Electrochemical EC Detection

Another important detection method is based on electrochemical measurements. These include the measurement of voltage (in potentiometry), solution resistance (in conductivity), current (in amperometry), and current-voltage profile (in voltammetry). These electroanalytical techniques are described in subsequent sections. 

---

Once the analyte has been separated from the matrix in LC, the best approach to the detection of the molecule must be determined. This section will discuss the detection techniques of ultraviolet/visible (UVA IS), fluorescence (FL), and electrochemical (EC) detection, with MS being addressed separately in Section 4.2. When deciding on the most appropriate detector for an LC separation, the appropriate chemical data on the analyte should be collected by using a spectrophotometer, fluorimeter, and potentiometer. 

Fig. 6.2.2a-bc High-performance liquid chromatography (HPLC) with electrochemical (EC) detection of neurotransmitter metabolites, a standard mixture b cerebrospinal fluid (CSF) sample - control c CSF sample - aromatic amino acid decarboxylase (AADC) deficiency. Peak identification 1 = 5HIAA (7.7 min), 2 = 3-MD (9.6 min), 3 = HVA (11.7 min)... 

The aim of this chapter is to show through the subsequent sections the present trends in capillary electrophoresis microchips paying special attention to the manufacturing and designs employed in those combined to electrochemical (EC) detection. Conductimetric and amperometric detection are considered. Clinical application of these devices is further revised. 

Brenes et al. employed two HPLC variants one equipped with the UV detection system and proposed the other with electrochemical (EC) detection. An effectiveness test was simultaneously performed on the three most applicable extraction methods, SPE, LEE, and DMF (proposed) based on the treatment of the extracted oil with 2NHC1 followed by the analysis of phenols in the aqueous phase. The conclusion was that 15%-40% of phenols remained unextracted when the liquid/liquid extraction method was applied with 80% methanol. Solid phase extraction (C,g cartridge) succeeded in retaining most phenols in the cartridge, but the recovery yield from the sorbent material was low. However, the new extraction method, based on the use of V,V-dimethylformamide (DME) as an extraction solvent, achieved a complete extraction of phenols from oils. 

When analyzing complex samples by HPLC, the selection of detection system is important. If chromatographic separation is incomplete or analytic concentration very low, then the more universal and insensitive ultraviolet absorbance (UV) detectors are not satisfactory, and other detection systems must be used. Fluorescense (FL) detection is a sensitive and selective alternative for those compounds that fluoresce. Electrochemical (EC) detection differs from UV and FL in that it is based on a chemical reaction rather than a physical phenomenon, and is the best choice for the many electroactive compounds. In this system analytes are either oxidized or reduced on the electrode surface. More precisely, this technique may be called ampero-metric or voltammetric detection, though in practice it is commonly referred to as EC. 

Electrochemical (EC) Detection In fact, most compounds are not naturally fluorescent and must be prebound with a fluorophore before being detected through fluorescence. Electrochemical (EC) detection is a universal label-free technique that is cost-effective and easy for integration with other functionalities in Lab-on-Chip devices. EC detection can be further divided into three categories ... 

Electrochemical (EC) detection is particularly well suited to capillary electrophoresis. In contrast to many optical methods, EC detection can be miniaturized without a loss of sensitivity. It is very selective because there are few compounds in biological samples that are electroactive. The selectivity is tunable and dependent on the electrode material and the applied potential. Modified electrodes and multiple electrode systems can be used to increase selectivity for certain classes of analytes. Electrodes and potentiostats can also be integrated into chip-based CE systems using procedures similar to those employed for the manufacture of the analytical chips. 

Detection limits reported for several HPLC methods are given in Table 1. With fluorometric (F) or electrochemical (EC) detection, it is possible to achieve much better sensitivity than with UV detection. However, UV detection has sufficient sensitivity for the determination of folic acid in fortified foods and vitamin... 

Electrochemically active compounds can be evaluated using a potentiometer to generate a cyclic voltammogram for the analyte. Cyclic voltammetry will allow the analyst to determine whether the compound can be oxidized or reduced, to choose the appropriate potential to use in the electrochemical detector, and to establish whether oxidation or reduction is irreversible. Irreversible oxidation or reduction of the analyte could be predictive of problems with electrode poisoning and reduced sensitivity of the electrochemical detector over time. Turberg et al. used EC detection at an applied potential of -1-600 mV to analyze for ractopamine. 

