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Biomedical subjects

R W Frei

Publications and source records attributed to R W Frei.

At least 55 records · Page 3Linked to original sources
PubMed1983

Automated liquid chromatographic analysis of the anti-tumorigenic drugs etoposide (VP 16-213) and teniposide (VM 26).

A method is described for the fully automated analysis of large numbers of 1--2 ml serum and plasma or urine samples containing the anti-tumorigenic drugs etoposide and teniposide and their aglycone. The blood samples are hydrolysed by a proteolytic enzyme, subtilisin A, prior to preconcentration on a small precolumn. The hydrolysis step serves both to release the strongly protein-bound drugs and to prevent clogging of the chromatographic system. On-line preconcentration is carried out with precolumns packed with PRP1, a micro-particulate divinylbenzene-styrene copolymeric sorbent. Chromatography takes place, after column switching, in a C18/methanol--water system. After a post-column clean-up step using continuous extraction with dichloroethane in an autoanalyzer system, native fluorescence of these analytes is used for detection of the drugs. Recovery of etoposide and teniposide from spiked serum and plasma samples was 100%. Calibration curves of etoposide and teniposide typically show correlation coefficients of 0.9994 over a two-to-three order linear range. The detection limit of etoposide is approx. 8 ng per sample. Repeatability was found to be excellent. Unattended overnight routine analysis is possible without any problems. This method, considering optimal sample throughput, reliability and selectivity, competes favourably with existing techniques for the analysis of etoposide and teniposide.

Autoanalysis↗
PubMed1983

Amperometric detection of amino acids in high-performance liquid chromatography with a copper electrode.

A copper electrode has been used as an amperometric detector for amino acids in high-performance liquid chromatography. The amino acids are separated in a reversed-phase system, using silica-based and polymer-type column materials. Neutral or alkaline buffer solutions of phosphate and carbonate can be used as mobile phases. Borate buffers are less suitable. The detection method is characterized by a high linear dynamic range, good reproducibility, the absence of electrode poisoning and a sensitivity comparable to that of UV absorption methods after derivatization of the amino acids. Detection limits with conventional-scale columns are in the range 10-100 pmoles. A reduction in the flow-rate in the flow-through cell improves the sensitivity for amino acids that give relatively low signals, such as proline. Therefore, the use of microbore columns is especially advantageous for these compounds. The absolute detection limits decrease by about one order of magnitude on changing to a miniaturized system.

Amino Acids↗
PubMed1983

Automated determination of drugs in blood samples after enzymatic hydrolysis using precolumn switching and post-column reaction detection.

Enzymatic hydrolysis of blood samples with subtilisin-A releases protein-bound drugs and permits the repeated (10-50 times) injection of up to 1-ml volumes on short (2-30 mm) precolumns without appreciable build-up of pressure or loss of performance of the precolumn. The principle of fully automated serum and plasma analysis is demonstrated with the drug secoverine as a model compound. After enzymatic hydrolysis of the sample with an equal volume of a 1 mg/ml solution of subtilisin-A for 15 min at 55 degrees C, the model compound is preconcentrated using a microprocessor-controlled column switching unit. Separation occurs in a reversed-phase liquid chromatographic system using a CN-type stationary phase and a buffered aqueous dioxane solution as mobile phase. Detection is done by UV spectrophotometry or fluorometrically after post-column ion-pairing reaction with dimethoxyanthracenesulphonate. The relative standard deviation of the procedure is less than +/- 6% (n = 10).

Autoanalysis↗
PubMed1983

Room temperature phosphorescence as a liquid chromatographic detection method for polychlorinated naphthalenes and biphenyls in complex matrices.

