H. Lingeman

Rapid Screening of a Large Group of Polar Pesticides in River Water by On-Line Trace Enrichment and Column Liquid Chromatography

Abstract Column liquid chromatography on a reversed-phase (C-18) analytical column using a linear acetonitrile-water gradient and diode-array detection is used in an on-line early-warning system of over 50 pesticides in surface water. It allows the separation of most compounds with detection limits of about 1–5 μg/1 after preconcentration of 30 ml of sample. Problems encountered for co-eluting pesticides with similar spectra and pesticides with weak ultraviolet absorption over 230 nm, are discussed. If the system is combined with on-line trace enrichment on a styrene-divinylbenzene copolymer (PLRP-S) precolumn, the overall resolution is only slightly affected, and the qualitative information obtained remains about the same. Chromatograms of spiked (2–10 μg/1) surface (river Rhine) water containing 54 test compounds are shown. Preliminary studies show the stability of several pesticides in water to be rather low. Some degradation products have been detected in 1-month-old methanolic and aqueous solutions.

Determination of polycyclic aromatic hydrocarbons in surface water by column liquid chromatography with fluorescence detection, using on-line micelle-mediated sample preparation

Abstract Column liquid chromatography with fluorescence and diode-array UV detection has been used for the trace-level determination of sixteen EPA-priority polycyclic aromatic hydrocarbons. The procedure involves on-line micellemediated preconcentration on selective sorbents. Using Brij-35 as the surfactant, unwanted adsorption of the analytes on inner walls or surfaces is prevented. The system has been used for the analysis of surface water samples, and detection limits typically are at the low- to sub-ng/l level. The system is robust and repeatability is excellent.

Derivatization trends in capillary electrophoresis

This survey gives an overview of recent derivatization protocols, starting from 1996, in combination with capillary electrophoresis (CE). Derivatization is mainly used for enhancing the detection sensitivity of CE, especially in combination with laser‐induced fluorescence. Derivatization procedures are classified in tables in pre‐, on‐ and postcapillary arrangements and, more specifically, arranged into functional groups being derivatized. The amine and reducing ends of saccharides are reported most frequently, but examples are also given for derivatization of thiols, hydroxyl, carboxylic, and carbonyl groups, and inorganic ions. Other reasons for derivatization concern indirect chiral separations, enhancing electrospray characteristics, or incorporation of a suitable charge into the analytes. Special attention is paid to the increasing field of research using on‐line precapillary derivatization with CE and microdialysis for in vivo monitoring of neurotransmitter concentrations. The on‐capillary derivatization can be divided in several approaches, such as the at‐inlet, zone‐passing and throughout method. The postcapillary mode is represented by gap designs, and membrane reactors, but especially the combination of separation, derivatization and detection on a chip is a new emerging field of research. This review, which can be seen as a sequel to our earlier reported review covering the years 1991—1995, gives an impression of current derivatization applications and highlights new developments in this field.

Derivatization in capillary electrophoresis

In recent years capillary electrophoresis (CE) has been developed into a versatile separation technique, next to gas and liquid chromatography (LC), well suited for the determination of a wide variety of e.g., pharmaceutical, biomedical and environmental samples. The main advantages of CE over chromatographic separation techniques are its simplicity and efficiency. It is well recognized, however, that the sensitivity and selectivity of the detection are relatively weak points of CE. One way to overcome these limitations is the conversion (derivatization) of the analytes into product(s) with more favourable detection characteristics. Although, in principle, almost any detection mode can be combined with a derivatization procedure, in practice, fluorescence monitoring is favoured in most cases. This paper aims to give a short overview on the various reagents that can be used for pre-, post- and on-column derivatization in CE. First, a short introduction is given on CE as an analytical technique, followed by a discussion of the pros and cons of the various modes of derivatization, a comparison of derivatizations in CE with derivatizations in LC, the principles of fluorescence and prerequisites for a good fluorophore and the potential of using diode lasers in combination with a labelling procedure. With respect to the derivatization reagents the emphasis is on the labelling of amino, aldehyde, keto, carboxyl, hydroxyl and sulfhydryl groups.

Fully automated on-line liquid chromatographic separation system for polar pollutants in various types of water

A fully automated column liquid chromatographic separation system using on-line trace enrichment, gradient elution and diode-array detection for the trace-level determination of polar pollutants is described. Automation of the system was achieved by means of an automated cartridge-exchange system (PROSPEKT). Relevant parameters such as pH, volume and ionic strength of the sample, flow-rate during the enrichment step and wavelengths and band widths during detection were optimized for eighteen pollutants in various types of water at concentration levels below 5 μg l−1. The determination limit for all test compounds in liquid chromatographic grade water was 0.1 μg l−1, and identification, via diode-array spectra, could be performed at the same level. The mean relative standard deviations of the peak areas and the retention times for all the test compounds were 10% and 0.3%, respectively, at the 5 μg l−1 level for river Rhine water.

