Potentiometric detectors

Nonspectroscopic detection schemes are generally based on ionisation (e.g. FID, PID, ECD, MS) or thermal, chemical and (electro)chemical effects (e.g. CL, FPD, ECD, coulometry, colorimetry). Thermal detectors generally exhibit a poor selectivity. Electrochemical detectors are based on the principles of capacitance (dielectric constant detector), resistance (conductivity detector), voltage (potentiometric detector) and current (coulometric, polarographic and amperometric detectors) [35]. 

These measure the potential difference between the transducing electrode and a reference electrode under conditions of zero current. Three types of potentiometric detectors are commonly employed ion-selective electrodes (ISE), gas-sensing electrodes and field effect transistors (FET). 

Fluorescence detection, because of the limited number of molecules that fluoresce under specific excitation and emission wavelengths, is a reasonable alternative if the analyte fluoresces. Likewise, amperometric detection can provide greater selectivity and very good sensitivity if the analyte is readily electrochemically oxidized or reduced. Brunt (37) recently reviewed a wide variety of electrochemical detectors for HPLC. Bulk-property detectors (i.e., conductometric and capacitance detectors) and solute-property detectors (i.e., amperometric, coulo-metric, polarographic, and potentiometric detectors) were discussed. Many flow-cell designs were diagrammed, and commercial systems were discussed. 

J.T. Colston, P. Kumar, E.D. Rael, A.T.C. Tsin, J.J. Valdes and J.P. Chambers, Detection of sub-nanogram quantities of Mojave toxin via enzyme immunoassay with light addressable potentiometric detector, Biosens. Bioelectron., 8(2) (1993) 117-121. 

R. Tantra and A. Manz, Integrated potentiometric detector for use in chip-based flow-cells, Anal. Chem., 72 (2000) 2875-2878. 

Electrochemical detectors are sometimes divided into the groups of potenliomctric. amperometric, and conductometric detectors, that is, according to the three parameters of electric measurements. Potentiometric detectors measure voltage, amperometric measure current, and conductometric measure resistance. 

Conductometric detectors respond to all ions, but the other detectors respond only to certain electroactive ions. In this book, electrochemical detection will refer to amperometric, or potentiometric detectors, but will not refer to conductometric detectors. 

The potentiometric detector operates on the same principles as ion-selective electrodes. An indicating electrode measures a change in the potential in the presence of certain sample ions. Schmuckler et al. [47] explored the use of potenliometric-type detectors for ion chromatography. Halides and pseudohalides were detected with a sil-ver/silver salt indicator electrode. Other indicating electrodes were also suggested, for example, lead/lead salicylate to detect sulfate type anions. Other workers have reported potentiometric detection [48-52]. 

P. R. Haddad, P. W. Alexander, and M. Trojanowicz, Ion chromatography of magnesium, calcium, strontium and barium ions using a metallic copper electrode as a potentiometric detector, /. Chro-matogr., 294,397,1985. 

K. Suzuki, H. Aruga, H. Ishiwada, T. Oshima, H. Inoue and T. Shirai, Determination of anions with a potentiometric detector for ion chromatography, Bunseki Kagaku, 32, 585, 1983 (Japanese). 

B. Saad, W.T. Wai, B.P. Lim, M.I. Saleh, Flow injection determination of anasidine value in palm oil samples using a triiodide potentiometric detector, Anal. Chim. Acta 591 (2007) 248. 

K+, H+ and Cl by means of serial potentiometric detectors, (b) Enlarged section of the valve used for selection of sample and standards. (Reproduced from [7] with permission of the American Chemical... 

Baseline drift [Fig. 6.10(7)]. In optical detectors this is due to deposition of material on the windows of the flow cell. Such depositions may often be removed by injecting a reagent that dissolves the material, or by washing the system with a suitable wash liquid or detergent. In potentiometric detector systems, the drift may be caused by a change in the standard cell potential (either drift in the value of the indicator electrode or the reference electrode or both) or by a change in junction potentials. 

P. W. Alexander, P. R. Haddad, and M. Trojanowicz, Response Characteristics of a Potentiometric Detector with a Copper Electrode for Flow-Injection and Chromatographic Determinations of Metal Ions. Anal. Chim. Acta, 111 (1985) 183. 

