Showing papers in "Hrc-journal of High Resolution Chromatography in 1988"

Effects of organic mobile phase modifiers in micellar electrokinetic capillary chromatography

Abstract: The addition of 1–20% (v/v) of methanol or acetonitrile as organic modifier to the mobile phase in a micellar electrokinetic capillary chromatographic (MECC) system, containing sodium dodecyl sulfate and a buffer, is shown to extend the elution range and thus increase the peak capacity of a given system. Although the net change in the elution range parameter, to/tmc, is essentially the same for both modifiers, the acetonitrile-modified system exhibits much faster elution times for the polar and non-polar test solutes employed in this study. Retention, as measured by the capacity factor, is generally decreased with the increase of an organic modifier, just as in conventional reversed phase chromatography. However, changes in selectivity as a function of the added modifiers are noted among polar and non-polar solutes as well. The efficiency of these MECC systems is increased with the addition of either organic modifier.

Cyclodextrins as chiral stationary phases in capillary gas chromatography. Part II: Heptakis(3-O-acetyl-2,6-di-O-pentyl)-ß-cyclodextrin

Abstract: Heptakis (3-O-acetyl-2,6-di-O-pentyl)-s-cyclodextrin is used as a chiral stationary phase in capillary gas chromatography. High enantioselectivity towards trifluoro-acetylated α and β-chiral amines, amino alcohols, α- and β-amino acid esters, and cyclic trans-diols is observed. In contrast to chiral polysiloxane phases, where hydrogen bonding interaction is essential for enantiomer separation, in cyclodextrins inclusion properties contribute to enantioselectivity. This can be concluded from the separation of N-alkylated amino compounds. The new chiral stationary phase exhibits a wide operating temperature range and is stable above 200°C.

Determination of column selectivity toward polycyclic aromatic hydrocarbons

Abstract: An empirical test is described for the evaluation of column selectivity in reversed-phase liquid chromatography. Using a test mixture of three polycyclic aromatic hydrocarbons (PAH), overall column selectivity toward PAH was assessed for over 20 different commercial C18 columns. Retention behavior was correlated to phase type (i.e., monomeric and polymeric surface modification chemistry) for custom synthesized phases. A classification scheme is proposed in which commercial C18 columns are grouped into three classes based on retention behavior: monomeric-like, polymeric-like, and intermediate phase selectivity toward PAH. Correlation of retention behavior of the test mixture with the separation of PAH mixtures and with more general column properties (e.g., phase thickness) is discussed.

Sample enrichment in high speed narrow bore capillary gas chromatography

Abstract: Summary Reduction of the column diameter has proved to be a highly efficient tool to increase the speed of analysis. Unfortunately, the requirementsfor instrumental design with respect tosample input band width, low dead volume interfacing, and time constants of detection and registration systems are the more critical the smaller the inside diameter. Recently we reported input band widths as low as 1 ms [l] for gaseous samples at ppm concentration levels, without any preconcentration, in a study with narrow bore columns and thermal conductivity detection. In this study a simple versatile micro on-column cold trap/ thermodesorption enrichment system for narrow bore columns is introduced and evaluated. The combination of considerable sample enrichment and presenration of the compatibility of the required input band width with column dimensions is critically examined. The process of thermodesorption (reinjection) which is the most critical step, is particulally emphasized. The system consists of a short aluminum coated fused silica or metal capillary with a low mass and a low cost electrical heating. Input band widths down to 1 ms are obtained without extreme demands on electrical power (300 watt). The potential of the system is illustrated with some extremely fast separations.

