Leading electrolyte

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. 

Marchandise H, Vandendriessche S (1985) The Certification of the Impurity Contents (Ag, As, Bi, Cd, Ni, Sb, Se, Sn, Te, T1 and Zn) in Three Grades of Lead, Electrolytically Refined Lead BCR No 286, Thermally Refined Lead BCR No 287, Lead with Added Impurities BCR No 288. European Commission Report EUR 9665 EN. Community Bmeau of Reference, Brussels. Merry f (1995) Reference materials for monitoring nutrients in sea water environment, approach, preparation, certification and their use in environmental laboratories. Fresenius J Anal Chem 352 148-151. 

Milk Isotachophoresis Conductivity 2ng 40% methanol, 10 mM sodium acetate pH 4.8, 0.2% hydroxy-ceUulose (leading electrolyte), 40% methanol, 20 mM acetic acid (terminating electrolyte) Prometryne, desmetryne, terbutryne, atrazine (OH metabolites), simazine (OH metabolites) 126... 

Isotachophoresis. In isotachophoresis (ITP), or displacement electrophoresis or multizonal electrophoresis, the sample is inserted between two different buffers (electrolytes) without electroosmotic flow. The electrolytes are chosen so that one (the leading electrolyte) has a higher mobility and the other (the trailing electrolyte) has a lower mobility than the sample ions. An electric field is applied and the ions start to migrate towards the anode (anions) or cathode (cations). The ions separate into zones (bands) determined by their mobilities, after which each band migrates at a steady-state velocity and steady-state stacking of bands is achieved. Note that in ITP, unlike ZE, there is no electroosmotic flow and cations and anions cannot be separated simultaneously. Reference 26 provides a recent example of capillary isotachophoresis/zone electrophoresis coupled with nanoflow ESI-MS. 

To satisfy the requirements for the properties of the leading electrolyte applied in the first stage and, consequently, to decide its composition, two facts had to be taken into account, i.e. the pH value of the leading electrolyte needs to be around 4 or less and at the same time the separations of the macro constituents need to be optimised by means other than adjusting the pH of the leading electrolyte (anions of strong acids). 

The choice of the leading electrolyte for the second stage, in which the micro-constituents were finally separated and quantitatively evaluated, was straightforward, involving a low concentration of the leading constituent (low detection limit) and a low pH of the leading electrolyte (separation according to pK values). 

In practice isotachophoresis is usually performed in narrow tubes with electrodes at either end and is one form of capillary electrophoresis. For the separation of a particular type of ion, e.g. an anion, two buffered electrolyte solutions are selected that have different anions but a common cation with a buffering capacity. One of the anions (termed the leading electrolyte) should show a greater mobility than the other anion and occupies the anodic end of... 

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. 

Capillary isotachophoresis (CITP) is an electromigration technique, which is performed using a discontinnous buffer system, formed by a leading electrolyte (LE) and a terminating electrolyte... 

FIGURE 6.12 Schematic view of the CITP separation mechanism. The sample is introduced into the capillary between two electrolyte systems a leading electrolyte (L), having electrophoretic mobility higher than any of the sample components to be separated and a terminating electrolyte (T), having electrophoretic mobility lower than any of the sample components (A). The sample components are separated according to the order of their individual mobility into distinct zones, which are sandwiched between T and L (B). The separated zones move with the same velocity toward the capillary end where they are detected as bands (C). 

Capillary isotachophoresis is usually performed in constant current mode, which implies the invariable ratio between concentration and electrophoretic mobility of ions. Therefore, bands that are less concentrated than the LE are sharpened, whereas those that are more concentrated than the LE are broadened to adapt their concentration to the requested constant value between concentration and electrophoretic mobility. The consequence of this unique property of CITP is that each sample component can be concentrated to an extent that depends on its initial concentration and the concentration of the leading electrolyte. Therefore, the opportune selection of composition and concentration of the leading electrolyte allows the enrichment of diluted analytes. 

The leading electrolyte was 2.8 pM ammonium acetate - acetic acid buffer (pH 4.9) containing 0.3% Triton X 100. 5 pM acetic acid served as the terminator. 

Procaine and other local anesthetics were separated by Chmela et al. with a VLD isotachophoresis apparatus, equipped with coupled PTFE separatory (23 cm x 0.8 mm) and analytical columns (23 cm x 0.8 mm) [151]. In one system, the leading cation was (0.01 M) containing 0.05 poly(vinylalcohol), with acetate as the counter-ion. The pH of the leading electrolyte was 4.75. The terminating electrolyte was 0.03 M beta-alanine. Two other systems were also reported. 

FIGURE 6.19 Fluorescence CCD images of tITP-ZE separation during injection (b) and after IIP concentration (c). (a) shows the general microfluidic channel configuration. Panels (b) and (c) show results obtained with a 250-pm injector. Conditions Sample was 1 mM fluorescein. Leading electrolyte 25 mM Tris+, 25 mM Cl. Trailing electrolyte 25 mM Tris+, 25 mM TAPS. Injection field 300 V/cm, current 18 pA. Separation field 200 V/cm, current 10 tl.A [634]. Reprinted with permission from the American Chemical Society. 

In the electrolyte system used in capillary isotachophoresis (cITP), the sample zone migrates between a leading electrolyte at the front and a different, trailing electrolyte at the end. The leading electrolyte contains a coion with mobility greater than that of any of the analyte ions. The trailing electrolyte contains a coion with mobility that is lower than that of any of the analyte ions. In isotachophoresis, it is possible to analyze for anions or cations, but not both simultaneously. Analyses are usually performed in the constant-current mode. 

Figure 5.20 shows the separation of ephedrine, procaine, and cycloserine using cationic cITP. The leading electrolyte consisted of 10 mM potassium acetate and acetic acid (pH 4.75), and the terminating electrolyte consisted of 10 mM acetic acid no additives were used. 

The second characteristic is that at equilibrium the concentration of the sample ions is related to the concentration of the leading ion. The concentration in each zone is constant and is determined by the concentration of the leading electrolyte. If the concentration of the leading electrolyte is high relative to the sample zones, the sample zones will narrow to approach the concentration of the leading ion. On the other hand, if the concentration of the leading electrolyte is low compared with the sample zones, the sample zones will broaden. Thus, concentration or dilution of the sample ions will occur, depending on the concentration of the analyte... 

In cITP, the sample zone migrates between a leading electrolyte and a trailing electrolyte. It is possible to analyze for anions or cations, but not for both simultaneously. 

The use of a column coupling configuration of the separation unit provides the possibility of applying a sequence of two leading electrolytes in one analytical run. Therefore, the choice of optimum separation conditions can be advantageously divided into two steps ... 

System No. Parameter Leading electrolytes 1st stage 2nd stage Terminating electrolyte... 

Better results were achieved when a divalent organic cation was used as a co-counter ion in the leading electrolyte [33,34] employed in the first-separation stage when, simultaneously, the pH of the leading electrolyte was 4 or less, and the steady state configuration of the constituents to be separated was chloride, nitrate, sulphate, nitrite, fluoride and phosphate. The detailed composition of the operational system of this type used for quantitative analysis is given in Table 1.1 (system No. 1.)... 

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