J. Georges and M. Khalil, A Potentiometric Detector for Following pH Shifts in Liquid Chromatography. Anal. Chim. Acta, 182 (1986) 281. 

In 1981, Meyerhoff and Fraticelli [35] appear to be the first to report on the coupling of a gas-diffusion separation system to a flow-through potentiometric detector. Ammonia was isolated from the donor stream containing the sample by penetrating a microporous membrane and collection in an acceptor buffer stream, and then determined selectively using a tubular nonactin p>olymer membrane electrode. The precision (<7% r.s.d.) and sample throughput (30 h ) of this early application was rather low. 

Ion-selective electrodes are used in potentiometric detectors. Early designs inserted micropipette electrodes into the outlet of the separation capillary. The capillary outlet was widened using etching to make placement easier and decrease the electric field at the electrode. As such it was foimd that electrical decoupling was unnecessary. More recent designs have used coated wire electrodes where a solid wire is coated with a polyvinyl chloride membrane. The electrode is placed 50pm from the outlet of the capillary. Coated wire electrodes are much less fragile and easier to position than micropipette electrodes. 

The development of potentiometric detectors based on ion-selective electrodes continues to expand the scope of clinical and pharmaceutical applications of FIA. The small surface area of the sensor avoids adsorption problems and extends the service life of the electrode. The surface can be readily renewed periodically by alternating the washing cycles with the sampling cycles. The selectivity thus achieved is usually very good as it relies on differences in the... 

Potentiometric titrations may be also performed in the flow injection mode. This is attractive because small amounts of sample are needed, the time of determination is short, and the results for low concentration are reliable. Besides, the potentiometric signal depends linearly on the concentration of the analyte, enabling determination in a broad concentration range. There are several types of such titration, e.g., a fixed sample volume is mixed with a titrant solution of constant concentration. The width of the potentiometric signal, measured on the time scale as At, is proportional to the logarithm of the analyte concentration. The quantitative evaluation is based on a calibration graph. The important factors are the linear range of determination and the time constant of the potentiometric detector. The latter must be compatible to the flow rate of the solution in the system. 

The main disadvantages of potentiometric detectors in IC are slow response of many electrodes and the fact that they respond well to only a few different species. ... 

In the case of FIA potentiometric detectors (see section 2.1), various configurations and different electrode materials, in particular platinum (wire or tubular) and graphite, with platinum or calomel as reference, have been studied [341]. Identical results with either type of indicating... 

In FIA, chemical-sensing field-effect transistors (Chem. FETs) and ion-selective field-effect transistors (ISFETs) (see sections 2.1.3 and 2.1.4) can be used with notable advantages over the more familiar membrane electrodes - namely miniaturization capability, high signal-to-noise ratio, faster response, decreased contribution of the detector to the creation of sample dispersion, etc. [346]. These types of potentiometric detectors can be reduced in size, enabling their incorporation into a hypodermic needle or other suitable probe, for making in vivo measurements [347]. 

Potentiometric detectors measure potential in volts (V) under conditions where I essentially equals zero and amperomteric and coulometric detectors measure current in amperes (A) as a function of applied potential. Conductometric detectors can be considered universal whereas potentiometric amperometric and coulometric detectors are selective detectors. 

Potentiometric detectors typically measure the potential difference (A ) across a membrane, which originates from the difference in analyte concentration in the eluent versus an internal reference solution. The most common potentiometric measuring device is a pH electrode, in which a glass membrane responds to hydronium ion concentration in the test solution. Other ion-selective, or indicator, electrodes are also available commercially. The attribute of an indicator electrode to be highly selective for a particular species is also its drawback, in that a different electrode is needed for each type of ion. Halides and sulfates can be monitored using silver/silver salt and lead/lead salt electrodes, respectively [50]. 

Major characteristics of potentiometric detectors are high specificity, low sensitivity, and slow response. 

The design of a picoliter-volume cell potentiometric detector for open tubular column liquid chromatography is shown in Figure 16. The microelectrode has a tip diameter of I pm and is inserted in the open tubular column. A cation-exchange solution can be used as membrane. The... 

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