Amperometric detection amines

Hydrophilic interaction chromatography on Asahipak NH2P or Excel-pak CHA-P44 with pulsed amperometric detection has been used to fractionate malto-oligosaccharides.266 The Asahipak NH2P is a polyvinyl alcohol support with a polyamine bonded phase, and the Excelpak is a sulfonated polystyrene in the Zn+2 form. Amine adsorption of sialic acid-containing oligosaccharides was performed on a Micropak AX-5 column (Varian) using acetonitrile-water-acetic acid-triethylamine.267... 

Voltammetry has been adapted to HPLC (when the mobile phase is conducting), and CE as a detection technique for electroactive compounds. In this usage, the voltammetric cell has been miniaturised (to about 1 p.L) in order not to dilute the analytes after separation. This method of amperometric detection in the pulsed mode is very sensitive. However, this device makes it possible to detect few analytically important molecules besides phenols, aromatic amines and thiols. 

Most aliphatic amines and alcohols are considered to be nonelectroactive. The reason for this is that the product of the oxidation adsorbs to the electrode surface, fouling the electrode. Therefore, most reactions of these compounds at noble metal electrodes have been transient and not amenable to direct amperometric detection. In voltammetry experiments, electrodes are cleaned between experiments by electrochemical or chemical treatment to restore the electrode response. 

F. Ricci, F. Arduini, C.S. Tuta, U. Sozzo, D. Moscone, A. Amine and P. Palleschi, Glutathione amperometric detection based on a thiol-disulfide exchange reaction, Anal. Chim. Acta, 558 (2006) 164-170. 

Pre-column OPA derivatization was also employed to analyze biogenic amines prior to MEKC separation on a PDMS chip [654]. Pre-column OPA derivatization and MEKC were also performed on a glass chip to analyze amino acids. Usually, OPA was used for fluorescent detection. However, in this report, amperometric detection was used as the OPA derivatives were also electroactive. Voltage (needed for separation) programming was used to decrease the migration time of late migrating species [655]. 

A replaceable C disk electrode (two-electrode system) was used for end-column amperometric detection of biogenic amines. The electrode was inserted through a guide tube and was 30 5 [tm away from the capillary exit. This configuration allowed easy replacement of the electrode, especially after biofouling. The electrode was situated at a central position to maximize coulombic... 

Iodide in vitamin tablets can be found by amperometric detection [88]. Nonaqueous eluents of methanol-containing ammonium perchlorate, which are relatively oxidant resistant, have been used in conjunction with a silica column to detect a wide range of drugs [89]. The use of higher potentials not possible in totally aqueous mobile phases allows for the detection of secondary and tertiary aliphatic amines 462 drugs have been detected in this manner. For compounds that are not electroactive, a procedure using a postcolumn photolysis can generate electroactive species [90] for penicillins [91], proteins [92], and barbiturates [93]. 

Because the sensitivity of the detector decreases with decreasing analyte ionization, the pH of the mobile phase should be chosen to maximize solute dissociation. For example, anions with pKa values above 7 are not detectable by conductivity detection. However, conductivity detection is often the preferred method for organic acids with carboxylate, sulfonate, or phospho-nate functional groups, since the pKa values are below 5. For cations, most aliphatic amines have pKa values around 10 and are readily detected by conductivity detection. The pKa values of aromatic amines, however, are in the range 2 to 7, which is too low to be detected by suppressed conductivity. Sensitivity by nonsuppressed conductivity is also poor, so these amines are monitored by UV absorption or pulsed amperometric detection. 

Pulsed amperometric detection is used for the analysis of carbohydrates and other nonchromophoric molecules such as alcohols, aldehydes, and amines. In the DC mode the products of the oxidation reaction of those compounds poison the surface of the working electrode, and further analyte oxidation is inhibited. This results in peaks that decrease rapidly in height. To maintain a stable and active electrode surface, alternatively positive and negative potentials are repeatedly pulsed. 

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. 

Because carbon dioxide is nonpolar, the separation of polar compounds by supercritical carbon dioxide is difficult. Thus, polar modifiers are often used for the separation of phenols and amines. Derivatization has also been employed to obtain nonpolar analytes in some applications. The UV detector has mainly been used for the detection of polar compounds. Oxidative and reductive amperometric detection was also utilized with a detection limit of 250 pg for oxidative detection of 2,6-dimethylphenol. The detection of amines has generally been achieved by FID. Other detectors used for the detection of polar analytes include Fourier transform infrared (FTIR), photodiode array, and flame photometry. 

Fig. 8-100. Gradient elution of catecholamines with amperometric detection. — Separator column Zorbax ODS eluent (A) 0.005 mol/L triethylamine / methanol, (B) 0.07 mol/L KH2P04 + 0.005 mol/L triethylamine + 0.001 mol/L sodium butanesulfonate (pH 3) gradient linear, 5% A in 20 min to 56% A flow rate 1 mL/min detection amperometry at a GC working electrode oxidation potential +0.8 V injection volume 50 pL solute concentrations 4 ppm each of norepinephrine (1), epinephrine (2), 3,4-dihydroxybenzyl-amine (3), dopamine (4), seratonine (5), 5-hydroxy-3-indolylacetic acid (6), and homovanillic acid (7).

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.

Chlorinated phenols have been analyzed using amperometric detection (Hilmi et al. 1997), as well as a range of amines at pH 2.35 in phosphate buffer containing 1,3-diamino propane (Cavallo et al. 1995). 

HA, heterocyclic amine AA, aromatic amine PA, polyamine Al, aliphatic amine N, nitrosamine CZE, capillary zone electrophoresis MEKC, micellar electrokinetic capillary chromatography LIE, laser induced huorescence ED, electrochemical detection CD, conductivity detection AD, amperometric detection PD, potentiometric detection ALA, alkylamines AQA, alkyl and aUcylbenzyl quaternary ammonium compound A, air H, water S, soil W, waste. 

B. S. Hui and C. O. Huber, Amperometric Detection of Amines and Amino Acids in Flow Injection Systems with a Nickel Oxide Electrode. Anal. Chim. Acta, 134 (1982) 211. 

D. C. Johnson, D. Dobberpuhl, R. Roberts, and P. Vandeberg, Pulsed amperometric detection of carbohydrates, amines and sulfur species in ion chromatography—the current state of research, J. Chromatogr., 640 (1993) 79-96. 

Despite the high sensitivity of amperometric detection, its use in environmental applications has not been exploited accordingly. Nevertheless, a few examples include the MEKC analysis of herbicides in tap water and the CZE and MEKC analysis of aromatic amines in contaminated water samples. ... 

Oxidative EC detection offers advantages over UV and FL for many alkaloids, especially for those that contain a phenol group, indole nucleus or a tertiary aliphatic amine group. Aliphatic alkaloids resembling sugars can be detected with pulsed amperometric detection, and this technique may be applicable to some other aliphatic alkaloids which are difficult or impossible to detect by conventional EC. 

Reducing-sugars, separated on amine-bonded silica, have been determined by post-column reaction with copper bis(phenanthroline) and amperometric detection (in which the reduced copper complex is re-oxidized). The method was compatible with organic eluants, could detect down to 1 ng of glucose, and was applied without... 

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