Simultaneous separation and detection of cations and anions on a microfluidic device with suppressed electroosmotic flow and a single injection point

Abstract

A rapid and simultaneous separation of cationic and anionic peptides and proteins in a glass microfluidic device that has been covalently modified with a neutral poly(ethylene glycol) (PEG) coating to minimize protein adsorption is presented. The features of the device allow samples that contain both anions and cations to be introduced from a central flow stream and separated in different channels with different outlets-all in the presence of low electroosmotic flow (EOF) imparted by the PEG coating. The analytes are electrophoretically extracted from a central hydrodynamic stream and electrophoretically separated in two different channels, in which pressure driven flow has been suppressed through the use of hydrodynamic restrictors. Having different outlets for the electrophoretic separation channels that are spatially separated from the injection enables coupling with further downstream functionalities or off-chip detection, such as mass spectrometry. A plug of charged analyte is hydrodynamically pumped to the sampling intersection and anions from the plug migrate electrophoretically toward the anode in one channel while cations migrate toward the cathode in the other channel due to suppressed EOF from the PEG coating. The separations presented here required less than a minute to complete and produced average separation efficiencies of up to about 3,500 plates from a separation length of 2 cm. The extraction efficiency of both cations and anions from the hydrodynamic stream is determined experimentally and compared with a previously reported model that was used to determine anion extraction efficiency. The extraction efficiency is determined to be 87% and 98% for the two sample mixtures analyzed, and the values predicted by the model are within 3.5% of the experimental data. It is anticipated that this basic approach for simultaneous separation of anions and cations with reduced EOF will be integrated into larger microfluidic systems because the design provides separate outlets that can feed downstream processes or linked to off-chip detection.

Recent developments in electrophoretic separations on microfluidic devices

“…In a subsequent article by Reschke et al [30], the glass microfluidic device for simultaneous cation and anion detection was modified. The device retained the single, hydrodynamically pumped sample stream, but separation and detection occurred in two separate channels, one for anions and one for cations.…”

“…Reschke et al from West Virginia University described devices that were used for the separation of ions [29,30]. In the first article, a glass microfluidic device where flow is controlled by electrophoresis was used for anion and cation separation and detection [29].…”

Velocity plateaus in traveling-wave electrophoresis

“…We review the sinusoidal model that has been studied previously [2,6,7], introduce traveling-electrode and stationary-electrode models, and show that only the stationary-electrode model produces plateaus. Experiments [3][4][5] have observed localization and trapping thresholds as predicted by the 2D model [2,3], but have not yet observed velocity plateaus. These plateaus might just be observable in careful experiments, but their physical origin is unclear.…”

“…For low mobilities R < R l , particles are confined to the locality of electrodes and u = 0. For high mobilities R > R t , particles are trapped by the wave and u = c. As R increases from the localization threshold R l to the trapping threshold [2][3][4][5], with channel height h and four electrodes per wavelength, λ. These electrodes are held at the AC potentials Φ0(t), Φ1(t), Φ2(t), and Φ3(t), each potential differing in phase by 90…”

“…Such methods might be useful in a wide variety of applications [1]. Traveling wave electrophoresis is a separation method that has been investigated recently [2][3][4][5]: four interdigitated electrode arrays alternate along a channel as shown in Fig. 1.…”