Valveless gated injection for microfluidic chip-based liquid chromatography system with polymer monolithic column

In this work, we developed a microfluidic chip-based liquid chromatography (LC) system with valveless gated sample injection method and monolithic columns. The valveless LC system consisted of a microchip with a separation column and a balance column, two syringe pumps and a laser induced fluorescence detector. Nanoliter-scale sample injection was achieved by simply switching the ON and OFF states of the syringe pumps under gated injection mode, without the need of any mechanical valve. The balance column facilitated the gated injection process by balancing the back pressure in the sample channel with that in the separation channel. Under the gated injection mode, the sample injection volumes could be varied in the range of 0.4–2.4 nL. We investigated the effects of the mobile-phase composition, flow rate, and the sample injection volume on the performance of the LC system. Under the optimized conditions, two fluorescently labeled amines were successfully separated in less than 220 s with a column length of 1 cm. Theoretical plates ranging from 35,700 to 29,400 per meter, corresponding to plate heights ranging from 28 to 34 μm were obtained. The advantages of the present system include simple system structure, ease of operation, convenience for varying injection volume, and high sample loading capacity.