Potential-gradient detectors

Capillary tube isotachophoresis using a potential gradient detector is another technique that has been applied to the analysis of alcohol sulfates, such as sodium and lithium alcohol sulfates [303]. The leading electrolyte solution is a mixture of methyl cyanate and aqueous histidine buffer containing calcium chloride. The terminating electrolyte solution is an aqueous solution of sodium octanoate. 

Isotachophoresis is carried out using a Shimadzu IR-2A isotachopho-retic analyzer with a potential gradient detector. Conditions for the analysis comprise a leading electrolyte of 5 mM potassium acetate (pH 6.0) containing 0.2% Triton X-100 and a half volume of dioxane, and a terminating electrolyte of 10 mM p-alanine adjusted to pH 4.5. 

A3. Akiyama, J., and Mizuno, T., Sensitivity of a newly designed potential gradient detector for isotachophoresis. J. Chromatogr. 119, 605-608 (1976). 

Figure 2. Schematic diagram of an ITP-apparatus. a = PT-electrode b = terminating electrolyte c = drain d = silicone septum e = UV-detector f = conductivity (potential gradient) detector g = silicone septum h = semi-permeable membrane (e.g. cellulose acetate) i = Pt-electrode. p and q lead to a current-stabilized power supply (20 kV). The separation compartment is a PTFE-capi1lary (I.D. =...

Ing cellulose acetate strip. This Instrument Is marketed by LKB Pro-dukter under the name Tachofrac. Kobayashl et al. (30) have also described preparative capillary ITP experiments using a syringe to extract fractions after migration across a potential gradient detector. Hjert n (31) has described a micropreparative zone electrophoretic technique In which the separation takes place In a capillary filled with a polyacrylamide gel. A flow of buffer past the end of the capillary sweeps the samples through a UV detector after which collection Is possible. This last method would seem to be the least useful because the smaller diameter tube limits the protein capacity to the ng to ug range. 

Instrumentation developed in 1970 and in subsequent years has been used till nowadays and has created the basis of the present instrumentation. Intensive efforts have been devoted to the development of more sensitive detectors than the thermocouple or the thermistor detector . Thus, the currently available very sensitive high-resolution detectors have been obtained. These include, the contact conductivity detector which senses the electric conductivity of zones by means of two metallic (Pt) microelectrodes protruding into the separation capillary and being in direct contact with the electrolyte the potential gradient detector in which the electric gradients in zones are sensed with the aid of two similar electrodes placed in at certain small distance from one another in the direction of migration the UV detector where a narrow beam of UV light passes perpendicularly... 

Isotaphoresis is therefore an enrichment method in those cases where the original concentration of the analyte in the sample is lower than the concentration in the steady state. In isotaphoresis identification cannot be carried out on the time axis, because the parameter that is measured in time units corresponds to the length of the analyte zone passing the detector, and is dependent on the amount of the analyte. Identification is mainly carried out on the axis of the measured electrical resistance of the analyte zones, when the universal electrical conductivity (or potential gradient) detector is applied. In this way, the same analyte properties (the ionic mobilities) are as decisive for separation as they are for identification [29]. 

Faraday cup is used to collect ions and thus provide a direct measure of the ion current. For higher sensitivity and rapid response, an electron multiplier detector is employed. In this device the accelerated ions impinge on a surface with the emission of secondary electrons. These are multiplied by a cascade along either a channel or set of plates having a large potential gradient. 

The technique of hpce, or ce, involves high-vOltage electrophoresis in narrow-bore fused-silica capillary tubes or columns and on-line detectors similar to those used in hplc (p. 123). Components of a mixture injected into one end of the tube migrate along it under the influence of the electric field (potential gradient) at rates determined by their electrophoretic mobilities. On passing... 

In the article presented capillary Isotachophoresis Is described. Basic principles are outlined and attention Is given to qualitative and quantitative evaluation of Isotachophoretlc analysis In which conductlmetrlc, potential gradient, photometric (UV-absorptlon and fluorlmetrlc) detectors are used. 

Briefly, a solution of the polymer s uIlple is introduced at the interface between a leading electrolyte of high effective mobility and a trailing electrolyte of low effective mobility. Ssunple ions of different electrophoretic mobilities separate into individual zones with the zone order in direct relation to their effective mobilities. The zones migrate past an on-column potential-gradient or conductivity detector with a response that is proportional to the effective mobility and eunount of Ion present. 

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