Novel volumetric method for highly repeatable injection in microchip electrophoresis

Abstract

A novel injector for microchip electrophoresis (MCE) has been designed and evaluated that achieves very high repeatability of injection volume suitable for quantitative analysis. It eliminates the injection biases in electrokinetic injection and the dependence on pressure and sample properties in hydrodynamic injection. The microfluidic injector, made of poly(dimethylsiloxane) (PDMS), operates similarly to an HPLC injection valve. It contains a channel segment (chamber) with a well-defined volume that serves as an “injection loop”. Using on-chip microvalves, the chamber can be connected to the sample source during the “loading” step, and to the CE separation channel during the “injection” step. Once the valves are opened in the second state, electrophoretic potential is applied to separate the sample. For evaluation and demonstration purposes, the microinjector was connected to a 75 µm ID capillary and UV absorbance detector. For single compounds, a relative standard deviation (RSD) of peak area as low as 1.04% (n=11) was obtained, and for compound mixtures, RSD as low as 0.40% (n=4) was observed. Using the same microchip, the performance of this new injection technique was compared to hydrodynamic injection and found to have improved repeatability and less dependence on sample viscosity. Furthermore, a non-radioactive version of the positron-emission tomography (PET) imaging probe, FLT, was successfully separated from its known 3 structurally-similar byproducts with baseline resolution, demonstrating the potential for rapid, quantitative analysis of impurities to ensure the safety of batches of short-lived radiotracers. Both the separation efficiency and injection repeatability were found to be substantially higher when using the novel volumetric injection approach compared to electrokinetic injection (performed in the same chip). This novel microinjector provides a straightforward way to improve the performance of hydrodynamic injection and enables extremely repeatable sample volume injection in MCE. It could be used in any MCE application where volume repeatability is needed, including the quantitation of impurities in pharmaceutical or radiopharmaceutical samples.

Toward miniaturized analysis of chemical identity and purity of radiopharmaceuticals via microchip electrophoresis

“…In HPLC, the amount of sample is measured volumetrically (by the injection loop), resulting in very high injection repeatability. Recently, we explored a novel volumetric injection technique for MCE and showed that a peak area RSD as low as 1.04% (n=4) [37] could be achieved, even using an external capillary for separation. We are thus confident that a next-generation device incorporating a PDMS-based injector will achieve sufficient repeatability for radiopharmaceutical analysis.…”

“…The dead-volume could be reduced by various approaches such as precise drilling [51] or molding the capillary port [52], or by tapering the capillary to fit directly in an in-plane microchannel [37, 53]. The geometry issue could also be addressed by integration of the separation channel directly into the chip (instead of using a capillary); this would eliminate the junctions altogether and simplify the overall setup, enabling a single integrated microfluidic device for injection, separation, and detection.…”

“…While the separation performance of the integrated device was lower than desired, a detailed analysis identified the capillary-chip junctions as the problem. Extrapolating from the performance when junctions are eliminated, we argue that a device with optimized junctions [37] could achieve the requisite performance. Furthermore, the optimized MCE device would be very much smaller than an HPLC system.…”

“…The solenoid valves switched between two states: (i) supplying pressurized nitrogen (35 psi) to close the on-chip microvalve, and (ii) venting to atmosphere to allow the on-chip microvalve to open via elastic restoration of the PDMS. To avoid the generation of air bubbles inside the sample-containing channels of the chip, the valve control channels were filled with water prior to use as previously described [37]. …”

Recent Progress toward Microfluidic Quality Control Testing of Radiopharmaceuticals

“…By replacing conventional HPLC methods with microchip electrophoresis (MCE), there is much potential to significantly reduce the size, cost, and complexity of chemical purity testing systems in the future. Furthermore, sample volume is generally reduced by orders of magnitude, and very short analysis times (seconds) have been reported [ 68 , 74 , 75 ].…”