Alkaloids, electrochemical detection

R. S. Schwartz and K. O. David, Liquid chromatography of opium alkaloids heroin cocaine and related compounds using electrochemical detection, Anal. Chem., 57 (1985). 

As already reported by Drager in 2002 [45], the low UV absorption of tropane alkaloids means that other detection modes have been investigated. Besides mass spectrometry, electrochemical detection was tested by different authors, but sensitivity was not significantly improved since tropane alkaloids are only moderately... 

Also, electrochemical detection has been applied in a few cases to alkaloids (see Chapters... 

Based on these electrochemical studies we developed a method for the quantitation of ajmalicine and catharanthine in cell cultures. These alkaloids were extracted from freeze-dried cells and purified by the solid-phase procedure described by Morris et al. (1985), except that ethanol was used as the extracting solvent instead of methanol. A dual-electrode coulometric cell was used in the screen mode. The potential of the first electode was set at +0.2 V (vs. Pd), which was at the base of catharanthine s voltammogram. The alkaloids were detected by the second electrode at +0.8 V, as this offered the best S/N ratio. Higher potentials led to lower S/N ratio, since the background current and noise started to increase exponentially above +0.85 V, due to the oxidation of water. The mobile phase was purified by a guard cell between the pump and injector. The guard cell operated at +0.8V. 

Naaranlahti T, Ranta V-P, Jarho P, Nordstrom M, Lapinjoki SP (1989) Electrochemical detection of indole alkaloids of Catharanthus roseus in high-performance liquid chromatography. Analyst 114 1229-1231... 

Pianezzola E, Bellotti V, La Croix R, Strolin Benedetti M (1992) Determination of cabergoline in plasma and urine by high-performance liquid chromatography with electrochemical detection. J Chromatogr 574 170-174 Popl M, Fahnrich J, Tatar V (1990) Chromatographic analysis of alkaloids. Marcel Dekker, New York... 

For detection, UV absorption is most widely used. Some alkaloids can also be detected by means of their fluorescence. Some type of alkaloids have poor UV-absorptive properties, e.g., tropane alkaloids, pyrro-lizidine alkaloids, and steroidal alkaloids, necessitating detection at a lower wavelength (200-220 nm). Electrochemical detection has been applied for alkaloids, enabling selective detection in the presence of interfering compounds. 

D.E.M.M. Vendrig, J. Teeuwsen and J.J.M. Holthuis, Analysis of vinca alkaloids in plasma and urine using high-performance liquid chromatography with electrochemical detection, J. Chromatogr., 1988, 424, 83-94. 

Selective and sensitive detection of p-carbolines is possible using HPLC methods in combination with UV, chemiluminescence, and fluorometry. Besides this, LC-MS and GC-MS are the techniques predominantly used for identification, separation, and quantitation of p-carbolines and tetrahydro-p-carbolines [13,42]. These alkaloids are detected in foods and beverages by HPLC with electrochemical detection at carbon nanotubes-modified glassy carbon electrodes (CNTs-GCE) [43]. In the seeds of Peganum harmala L., the alkaloids harmol, harmalol, harmine, and harmaline were separated using a Metasil ODS column by isocratic elution with isopropyl alcohol acetonitrile water formic acid (100 100 300 0.3) (v/v/v/v pH adjusted 8.6 with triethylamine) and detected at 330 nm [13]. These alkaloids can be detected by HPTLC method [44]. 

Various strategies have been applied during recent years to overcome the drawbacks associated with the analysis of tropane alkaloids by CE. Using atropine and scopolamine as model analytes, a comparison of UV and electrochemical detection (ED) in conjunction with non-aqueous CE (NACE) has been performed for capillaries with different internal diameters by Blasco et al. [108]. The results showed a significant increase in relative LODs with UV detection for capillaries... 

Ergot alkaloids are easily oxidized in the indole moiety by a two-electron transfer process affording a deep purple highly conjugated dimer as the principal product (Dankhazi et al., 1993). This feature can be used, e.g., for the electrochemical detection of EA after their HPLC separation (Pianezzola et al.. 

These analytical dilemmas interfere with the methods of alkaloid analysis. Each group of alkaloids has its own methods of extraction, isolation and crystallization, as well as detection in structure, molecule and dynamicity. Not all these stages are still possible in the majority of alkaloids. In recent years, many techniques have been used in alkaloid detection. There are atomic and molecular electronic spectroscopy, vibration spectroscopy and electron and nuclear spin orientation in magnetic fields, mass spectroscopy, chromatography, radioisotope and electrochemical techniques. Although important developments in methodology and... 

The constant potential amperometric detector determines the current generated by the oxidation or reduction of electoactive species at a constant potential in an electrochemical cell. Reactions occur at an electrode surface and proceed by electron transfer to or from the electrode surface. The majority of electroactive compounds exhibit some degree of aromaticity or conjugation with most practical applications involving oxidation reactions. Electronic resonance in aromatic compounds functions to stabilize free radical intermediate products of anodic oxidations, and as a consequence, the activation barrier for electrochemical reaction is lowered significantly. Typical applications are the detection of phenols (e.g. antioxidants, opiates, catechols, estrogens, quinones) aromatic amines (e.g. aminophenols, neuroactive alkaloids [quinine, cocaine, morphine], neurotransmitters [epinephrine, acetylcoline]), thiols and disulfides, amino acids and peptides, nitroaromatics and pharmaceutical compounds [170,171]. Detection limits are usually in the nanomolar to micromolar range or 0.25 to 25 ng / ml. 

Electrochemical and UV (The limit of detection being 4 ng/injection for each alkaloid). 

Agui L, Pena-Farfal C, Yanez-Sedeno P, Pingarron JM (2007) Determination of P carboline alkaloids in foods and beverages by high-performance liquid chromatography with electrochemical P detection at a glassy carbon electrode modified with carbon nanotubes. Anal Chim Acta 585 323-330... 

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