Abstract: Tri-isobutylphosphine sulfide - Hg complex-based ISE were prepared (described) and were used in conjunction with an Orion 90-02-00 double-junction reference electrode. Batchwise potentiometric measurements were made in 0.1 M 2-morpholinoethanesulfonic acid (I)/NaOH buffers of pH 5.5, 6 or 6.5 or 0.1 M NaH2PO4/NaOH buffers of pH 6 or 6.5. Various Hg complex/PVC/plasticizer/DMF/THF mixtures were also used to prepare a membrane within a flow-through tubular electrode made from Epo-Tek 410 Ag-based conductive epoxy. This membrane electrode was used for detection in FIA. ISE containing 4% of ionophore showed near-Nernstian response to thiocyanate in all three I buffers and the detection limit was 2-4 µM, slightly better than in the phosphate buffers (detection limit of 5 µM-thiocyanate). Response times averaged 1 min for thiocyanate and 12 min for iodide. In FIA, the best signals were obtained with 100 µL sample injections and a carrier flow rate of 3.3 ml/min; with a membrane containing 4% of ionophore, the peak heights were reproducible to within ±1 mV for three injections at a given concentration.

Abstract: The development of an integrated analysis system for small ions based on ion-selective optodes and centrifugal microfluidics is reported. The performance of this system was evaluated through five-point calibration plots for two types of optode membranes, one being cation-selective and the other anion-selective, which were incorporated into a microfluidics platform on which fluid motion is induced via angular rotation. Additionally, the application of the microfluidic platform to ion analysis is studied via a two-point calibration protocol used to quantify an unknown sample. Calibrant solutions are delivered from reservoirs fabricated onto the platform to a measuring area that contains the optode membrane, with a change in membrane fluorescence being monitored. This work demonstrates the first instance of a microfluidic-based analysis system with detection based on ion-selective optode membranes monitored with fluorescence transduction. Furthermore, in addition to employing a standard excitation source where a fiber-optic probe is coupled to a tungsten-halogen lamp, laser diodes such as those employed in portable CD/DVD players were studied as excitation sources to enhance the observed fluorescence signals.

Abstract: The assay described was based on the enhancement of the emission intensity of the fluorescein-labelled conjugate of streptavidin by the biotin moiety. A merging-zone FIA system was used in which 70 µL of streptavidin-fluorescein isothiocyanate reagent (1.0 mg/l) was injected into one buffer stream and 70 µL of sample was injected into a second buffer stream. Both streams consisted of 50 mM phosphate buffer of pH 8 (1 ml/min). The streams were merged and passed through a 2-m knitted open-tubular reactor coil into the detection cell were the emission signal at 518 nm was recorded (excitation at 495 nm). The calibration graphs for biocytin, biotin, biotin-thyroxine and biotinylated BSA were linear at low concentration. (0.4 µM) but the gradients varied considerably. The detection limit was 2 nM-biotin.