Reductive Lcec of Organic Compounds

Practice and theory of high temperature liquid chromatography

High temperature liquid chromatography (HTLC) exists in a temperature region beyond ambient (ca. 40 degrees C) and below super critical temperatures. The promises of HTLC, such as increased analysis speed, enhanced separation productivity, "green" LC with pure water mobile phases coupled to universal FID detection, and fast analysis of complex samples by combination with fast 2-D techniques, have become an option for routine practice. The focus of this paper is to review the key developments that have made the application of HTLC a practical technique and draw attention to new developments in 2-D techniques that incorporate HTLC that offer an opportunity to vastly increase the usefulness of HPLC for the analysis of complex samples.

High performance liquid chromatography equipped with a cathodic detector and column-switching device as a high-throughput method for a phosphatase assay with p-nitrophenyl phosphate

LC coupled to an electrochemical detector (LC-EC) operating in cathodic mode with a column-switching system realizes a high-throughput detection of p-nitrophenol (NP). The measurement-time for each NP sample was shortened to 20 s, and the successive analyses of 39 samples was completed within 13 min. In the present system, the limits of detection and quantification were 0.15 and 0.20 microM, respectively, and further, up to 25 microM, a linear calibration curve was afforded. Relative standard deviations for standard solutions of 0.20, 1.0, and 25 microM NP were 4.3, 2.0, and 1.1% (n = 5), respectively. Between-run precisions of the analysis of 5.0 and 25 microM NP over 6 days were 4.8 and 1.3%, respectively. A comparison with the commonly used Bessey-Lowry-Brock method indicates that the present LC-EC is useful for the high-throughput assay of acid and alkaline phosphatases in urine and blood samples with a p-nitrophenyl phosphate substrate.

High Performance Liquid Chromatography with an Electrochemical Detector in the Cathodic Mode as a Tool for the Determination of p-Nitrophenol and Assay of Acid Phosphatase in Urine Samples

Utilizing a commercially available helium-purging device and PEEK tubes for all tubing, especially for connection between the mobile phase and pump, high performance liquid chromatography with an electrochemical detector (ECD/HPLC) at the cathodic mode is a simple and precise method for the determination of p-nitrophenol (NP). Studies with cyclic and hydrodynamic voltammetry indicated that 25% aqueous MeOH containing 0.1% (v/v) CF(3)CO(2)H and -0.8 V vs. Ag/AgCl are the best mobile phase and detection potential for cathodic ECD/HPLC. With the present system, the limits of detection and determination were 0.2 and 0.25 microM, respectively, and up to 50 microM, a linear calibration curve was afforded. Within-day precisions for the analysis of 5 and 50 microM NP were 0.8 and 0.7% (n=6), respectively, and between-day precisions (n=6) for these samples were 3.5 and 2.2%, respectively. Compared with the commonly used Bessey-Lowry-Brock method, cathodic ECD/HPLC was useful for the assay of acid phosphatase in urine samples with p-nitrophenyl phosphate disodium salt as a substrate.

Biomedical applications of liquid chromatography-electrochemistry

Liquid chromatography-electrochemistry has proven to be a viable tool for solving a wide variety of practical analytical problems, primarily in biomedical research. Over 2000 papers have appeared and over 20 manufacturers participate in this area. In spite of these obvious measures of acceptance, there continues to be a general lack of understanding of how these systems work. This naturally stems from the unfamiliarity of chromatographers with electrochemistry and electrochemists with chromatography. Furthermore, there are aspects of both which are not yet understood in molecular detail by anyone. While there is more work to do to optimize the technique, liquid chromatography-electrochemistry clearly is very useful at its present state of development. This article presents an overview of the technology and more recently developed applications. Emphasis is placed on the chromatograph as a whole, dual-channel detectors, microdialysis sampling for in vivo determinations, derivatization and post-column reactions, and studies of xenobiotic metabolism.

Oxidative detection of coulometrically reduced organonitro pesticides in reversed-phase high-performance liquid chromatography

A high-performance liquid chromatographic system is presented for determination of organonitro pesticides, metabolites and industrial chemicals having a variety of chemical structures. The compounds are chromatographed on a C8 column with an acetonitrile-aqueous ammonium monochloroacetate gradient. The chromatographic responses of the compounds are monitored indirectly by oxidative detection of the coulometrically reduced organonitro functionality by means of a porousgraphite detector. Three nitrophenols (2,4-dinitrophenol, 4,6-dinitro-o-cresol and dinoseb), a dinitrophenyl aliphatic ester (binapacryl) and a nitroaniline (dicloran) were detected at low nanogram levels (3-10 ng for 50% recorder full-scale deflection). This technique overcomes the problems of high background currents and oxygen interference, observed in the direct reductive detection of organonitro compounds.

