product review
Electrochemical Detectors for Liquid Chromatography Bioanalytical applications keep a mature market strong. Britt E. Erickson
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lthough electrochemical (EC) detectors make up only a small fraction of the LC detector market, their importance cannot be overlooked. For many applications, ranging from the analysis of catecholamines and other neurochemicals to the detection of DNA adducts, antioxidants, carbohydrates, and amino acids, EC detection offers better selectivity and sensitivity than other modes of LC detection. Because only compounds that undergo an electrochemical reaction at an applied potential are detected, EC detectors are inherently more selective than other LC detectors, such as UV–vis and refractive index detectors. In addition, detection limits in the nanomolar and even picomolar range have been achieved with EC detection, making it well suited for the analysis of extremely low levels of biological compounds.
As far as the technology is concerned, little has changed since the last time Analytical Chemistry reviewed commercially available EC detectors for LC (Anal. Chem. 1994, 66, 601 A– 606 A). New products, however, which claim to be more flexible and userfriendly, have emerged. The new epsilon introduced by Bioanalytical Systems (BAS) at Pittcon 2000 even incorporates Internet technology so that it can be operated at home or on business trips and upgraded via the Web. Dionex
also introduced a new EC detector (ED50) at Pittcon 2000, specifically designed for detection of amino acids using integrated amperometry on a gold electrode. Multiple channel EC detectors are now available, and cells designed to accommodate microbore LC columns are relatively common. A summary of selected commercially available LC-EC detectors is presented in Table 1. Several manufacturers offer EC detectors other than those shown in Table 1. Readers are encouraged to
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product review
Table 1. Summary of selected electrochemical detectors for LC. Model
Agilent 1049A
DECADE†
epsilon
800A Series
Manufacturer
Agilent Technologies 2850 Centerville Rd. Wilmington, DE 19808 800-227-9770
Antec Leyden P.O. Box 3091 2301 DB Leiden, The Netherlands 31 71 581 3333
Bioanalytical Systems 2701 Kent Ave. West Lafayette, IN 47906 317-463-4527
CH Instruments 3700 Tennison Hill Dr. Austin, TX 78733 512-402-0176
URL
www.chem.agilent.com
www.antec-leyden.nl
www.bioanalytical.com www.epsilon-web.net
www.chinstruments.com
Operating modes
DC, pulse, and scan
DC, pulse, and scan
DC, pulse, difference, and CV
DC, pulse (differential, triple), and scan
Cell Design
Thin-layer amperometric
Confined wall-jet or thin-layer amperometric
Cross-flow thin-layer (CC-5) or radial flow thin-layer (UniJet) amperometric
Thin-layer amperometric
Working electrode
gc, Au, Pt
gc, Pt, Au, Ag, Cu
gc, Au, Pt, Ni, Cu, Ag, Pd, carbon paste, combinations, chemically modified (CC-5); gc, carbon paste, Au, Pt (UniJet)
gc, Au, Pt
Volume
0.5 µL
0.005 µL (minimum, wall-jet); 0.5 µL (thin-layer)
0.3 µL (13-µm gasket, CC-5); 0.05 µL (2-mm electrode and 16-µm gasket, UniJet)
0.5 µL
±2.0 V in 1-mV steps
±2.0 V
±2.55 V
±3.275 V
Current range
±500 nA or ±500 µA
10 pA–5 µA (dc mode); 10 nA– 5 µA (pulse and scan modes)
100 pA–5 mA
10 pA–0.01 A
Maximum number of channels
1
1
4
2
Built-in VALCO electrically actuated injector (standard or microversion with 60–200 nL injection volume) or Rheodyne manual injector; Dialogue software for computer control; reactor flow cell control
Column temperature control; standard and microbore injectors and columns
Various instrument models available to meet different needs
Integrated oven accomodating column and flow cell; 5-year warranty on gc flow cells; detection sensitivity of 20 pM for catecholamine; time file programming
Optically isolated circuitry for noise reduction; up to four-channel EC capability; provision for inputs from two external LC detectors; upgradeable over the Internet; built-in data processing; remote monitoring via the Internet
Single- or dual-channel measurements; conventional electroanalytical techniques, including CV, LSV, DPV, NPV, SWV, BE, and stripping
Electronics Potential range
Options
Special features
Autoincrement mode; pulse mode; pretreat mode; solvent thermostatting; GLP compliance and automation features; diagnostics with sweep mode; complete control from ChemStation
†DECADE is distributed in North America by GBC Separations, 978-928-1165, www.gbcseps.com, and is privately labeled as STAR 9080 by Varian, Inc., 2700 Mitchell Dr., Walnut Creek, CA 94705, 800-926-3000, www.varianinc.com. GLP = good laboratory practices; CV = cyclic voltammetry; LSV = linear sweep voltammetry; DPV = differential pulse voltammetry; NPV = normal pulse voltammetry; SWV = square wave voltammetry; BE = bulk electrolysis with coulometry; NA = not applicable
contact the manufacturers for their complete product lines.
Modes of detection EC detection is typically divided into four categories—amperometric, coulometric, conductometric, and potentiometric. This review will focus
354 A
exclusively on the two most widely used EC modes for LC detection— amperometric and coulometric. In the amperometric mode, compounds are either oxidized or reduced at a working electrode surface. Although only a small percentage (typically 5–10%) of the analyte is actually oxidized or re-
A N A LY T I C A L C H E M I S T R Y / M A Y 1 , 2 0 0 0
duced at the electrode, the amount of current resulting from the reaction is proportional to the concentration of the analyte in the sample. Similarly, in the coulometric mode, compounds undergo a redox reaction at the surface of a working electrode; however, because of the high efficiency of
product review
Table 1. Summary of selected electrochemical detectors for LC. Model
ED50
CoulArray
Coulochem II
L-ECD-6A
Waters 464
Manufacturer
Dionex 1228 Titan Way P.O. Box 3603 Sunnyvale, CA 94088-3603 408-737-0700
ESA 22 Alpha Rd. Chelmsford, MA 018244171 978-250-7000
ESA 22 Alpha Rd. Chelmsford, MA 018244171 978-250-7000
Shimadzu 7102 Riverwood Dr. Columbia, MD 21046 800-477-1227
Waters 34 Maple St. Milford, MA 01757 508-478-2000
URL
www.dionex.com
www.esainc.com
www.esainc.com
www.shimadzu.com
www.waters.com
Operating modes
Conductivity, DC, and integrated amperometry
DC coulometric array
DC, pulse, and scan coulometric or amperometric
DC
DC, pulse, and scan
Cell Design
Thin-layer amperometric
4, 8, 12, or 16 coulometric electrodes in series
Dual coulometric in series or thin-layer amperometric
Carbon plate
Thin-layer amperometric
Working electrode
Au, Pt, Ag, gc (1-mm diam)
Porous graphite
Porous graphite (coulometric); gc, Au, Ag, Pt (amperometric)
gc
gc, Ag, Au, Pt
Volume