Retention characteristics of volatile compounds on Tenax TA

Abstract: The chromatographic separation properties of long, thin adsorption tubes enable substance-specific quantitative enrichment and reduction to be achieved when sampling and thermal desorption are carried out in the same flow direction. The specific retention volumes, and also the breakthrough and peak end volumes, of 69 compounds in the boiling range between-164 and 126°C and of a relative molecular mass between 16 and 119 were determined at temperatures between 30 and 130°C: normal alkanes, isoalkanes, cycloalkanes, alkenes, polyenes, alkynes, aromatics, ethers, alcohols, aldehydes, ketones, carboxylic acids and their esters, nitroalkanes, O-heterocycles, S-heterocycles, chloroalkanes, water, nitrogen monoxide, nitrogen dioxide, carbon dioxide, and sulfur dioxide. The correlation between adsorbent temperature and specific retention volume of these components, presented in the form of diagrams, permits the required quantity of adsorbent to be determined for a given sample volume. Contrary to literature sources, even extremely volatile compounds such as propane, propene, methanol, formaldehyde, formic acid, and chloromethane can be quantitatively retained on Tenax provided the operating conditions are appropriately selected.

On-line combination of liquid chromatography and capillary gas chromatography. Preconcentration and analysis of organic compounds in aqueous samples

Abstract: This paper describes the design of a new, versatile, and low-cost on-line LC-GC interface that allows the fast and reliable introduction of large sample volumes onto a capillary GC column. The sample introduction procedure consists successively of: evaporation of the entire sample (LC fraction), selective removal of the solvent and simultaneously cold-trapping of the solutes, splitless transfer of the solutes to the GC column, on-column focusing, GC separation and detection. Quantitative and qualitative aspects of various experimental parameters are evaluated and optimum conditions are reported. The applicability of the method is demonstrated on a synthetic aqueous sample of chlorinated pesticides.

On‐line extraction and separation by supercritical fluid chromatography with packed columns

Abstract: The application of fluid extraction in combination with fluid chromatography with packed column and flame ionization detection is described. Fluid chromatographic equipment is shown. Applications of this system to drug characterization are demonstrated.

Supercritical fluid extraction / capillary supercritical fluid chromatography / Fourier transform infrared microspectrometry of polycyclic aromatic compounds in a coal tar pitch

Abstract: The combination of supercritical fluid extraction, high resolution capillary supercritical fluid chromatography, and Fourier transform infrared microspectrometry is described for the separation and identification of polycyclic aromatic hydrocarbons in a coal tar pitch. The variable solvating power of the supercritical fluid was utilized to selectively fractionate the sample. The fluid extract was decompressed through a frit restrictor into the sample cavity of a cooled microvalve injector, where the analytes were deposited and concentrated for subsequent chromatographic analysis. Several of the analytes separated in the chromatograph were collected on a potassium bromide disc at a solvent elimination inter-face for subsequent infrared analysis involving the use of an infra-red microscope accessory. The spectra obtained show the power of this detection technique for distinguishing between isomers.

PTV splitless injection of sample volumes up to 20 μl

Abstract: PTV splitless injection cannot compete with on-column injection as far as simplicity, reliability, and accuracy of quantitative analysis is concerned. However, PTV splitless injection is attractive for trace analysis of samples containing high concentrations of involatile sample by-products. Maximum injection volumes are limited by the amount of liquid that can be retained within the PTV injector chamber and are around 20–30 μl injected at once. Solvent evaporation must be carried out in such a way that injector overflow is avoided.

Splitless injection in capillary supercritical fluid chromatography

Abstract: Elimination du solvant par separation dans une precolonne, et systeme permettant de l'eliminer avant qu'il ne passe dans la colonne proprement dite

New method for the preparation of small diameter columns with polymeric stationary phases for open tubular liquid chromatography

Abstract: The purpose of this report is to introduce a new method for use in coating polymeric stationary phase films on the inside wall of small bore diameter fused silica capillary tubing. This technique is being developed for use in fabricating capillary columns for open tubular liquid chromatography.

Multi-element selective radio frequency plasma detector for capillary gas chromatography

Abstract: A radio frequency plasma detector for element specific detection in gas chromatography is described. The detector is comprised of a radio frequency (300 kHz) discharge between electrodes in helium, and utilizes a low-resolution emission spectrometer to monitor selected spectral emission lines produced when the helium discharge decomposes and excites the atomic constituents in the chromatographic column effluent. The spectrometer is tuned to an atomic emission line in the near-infrared portion of the spectrum, and the emission intensity from the discharge region of a selected line is used to monitor the concentration of the element producing that line. Acceptable detector sensitivity was achieved by the use of a high-throughput optical system. Selectivity was achieved by a combination of correct line selection, plasma and carrier gas purification, and plasma gas doping.