Reduction of quinones with zinc metal in the presence of zinc ions: application of post-column reactor for the fluorometric detection of vitamin K compounds

A simple and effective chemical method has been developed for quantitatively reducing quinones, which is based on their reaction with zinc metal and zinc ions. Comparison of this method with conventional electrochemical reduction revealed the chemical method to be considerably superior. The reduction of quinones to their corresponding hydroquinones was verified by ultraviolet spectrophotometry. The reduction methodology has been applied to derivatize phylloquinone and its metabolite, namely phylloquinone 2,3-epoxide, 'on-line', with subsequent fluorometric detection of the generated hydroquinones.

Determination of cis-diamminedichloroplatinum(II) in human plasma using ion-pair chromatography with electrochemical detection

A thin-layer gold/mercury electrode and a hanging mercury drop electrode are compared as part of an evaluation of liquid chromatography (LC) with electrochemical detection (ED) for the determination of platinum species in human plasma ultrafiltrate. The platinum species, derived from an aged aqueous solution of the antineoplastic agent cis-diamminedichloroplatinum(II) (CDDP), can be separated by ion-pair chromatography. Variation of a number of parameters is described along with the limitations and advantages of each kind of electrode system. We have used our LC-ED technique to separate CDDP from its hydrolysis products and other non-platinum-containing species in human plasma ultrafiltrate with a detection limit of 62 ng/ml (ppb).

Determination of misonidazole and desmethylmisonidazole in plasma by high-performance liquid chromatography with reductive electrochemical detection

A reversed-phase high-performance liquid chromatographic method with reductive mode electrochemical detection was developed for the determination of misonidazole and desmethylmisonidazole in plasma. A thin-layer amperometric detector with glassy carbon working electrode was used to detect the nitroimidazoles at a potential of -0.60 V. The calibration curves were linear. The within-day and day-to-day coefficients of variation were below 3% for plasma misonidazole concentrations of 6-60 mg/l and 1-15 mg/l for desmethylmisonidazole. Electrochemical detection limits were between 2 and 4 pg, which is about 10-20 times lower than that obtained by detection at 323 nm. Limits of quantitation of the nitroimidazoles in plasma were in the order of 1-2 micrograms/l. Under the described conditions other nitroimidazoles and nitro compounds can also be detected with ultimate sensitivity by reductive mode electrochemical detection.

Determination of aniline and metabolites produced in vitro by liquid chromatography/electrochemistry

A method utilizing reverse-phase liquid chromatography/electrochemistry (LC/EC) was developed for the simultaneous determination of aniline and its hydroxylated derivatives, p-aminophenol, o-aminophenol, m-aminophenol, and N-phenylhydroxylamine. To achieve separation of these compounds, a mobile phase of 3.0% dimethylformamide and 97.0% 0.05 M piperazine acetate, pH 5.4, containing 0.05 M KNO3 was developed. A procedure is also presented for the determination of p-nitrophenol, nitrobenzene, and nitrosobenzene, possible aniline metabolites in higher N-oxidation states, using reductive amperometric detection. The hydroxylated compounds, including the hydroxylamine, and nitrosobenzene are easily detected as metabolites of aniline in mouse liver slice or microsomal preparations. No prior extraction, preconcentration, or derivatization steps are needed for the determinations, which can be accomplished by a direct injection of the incubation mixture. The Km value for the hepatic aniline 4-hydroxylase activity in male Cox-Swiss mice microsomal preparations has been determined to be 0.52 mM; the Vmax value is 2.90 +/- 0.64 nmol min-1 mg microsomal protein-1. Detection limits for all compounds of interest are in the picomole range.

Evaluation of reductive amperometric detection in the liquid chromatographic determination of antineoplastic platinum complexes

The usefulness of reductive electrochemical detection at mercury drop electrodes has been determined for platinum complexes separated by solvent-generated anion-exchange high-performance liquid chromatography. Both current-sampled dropping mercury and hanging mercury drop electrodes (DME and HMDE) provide significant advantages over UV absorbance and off-line non-flame atomic absorption detection. The effects of chromatographic and polarographic parameters on analytical system performance have been investigated. By raising the detector cell temperature, the detector response to cis-dichlorodiammineplatinum(II) (DDP) can be shifted anodically to 0.0 V vs. Ag/AgCl, thereby increasing detector selectivity for this compound. The noise-limited minimum detectable quantities of DDP with DME and HMDE are 1.8 ng and 70 pg injected, respectively. DDP can be determined in untreated urine at levels below 100 ng/ml.

The determination of pterins in biological samples by liquid chromatography/electrochemistry

A method is presented for determining the concentrations of several oxidized pterins in a variety of biological samples. This methodology employs reverse-phase "ion-pair" liquid chromatography with electrochemical detection. Detection limits below a picomole have been achieved with responses linear over four orders of magnitude. This methodology was employed to study the excretion pattern of pterins in human urine. The concentration of several pterins in mouse brain and liver samples were also determined. The direct detection of tetrahydrobiopterin was demonstrated.