Liquid chromatography electrochemical detectors

Fig. 14.8 A thin-layer cell for use as a high pressure liquid chromatography electrochemical detector (courtesy of Bioanalytical Systems).

Aminosalicylic acid in assessment of reactive oxygen species formation by in vitro Fenton and ozonation reactions and by in vivo ozone-exposure experiments in rats revealed oxidation products as follows salicylic acid, by deamination 2,3-dihydro-xybenzoic acid and 2,5-dihydroxybenzoic acid, from radical or enzymatic hydroxylation 5-amino-2 - hydroxy-N,W-bis(3 - carboxy- 4- hydroxyphenyl)-1,4-benzoquinonediimine, a condensation product of oxidised 5-aminosalicylic acid and 5-amino-2,3,4,6-tetrahydroxybenzoic acid, attributed to hydroxyl radical attack without deamination, identified by high-pressure liquid chromatography electrochemical detector system analysis and by gas chromatography-mass spectrometry analysis of trimethyl silyl derivatives (Kumarathasan et al. 2001). 

Typically, in gradient elution liquid chromatography, electrochemical detection has been difficult due to base-line shifts that result as a consequence of the altered mobile phase composition. However, a unique property of micelles allows for much improved compatibility of gradients (i.e. gradient in terms of micellar concentration or variation of small amount of additive such as pentanol) with electrochemical detectors. This has been demonstrated by the separation and electrochemical detection of phenols using micellar gradient LC (488). A surfactant (apparently non-micellar) gradient elution with electrochemical detection has also been successfully applied for the assay of some thyroid hormones by LC (491). 

Virtually every type of high-performance liquid chromatography (HPLC) detector can be combined with SCIC refractive index, UV absorbance (direct and indirect), electrochemical, and so forth. 

Electrochemical detectors are a very interesting example of detectors used in liquid chromatography. These detectors can detect only compounds that are able to react by oxidation or reduction reactions. Usually, this technique functions by measuring the gain or loss of electrons from the flowing sample as it passes between two electrodes maintained at an electrical potential difference. 

Fig. 1 One example of a sandwich type thin-layer LCEC detector with adjustable dead volume, flow pattern, and up to four channels. Source From Four channel liquid chromatography/electrochem-istry, in Curr. Sep. ...

In liquid chromatography, electrochemical detection is a superior alternative to optical detection for many analytical problems [29]. Electrochemical detectors monitor currents, electrode potentials, or electrolytic conductivities. In all cases the signal is directly proportional to the concentration or mass flow rate over relatively wide linear ranges. With electronic amplification and simple... 

Despite their importance, gas chromatography and liquid chromatography cannot be used to separate and analyze all types of samples. Gas chromatography, particularly when using capillary columns, provides for rapid separations with excellent resolution. Its application, however, is limited to volatile analytes or those analytes that can be made volatile by a suitable derivatization. Liquid chromatography can be used to separate a wider array of solutes however, the most commonly used detectors (UV, fluorescence, and electrochemical) do not respond as universally as the flame ionization detector commonly used in gas chromatography. 

Liquid chromatography was performed on symmetry 5 p.m (100 X 4.6 mm i.d) column at 40°C. The mobile phase consisted of acetronitrile 0.043 M H PO (36 63, v/v) adjusted to pH 6.7 with 5 M NaOH and pumped at a flow rate of 1.2 ml/min. Detection of clarithromycin and azithromycin as an internal standard (I.S) was monitored on an electrochemical detector operated at a potential of 0.85 Volt. Each analysis required no longer than 14 min. Quantitation over the range of 0.05 - 5.0 p.g/ml was made by correlating peak area ratio of the dmg to that of the I.S versus concentration. A linear relationship was verified as indicated by a correlation coefficient, r, better than 0.999. 

Enzyme linked electrochemical techniques can be carried out in two basic manners. In the first approach the enzyme is immobilized at the electrode. A second approach is to use a hydrodynamic technique, such as flow injection analysis (FIAEC) or liquid chromatography (LCEC), with the enzyme reaction being either off-line or on-line in a reactor prior to the amperometric detector. Hydrodynamic techniques provide a convenient and efficient method for transporting and mixing the substrate and enzyme, subsequent transport of product to the electrode, and rapid sample turnaround. The kinetics of the enzyme system can also be readily studied using hydrodynamic techniques. Immobilizing the enzyme at the electrode provides a simple system which is amenable to in vivo analysis. 

High-performance liquid chromatography (HPLC) with a micellar mobile phase or with a selective pre-column or reaction detection system has also been used to determine alkylenebis(dithiocarbamaes). ° Zineb and mancozeb residues in feed were determined by ion-pair HPLC with ultraviolet (UV) detection at 272 nm. These compounds were converted to water-soluble sodium salts with ethylenediaminetetra-acetic acid (EDTA) and sodium hydroxide. The extracts were ion-pair methylated with tetrabuthylammonium hydrogensulfate (ion-pair reagent) in a chloroform-hexane solvent mixture at pH 6.5-8.S. The use of an electrochemical detector has also been reported. ... 

Brunt, K., Electrochemical detectors for high-performance liquid chromatography and flow analysis systems, Trace Analysis, Vol. 1, Lawrence, J. F., Ed., Academic Press, New York, 1981, 47-120. 

Bollet, C., Oliva, P., and Caude, M., Partial electrolysis electrochemical detector in high-performance liquid chromatography, /. Chromatogr., 149,625,1977. 

Weber, S. G., The dependence of current on flow rate in thin-layer electrochemical detectors used in liquid chromatography. A clarification, /. Electroanal. Chem., 145, 1, 1983. 

ECD = electrochemical detection FID = flame ionization detector GC = gas chromatography HPLC = high performance liquid chromatography M = molar NaOH = sodium hydroxide NR = not reported rpm = revolutions per minute... 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

Nagaosa et al. [839] simultaneously separated and determined these elements in seawater by high-performance liquid chromatography (HPLC) using spec-trophotometric and electrochemical detectors. 

Electrochemical detectors (ECD) gas chromatography, 4 615 liquid chromatography, 4 622 6 387, 449 supercritical fluid chromatography, 4 631... 

With the introduction of modern electronics, inexpensive computers, and new materials there is a resurgence of voltammetric techniques in various branches of science as evident in hundreds of new publications. Now, voltammetry can be performed with a nano-electrode for the detection of single molecular events [1], similar electrodes can be used to monitor the activity of neurotransmitter in a single living cell in subnanoliter volume electrochemical cell [2], measurement of fast electron transfer kinetics, trace metal analysis, etc. Voltammetric sensors are now commonplace in gas sensors (home CO sensor), biomedical sensors (blood glucose meter), and detectors for liquid chromatography. Voltammetric sensors appear to be an ideal candidate for miniaturization and mass production. This is evident in the development of lab-on-chip... 

ECO = electron capture detector ED = electrochemical detector FID st flame ionization detector GC = gas chromatography HECD = Hall s electrolytic conductivity detector HPLC = high performance liquid chromatography MEC = molecular emission cavity analysis MS - mass spectrometry HD = photo-ionization detector... 

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