Optimization of precolumn design in liquid chromatography

New directions of miniaturization within the biomarker research area

An overview of the current trends within protein expression profiling is given where multidimensional separation of both gel and liquid phase techniques linked to mass spectrometry is viewed as a major route in the global proteome mapping. A clear trend in these biochemical developments is the effort to sequence and identify low-abundant protein expressions where assay miniaturization and integrated sample processing play a central role. Two areas of miniaturization within the proteomics field are addressed: (i) sample cleanup and enrichment, and (ii) silicon microstructure developments for protein chip microarrays.

Fifty years of solid-phase extraction in water analysis--historical development and overview

The use of an appropriate sample handling technique is a must in an analysis of organic micropollutants in water. The efforts to use a solid phase for the recovery of analytes from a water matrix prior to their detection have a long history. Since the first experimental trials using activated carbon filters that were performed 50 years ago, solid-phase extraction (SPE) has become an established sample preparation technique. The initial experimental applications of SPE resulted in widespread use of this technique in current water analysis and also to adoption of SPE into standardized analytical methods. During the decades of its evolution, chromatographers became aware of the advantages of SPE and, despite many innovations that appeared in the last decade, new SPE developments are still expected in the future. A brief overview of 50 years of the history of the use of SPE in organic trace analysis of water is given in presented paper.

Ion-pair chromatography of methotrexate in a column-switching system using an alkyl-diol silica precolumn for direct injection of plasma

The retention behaviour of methotrexate as an ion-pair with tetrabutylammonium in a column-switching system, based on an alkyl-diol silica C8 precolumn, combined with an analytical column, LiChrospher RP 18, was studied. Methotrexate is mainly present as a divalent anion at pH 7.4, however, the retention data was consistent with the formation of a 1 + 1 ion pair with the counter ion. The concentration of the tetrabutylammonium and the acetonitrile in the mobile phase could be used to regulate the retention in the system. Relevant chromatographic parameters to estimate the enrichment effect in the column-switching system are also identified and discussed. The column-switching system was applied to direct injection of plasma (100 microliters) giving a limit of detection of 10 ng/ml for methotrexate using UV detection at 307 nm.

Column-switching techniques for high-performance liquid chromatography of drugs in biological samples

In recent years, an increasing number of publications have demonstrated the potential of column-switching techniques for the chromatographic separation, determination and preparative isolation of analytes from biological matrices. Column-switching systems greatly facilitate drug analysis, by on-line sample clean-up and trace enrichment, or by improving the analytical separative process. In this paper, the main applications of column-switching techniques to drug analysis in biological samples, are reviewed.

Rapid on-line precolumn high-performance liquid chromatographic method for the determination of benomyl, carbendazim and aldicarb species in drinking water

A reversed-phase high-performance liquid chromatographic (HPLC) method has been developed for the determination of trace concentrations of benomyl, carbendazim, aldicarb, aldicarb sulphoxide and aldicarb sulphone in drinking water. A 10-ml sample of water is passed through a 3-cm precolumn, packed with 5-microns C8 sorbent, at a flow-rate of 5 ml/min. The HPLC system is then switched to an acetonitrile-water gradient elution program. The preconcentrated analytes are eluted from, and separated by, the 3-cm C8 precolumn and determined by UV absorption. The total analytical time is 25 min. The lowest detectable concentrations are in the range of 2.5 x 10(-9)-11.0 x 10(-9) g/ml for the five analytes investigated with 10 ml of sample.

Development of an automated high-performance liquid chromatographic method for the on-line pre-concentration and determination of trace concentrations of pesticides in drinking water

An automated reversed-phase high-performance liquid chromatographic (HPLC) method has been developed for the determination of trace concentrations of propoxur, carbofuran, carbaryl, propham, captan, chloropropham, barban and butylate in drinking water. A 100-ml of sample water is passed through a 3-cm precolumn, packed with 5-microns ODS sorbent, at a flow-rate of 5 ml/min. The HPLC system is then switched to an acetonitrile-water gradient elution program. The analytes, which are concentrated on the precolumn, are eluted and separated on a 25-cm C8 analytical column and determined by measuring the UV absorption at 220 nm. The resolution of analytes is excellent regardless of whether the elution from the precolumn is done unidirectionally or with backflushing. The precolumn can be used repeatedly for at least 30 samples without a significant decrease in efficiency. The total analytical time is 60 min. Tap, distilled, deionized, commercial spring and HPLC-grade waters were analyzed. The lowest detectable concentrations are in the range of 10.10(-12)-460.10(-12) g/ml for the eight pesticides with 100 ml of sample.

