Detection coulometric

In coulometric mode, in which EC conversion is considered to be complete, it can be shown that the steady state current (i) is dependent on the rate at which analyte is delivered to the electrode and is given by  

Thus increasing the flow-rate will increase the peak height but the peak area remains constant over a wide range of flow-rates. Coulometric detection is therefore an absolute method, such that peak area can provide a means of quantification by relating area directly to sample mass using Faraday s Law, provided the number of electrons being transferred is known. 

When an electroactive analyte passes through a coulometric cell its concentration decreases with time according to first order kinetics  

To achieve coulometric operation it is necessary to have a very large rate constant. Since this reflects primarily mass transport in the cell it can be shown that  

D is the diffusion coefficient (cm s ) d is the diffusion layer thickness (cm) V is the cell volume (cm ) 

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PEYRAT-MAiLLARD M N, BONNELY s and BERSET c (2000) Determination of the antioxidant activity of phenolic compounds by coulometric detection, Talanta, 51, 709-16. 

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. 

Jorg, E. and Sontag, G. (1993). Multichannel coulometric detection coupled with liquid chromatography for determination of phenolic esters in honey. /. Chromatogr. A 635, 137-142. 

Coulometric detection describes amperometry in which the reaction of the analyte proceeds to completion.59 The extent of reaction is a function of cell geometry, flow rate, and analyte concentration, so all cells can be made coulometric by stopping the flow.59 Several designs for coulometric cells that are independent of flow rate have been described.46... 

The multichannel coulometric detection system serves as a highly sensitive tool for the characterization of antioxidant phenolic compounds because they are electroactive substances that usually oxidize at low potential. The coulometric efficiency of each element of the array allows a complete voltammetric resolution of analytes as a function of their oxidation potential. Some of the peaks may be resolved by the detector even if they coelute (Floridi and others 2003). 

Careri M, Elviri L, Mangia A and Musci M. 2000. Spectrophotometric and coulometric detection in the high-performance liquid chromatography of flavonoids and optimization of sample treatment for the determination of quercetin in orange juice. J Chromatogr A 881 449-460. 

Detection with 100% efficiency is called coulometric detection, detection with <100% efficiency is called amperometric detection. Typical efficiency for... 

Ary K, Rona K, Ondi S, Gachalyi B. 1998. High-performance liquid chromatographic method with coulometric detection for the determination of buspirone in human plasma by means of a column-switching technique. J Chromatogr A 797(1-2) 221-226. 

Foglia JP, Sorisio D, Kirshner MA, Mulsant BH, Perel JM. 1995. Quantitative determination of perphenazine and its metabolites in plasma by high-performance liquid chromatography and coulometric detection. J Chromatogr B Biomed Appl 668(2) 291-297. 

Luo JP, Hubbard JW, Midha KK. 1997. Sensitive method for the simultaneous measurement of fluphenazine decanoate and fluphenazine in plasma by high-performance liquid chromatography with coulometric detection. J Chromatogr B Biomed Sci Appl 688(2) 303-308. 

Odontiadis J, Franklin M. 1996. Simultaneous quantitation of buspirone and its major metabolite l-(2-pyrimidinyl)pi-perazine in human plasma by high-performance liquid chromatography with coulometric detection. J Pharm Biomed Anal 14(3) 347-351. 

Recently flow coulometry, which uses a column electrode for rapid electrolysis, has become popular [21]. In this method, as shown in Fig. 5.34, the cell has a columnar working electrode that is filled with a carbon fiber or carbon powder and the solution of the supporting electrolyte flows through it. If an analyte is injected from the sample inlet, it enters the column and is quantitatively electrolyzed during its stay in the column. From the peak that appears in the current-time curve, the quantity of electricity is measured to determine the analyte. Because the electrolysis in the column electrode is complete in less than 1 s, this method is convenient for repeated measurements and is often used in coulometric detection in liquid chromatography and flow injection analyses. Besides its use in flow coulometry, the column electrode is very versatile. This versatility can be expanded even more by connecting two (or more) of the column electrodes in series or in parallel. The column electrodes are used in a variety of ways in non-aqueous solutions, as described in Chapter 9. 

Figure 27.16B shows the method known as pulsed coulometric detection (PCD). In this case, the current is integrated over a longer period and the time period is an integral number of 16.7-ms segments with typical total integration times of greater than 200 ms. The use of this type of waveform eliminates the most common electrical interference (60 Hz sinusoidal) encountered in pulsed electrochemical detection, and thereby increases the detection limits for most compounds. 

Fats, oils, and dairy products digest sample with lipase, extract with hexane. Purify hexane extracts obtained from either extraction technique by silica solid-phase extraction in the sample cleanup mode. The internal standard is phylloquinone 2,3-epoxide (unlabeled for UV detection and tritium labeled for coulometric detection). 