Quenched and sensitized room temperature phosphorescence techniques have been used for the detection of PCNs and PCBs after liquid chromatographic separation. The usefulness of these techniques to fingerprinting of commercial Aroclor and Halowax mixtures in complex matrices has been shown. The complementary nature of these detection modes yield valuable information in addition to UV detection. a signal inverter is proposed for linearization of the quenched RTPL signals. In this way linear calibration plots over more than two orders can be obtained. Detection limits are generally in the low nanogram or subnanogram concentration region. The application of RTPL detection techniques to the analysis of commercial PCN and PCB mixtures in surface water and urine is demonstrated. Pre-columns can be used to advantage for pre-concentration and clean-up of this type of samples.

Chemical Phenomena↗
PubMed1981

Liquid chromatography of demoxepam and phenothiazines using a post-column photochemical reactor and fluorescence detection.

Upon irradiation with short-wavelength UV light the tranquillizer demoxepam is converted into a highly fluorescent product. This reaction serves as a sensitive and selective means of detection of low levels of demoxepam in e.g., serum after separation by means of reversed-phase high-performance liquid chromatography. The effect of mobile phase composition and time of irradiation on the intensity of the fluorescence signal has been studied. A residence time of about 2 min in the post-column photochemical reactor is optimal, band broadening being efficiently suppressed (delta t ca. 1 sec) by means of air segmentation. Linear calibration graphs are obtained over a three-orders of magnitude concentration range; the detection limit for demoxepam is about 100 pg. Further work has demonstrated that detection limits of between 40 and 100 pg can also be obtained for the photoproducts of the phenothiazines fenergan, largactil, levopromazine and nedaltran. The calibration graphs show good linearity and the analysis of spiked serum samples was successful.

Anti-Anxiety Agents↗
PubMed1981

Sample extraction detector for the liquid chromatographic determination of secoverine in biological samples.

A simple detection system for liquid chromatography based on post-column ion-pair extraction in a solvent-segmented stream is described. The fluorescent ion-pairing reagent 9,10-dimethoxyanthracene-2-sulphonate is added to the aqueous mobile phase prior to the column. An organic solvent, used for extraction of the ion pairs and for segmentation purposes, is added to the column effluent by gravity; about 70% of this phase is drawn through the detector cell using the siphon principle. The noise is significantly less than that of system with a pulsating peristaltic pump. The system has been used for the detection of the prospective drug secoverine. Under optimized conditions, band broadening due to the extraction detection system is sigma t = 7 sec. The detection limit of secoverine is ca. 20 pg, the repeatability in the nanogram range is 2% (relative standard deviation) (n = 6) and the linear range is at least 4 decades. The use of pre-column containing a 1--2-mm layer of stationary phase for clean-up and trace enrichment allows the direct injection of 200--1000 microliters of plasma or serum. In this way, secoverine can be determined in the parts per 10(12) range.

Chromatography, Liquid↗
PubMed1981

Liquid chromatographic detector for organosulphur compounds based on a ligand-exchange reaction.

A method is described for the post-column reaction detection of organosulphur compounds in liquid chromatography, which is based on a ligand-exchange reaction between the palladium(II)-calcein complex and the sulphur-containing compounds. The release of free calcein provides an indirect measure of the amount of organosulphur compounds via fluorescence detection. The kinetics of the reaction and the signal intensity have been studied as a function of the structure of selected organosulphur compounds and of parameters such as carrier- and reagent-stream composition and temperature. Optimal conditions are as follows: an aqueous mobile phase of pH 5-7, to which may be added up to 20% of methanol; an elevated reaction temperature of around 60 degrees C and a palladium(II)-calcein reagent solution of ca. 10(-5) M, which should contain some zinc(II) to enhance the fluorescence intensity. Detection limits, which are dependent on the structure of the compounds, typically are between 0.5 and 1.0 ng. Good linearity is observed over a two- to three-order concentration range. The method has been applied to the determination of ethylene thiourea in e.g. wash-water of apples and tomatoes, and to the detection of penicillamine in spiked serum and urine.

Chromatography, Liquid↗
PubMed1980

Use of PTFE coils in post-column photochemical reactors for liquid chromatography--application to pharmaceuticals.