A technique for coating capillary columns with a very thick film of cross-linked stationary phase

Abstract: A method for preparation of capillary columns and traps with a very thick film (up to 100 μm) of stationary phase is described. The principle of this method is based on an immediate fixation of a film of prepolymer, formed during dynamic coating. Thus, the development of film irregularities, such as are caused by Rayleigh instability is avoided. Fixation of the film is conveniently accomplished by heat-accelerated crosslinking as was demonstrated in this work, where a commercially available silicone prepolymer (Sylgard 184, Dow Corning) was employed. The low phase ratio columns which thus can be prepared are interesting both in chromatography with dense mobile phases and as enrichment devices. Examples of the latter application are shown, where trace organic components from air and water were concentrated.

Instrumentation for SFC systems: Different sampling and restriction designs

Abstract: Different designs of injection and restriction devices for capillary supercritical fluid chromatography (SFC) have been investigated with respect to their practical applicability and usefulness for reproducible and accurate qualitative and quantitative analyses. In combination with a self-made instrument a fast switching valve is preferable as an injection device compared to a split-injector, and an integral restrictor made from the end of the fused silica (FS) separation column was superior to a linear restrictor made by coupling a small diameter FS-column to the separation column.

Glass capillary gas chromatography-mass spectrometry at high temperatures. direct analysis of free base porphyrins and metal porphyrin complexes extracted from the serpiano oil shale

Abstract: This contribution describes GC/MS analysis of natural petroporphyrin extracts containing alkylporphyrins either as vanadyl complexes or as demetalated free bases. The combination of high temperature glass capillary gas chromatography with mass spectrometry allows, for the first time, direct determination of electron impact mass spectra of separated alkylporphyrins, making additional purification and derivatization unnecessary. The separation is carried out on glass capillary columns coated with the high temperature-stable medium polar, OH-terminated, polysiloxane phases PS 086 and OV-225-OH. The paper gives detailed working directions for the preparation of the high temperature GC/MS-interface, and of the high temperature stable OV-225-OH columns (max. working temperature 390°C).

Co-solvent effects for preventing broadening or loss of early eluted peaks when using concurrent solvent evaporation in capillary GC. Part 1: Concept of the technique

Abstract: The concept and some first results of a method are described for evaporating large volumes of solvent in a relatively short pre-column (retention gap) in such a way that solvent trapping retains volatile components in the inlet up to completion of solvent evaporation. The method was developed for transferring large volumes (easily exceeding 1 ml) of HPLC eluent to GC when using on-line coupled HPLC-GC, but is equally suited for injecting large sample volumes (at least some 50 μl) and could be particularly useful for introducing aqueous solutions. Concurrent solvent evaporation allows introduction of very large volumes of liquid into GC. However, peaks eluted up to some 40–80° above the column temperature during introduction of the liquid are strongly broadened due to the absence of solvent trapping. On the other hand, previous retention gap techniques involving solvent trapping were not suited for transferring very large volumes of liquid into GC. Using a relatively high boiling co-solvent added to the sample or the HPLC eluent, advantages of concurrent solvent evaporation can be combined with solute reconcentration by solvent effects, allowing elution of sharp peaks starting at the column temperature during introduction of the sample.

Reproducibility in capillary supercritical fluid chromatography: Comparison of injection techniques

Abstract: An analysis of the precision obtained using commercially available microvalve injectors is reported for three modes of injection: conventional split; timed-split; and direct. Results from this study show that good precision (< 3% RSD for external standard and < 1% RSD for internal standard methods) can be obtained with capillary supercritical fluid chromatography (SFC). However, particular attention must be paid to the type of valve used, the orientation of the column relative to the valve, the mode of interfacing or connecting the column to the valve, and the type of pressure or density programming used for the analysis as all of these factors will affect the reproducibility.