Fig. 14 Analytical HPLC of the phylloquinone fraction from an extracted sample of brown rice isolated by semipreparative HPLC. Column, Spherisorb C8 (octyl) mobile phase, methanol/50 mM acetate buffer pH 3.0 (97 3) containing 0.1 mM EDTA, dual-electrode coulometric detection (redox mode), porous graphite electrodes, — 1.5 V (generator electrode), +0.05 V (detector electrode). The arrows signify the fraction containing tritiated phylloquinone 2,3-epoxide (internal standard) and phylloquinone (analyte) that is collected for quantitation by radioisotopic dilution. (Courtesy of M. J. Shearer.)...

MeOH/50 mM acetate buffer pH 3.0, 97 3 containing 0.1 mM EDTA Dual-electrode coulometric detection (redox mode), porous graphite electrodes,... 

Dual-electrode coulometric detection (redox mode), porous graphite electrodes ... 

Sabbioni, C., Saracino, M. A., Mandrioli, R., Pinzauti, S., Furlanetto, S., Gerra, G., and Raggi, M. A. (2004). Simultaneous liquid chromatographic analysis of catecholamines and 4-hydroxy-3-methoxyphenylethylene glycol in human plasma. Comparison of amperometric and coulometric detection. J. Chromatogr. A 1032 65 -71. 

It is also possible to employ detectors with solutions flowing over a static mercury drop electrode or a carbon fiber microelectrode, or to use flow-through electrodes, with the electrode simply an open tube or porous matrix. The latter can offer complete electrolysis, namely, coulometric detection. The extremely small dimensions of ultramicroelectrodes (discussed in Section 4.5.4) offer the advantages of flow-rate independence (and hence a low noise level) and operation in nonconductive mobile phases (such as those of normal-phase chromatography or supercritical fluid chromatography). 

With the application of electron-capture (EC) and micro-coulometric detection to gas chromatograph effluents from i960, the era of the measurement of nothing in everything had arrived and the environmental controversy was truly on. It was easier to make an effective EC detector than to interpret the analytical results correctly and many of the identifications of chlorinated insecticide (OC) residues made in the early 1960s are undoubtedly suspect, especially since it was found in 1966 that widespread polychlorobiphenyl (FCB) contamination in the bio-sphere can simulate OC in gas chromatographic analysis. 

HPLC is less commonly used by laboratories performing hair tests. HPLC with fluorescence detection has been evaluated by Marigo et al." Following LLE, extracts were derivatized with a solution of dansyl chloride. The derivatized extract was back-extracted into toluene, evaporated, and reconstituted in chromatographic eluent. The limit of detection was <1 ng/mg. Also, HPLC with coulometric detection was evaluated by Kintz for the analysis of buprenorphine. The detection limits were about 0.01 and 0.02 ng/mg for buprenorphine and norbuprenorphine, respectively. 

Hair samples were collected from subjects who reported daily use of buprenorphine, a partial opioid agonist. Buprenorphine and its metabolite, norbuprenorphine, were identified in the samples using HPLC with coulometric detection. The concentrations of norbuprenorphine were lower than buprenorphine." ... 

Substrates (p-octopamine, dopamine, or 5-hydroxytryptamine) were separated from their N-acetylated products on an Ultrasphere I.P. C18 column (4.6 mm x 250 mm, 5 pm). The mobile phase was comprised of 75 mAf monobasic sodium phosphate, 1 pAf EDTA, 0.35 mAf 1-octanesulfonate (sodium salt), 11% methanol, and 4% acetonitrile. Coulometric detection was used. Detection of p-octopamine and its N-acetylated product was achieved at a potential of +0.75 V, while dopamine, 5-hydroxytryptamine and their products were detected at +0.50 V. 

Mancini, F. et al. Monolithic stationary phase coupled with coulometric detection development of an ion-pair HPLC method for the analysis of quinone-bearing compounds. J. Sep. Sci. 2007, 30, 2935-2942. 

Iodide in urine [39] and catecholamines [40,41] are example of analytes recently detected electrochemically and studied under IPC conditions. Pulsed amperometric detection on a gold electrode was used to detect etimicin [42] and gentamicin [43] in commercial samples, thus avoiding tedious pre-column derivatization. Heterocyclic aromatic amines in soup cubes [44] were determined by a coulometric electrode array detector, and the coulometric detection of a quinone-bearing drug candidate [45] allowed the study of electrochemical properties. 

This detector responds to substances that are either oxidizable or reducible and the electrical output results from an electron flow caused by the reaction that takes place at the surface of the electrodes. If the reaction proceeds to completion, exhausting all the reactant, then the current becomes zero and the total charge that passes will be proportional to the total mass of material that has been reacted. For obvious reasons, this process is called coulometric detection. If, on the other hand, the mobile phase is flowing passt the electrodes, the reacting solute will be continuously replaced as the peak passes through the detector. While there is solute present between the electrodes, a current will be maintained, albeit varying in magnitude. Until relatively recently, this procedure was the most common employed in electrochemical detection and is called amperometric detection. 

Three Dimensional Graphs Demonstrating the Difference between Amperometric and Coulometric Detection Employing an Electrode Array Courtesy of the Analyst. 

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