Determination of disulfiram and two of its metabolites in urine by reversed-phase liquid chromatography and spectrophotometric detection after post-column complexation

A selective method was developed for the determination of disulfiram and two of its metabolites, diethyldithiocarbamate (DTC) and copper (II) diethyldithiocarbamate [Cu(DTC)2], in complex (biological) samples by reversed-phase liquid chromatography (RP-LC) with post-column derivatization. In the first step, DTC is converted into lead (II) diethyldithiocarbamate [Pb (DTC)2] by adding lead (II) acetate. Disulfiram, Pb (DTC)2 and Cu (DTC)2 can be easily pre-concentrated on C18-bonded silica. After separation by isocratic RP-LC, derivation takes place in two solid state post-column reactors packed with metallic copper and copper (II) phosphate. Disulfiram reacts with metallic copper to form Cu (DTC)2. The same product is obtained by the ligand-exchange reaction between Pb (DTC)2 and copper (II) phosphate. Cu (DTC)2 can be detected selectively at 435 nm with good sensitivity (molar absorptivity, epsilon = 13,000). The derivatization reactions proceed rapidly and quantitatively, which was confirmed by comparison of absorption spectra. The applicability of this method is demonstrated for undiluted urine samples which, apart from the addition of lead (II) acetate and pre-concentration of C18-bonded silica, require no clean-up procedure.

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.

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.

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.

Use of enzymatic solubilization of tissues and direct injection on pre-columns of large volumes for analysing biological samples by high-performance liquid chromatography

A pre-column technique, which can be used to introduce all types of biological samples into high-performance liquid chromatographic systems, is described. Fluids are injected directly, whereas faeces are homogenized and centrifuged and tissue samples are solubilized by the enzyme subtilisin A. Acetonitrile is added to all samples (no precipitation of proteins is seen), fluids (10%), solubilized tissue (25%) and faeces (20%), to obtain better wettability of the packing and to counteract binding. Samples of up to 50 ml are injected onto a 6 X 4 mm I.D. pre-column from which the compounds are backflushed onto the analytical column. Different packing materials for the pre-column have been used; LiChroprep RP-18 gave the best results. This system has been used for a series of different drugs, using different analytical columns and different detectors. Both gradient and isocratic elution have been used. High recoveries, good reproducibility and low detection limits are seen for routine analysis. For metabolic work, a very large enrichment factor is obtained.

Determination of Ro 14-1761, a new third-generation cephalosporin, in the plasma and milk of cattle by column switching high-performance liquid chromatography

A sensitive and specific high-performance liquid chromatographic procedure was developed for the determination of the third-generation cephalosporin Ro 14-1761 in cow plasma and milk. The molecular structure of the new antimicrobial was very close to that of ceftriaxone, but the high-performance liquid chromatographic methods available for the latter could not be used as Ro 14-1761 adsorbed and/or degraded during the chromatographic process. Furthermore, the high-performance liquid chromatographic technique derived for ceftriaxone was not sensitive enough for our purposes. In the new assay, the plasma (milk) protein was precipitated with acetonitrile after dilution of the sample with water. For low concentrations (less than or equal to 10 micrograms/ml), the supernatant obtained after centrifugation was concentrated by extracting acetonitrile with methylene chloride. Quantification was performed by column switching high-performance liquid chromatography with UV detection (274 nm) using ion-pair reversed-phase chromatography. Ethylenediaminotetraacetic sodium salt had to be added to the mobile phase (1.2 mM) to prevent adsorption and/or degradation of the cephalosporin on the analytical column. The selectivity of the chromatographic separation was enhanced by heating the column to ca. 50 degrees C. The drug recovery was better than 85%. The limit for quantitative determination in both milk and plasma was 0.1 microgram of Ro 14-1761 per millilitre with an accuracy of 1% (coefficient of variation 10%). The overall accuracy and precision were 1-10% in the 0.1-100 micrograms/ml concentration range.

Useful sample handlings for reversed phase high performance liquid chromatography in emergency toxicology

Some physicochemical treatments of biological samples, before being injected into a liquid chromatograph, are discussed. The advantages of dilution, liquid and solid extraction are compared referring mainly to unpublished results. Assays of antiepileptic drugs, caffeine, theophylline, tricyclic antidepressants, valproic acid, and meprobamate are used to demonstrate the importance of sample handlings in toxicological analyses in which reversed phase high performance liquid chromatographies are applied.