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Biomedical subjects

R W Frei

Publications and source records attributed to R W Frei.

At least 37 records · Page 2Linked to original sources
PubMed1987

Screening of amphetamines by gradient microbore liquid chromatography and pre-column technology.

Amphetamine-type drugs with a wide polarity range have been screened in both human and horse urine using on-line pre-concentration on pre-columns packed with hydrophobic and cation-exchange sorbents in series and gradient microbore high-performance liquid chromatography. The underivatized amphetamines were identified by UV detection at 210 nm. The method has potential for the automated liquid chromatographic screening of amphetamines in urine, e.g., for doping control.

Amphetamines↗
PubMed1987

Fully automated sample handling system for liquid chromatography based on pre-column technology and automated cartridge exchange.

The design of an automated cartridge exchange module for on-line sample handling in liquid chromatography is described. When combined with a low-cost purge pump, a solvent selection valve and an auto-sampler, a fully automated sample handling system is obtained. Samples are sorbed on a disposable cartridge packed with 40 micron octyl-bonded silica, purged for clean-up and eluted on-line to the analytical column. Unattended operation of the system is demonstrated for various examples, i.e., the determination of anti-epileptic drugs in serum, an anti-cancer drug in plasma, barbiturates in urine, phenylurea herbicides in river water and caffeine in a soft drink.

Autoanalysis↗
PubMed1987

On-line low-level radiometric detection of [14C]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.

Antipsychotic Agents↗
PubMed1987

HPLC detection of choline and acetylcholine in serum and urine by an immobilized enzyme reactor followed by chemiluminescence detection.

A method using HPLC has been developed for the detection of choline (Ch) and acetylcholine (ACh) using an immobilized enzyme reactor which converts Ch and ACh into hydrogen peroxide and betaïne. The formed H(2)O(2) is quantified by means of a solid-state peroxyoxalate chemiluminescence detector based on an immobilized fluorophore and addition of oxalate from a solid bed. The conditions necessary for chemiluminescence detection are obtained by using a make-up flow of acetonitrile after the enzyme reactor. Precipitation problems due to the poor solubility of salts in the final acetonitrile-water mixture are circumvented by adding a crown ether to the make-up flow. The reproducibility of the method was calculated to be 3.4-3.7% RSD. Detection limits are in the sub-picomole range and a linear range of at least three orders of magnitude is found. Measurements in urine and serum reveal no matrix effects.

Journal Article↗
PubMed1986

Metallic copper-containing post-column reactor for the detection of thiram and disulfiram in liquid chromatography.

A reaction detector has been developed for the selective detection of thiram and disulfiram. The detection is based on the post-column complexation of these analytes on a solid-state reactor packed with finely divided metallic copper to form a coloured copper complex, copper(II) N,N-dimethyldithiocarbamate, with an absorption maximum at 435 nm. The method is combined with a pre-concentration and clean-up step on a pre-column to permit the sub-ppb determination of, e.g., thiram in surface water samples or disulfiram in urine. Separation is achieved by reversed-phase liquid chromatography.

Chromatography, Liquid↗
PubMed1986

Parallel column ion exchange for post-separation pH modification in liquid chromatography. Application to barbiturates and miniaturization.

A new approach to the UV detection of barbiturates in high-performance liquid chromatography is demonstrated. The analytical system comprises an anion-exchange column inserted parallel to the injection valve and analytical column. One part of the acetate-containing mobile phase flows through the injection valve and analytical column to achieve the separation, the other part flows through the anion-exchange column where the acetate ions cause the release of hydroxide ions from this column. Finally, the alkaline stream from the anion-exchange column is recombined with the analytical column effluent. This results in an alkaline medium which can be favourable for many detection processes. A series of barbiturates, which show enhanced UV detectability at 254 nm in alkaline solution, was chosen to demonstrate the potential of such a method. Applications of this principle to the analysis of urine and plasma samples are described. In the system only one pump is needed for the separation and the post-column pH modification. A critical comparison between conventional scale and narrow-bore systems is made.

Barbiturates↗
PubMed1986

Comparison of electrochemical detection methods in liquid chromatography for the determination of N-acetylcysteine in plasma.

Three electrochemical detection systems for the liquid chromatography of N-acetylcysteine in plasma have been compared: a dropping-mercury electrode detector, an amalgamated-gold electrode detector and a system in which iodine is generated electrochemically in the column effluent. The last detector approaches the reproducibility of the first (R.S.D. = 2%) and is as sensitive as the second, with a detection limit of 0.1 micrograms/ml plasma.

Acetylcysteine↗
PubMed1986

Handling of environmental and biological samples via pre-column technologies.

Sample handling is still a weak point in chromatography and in analytical chemistry in general. One consideration is the automation potential of new procedures. Solid-liquid extraction techniques in combination with pre-column technology are particularly promising in this regards. The construction and geometry of pre-columns both for conventional and narrow-bore HPLC are of major importance, since band broadening should be kept at a minimum for an optimal functioning of the analytical system. The various operations that can be carried out with such a pre-column are trace-enrichment, clean-up of the sample which depends on the type of adsorbents used in the precolumn, i.e., polar or apolar materials, ion exchangers or metal covered surfaces, etc., protection of the analytical column, field sampling and storage of samples and as a substrate for on-column chemical derivatizations. These various operations are demonstrated with practical examples from the fields of environmental and biological analysis. The selectivity can be further enhanced by coupling pre-column technology with selective detection models such as diode array UV, electrochemical or fluorescence detection. This enables the construction of optimal and integrated analysis systems which are fully automated and microprocessor controlled. They can also be made compatible with miniaturized LC-technology.

Autoanalysis↗
PubMed1985

Analysis of neomycins A, B and C by high-performance liquid chromatography with post-column reaction detection.

The analysis of the antibiotics neomycins A, B and C was investigated. The separation of the components was studied using reversed-phase and reversed-phase ion-pair chromatography. The optimum separation was obtained utilizing a Lichrosorb RP-2 column with a mobile phase consisting of 75 mg/l sodium dodecyl sulphate, 0.5M Na2SO4 and 0.015 M sodium acetate buffer at pH 7.0. Using this mobile phase, baseline separation was obtained for all three compounds in approximately 20 min. Detection was via post-column derivatization of the analytes with ortho-phthalaldehyde in the presence of mercaptoethanol to form fluorescent iso-indole products. This system is applied to the analysis of a number of formulated products containing neomycin.

Journal Article↗
PubMed1984

The use of various chromatographic techniques for the determination of phenylurea herbicides and their corresponding anilines in environmental samples. II. Applications.

In earlier work, various strategies have been developed for the trace-level determination of phenylurea herbicides and the anilines which are their main degradation products. They include catalytic hydrolysis of the phenylureas on silica, liquid chromatographic fractionation of complex mixtures of herbicides and anilines, derivatization of anilines and herbicides with electron-capture-sensitive reagents, and final analysis by means of capillary gas chromatography. In the present paper, the application of these principles to trace-level analysis of surface water, soil and crop samples is demonstrated.

Aniline Compounds↗