The advantages and the performance of PTFE reaction coils in photochemical reaction detectors for high-performance liquid chromatography (HPLC) are discussed. The excellent performance of these materials at irradiation wavelengths below 300 nm is based on a diffuse radiation transfer and an internal reflectance (light-tube) effect. Optimal coil designs can be obtained from signal vs. flow-rate curves for a particular application. Lamp performance has to be tested periodically and the reactor should be equipped with an efficient cooling system. Application of the principle to the fluorimetric detection of clobazam and phenothiazines after HPLC separation is discussed. The sensitive detection of these pharmaceuticals and their metabolites in serum with a minimum of sample handling demonstrates the potential of photochemical reaction detectors.

Anti-Anxiety Agents↗
PubMed1977

Quantitative analysis of nonapeptides in pharmaceutical dosage forms by high-performance liquid chromatography.

A high-performance liquid chromatographic (HPLC) method is described for determining oxytocin, lypressin and other nonapeptides and their by-products in liquid and solid pharmaceutical dosage forms. The use of injection volumes up to 750 microliter permits accurate determination of, e.g., oxytocin in injection solutions containing only 1 I.U. (international unit) per ml. Reproducibilities between 1.0 and 1.5% (relative S.D) have been obtained for liquid dosage forms and up to 3% (relative S.D.) for tablets. The correlation between results obtained by bioassay and by the proposed method is highly significant and suggests the use of HPLC as an alternative technique for stability and content-uniformity tests on dosage forms and concentrates.

Animals↗
PubMed1977

The separation of nonapeptides by reversed-phase high-performance liquid chromatography.

The separation properties of five nonapeptides on commercial reversed-phase materials have been investigated and the effects of pH, salt concentration and solvent composition have been studied. With appropriate variation of the pH and salt concentration in the mobile phase, it is possible to resolve all of the peptides investigated and their by-products. Mixtures of water and organic solvents (acetonitrile, dioxan, methanol and n-propanol) have been used. The choice of the organic solvent does not strongly influence the separation pattern. The simplicity, speed and quality of the separations and the favourable detection limits (ca. 30 ng) at 220 nm render this technique suitable to routine quantitative analysis.

Chromatography, High Pressure Liquid↗
PubMed1977

A comparison of reversed-phase and partition high-performance liquid chromatography of some digitalis glycosides.

Comparison of the data for adsorption and reversed-phase chromatography of digitalis glycosides shows the complementary nature of the two modes of separation. The correct choice for a particular problem should make possible a rapid and good separation with simple isocratic systems. Detection limits vary between 10 and 100 ng per injection and permit the analysis of by-products even in low-dosage pharmaceutical formulations. Quantitation is easily possible with both chromatographic techniques using external standardization. The reproducibility for repetitive chromatograms is about 1% relative standard deviation for manual injections by loop injectors and is even significantly better for automatic injection. Reversed-phase chromatography can offer some advantages with regard to sample preparation of pharmaceutical formulations.

Cardenolides↗
PubMed1976

Post-column fluorescence derivatization of peptides. Problems and potential in high-performance liquid chromatography.

Some critical parameters such as the pumping system, mixing devices and detector design in instrumentation for post-column derivatization in high-performance liquid chromatography are discussed. The derivatization was studied with pharmaceutically important nona-peptides containing primary amino groups which react with Fluram and reaction parameters such as pH, solvent and reagent concentration were investigated. Both adsorption systems and reversed-phase systems were used to separate the peptides prior to the post-column reaction. Reversed-phase chromatography has the advantage of simpler sample preparation, better reaction control and optimization of solvent conditions. As a result, detection limits of between 5 and 10 ng per injection can be obtained and the reproducibility of the results is better than +/- 2% (relative standard deviation). The method has been applied to the analysis of injection solutions (ampoules).

Amino Acid Sequence↗