Amperometric electrode HPLC detector

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

The comparison of analytical characteristics HPLC methods of determination of phenols with application amperometric and photometric detectors was caiiy out in this work. Experiment was executed with use liquid chromatograph Zvet-Yauza and 100 mm-3mm 150mm-3mm column with Silasorb C18 (5 10 p.m). With amperometric detector phenols were detected in oxidizing regime on glass-cai bon electrodes. With photometric detector phenols were detected at 254 nm. 

Many published articles on HPLC-ECD refer to the use of one of three voltammetric detectors (amperometric, coulometric, or polarographic). More detailed information on principles and techniques of various electrochemical detection modes can be obtained from the recent book, Coulometric Electrode Array Detectors for HPLC (34). There are also two electrode array detectors, the coulometric electrode array system and the CoulArray detector, currently available. Both detectors offer the qualitative data of PDA and the extreme sensitivity of ECD (34). The... 

As noted earlier, one of the shortcomings of flow techniques is the lack of flow cells which would allow the instruments currently available to be coupled to flow systems. However, especially common among available cells are amperometric and coulometric cells for solid electrodes. Some have been designed for HPLC detectors, not being all useful for flow systems owing to the high pressure required to pass liquids through them. 

For selective estimation of phenols pollution of environment such chromatographic methods as gas chromatography with flame-ionization detector (ISO method 8165) and high performance liquid chromatography with UV-detector (EPA method 625) is recommended. For determination of phenol, cresols, chlorophenols in environmental samples application of HPLC with amperometric detector is perspective. Phenols and chlorophenols can be easy oxidized and determined with high sensitivity on carbon-glass electrode. 

An ECD measures the current generated by electroactive analytes in the HPLC eluent between electrodes in the flow cell. It offers sensitive detection (pg levels) of catecholamines, neurotransmitters, sugars, glycoproteins, and compounds containing phenolic, hydroxyl, amino, diazo, or nitro functional groups. The detector can be the amperometric, pulsed-amperometric, or coulometric type, with the electrodes made from vitreous or glassy carbon, silver, gold, or platinum, operated in the oxidative or reductive mode. Manufacturers include BSA, ESA, and Shimadzu. 

The diffusion-limited electrochemical oxidation of V-nitrosamines in an aqueous pH 1.5 buffer was demonstrated at a GCE coated with a film of mixed valence ruthenium oxides, stabilized by cyano crosslinks. This electrode was used in a potentiostatic amperometric detector for FIA and HPLC, to allow the determination of representative N-nitrosamines (278a, 278c and 278d) for 278c, LOD was 10 nM and RSD 2% at 0.80 pM... 

Because of its advantages (high sensitivity and selectivity, low cost and miniaturization) amperometric detection has been frequently used in flow injection analysis (FIA) and RP-HPLC. However, it has been established that the peak area (detector response) considerably depends on the flow rate. A general approach has been proposed to predict the effect of flow rate on the peak area in FIA and RP-HPLC. The general form of the correlation describing the flow in a parallel plate cell with short rectangular electrodes is... 

All of the fat-soluble vitamins, including provitamin carotenoids, exhibit some form of electrochemical activity. Both amperometry and coulometry have been applied to electrochemical detection. In amperometric detectors, only a small proportion (usually <20%) of the electroactive solute is reduced or oxidized at the surface of a glassy carbon or similar nonporous electrode in coulometric detectors, the solute is completely reduced or oxidized within the pores of a graphite electrode. The operation of an electrochemical detector requires a semiaqueous or alcoholic mobile phase to support the electrolyte needed to conduct a current. This restricts its use to reverse-phase HPLC (but not NARP) unless the electrolyte is added postcolumn. Electrochemical detection is incompatible with NARP chromatography, because the mobile phase is insufficiently polar to dissolve the electrolyte. A stringent requirement for electrochemical detection is that the solvent delivery system be virtually pulse-free. 

Acetylcholineesterase and choline oxidase 300 pL 0.1 M phosphate buffer (pH 6.5) containing 16 mg BSA and 1 mg each of ChO and AChE were mixed with 30 pL 25% glutaral-dehyde diluted 10 fold with phosphate buffer. The solution was used to coat the surface of a Pt electrode. This enzyme electrode was used for the amperometric measurement of ACh and Ch. Calibration graphs were linear upto 0.09 and 0.08 mM Ch and ACh, respectively. Detection limits were 0.1 pM of both Ch and ACh. Response time was 1 s for both Ch and ACh. The use of the sensor as detector for HPLC analysis for both Ch and ACh was demonstrated. [91]... 

Electrochemical Detectors. In amperometric electrochemical detectors (see Chapter 4), an electroactive analyte enters the flow cell, where it is either oxidized or reduced at an electrode surface under a constant potential. Electroactive compounds of clinical interest conveniently analyzed by HPLC with electrochemical detection include the urhiary catecholamines (see Chapter 29). In addition, electrochemi-cally active tags (e.g., bromine) are added to compounds such as unsaturated fatty acids or prostaglandins. 

HPLC with amperometric detector wax impregnated carbon paste detector electrode 270... 

Figure 20.5 Amperometric detection in HPLC and HPCE. (a) Two models of detector cells. The porous graphite working electrode have a large surface and operates under coulometric conditions. The flow of the mobile phase at the working electrode ensures renewal of the electroactive species (b) Detail of the end of a capillary in HPCE. The working electrode, placed on the cathodic side of the apparatus, is bathed by ions exiting the capillary. Apart from phenols, aromatic amines and thiols, few analytically important molecules are electroactive.

---