Supercritical‐fluid chromatography: Open columns vs packed columns

Abstract: Supercritical-fluid chromatography (SFC) may be performed either in open (capillary) columns or in packed columns. Both approaches have been demonstrated numerous times in the literature. In this contribution it will be attempted to discuss some aspects of columns for SFC. Some advantages of both types of columns will be identified. Attention is paid to the stationary phase film thickness, the speed of analysis, and to the maximum number of theoretical plates (effect of column pressure drop). In this brief contribution many questions will be left unanswered and many significant aspects will be left undiscussed, illustrating that much research remains to be done in this area.

Coupling miniaturized liquid chromatography to capillary gas chromatography (micro-LC-CGC): Possibilities of reversed phase LC

Abstract: Differentes phases stationnaires et mobiles sont etudiees afin d'observer les retentions d'alcools primaires et de composes benzeniques. Une analyse qualitative du diazepam et de son metabolite dans l'urine est faite

High sensitivity laser induced fluorescence detection in capillary zone electrophoresis

Abstract: Ce type de detection est utilise pour les aminoacides marques. Une comparaison de trois reactifs est faite: phtalaldehyde, naphtalenedicarbaldehyde-2,3 et l'isothiocyanate tetrahydro fluoresceine. La detection de la phenylalanine est effectuee

Determination of polychlorinated biphenyls in sediment by on-line narrow-bore column liquid chromatography/capillary gas chromatography

Abstract: Narrow-bore column liquid chromatography coupled on-line with capillary gas chromatography (LC/GC) is used for the determination of polychlorinated biphenyls (PCBs) in sediment via a heart-cutting technique. This method is compared with a method in which two off-line column clean-up steps are used with subsequent analysis by capillary gas chromatography. For the LC/GC analysis the recovery of PCBs was 90–100%. For two sediment samples from the river Meuse the LC/GC and the other, more laborious method showed good agreement.

Unified capillary chromatography

Abstract: Separation dans un melange artificiel de composes benzeniques et d'oligomeres du styrene sur une colonne capillaire recouverte de gel de silice

Splitless injection–development and state of the art. Including a comparison of matrix (“dirt”) effects in conventional and PTV splitless injection

Abstract: Kurt Grob introduced splitless injection in 1969. He elaborated most of the working guidelines including the techniques required for reconcentrating the broad intial bands, i.e. the solvent effects and cold trapping. He also designed a vaporizing injector suited for splitless injection. Nevertheless, splitless injection is still often carried out using inappropriate conditions, and many of today's vaporizing injectors are not suited for splitless injection. No autosampler is available that introduces the sample at the appropriate position. Conventional splitless injection is compared to PTV splitless injection for the range of samples that cannot be handled by the anyway superior oncolumn injection, i.e. sample with high loads of involatile byproducts. There is a clear preference for PTV splitless injection as matrix effects observed in conventional splitless injection were found to be substantially reduced or even eliminated.

Automated on‐line HPLC‐HRGC: Instrumental aspects and application for the determination of heroin metabolites in urine

Abstract: A fully automated on-line HPLC-HRGC instrument is described. Samples are loaded into an HPLC autosampler. Pre-separation is carried out, automatically transferring the previously determined HPLC fraction to GC. Total HPLC fractions are introduced into GC, using the on-column or the loop-type interface, depending on the solvent evaporation technique applied. The HPLC column is automatically backflushed with a suitable solvent during GC analysis. The instrument was used for analyzing heroin metabolites, particularly morphine, in urine samples. Raw urine extracts were injected into HPLC and analyzed by GC using FID.

Direct sample injection in supercritical fluid chromatography with packed fused silica column

Abstract: A direct sample injection technique was developed for supercritical fluid chromatography in a packed capillary column, with carbon dioxide as mobile phase and a flame ionization detector. The method allowed solutions, neat liquids, and even solids to be introduced as samples. Also, extraction with supercritical carbon dioxide was combined with this method to separate polymer additives.