Polyion selective polymeric membrane-based pulstrode as a detector in flow-injection analysis

Abstract

A method for the detection of polyions using fully reversible polyion selective polymeric membrane type pulstrodes as detectors in a flow-injection analysis (FIA) system is examined. The detection electrode consists of a plasticized polymeric membrane doped with 10 wt % of tridodecylmethylammonium-dinonylnaphthalene sulfonate (TDMA/DNNS) ion-exchanger salt. The pulse sequence used involves a short (1 s) galvanostatic pulse, an open-circuit pulse (0.5 s) during which the EMF of the cell is measured, and a longer (15 s) potentiostatic pulse to return the membrane to its original chemical composition. It is shown that total pulse sequence times can be optimized to yield reproducible real-time detection of injected samples of protamine and heparin at up to 20 samples/h. Further, it is shown that the same membrane detector can be employed for FIA detection of both polycations at levels ≥10 μg/mL and polyanions at levels of ≥40 μg/mL by changing the direction of the galvanostatic pulse. The methodology described may also be applicable in the detection of polyionic species at low levels in other flowing configurations, such as in liquid chromatography and capillary electrophoresis.

Figure 1

Schematic diagram of the flow-injection system coupled with polyion selective polymeric membrane-based pulstrode used for the experiments reported.

Figure 2

Movement of salt and ions within a membrane and at interfaces during pulse sequence employed for detection of polyanions using anodic current pulse in FIA arrangement. The polyion is represented by P^z–, the interfering ions by I⁺ and A^–, and R⁺ and R^– are the TMDA⁺ and DNNS^– ions of the lipophilic ion-exchanger salt within the membrane phase. It should be noted that smaller anions (A^–) also enter the outer surface of the sensing membrane during pulse 1 period but are eventually outcompeted for serving as the counteranions to R⁺ sites by the polyanions (P^z–).

Figure 3

Dynamic potentiometric response of polyion pulstrode to (A) protamine; duplicate sequential injections of standards in the range of 10–100 μg/mL; and (B) heparin; duplicate sequential injections of standards in the range of 40–200 μg/mL in 10 mM phosphate buffer, pH 7.4, with 10 mM NaCl present. The diluent stream was the same buffer solution, and flow rates were set at 0.6 mL/min. Numbers above and below peaks represent concentrations in micrograms per milliliters. Every 100 pulses is ∼30 min.

Figure 4

Open circuit potentiometric response of polymeric membrane electrode to 1 mM KBr (A) before pulstrode measurements, (B) after 500 pulse sequences in flow mode, where anodic current was applied during galvanostatic pulse, (C) after 3 d in the flow system with no galvanostatic pulsing, and (D) after 500 pulse sequences in the flow mode with cathodic current applied during the galvanostatic pulse. All measurements were conducted in a 10 mM phosphate buffer, pH 7.4, containing 10 mM NaCl.

Figure 5

(A) Calibration of ΔEMF response (peak voltage – baseline voltage) to protamine in the flow-injection mode in 10 mM phosphate buffer, pH 7.4, containing 100 mM NaCl. It should be noted that the standard deviation for n = 3 injections at each concentration is so small that it cannot be observed for some data points. (B) Response to increasing concentrations of heparin with a constant concentration of 60 μg/mL protamine present in the test sample in the 10 mM phosphate buffer, pH 7.4, containing 100 mM NaCl. Data in (B) represents testing with three different sensors with the same membrane composition in the FIA system, where standard deviation was calculated using data from all three sensors. Flow rates for both A and B were set at 0.6 mL/min.

Figure 6

Dynamic potentiometric response to repeated injections (n = 20) of 60 μg/mL protamine in 10 mM phosphate buffer, pH 7.4, containing 100 mM NaCl. A sampling rate of approximately 20 samples per hour has been achieved. For data shown, the average ΔEMF = 33.7 mV ± 1.42 mV (n = 20). Flow rate was set at 0.93 mL/min.