In most applications, the electrochemical compounds are usually oxidized, yielding one or more electrons per molecule reacted. The oxidized form is usually unstable and reacts further to form a stable compound that flows past the carbon electrode surface. Unfortunately, this is not always the case, with the stable oxidized form occasionally building up at the surfaces of the carbon electrode. This creates sensitivity problems and decreases the efficiency of the detector. However, the problem is usually overcome by regularly cleaning the carbon electrode surfaces, removing any oxidizable products. Eluents for EC detection must be electrochemically conductive, which is achieved by the addition of inert electrolytes (to maintain a baseline current) such as phosphate or acetate. All solvents and buffers used in preparation of an eluent must be relatively pure and selected so as to not undergo electrochemical changes at the applied electrode potentials. 

Electrochemical (EC) detectors have been used for detection in CE. EC methods offer an advantage over the spectroscopic detection methods because electrochemistry that occurs directly at an electrode surface is not limited by the small dimensions inherent in The... 

MTHF was initially measured by microbiological and radioisotope dilution assay [12, 13], and later by high-pressure liquid chromatography (HPLC) using electrochemical (EC), ultraviolet, or fluorescence detection [14-16]. Compared to other methods, EC detection is more sensitive. 

Catecholamines, nerve transmitters monitored in brain and heart patients, are separated on C18 using octane sulfonate ion pairing in 6% An/water (pH 3) with added EDTA and phosphate. Detection can be at UV, 270 nm, or by electrochemical detection at +0.72 V for maximum sensitivity. Other tyrosine and tryptophan metabolite neurotransmitters such as serotonin, VMA, and HMA can be analyzed with ion pairing and EC detection. 

It was reported that in-channel amperometric detection was possible without using a decoupler. This has been achieved using an electrically isolated (non-grounded, floating) potentiostat [750]. On the other hand, when the CE power supply was battery operated (12-V battery to power 3-kV HV module) and therefore it was electrically isolated from the electrochemical detection system [765], In another report, a portable HV power supply (using 6 V battery for the HV module and 9 V battery for the electrochemical detection circuit) was constructed for CE-EC detection on a glass chip [751],... 

Electrochemical (EC) techniques provide an alternative way to detect sulfur containing molecules. Earlier methods of EC detection involve the application of a gold/mercury electrode.15 Platinum and gold electrodes have also been used for anodic detection of thiols,16 but this requires high oxidation potentials, which complicates analytical applications. Thus, chemically modified electrodes with inorganic or organic mediators have been employed to facilitate electron-transfer between the electrode and the analyte, and therefore reduce the oxidation potential. Recently, pyrroloquinoline quinone (PQQ) modified electrodes have been developed for detection of endo- and exogenous thiols.17... 

IMPROVED SENSITIVITY FOR EXPLOSIVE VAPOURS AND ICAO TAGGENTS DETECTION USING ELECTROCHEMICAL (EC) SENSORS... 

Ilible 19-2. Detection limits, range, and response times for flame emission spectrometry (FES), Fburier transform infra red (FTIR) spectrometry, mass spectrometry (MS), paper tape (FT), solid state (SS) sensors, and electrochemical (EC) sensors. Threshold limit value (TLV) maximum exposure in 8 h period in 40 h work week [7]. 

Procedures for the production of large quantities of fused silica capillaries were developed to fill the growing demand for capillary gas chromatography columns. The advent of laser induced fluorescence (LIF) and electrochemical (EC) detectors provided the high mass sensitivity necessary for detection in nanoliter volumes. Jorgenson and Lucas published several key papers on modern CE in 1981. Since then CE has experienced exponential growth to the point where there are well over 2000 articles published annually involving CE. 

As the system is aqueous based, it is routinely used for the analysis of nonvolatile analytes with no need for derivatization of sample to maintain analyte solubility. A typical LC system experiences lower diffusion rates than GC because the mobile phase has a higher viscosity and density. LC can be coupled online to sensitive electrochemical and fluorescence detection units. This is an important consideration for the analysis of EDCs present at low concentrations in environmental matrices. Both UV and EC detection were applied to the determination of estrogenic compounds in water samples using LC. 

---