Electrode designs,conductivity detector

A well-known fact of fundamental solution science is that the presence of ions in any solution gives the solution a low electrical resistance and the ability to conduct an electrical current. The absence of ions means that the solution would not be conductive. Thus, solutions of ionic compounds and acids, especially strong acids, have a low electrical resistance and are conductive. This means that if a pair of conductive surfaces are immersed into the solution and connected to an electrical power source, such as a simple battery, a current can be detected flowing in the circuit. Alternatively, if the resistance of the solution between the electrodes were measured (with an ohmmeter), it would be low. Conductivity cells based on this simple design are in common use in nonchromatography applications to determine the quality of deionized water, for example. Deionized water should have no ions dissolved in it and thus should have a very low conductivity. The conductivity detector is based on this simple apparatus. 

A typical design for a conductivity detector uses electrically isolated inlet and outlet tubes as the electrodes. This design is illustrated in Figure 13.12. 

The first conductivity detector was developed by Martin and Randall as long ago as 1951 [1]. Improved cell designs have been described by Harlan [2], Sjoberg [3] and more stable and sensitive electronic circuits have been Avinzonis and Fritz [4] and Berger [5]. Scott et al. [6] inserted electrodes in the wall of a column to monitor the progressive band dispersion along a packed LC column. Keller [7] described a bipolar electrical conductivity detector and Kornilova et al. [8] describe a electric conductivity sensor for use in LC having a volume of 0.1 pi. 

Figure 6.4. Detailed diagram of hardware configuration for post-column addition of SPR. (1 = Conductivity detector Waters 431 detector, four electrode cell design 2 = waste line 4 x 0.009 in. stainless connected to 431 + 24 X 1/16 X 0.060 in PTFE tubing 3 = tee to 431 15 x 1/16 x 0.010 in PTFE to 431 inlet 4 = column to lee shortest 1/16 x 0.010 in PTFE from column to tee 5 = tee Unmount tee from check valve block for shortest path length 6 = analytical colunm Waters 1C PAK A or 1C PAK A HR 7 = check valve to tee 2 x 1/8 in o.d. PTFE 8 = check valve 9 = polisher column to check valve 3 x 1/8 in o.d. PTFE 10 = polisher column 8 x 25 mm containing AGI x 8, 200 mesh 11 = reservoir to polisher column 12 x 1/8 in. o.d. PTFE 12 = air supply minimum of 90 p.s.i. compressed air supply 13 = reservoir for SPR reconfigure with outlet on left side. From Ret [9] with permission.)...

Fig. 7 Schematic diagrams of the microchip electrophoretic system with the movable contactless-conductivity detector (A) along with a detailed design of the movable electrode system top (B) and bottom (C) views as well as cross-sectional views without (D) and with (E) the PMMA separation chip, (a) Run buffer reservoir, (h) sample reservoir, (c) unused reservoir, (d) movable electrodes, (e) separation chaimel, (f) sample waste reservoir, (g) PMMA chip, (h) conductive silver epoxy, (0 PVC clamps, (/ ) copper wires, (k) aluminum foil electrodes, and (/) Plexiglas plate (Reprinted with permission from Ref. [9])...

Conductivity detector Waters 431 detector, four electrode cell design 2 = waste line ... 

The infrared-electrochemical cell, originally designed by Bewick and his coworkers, was partly modified to introduce an electrode from the upper part of the cell. The front side of the cell is attached with a CaFg optical window, and the backside with a glass syringe which pushes the electrode against the window. The Fourier transform infrared measurements were conducted at 0 °C for Cu electrodes and at ambient temperature for Ni and Fe electrodes by JIR-6000 (Nihon Densi, Co. Ltd.) externally equipped with an MCT (mercury-cadmium-telluride) detector. Infrared spectra were acquired by the subtraction of two spectra reflected from the electrode at different potentials (SNIFTIRS). The other details were described previously. [9]... 

Figure 12.13 illustrates a versatile experimental set-up for microwave conductivity measurements with the microwave source (8 0 GHz), a circulator and a detector, which monitors the microwave energy reflected from the electrochemical or photovoltaic cell. The cell and electrode geometries are designed in such a way that the microwave power can reach the energy-converting interface (losses in metal contacts or aqueous electrolyte should be minimised). Depending on the experimental conditions, time-resolved, space-resolved or potential-dependent measurements are possible as well as combinations (for further details, see Schlichthbrl and Tributsch, 1992 Wiinsch et al., 1996 Chaparro and Tributsch, 1997 Tributsch, 1999). 

Recently, electrochemical detection methods, namely, conductimetry, amperometry, and potentio-metry, have also become accessible. All three variants of electrochemical detection are intrinsically simpler than the optical methods, and their success depends highly on the electrode materials and designs used. Conductivity detection relies on measurement of the differences between the conductivities of the analyte and the separation electrolyte this provides a direct relationship between migration times and response factor, and makes this detector universal. On the contrary, amperometric detection is restricted to electroactive species and potentiometric detection is not possible for certain small ions with multiple charges. Conductimetric detection works better for inorganic compounds since the higher mobility of... 

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