Electrochemical detectors for HPLC

Roe, D. K., Comparison of amperometric electrochemical detectors for HPLC through a figure of merit, Anal. Letts., 16, 613, 1983. 

Fluorescence detection, because of the limited number of molecules that fluoresce under specific excitation and emission wavelengths, is a reasonable alternative if the analyte fluoresces. Likewise, amperometric detection can provide greater selectivity and very good sensitivity if the analyte is readily electrochemically oxidized or reduced. Brunt (37) recently reviewed a wide variety of electrochemical detectors for HPLC. Bulk-property detectors (i.e., conductometric and capacitance detectors) and solute-property detectors (i.e., amperometric, coulo-metric, polarographic, and potentiometric detectors) were discussed. Many flow-cell designs were diagrammed, and commercial systems were discussed. 

Fleet, B. Little, C.J. Design and evaluation of electrochemical detectors for HPLC [high-pressure liquid chromatography]. J. Chromatogr. Sci. 1974,12, 747-752. 

Thus electrochemical pretreatment is seen to be an important means of modifying the surface of carbon electrodes prior to their use in electrochemical detectors for HPLC and FIA. This is mostly probably due to the formation of more surface oxygen-containing sites which are capable of promoting oxidation of various organic species. 

G.V. Melzi d Eril, G. Achilli and G.P. Cellerino, A new microprocessor controlled multi-mode multi-electrode electrochemical detector for HPLC Sensitivity and selectivity enhancements in neurochemical measurements. Inti. LabMate, 1992 (February), 17-18. 

Basically, an electrochemical detector for HPLC is a flow electrochemical cell which contains all of its basic elements (working, reference and auxiliary electrodes, holder, and connections) adapted to the experimental conditions expected in the chromatographic separations. The characteristics of the materials used for the holder and the electrodes construction must be compatible with typical mobile phases, working pressures, and temperatures employed in HPLC whereas the geometric configuration must provide an easy coupling... 

Data and chromatograms for four antibiotics will be used to help illustrate and characterize representative approaches to real situations. The work on cefadroxil, cefmenoxime, cefsulodin, and clarithromycin are all HPLC assays. The three cephalosporins used a UV finish, while the clarithromycin being a macrolide antibiotic and having a low chro-mophoric response, required an electrochemical detector for quantitation. 

Although this section provides a brief description of most commonly nsed detectors for HPLC, most of the focus is on a few detection modes. Optical absorbance detectors remain the most widely nsed for HPLC, and are discnssed in some detail. We also focns on flnorescence, condnctivity, and electrochemical detection, as these methods were not widely nsed for HPLC in the past, bnt are especially well suited to micro- and nano-flow instrnments becanse of their high sensitivity in small sample volumes. Mass spectrometry has also come into wide and rontine nse in the last decade, but as it is the subject of another chapter, it will not be fnrther discnssed here. Miniaturization has been particularly important for capillary and chip-based electrophoresis, which often employs sub-nanoliter detection volnmes [36,37]. 

Electrochemical detection of carbohydrates at nickel-copper and nickel-chromium-iron alloy electrodes has been reported for sorbitol, and has been used as a detector for HPLC analysis [36]. Oxidation of various carbohydrates at the electrodes was used for detection, and baseline separation was achieved for mixtures of sorbitol, rhamnose, glucose, arabinose, and lactose. 

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... 

S. J. Lyle and M. I. Saleh, Observations on a Dropping-Mercury Electrochemical Detector for Flow Injection Analysis and HPLC. Talanta, 28 (1981) 251. 

A recent advanced analytical application of LSV and CV is their introduction in electrochemical detectors for flow analysis (HPLC, EC). Fast-scan LSV and CV (20-1000Vs ), performed at the rising portion of the typical peaks afforded by these hydro-dynamic methods with microelectrodes suitably positioned at the outlet of the flowing system, provide... 

R.J. Driebergen, and A.J.T.C. Benders, Comparative performance of electrochemical detectors for FI A and HPLC. Chromatogr Anal 1990 (February) 13-4. Response Andrews RW. Chromatogr Anal 1990 (June) 17. 

