaries that continuously removes sample from a reactor at low flow rates (μL/min) and mixes the sample rapidly with a quenchant to preserve the reaction conditions before analysis. Finite element analysis showed that reducing the pull capillary ID resulted in faster mixing, while flow rate had a minimal effect on mixing time. This sampler was used with samples spanning a range of viscosities (1.5 – 4.8 Pa*s), and with samples containing spherical polystyrene beads 10 – 500 μm in diameter. The sampler was demonstrated by monitoring an aqueous enzymatic reaction as well as an organic reaction. A “droplet” sampler was then developed that uses segmented flow to remove sample from a reactor on demand. This sampler can remove and run very small (0.1 – 2 μL) samples with on-demand sampling. A third device was developed specifically for heterogeneous samples that contain solid particles. This “soup pot” sampler was a custom-built borosilicate glass chamber for mixing sample with quenchant and diluent. A syringe was used to obtain sample and to mix the sample with quenchant/diluent in the chamber. A Teflon stopcock was modified to allow access to the reactor vessel while also providing a leak-free seal during mixing. The sampler was coupled to an analytical-scale HPLC and was demonstrated for monitoring dibutyl phthalate hydrolysis. These samplers represent several prototypes for further development of automated process monitoring of small reaction volumes and reactions containing particulates.