H. A. Das

On-line low-level radiometric detectin of [14]remoxipride in liquid chromatographic effluents: Application to urine samples

A method for on-line radiometric detection in liquid chromatography (LC) is described that permits the detection of low levels of radioactivity in LC effluents by using solvent segmentation and storage of the segmented effluent in a capillary storage loop. The object of the method is to make the flow-rate in the on-line radioactivity monitor (and hence the mean residence time in the monitor) independent of the separation process. Therefore, after storage of the complete chromatogram, the segmented effluent is led through the monitor at flow-rates that can be chosen according to the residence time desired for accurate and precise radioactivity determination. In this system, it is possible to use a flow-cell volume small enough to preserve the chromatographic integrity, while maintaining the possibility of increasing the counting time. Tests have been performed on the reproducibility of 14C detection and the influence of the flow-rate through the monitor on the standard deviation in 14C peak area and, thus, on the detection limit, using 14C-labelled remoxipride. As an application, analyses of urine samples for [14C]remoxipride and one of its potential metabolites are reported.

Low-level radiometric detection in LC using solvent segmentation and an effluent storage loop

SummaryIn this study a new approach is presented for on-line radiometric detection in reversed phase LC of medium to low polarity compounds labelled with14C.The test compounds,14C-carbaryl and14C-parathion, are extracted post-column into a non-water miscible liquid scintillation cocktail. The segmented two-phase system formed is introduced into the beta-detector without phase separation and collected in a capillary storage tube.After completion of the LC separation and detection process, the direction of the flow in the storage system is reversed and the segmented contents of the loop led at lower flow-rates through the beta-detector again. An enhanced signal, corresponding to the increase in counting time is obtained without measurable peak broadening.The lowest possible detection limit of the system is 9 counts per peak corresponding to subnanogram quantities of tested pesticides. Calibration curves are linear over at least 2 orders of magnitude and have the expected theoretical slopes.The reproducibility of the system is better than 4 % rel. S.D. An application to a recovery study of parathion shows the practical potential of this technique.

On-line radiometric determination of [14C]-urapidil and its main metabolises in rat plasma, using post-column ion-pair extraction and solvent segmentation techniques.

Post-column ion-pair extraction has been applied to the on-line radiometric determination of [(14)C]-urapidil and its main metabolises in reversed-phase liquid chromatography (LC). [(14)C]-remoxipride was used as internal standard. Chromatography was performed isocratically, using acetonitrile-water (pH = 2.2) as eluent on a cyanobonded column. Various ion-pair reagents were tested for their extraction efficiency of the analytes mentioned from the eluent into chloroform. Quantitative extraction was possible using sodium dodecylbenzenesulphonate as ion-pair reagent. The influence of the extraction solvent composition, the percentage organic modifier in the eluent and the flow-rates of eluent and scintillator on the extraction percentage and the (14)C-counting efficiency has been studied. In addition, the enhancement of sensitivity is possible using an on-line radiometric procedure. Finally, the method has been applied to the analysis of rat plasma for [(14)C] -labelled urapidil and its metabolises.

Characterization of a β-detector for on-line radiometry in high performance liquid chromatography

The characteization of a β-detector for on-line radio-HPLC in terms of sensitivity, effective volume, peak efficiency and limits of determination, detection and decision is dissussed.

Flow-through radioactivity detection of 3H- and 14C-labelled compounds in reversed-phase liquid chromatography by a liquid scintillation counting technique based on post-column extraction

Abstract A method for flow-through liquid scintillation counting in reversed-phase liquid chromatography (RP-LC) based on the continuous extraction of aqueous column eluates with a water-immiscible liquid scintillator is evaluated in terms of sensitivity, reproducibility and extra-column band broadening. 3H- and 14C-labelled phenylthiohydantoinamino acid derivatives of widely different polarity serve as model compounds. For extractable derivatives, counting efficiencies of over 30 and 80% can be obtained for 3H and 14C, respectively. The reproducibility and extra-column band broadening depend on the mixing ratio of scintillator and LC eluent; relative standard deviations (peak areas) of less than 3% can be obtained. The sensitivity of flow-through counting can be increased at least 150-fold by storing the segmented scintillator/eluate stream in a capillary storage loop. After the separation is complete, the stream is re-introduced into the radioactivity detector at reduced flow-rates to increase the mean residence time, i.e., the counting time, in the detector.

Multielement isotope dilution analysis by means of radiometric titration

A theoretical concept is derived for multielement isotope dilution analysis in a liquid-liquid extraction system. The practical performance is based on radiometric titration. To that purpose a prototype of a titration/extraction vessel with flow injection has been constructed. For calculations of the initial concentrations of the elements of interest from the experimental data, a computer program has been developed.

Radiometric detection of beta-labelled polar compounds using an effluent storage principle based on solvent segmentation

Abstract A new method is presented for the on-line detection of beta-labelled compounds of high polarity in reversed-phase column liquid chromatography. The aqueous column effluent containing the 14C-labelled amino acids alanine, valine, isoleucine and/or leucine is segmented with a non-water-miscible liquid scintillator. This two-phase system is introduced into the beta-detector without phase separation, where the betas emitted in the aqueous phase are detected in the scintillator phase. The detector effluent is collected in a storage capillary tube. At the end of the chromatographic process, the contents of the storage capillary tube are introduced into the beta-detector by reversing the direction of the flow. The two phases are homogenized by adding a water-miscible liquid scintillator. By lowering the flow-rates the signal increases, depending on the flow-rates used.

On-line and low-level measurements of β-radiation in HPLC

The use of on-line β-detection in HPLC of radioactive-labelled samples is hampered by the time lag between the (relatively) fast separation and the slow counting. In the present approach, this difficulty is overcome by introducing a storage loop in which the eluate is temporarily stored, interspaced by a non-mixing liquid which may be the scintillator itself. The apparatus developed enables the determination of radiochromatograms which contain a total activity of ∼10 Bq.

Application of displacement- and exchange reactions in neutron activation and isotope dilution analysis

The efficiency of a separation of metal ions by combining a displacement or exchange reaction and a liquid-liquid extraction procedure is calculated as a function of parameters such as pH, the ratio of the concentrations of reagent and analyte and the ratio of the concentrations of reagent and interfering ions. The resulting equations can be used to optimize separation conditions in activation analysis and to calculate favourable conditions for multielement isotope dilution analysis, the application of which is discussed.

Direct and reverse flow-through heterogeneous scintillation counting of radiolabelled amino acids using post-column solvent segmentation

Abstract A method for flow-through heterogeneous scintillation counting of 14C-labelled amino acid is described. It is based on post-column solvent segmentation of the aqueous column eluate with hexane before the eluate enters the flow cell of the radioactivity monitor. Using yttrium silicate scintillator granules of 60–80 μm diameter, counting efficiencies of 0.71(14C) and 0.01(3H) are obtained. Segmentation of the column eluate allows the temporary storage of the column eluate in a capillary storage loop without additional band broadening and, after the separation is completed, re-introduction of the segmented stream through the radioactivity monitor at reduced flow-rate so as to increase the sensitivity of flow-through radioactivitydetection.