Closed-loop peristaltic micro-pump for flow-through sample injection in capillary electrophoresis

Capillary electrophoresis (CE) is increasingly used in distributed field environmental monitoring, teaching, and citizen science. Consequently, the development of low-cost CE instruments using inexpensive components and do-it-yourself approaches has gained significant attention. Peristaltic micro-pumps (PMPs), with dimensions as small as 15 × 17 × 34 mm³, offer advantages such as low cost, compact size, and energy efficiency, making them highly suitable for miniaturized CE instruments with automated flow-through injection. This study presents the development and application of a closed-loop PMP integrated into a CE system to enhance flow-through injection precision and analytical performance. The closed-loop control design incorporates a Hall sensor to monitor pump rotor positions, ensuring stable operation. The modified PMP achieved a pressure relative standard deviation (RSD) of 0.7 % (n = 10), representing a threefold improvement over time-based and back electromotive force feedback controls. The system was applied to detect common cations and anions in drinking water. Experimental validation confirmed the reliability and precision of the CE system, with injection RSDs below 4.0 % (n = 42 for anions and 58 for cations). The calibration ranges were 0-500 μM with R² values ≥ 0.994. Detection limits ranged from 0.9 to 3.6 μM. The RSDs (n = 10) of peak heights for ions in tap water were between 1.1 % and 7.7 %. Additionally, the CE system was employed to analyze total nitrogen and total phosphorus in water following persulfate oxidation. The total cost of the system was estimated to be approximately 6000 USD, including a ∼4000 USD TraceDec contactless conductivity detector. These findings demonstrate that the modified CE system with a closed-loop PMP significantly enhances injection precision, analytical performance, and versatility for environmental ion and nutrient detection. This system provides a reliable and cost-effective tool for on-site water quality monitoring.