Electrophoretic separations in poly(dimethylsiloxane) microchips using mixtures of ionic, nonionic and zwitterionic surfactants

Abstract

The use of surfactant mixtures to affect both EOF and separation selectivity in electrophoresis with PDMS substrates is reported, and capacitively coupled contactless conductivity detection is introduced for EOF measurement on PDMS microchips. First, the EOF was measured for two nonionic surfactants (Tween 20 and Triton X-100), mixed ionic/nonionic surfactant systems (SDS/Tween 20 and SDS/Triton X-100), and finally for the first time, mixed zwitterionic/nonionic surfactant systems (TDAPS/Tween 20 and TDAPS/Triton X-100). EOF for the nonionic surfactants decreased with increasing surfactant concentration. The addition of SDS or TDAPS to a nonionic surfactant increased EOF. After establishing the EOF behavior, the separation of model catecholamines was explored to show the impact on separations. Similar analyte resolution with greater peak heights was achieved with mixed surfactant systems containing Tween 20 and TDAPS relative to the single surfactant system. Finally, the detection of catecholamine release from PC12 cells by stimulation with 80 mM K(+) was performed to demonstrate the usefulness of mixed surfactant systems to provide resolution of biological compounds in complex samples.

Figure 1

EOF measurements using both current monitoring and C⁴D methods. The mark* denotes the time point at which the polarity is reversed. Field strength: 200 V/cm; BGEs: 20 mM and 18 mM TES buffer, pH 7.0.

Figure 2

EOF as a function of concentration for Tween 20 and Triton X-100 in boric acid buffer (20 mM and 18 mM) at pH 9.2.

Figure 3

EOF as a function of SDS concentration using (A) Tween 20 and (B) Triton X-100 in boric acid buffer (20 mM and 18 mM) at pH 9.2.

Figure 4

EOF as a function of TDAPS concentration using (A) Tween 20 and (B) Triton X-100 in boric acid buffer (20 mM and 18 mM) at pH 9.2.

Figure 5

Example electropherograms for 20 µM DA, NE, E and 40 µM CA and L-DOPA in 20 mM TES buffer at pH 7.0 as a function of surfactant composition. Field strength: 150 V/cm; 10-s hydrodynamic injection; Detection: DC Amp, E_det = 1.2 V.

Figure 6

Electropherograms of catecholamine release from PC12 cells by stimulation with 80 mM K⁺ in 2× diluted sample and with the standard solution containing 5 µM DA, NE, E and 10 µM CA and L-DOPA. BGE: 20 mM TES, 0.5 mM Tween 20, 4 mM TDAPS, pH 7.0; Field strength: 150 V/cm; 10-s Hydrodynamic injection; Detection: DC Amp., E_det = 1.2 V.