R. Alan Wheatley

Some recent trends in the analytical chemistry of lipid peroxidation

The analytical chemistry of lipid peroxidation is reviewed with special attention to literature that has appeared in the last four years. The pathways of the cascade of reactions are first described and current analytical practice discussed. Selected assays, classified by analyte, are then considered as indices of lipid peroxidation; these mainly involve ultraviolet and visible spectrophotometry, fluorescence and chemiluminescence.

Ultrasound-enhanced flow injection chemiluminescence for determination of hydrogen peroxide.

A novel ultrasonic flow injection chemiluminescence (FI-CL) manifold for determining hydrogen peroxide (H2O2) has been designed and evaluated. Chemiluminescence obtained from the luminol-H2O2-cobalt(II) reaction was enhanced by applying 120 W of ultrasound for a period of 4 s to the reaction coil in the FI-CL system and this enhancement was verified by comparison with an identical manifold without ultrasound. The system was developed for determining ultra-trace levels of H2O2 and a calibration curve was obtained with a linear portion over the range of 10-200 nmol L(-1) H2O2 (correlation coefficient 0.9945). The detection limit (3sigma) and the quantification limit (LOQ) were found to be 1 x 10(-9) and 3.3 x 10(-9) mol L(-1) respectively and the relative standard deviation was 1.37% for 2 x 10(-7) mol L(-1) H2O2 (n = 10). The method was applied to the determination of trace amounts of H2O2 in purified water and natural water samples without any special pre-treatments.

Flow Injection Analysis for Monitoring Antioxidant Effects on Luminol Chemiluminescence of Reactive Oxygen Species

Abstract Knowledge of the antioxidant capacity of specific chemicals is essential to understanding the susceptibility to oxidative stress. Various assays have been developed for measuring the scavenging capacity of molecules. In the present study we used a continuous flow system for monitoring chemiluminescence (CL) reactions initiated by superoxide ( ) (derived from xanthine–xanthine oxidase reaction), hydrogen peroxide (H2O2), hypochlorite anion (−OCl) (derived from NaOCl), hydroxyl (·OH) (generated from O2–FeSO4-buffer), or peroxynitrite (ONOO−) (freshly synthesized). By adapting the flow injection analysis (FIA) method, inhibition of luminol-CL responses of these reactive oxygen species (ROS) by ascorbic acid (well-characterized, chain-breaking antioxidant) and other antioxidants were also investigated. Data showed that to monitor the antioxidant sensitivity of CL responses initiated by ROS is possible by using FIA method.

Oxidative chemiluminescence of some nitrogen nucleophiles in the presence of formic acid as an ancillary reductant

Chemiluminescence signals were obtained when aqueous propan-2-ol solutions of phenylhydrazine, 2-nitrophenylhydrazine, 4-nitrophenylhydrazine, 2,4-dinitrophenylhydrazine and hexanal oxime or aqueous solutions of hydroxylamine and dimethylglyoxime were injected into a carrier stream of formic acid which merged with acidified potassium permanganate solution. Rhodamine-B was present in the carrier stream as a sensitiser. The chemiluminescence was deduced to originate from the oxidation of the hydrazine or amine functional groups. For phenylhydrazine oxidation, dinitrogen was proposed as the emitting molecule; in the oxidation of hydroxylamine, a nitrogen oxide emitter was proposed. The role of the formic acid was as a source of formate ions, which are oxidised to carbon dioxide in a relatively slow reaction. It is further proposed that excited carbon dioxide molecules, as well as producing a feeble emission in what seems to be an autocatalytic reaction, transfer energy to the emitting products of analyte oxidation, thus enhancing their chemiluminescence. Injection of formate ions, or their massive production by injections of strong alkali, cause chemiluminescence signals which would interfere with those from the analyte. Iron salts, which catalyse the permanganate–formic acid reaction, have a similar effect but at much lower concentrations.

Enhancement of luminol chemiluminescence by cysteine and glutathione

Cysteine enhancement of cobalt(II)-catalysed chemiluminescence of hydrogen peroxide and luminol occurs in carbonate buffer (but not in borate buffer), whether cysteine mixes with hydrogen peroxide before it mixes with luminol-cobalt(II) or vice versa. Enhancement was measured by the ratio of the signals in the presence and absence of cysteine; standard errors were generally < 5% of the mean ratio. Cystine in sufficiently acidic solution also enhances the chemiluminescence but otherwise diminishes the emission. The emission is also inhibited by glutathione. A mixed solution of cysteine and cystine gives rise to enhanced signals. In all the above cases, enhancement occurs only in the presence of a cobalt(II) catalyst. Luminol-peroxynitrite chemiluminescence is enhanced by cysteine and by glutathione without the presence of a catalyst.

Using flow injection analysis to time-resolve rhythmic and pulsatile signalsElectronic supplementary information (ESI) available: schematic representations of low and high dispersion FIA manifolds, and injection protocol for subsampling at 12 s after the start of sample injection. See http://www.rsc.org/suppdata/an/b2/b201504h/

Continuous monitoring can be used to detect rhythms, an important aspect of biology. But peaks of concentration are broadened by dispersion so that they overlap their neighbours and obscure high frequency chemoperiodicities. In this study, flow injection was found experimentally to be useful in resolving these. A rhythmically varying pattern of permanganate concentration was measured spectrophotometrically. The rhythm (frequency 0.08 Hz) was observable at a dispersion coefficient of 3.0 but not at 3.9 (when only a single peak was recorded). It was again observable using the same high dispersion manifold but positioned after an injection valve that subsampled the stream at intervals. A design based on this work is proposed for an automated instrument that outputs a time series of concentration measures.