Electrochemical coulometric detector

While the terms amperometric detection and coulometric detection have come into use to describe detectors of less than 100% efficiency and 100% efficiency respectively, these terms are actually misnomers. An amperometric detector is any electrochemical detector where current is plotted as a function of time, regardless of the conversion efficiency. A coulometric detector is any electrochemical detector where charge is plotted as a function of time, again regardless of the conversion efficiency. Preferred terminology should be high efficiency and low efficiency detectors to describe the two situations. 

Microcoulometric titration is used as the detection mode in some commercial sulfur-specific analysers. Sulfur in PP and waxes (range from 0.6 to 6 ppm S) were determined by means of an oxidative coulometric procedure [537]. The coulometric electrochemical array detector was used for determining a variety of synthetic phenolic antioxidants (PG, THBP, TBHQ, NDGA, BHA, OG, Ionox 100, BHT, DG) in food and oils [538],... 

Ferruzzi, M.G., Sander L.C., Rock C.L., and Schwartz S.J. 1998. Carotenoid determination in biological microsamples using liquid chromatography with a coulometric electrochemical array detector. Anal. Biochem. 256 74-81. 

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. 

The first combined HPLC-electrochemical measurements of vitamin K used the reductive mode, but this technique suffered from interference from the reduction of oxygen. A redox method was later developed that eliminated this interference, and provided a 10-fold increase in sensitivity over photometric detection and an improved selectivity. The coulometric detector employed in the redox mode is equipped with a dual-electrode cell in which phylloquinone is first reduced upstream at the generator electrode and the hydroquinone is reoxidized downstream at the detector electrode. 

Achilli, G., Cellerino, G. P., Melzi d Eril, G. V., and Tagliaro, F. (1996). Determination of illicit drugs and related substances by high-performance liquid chromatography with an electrochemical coulometric-array detector. J. Chromatogr. A 729 273-277. 

Eluted peaks were detected by electrochemical oxidation using the ESA 5100A coulometric detector equipped with an ESA 5010 dual electrode detector cell and a guard cell (ESA, Bedford, MA). The guard cell was placed between the pump and injector (19) and set at a potential of 0.75V. The first electrode of the analytical cell was set at a potential of 0.5V and the second electrode at which OA and N-acetyl OA are oxidized was set at 0.7V. 

SPE, solid-phase extraction LLE, liquid-liquid extraction SPME, solid-phase microextraction DAD, diode-array detector CD, coulometric detector ED, electrochemical detector FD, fluorescence detector MS, mass spectrometry IT, ion trap TOP, time-of-flight. 

Metal meshes (e.g.. Mo) have also been coated. Such electrodes could eventually be useful for electrosynthesis, coulometric detectors, and as supports for electrocatalysts. For many of the envisioned electrochemical applications of diamond, the electrode needs to be in a nonplanar, high surface area form. The low-magnification SEM image in Fig. 4B shows a junction in the mesh between two wires coated with diamond. The film was deposited using a CH4/H2 source gas mixture (—0.5%). The mesh wires are about 200 pm in diameter, and, although not easy to see, the diamond film thickness is several micrometers. 

Five major tea catechins (e.g., epicatechin, epicatechin gallate, epigallocatechin 3-galIate) were isolated from plasma and baseline resolved on a C g column (coulometric detector at 70 mV) using a complex 44-min 96/4 -> 67/33 water (1.75% acetonitrile, 0.12% THF, and lOOmM sodium phosphate buffer at pH 3.35)/(58.5/29/12.5 acetonitrile/water/THF with 15mM sodimn phosphate buffer at pH 3.45 ) gradient [1157]. The authors noted that bacterial growth in the weak solvent was foxmd at phosphate levels below lOOmM. They also noted that acetic acid was not compatible with the electrochemical detection system setup used here. Standards from 1 to 10,000 ng/mL were used. Detection limits of 5ng/mL (S/N = 3) were reported. 

Electrochemical detectors can be classified according to the three fundamental parameters of voltage or potential (V), resistance (R), or current (i). These terms are related via Ohm s Law, which is V = I R. Electrochemical detectors are considered to be conductometric, potentiometric, amperometric or coulometric detectors. Conductometric detection has been discussed earUer in this chapter and there is only Umited discussion in this section. Coulometric detection is not commonly used and is discussed only briefly. 

Figure 24. Coulometric detector with dual porous electrodes and total cell volume < 5 pL (Coulochem 5100 A electrochemical detector)...

Importantly, King succeeded in the formation of a Fc-labeled 998-bp (base-pair) construct by PCR using T4 DNA polymerase in the presence of Fcl-dUTP. Incorporation of the redox label shows that Fcl-dUTP is suitable as a substrate for PCR. In contrast, Fc2-dUTP acts predominantly as a terminator in the PCR. The melting behavior of a 37-mer duplex containing five Fcl-dU residues reveals that the labeled nucleotide induces only a modest helix destabilisation, with T ,=71°C for a labeled duplex versus 75°C for the corresponding nonlabeled ds-DNA construct. King reports that the Fc-labeled DNA is detected at femtomolar levels by HPLC using a coulometric detector. Thus, it must be emphasized that the incorporation of the Fc label by PCR and its facile and cost-effective electrochemical detection should promote the use of this technique in nucleic acid analysis and may replace the more costly fluorescence-based detection systems. 

Nonspectroscopic detection schemes are generally based on ionisation (e.g. FID, PID, ECD, MS) or thermal, chemical and (electro)chemical effects (e.g. CL, FPD, ECD, coulometry, colorimetry). Thermal detectors generally exhibit a poor selectivity. Electrochemical detectors are based on the principles of capacitance (dielectric constant detector), resistance (conductivity detector), voltage (potentiometric detector) and current (coulometric, polarographic and amperometric detectors) [35]. 

UV detection, diode-array detector (DAD) and fluorescence have been the detection techniques used, coupled to HPLC for the analysis of OTC. UV detection is set at 355 nm [49-51], 350 nm [40], or at 353 nm [52], Using the diode array detector [49] offers advantages that the target peak can be identified by its retention time and absorption spectrum. Compared to UV detection, fluorescence detection is generally more specific and is less interfered by other compounds in the sample matrix [51]. A HPLC method with electrochemical detection has also been suggested recently. Zhao et al. [53] described HPLC with a coulometric electrode array system for the analysis of OTC, TC, CTC, DC, and methacycline (MC) in ovine milk. An amper-ometric detection coupled with HPLC was developed by Kazemifard and Moore [54] for the determination of tetracyclines in pharmaceutical formulations. 

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