An integrated LC-ESI chip with electrochemical-based gradient generation

This work reports the first on-chip generation of gradient elution using electrochemical pumping, for application in Liquid Chromatography-Electrospray Ionization (LC-ESI). The system consists of Parylene, SU-8 and PDMS parts to create two 3 μL on-chip solvent reservoirs, two electrochemical pumps and an ESI nozzle. Pumping is controlled galvanostatically (i.e., through electrolysis current). At low back pressures, pumping rate can reach 200 nL/min using just 40 μA (power consumption <100 μW). At 80 psi, 200 μA (power consumption <1 mW) is sufficient to deliver fluid at 75 nL/min. Higher electrical currents have been used to increase the flow rate or to achieve pumping at higher back pressures. For example, pumping at 200 psi back pressure has been demonstrated using a current of 800 μA. Electrically controlled on-chip gradient generation is achieved without any external fluidic connections to the chip. Continuous generation of gradients for 30 minutes at a total flow rate of ∼100 nL/min is also realized and confirmed with mass spectroscopy (MS) measurements. The developed on-chip gradient generation system meets the general requirements of LC applications.