Detectors, capillary isotachophoresis

In the 1970s a new analysis method was developed, referred to as isota-chophoresis [247]. Capillary isotachophoresis with conductivity detector is very useful in detergent analysis for the determination of all kinds of ionic species [248]. It is also useful for the determination of MCA in ether carboxylates. 

Fanali et al. have described a capillary isotachophoresis method for the determination of procaine in pharmaceuticals [ 150]. The drug was determined in a 6 pL sample of solution (Spofa product, obtained from Czechoslovakia, and diluted 180-fold) by cationic isotachophoresis in the single column mode. The system used a PTFE capillary column (20 cm x 0.3 mm) and a conductivity detector. The separation was carried out at room temperature, at 50 pA (but switched to 25 pA during detection). 

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. 

Fig. 6.32. The principle of preparative capillary isotachophoresis. (A) Schematic representation of the analytical equipment. (B) The preparative version with the additional flow of the leading electrolyte. L, Leading electrolyte T, terminating electrolyte D, UV detector S, sample v, migration velocity m,. mj, semipermeable membranes A, flow of L to elute the sample zones.

Capillary isotachophoresis is a rapid, accurate, and potential detection technique for TTX. A small amount of TTX in contaminated extracts can be determined by this method (Shimada et al, 1983). It is performed using a cationic system, as TTX exists as cation under acidic and neutral conditions. Conditions for capillary isotachophoresis composed of 5 mmol/liter potassium acetate (pH 6.0) as an electrolyte, containing 0.2% Triton X-100 and 0.5 volume of dioxane, and 10 mmol p-alanine adjusted to pH 4.5 with acetic acid as a terminating electrolyte. When TTX is applied to isotachophoretic analyzer (Shimadzu IR-2A) equipped with a potential gradient 0.32, it is eventually monitored by the detector. PU is expressed as (PGs-PGl), where PGs, PGl, and PGt stand for potential gradient values for sample, leading ion, and... 

In the development of the method itself, the increase in the temperature in the isotachophoretic zones played an important role since temperature measurement was the first universal detection procedure with sufficient sensitivity in the capillary isotachophoresis . In connection with the detection by means of a thermocouple, more attention was devoted to the study of interrelations between the longitudinal distribution of the temperature on the outside wall of the isotachophoretic column and the width and the position of the zone boundary inside the column. At present, the thermocouple detector lost greater significance however, the temperature regime in the isotachophoretic column still requires adequate attention. 

Kaniansky, D., Marak, J., Rajec, R, Svec, A., Koval, M., Lucka, M., and Sabanos, G, On-column radio-metric detector for capillary isotachophoresis and its use in the analysis of C-14-labeled constituents, J. Chromatogr., 470, 139, 1989. 

Figure 5 Configuration of capillary isotachophoresis. The arrangement shown is close to a real Instrument. The capillary is often made of plastic. The semipermeable membrane prevents electro-osmotic and hydrodynamic flows of the electrolyte but allows electromigration of ions. A conductivity detector is often used. It has miniature electrodes placed inside the capillary.

Figure 9 ITP-ITP analysis of 5 jl of untreated heparin plasma of patients with diabetes mellitus. (A) The record from the conductivity detector in the preseparation capillary, ID 0.8 mm, LE 10 mmol I HCI + -alanine, pH 4.2. (B) The record from the analytical capillary, ID 0.3 mm, LE 5 mmol I HCI -i- glycine, pH 3.0. L, leading ion - chloride T, terminating ion - nicotinate. (Reprinted with permission from Kfivankova L and Bocek P (1990) Determination of pyruvate, acetoacetate, lactate, and 3-hydroxybutyrate in plasma of patients with diabetes mellitus by capillary isotachophoresis. Journal of Microcolumn Separation 2 83.)...

AFID alkali flame ionization detector CITP capillary isotachophoresis... 

Kaniansky et al. [35] have also reported on a method for the determination of nitrate, sulphate, nitrite, fluoride and phosphate by capillary zone electrophoresis coupled with capillary isotachoelectrophoresis in the column coupling configuration. Such distributions of these anions are typical for many environmental matrices, and it is shown that capillary isotachophoresis-capillary zone electrophoresis tandem enables the capillary isotachoelectrophoresis determination of the macroconstituents, while capillary isotachophoresis-preconcentrated microconstituents cleaned up from the macroconstituents can be determined by capillary zone electrophoresis with a conductivity detector. This approach was effective when the concentration ratio of macro-microconstituents was less than (2-3) x 10. The limits of detection... 

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. 

Radioisotope detection of P, 14C, and Tc was reported by Kaniansky et al. (7,8) for isotachophoresis. In their work, isotachophoretic separations were performed using fluorinated ethylene-propylene copolymer capillary tubing (300 pm internal diameter) and either a Geiger-Mueller tube or a plastic scintillator/photomultiplier tube combination to detect emitted fi particles. One of their reported detection schemes involved passing the radiolabeled sample components directly through a plastic scintillator. Detector efficiency for 14C-labeled molecules was reported to be 13-15%, and a minimum detection limit of 0.44 nCi was reported for a 212 nL cell volume. 

Two kinds of conductivity detector are distinguished contact detectors and contactless detectors. Both types were originally developed for isotachophoresis in 0.2-0.5-mm-inner diameter (i.d.) PTFE tubes. Contactless detectors are based on the measurement of high-frequency cell resistance and, as such, inversely proportional to the conductivity. The advantage is that electrodes do not make contact with the buffer solution and are, therefore, outside the electric field. As these types of detectors are difficult to miniaturize down to the usual 50-75-jU.m capillar inner diameter, their actual application in capillary electrophoresis (CE) is limited. 

In isotachophoresis, electrical detection is most commonly used, although thermal and UV-absorption detection are also used(3). For capillary zone electrophoresis, UV-absorption and fluorescence detection have proven most useful so far. The principles behind the optical detection modes are fairly obvious. However, the electrical and thermal detectors deserve further explanation. As described earlier, in isotachophoresis, each zone is an individual "pure" band of sample ions. The zones travel in order of decreasing mobility. To compensate for each successive... 

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