Evaluation of microchip capillary electrophoresis with external contactless conductivity detection for the determination of major inorganic ions and lithium in serum and urine samples

Abstract

The determination of inorganic ions in clinical samples in less than 90 seconds was demonstrated for microchip capillary electrophoresis using capacitively coupled contactless conductivity detection (C(4)D). Bare electrophoresis chips were used in combination with external electrodes which were part of the chip holder. In order to achieve the required selectivity and sensitivity, an optimization of the electrode layout was carried out. Limits of detection (LOD) of 1 microM for K(+), 1.5 microM for Ca(2+), 3 microM for Na(+), 1.75 microM for Mg(2+) and 7.5 microM for Li(+) were achieved. The determination of inorganic cations (NH(4)(+), K(+), Na(+), Ca(2+), Mg(2+)) and anions (Cl(-), NO(3)(-), SO(4)(2-), phosphate) in blood serum and urine samples was possible in one common electrolyte solution containing 15 mM L-arginine, 10.75 mM maleic acid and 1.5 mM 18-crown-6 at pH 5.90 by simply switching the separation voltage from positive to negative polarity. Lithium, present at significant levels when used for therapeutic purposes, can also be determined in blood serum using a slightly modified background electrolyte solution.

A three-layer PMMA electrophoresis microchip with Pt microelectrodes insulated by a thin film for contactless conductivity detection

“…5, Na + and K + were baseline separated within 50 s and the resolution was 2.13. Theoretical plates for Na + and K + were 77 260 and 48 293 plates/ m, respectively, which are higher than those results reported in the literature, 8,16,[24][25][26] as shown in Table 1.…”

“…As expected, the LOD obtained by our chip is lower than those results obtained from chips with similar electrode geometry, thicker insulating layers and operated at low excitation frequencies, [24][25][26] as shown in Table 1. While it is higher than the results reported by Kubaň and Hauser,8,16,27 the excitation frequencies used in their work (500 kHz, 8 530 kHz, 16 600 kHz 27 ) are much higher than our applied frequency of 60 kHz and the range of the detector (0-300 kHz) used here, which is in line with the conclusion that the output of the detector increased sharply between 400 and 600 kHz. 13 Moreover, the LODs in their work were obtained when the optimized electrode geometry was used, and it has also been demonstrated that the electrode geometry had a significant effect on the sensitivity.…”

“…13 Moreover, the LODs in their work were obtained when the optimized electrode geometry was used, and it has also been demonstrated that the electrode geometry had a significant effect on the sensitivity. 8,16 Therefore, a better sensitivity could be expected after using a new contactless conductivity detector with a wider range of excitation frequencies and optimizing the electrode geometry in the near future.…”

“…8,16,[24][25][26][27] Based on the signal-to-noise ratio of 3, the LOD for Na + was 1.25 mg ml À1 by using our system. As expected, the LOD obtained by our chip is lower than those results obtained from chips with similar electrode geometry, thicker insulating layers and operated at low excitation frequencies, [24][25][26] as shown in Table 1.…”

“…Kubaň and Hauser placed the microelectrode on a separate electrode plate, and had the electrode plate in tight contact with the thin cover plate of a PMMA microchip. 8, 16 Chen 9 and Vazquez 10 employed a similar technique to fabricate glass and hybrid PDMS-glass microchips. While this method didn't require integrating microelectrodes on a thin film, the microelectrode was not fully integrated onto the chip and some assistant structures were needed, such as chip holders, screws or clips, which compromised the integration and reproducibility of the microchips.…”