Fully automated multi-residue method for trace level monitoring of polar pesticides by liquid chromatography

Abstract A fully automated liquid chromatographic method using on-line trace enrichment, gradient elution and diode-array detection for the trace level determination of polar pesticides in surface water is described. The automated system uses specially developed software in the form of “user macros”, allowing the on-line control of both the automated cartridge exchange unit for sample preparation and the liquid chromatograph with diode-array detector by means of the Pascal Workstation computer of that liquid chromatographic system. The collected data are automatedly evaluated, i.e., pollutants present in the sample at a concentration level above an input treshold level are identified/determined and a report is printed. Parameters such as the sampling interval of the spectra, temperature of the analytical column compartment, wavelength/bandwidth ratios and data handling were optimized. The validation results for 27 pesticides are presented. At an analyte concentration of 1 μg/1 the relative standard deviations of the retention times and peak areas in different types of water are in the range 0.2–1.5% and 1–15%, respectively. All calibration graphs are linear in the range 0.1–7 μg/1.

Use of membrane extraction disks for on-line trace enrichment of organic compounds from aqueous samples

SummaryMembrane extraction disks have been recently introduced for the solid-phase extraction of organic compounds from aqueous samples. The material consists of alkyl-modified silica particles enmeshed in an inert PTEE matrix. Aqueous samples containing polar pesticides and herbicides, are preconcentrated on-line from acidic solutions on membrane extraction disks containing immobilized octadecyl-modified silica and analyzed with an isocratic liquid chromatographic system using PLRP-S as the stationary phase and aqueous acetonitrile mixtures (pH 3) as the eluent. Data on the lifetime and the dimensions of the preconcentration disks, and the efficiency and the repeatability of the procedure are reported. For 10 ml samples, the detection limits of the analytes atrazine, simazine and 2,3,4-trichlorophenol in tap water are 0.1–1 ppb.

Determination of polar pollutants in water using an on-line liquid chromatographic preconcentration system

SummaryAn isocratic column liquid chromatographic system with UV absorbance detection at 230 nm has been developed for the rapid trace-level determination of a large number of polar pollutants in water. The systems contains two precolumns, in series, which are packed with a styrenedivinylbenzene (PLRP-S) polymer. The second precolumn is also loaded with sodium dodecylsulphate before use. Each precolumn is combined with a PLRP-S analytical column, and aqueous acetonitrile mixtures (pH 3) are used for the separation of neutral as well as acidic and basic pollutants. With 10-ml water samples the detection limits for all analytes, in tap water, are in the low to sub μg/l range. Relevant analytical data are reported and the advantage of using a dodecylsulphate-loaded precolumn over a cation-exchange precolumn is discussed.

Coupling of supercritical fluid extraction with chromatographic techniques

Abstract After a brief description of the basic principles of supercritical fluid extraction (SFE), this review extensively discusses the application of SFE via its off-line and on-line coupling to chromatographic techniques, such as thin-layer, high-performance liquid, gas and supercritial fluid chromatography. Aspects such as speed, selectivity, sensitivity, potential for automation and possibilities of fractionation of the supercritical extract are discussed. Further, SFE liquid-liquid and liquid-solid extraction procedures are compared. Until now, SFE has been applied almost exclusively to the extraction of apolar compounds from solid samples, but the method seems also to be attractive for liquid samples. Generally, SFE is more efficient (in terms of extraction times and recoveries) than Soxhlet extractions and more suitable for thermolabile compounds. Furthermore, efficient coupling to chromatographic techniques is possible, although much work still has to be done to optimize the necessary interfaces. The extraction of relatively polar compounds is possible only if high densities are used or if modifiers are added to the supercritical fluid. The interfacing with separation techniques is then less simple.

Switching valve with internal micro precolumn for on-line sample enrichment in capillary zone electrophoresis

Abstract The design of a switching valve containing a micro precolumn for on-line sample enrichment in capillary electrophoresis is described. Samples are loaded on to the precolumn with a micro liquid chromatographic pump, while desorption is performed by means of the electroosmotic flow. The influence of the precolumn on the capillary zone electrophoretic separation process was investigated. A zone-cutting procedure was introduced, by means of valve switching, to prevent excessive band broadening and shifting of the migration times. Using papaverine as a model compound, it was shown that sample volumes of up to 100 μl can be enriched on the precolumn. Calibration plots are linear over a concentration range of two orders of magnitude. The detection limit of papaverine is 5 - 10−8 M (5 pmol injected; UV detection at 254 nm). The potential of the sample-enrichment valve for on-line coupling to micro liquid chromatography is discussed.