Amperometric Detectors. Amperometric detectors are the most commonly used electrochemical detectors for highly sensitive and selective determinations in HPLC [30], [31 ]. The frequently used synonym electrochemical detector is not very precise in relation to the detection mode. 

Table 28-1 lists the most common detectors for HPLC and some of their most important properties. The most widely used detectors for LC are based on absorption of ultraviolet or visible radiation (see Figure 28-8). Fluorescence, refractive-index, and electrochemical detectors are also widely used.. Mass spectrometry (MS) detectors are currently quite popular. Such LC/MS systems can greatly aid in identifying the analytes exiting from the HPLC column as discussed later in this section. 

I. N. Acworth and M. Bowers, An Introduction to HPLC-Based Electrochemical Detection From Single Electrode to Multi-Electrode Arrays, in Coulometric Electrode Array Detectors for HPLC. Progress in HPLC-HPCE. 6, ed. I. N. Acworth, M. Naoi, S. Parvez and H. Parvez, 1997, VSP Publications, The Netherlands, pp. 3-50. 

Electrochemical detection in HPLC has become established for some, more specialized, applications such as catecholamine analysis though it can be exploited for a far wider range of compounds. The work in this paper has attempted to investigate some of the basic properties of an electrochemical detection system and some more difficult applications. The detector has a linear dynamic range and precision that are comparable with those of other detectors for HPLC. It is, however, more dependent on temperature than, for example, the UV absorption detector and must be operated in a temperature controlled environment to obtain the lowest detection limits. For many electroactive compounds with moderate oxidation potentials, the electrochemical detector can yield sub-nanogram detection limits. 

In the following part of this paper I am going to present a system of organic electroactive groups or of substances which can be subjected to voltammetric analysis at solid electrodes forming thus a base for construction of electrochemical detectors in HPLC. Since electroactivity is the most important property of the substance to be studied and we are interested here only in the analytical properties, i.e. in the characteristical potentials (Ep or Ep and in the shape of the i - E or ip - E plot it will be not necessary to stress Adams observatlon[7] "Electrochemistry of organic oxidations is mainly the chemistry of follow-up reactions" (of course, similar ideas hold true with reductions). Nevertheless, electroactivity will be discussed here jointly with probable or proved interpretations of mechanisms. This is done in contrast to Bond s view[9] who considers interpretations of electrode processes and of their follow-up or preceding reaction useless in analytical chemistry. 

The available detectors for HPLC involve either bulk properties of the mobile phase (such as refractive index, conductance, dielectric constant) or specific properties of the solute, e.g. ultraviolet, visible or infra-red absorbance, fluorescence, or electrochemical characteristics. The latter class are generally more selective and have a wider dynamic range. 

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. 

HPLC method with amperometric detection was applied for detenuination of phenols in sea sediment and some dmg preparation. Peaks of phenol, guaiacol, cresols, hydroquinon and resorcinol were identified on chromatogram of birch tai. The HPLC method with electrochemical detectors was used for detenuination of some drug prepai ation of aminophenol derivate. So p-acetaminophenol (paracetamol) was determined in some drug. 

The most common detectors in HPLC are ultraviolet, fluorescence, electrochemical detector and diffractometer. However, despite all improvements of these techniques it seems necessary to have a more selectivity and sensitivity detector for the purposes of the medical analysis. It should be therefore improvements to couple analytical techniques like infrared IR, MS, nuclear magnetic resonance (NMR), inductively coupled plasma-MS (ICP-MS) or biospecific detectors to the LC-system and many efforts have been made in this field. 

HPLC has been recommended as a cleanup and fiactionation procedure for food samples prior to analysis by GC/ECD (Gillespie and Walters 1986). The advantages over the AOAC-recommended Florisil colunrn are that it is faster, requires less solvent, and gives better resolution. HPLC coupled with various detectors MS, MS/MS, UV/electrochemical detector, or UV/polarographic detection has been tested as a rapid, simplified separation and detection system to replace GC (Betowski and Jones 1988 Clark et al. 1985 Koen and Huber 1970). Recoveries, detection limits, and precisions were generally good, but further work is needed before the techniques are adopted for general use. 

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