bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of ionic strength of eluent on retention behavior and on the peak broadening process in hollow fiber flow field‐flow fractionation
Authors: Carlshaf Alf , Jönsson Jan Åke
Published: May 2005

The influence of forces on the particles other than the applied cross flow is studied in hollow fiber flow field‐flow fractionation (HF5). Such forces can be of either attractive or repelling in nature. It is shown by injecting polystyrene latex beads (0.091 μm in diameter) at different cross flows and by using eluents with different ionic strengths, that there will be disturbances in the concentration profile of the sample irrespective of the type of interacting forces. The retention time and peak broadening were either increased or decreased compared to the theoretical value, depending on the nature of the dominating interaction force. It is, nevertheless, possible to minimize the total interaction energy between particles and between the particles and the channel wall by choosing a suitable ionic strength.
Molecular shape recognition of polycyclic aromatic hydrocarbons with a liquid‐crystal bonded phase in supercritical fluid chromatography
Authors: Jinno Kiyokatsu , Mae Hideo , Saito Yoshihiro , Pesek Joseph J. , Fetzer John C. , Biggs Wilt R.
Published: May 2005

The chromatographic characteristics of a liquid crystal bonded phase have been evaluated for the separation of polycyclic aromatic hydrocarbons (PAHs), especially focusing on its shape recognition capability in supercritical fluid chromatography (SFC). The results clearly indicate that the phase has a stronger molecular planarity and shape recognition capability than typical polymeric and monomeric octadecylsilica (ODS) phases because of the very ordered bonded phase structure. The recognition capability was slightly dependent on SFC conditions such as column pressure, column temperature, and mobile phase density. Modifier addition did not make a large difference on the recognition capability. The phase seems to be a very promising stationary phase in SFC because the recognition capability is slightly influenced by the SFC conditions. Selectivity enhancement for molecular shape and planarity recognition has been found in SFC compared to reversed‐phase LC separations with the same stationary phase.
Sample introduction in capillary supercritical fluid chromatography using sequential density gradient focussing and solvent venting
Authors: Liu Zaiyou , Farnsworth Paul B. , Lee Milton L.
Published: May 2005

A technique was developed for large volume sample introduction in capillary supercritical fluid chromatography. A 20‐cm length of 200‐μm i.d. capillary tubing was used as precolumn. The precolumn temperature could be easily controlled by passing an electrical current through an electrically conductive paint coated on its outer surface. During injection, the sample solvent was vented from the precolumn with CO2 (gas) at 32 atm, while the precolumn was kept at room temperature. Solutes were transferred onto the head of the analytical column as a narrow band by density gradient focussing, which was established with (a) a temperature gradient along the precolumn, (b) a rapid expansion of CO2 from supercritical fluid to gas, and (c) a temperature difference between the precolumn and the analytical column. This injection approach minimized solute mass discrimination and could be easily performed.
Fused silica microcolumn LC‐MS and MS/MS using an aeronebulized interface
Authors: Karlsson Karl‐Erik
Published: May 2005

An aeronebulized interface has been developed for use with 0.15–0.25‐mm i.d. slurry‐packed fused silica columns. The interface was built in a removable probe that uses tha direct inlet on a triple quadrupole mass spectrometer for easy switching between LC, GC, or the solids probe. The probe is indirectly heated by the ion source, and premature evaporation of the mobile phase, acetonitrile/water, is prevented by controlled water cooling. A standard EI/CI ion source was used in the chemical ionization mode using methane to control the pressure in the source. Several variables that influence the performance of the interface were evaluated using various drugs as model compounds. Typically, 50,000 to 70,000 theoretical plates could be generated. The minimum detectable quantities were generally in the low picogram range. The best result was 3 pg using negative ion SIM, corresponding to a concentration of 8 × 10−8 M in the injected sample.
Analysis of chemical warfare agents in soil samples by off‐line supercritical fluid extraction and capillary gas chromatography
Authors: Kuitunen Marja‐Leena , Hartonen Kari , Riekkola Marja‐Liisa
Published: May 2005

Nerve agents and two other chemical warfare agents were extracted from spiked soil with supercritical CO2. Supercritical fluid extraction (SFE) was used as an off‐line technique. The extracts were analyzed with a two‐channel capillary gas chromatograph, equipped with an autosampler and flame ionization and nitrogen‐phosphorus detectors. Compounds were identified and quantified automatically with the help of retention index monitoring (RIM). Recoveries were equal to or better than those obtained with solvent extraction by sonication.
Comparative study of “on‐column focusing” applied to micro‐LC and conventional LC
Authors: Ling Betty Lin , Baeyens Willy , Dewaele Chris
Published: May 2005

The technique of „on‐column focusing”︁ allows the injection of high sample volumes onto a conventional LC or micro‐LC column for improved sensitivity without severe peak broadening effects. For early eluting compounds, the application of this technique provided better results for micro‐LC studies than for LC assays. An increase in sensitivity of more than two times was obtained with injection volumes of up to 50 μL onto a capillary column. Therefore, the main drawback of reduced detectability in miniaturized systems could be overcome. For the late eluting compounds, this technique could be similarly applied to both types of chromatographic systems.
Use of SFC/MS for the confirmation of peak identities in the SFC/FID chromatogram of olestra
Authors: Pinkston J. David , Bowling Donald J.
Published: May 2005

Open tubular supercritical fluid chromatography (SFC) with flame ionization detection (FID) has been used for characterizing olestra, a mixture of fatty acid sucrose esters. In this publication, we describe SFC/mass spectrometry (SFC/MS) of olestra using the direct fluid introduction interface. Chemical ionization spectra were used in confirming SFC/FID peak identities. Electron ionization SFC/MS of olestra was also investigated. Both major and minor component peaks in the SFC chromatogram of olestra were characterized. A slight modification of the interface‐probe tip was necessary for operation at higher temperature. A procedure was developed for tuning and calibrating the instrument in the 10‐4000‐u mass range, chemical ionization mode using Ultramark 3200, a perfluorinated polyether. The use of additional pumping in the form of liquid nitrogen cooled cryopumps was key in providing good signal‐to‐noise ratios for the latereluting (i.e., eluting at higher mobile phase pressures and flow rates) olestra components.
Reproducibility of migration times and propagation of error in micellar electrokinetic chromatography
Authors: Vindevogel John , Szücs Roman , Sandra Pat
Published: May 2005

The contributions of several factors (electroosmosis, capacity factor, and elution range) to migration time reproducibility have been evaluated using the theory of error propagation. Experimentally obtained values confirm that a single factor, electroosmosis, can explain most of the observed variability.
Separations of proteins and proteolytic digests of proteins by capillary electrophoresis on superox‐coated open tubular columns
Authors: Zhao Zhongxi , Malik Abdul , Lee Milton L.
Published: May 2005

Fused silica open tubular columns treated with Superox 0.6 and 4 for capillary electrophoresis were evaluated. Excellent separations of proteins were obtained. Column effeiciencies of over 700,000 theoretical plates were achieved for basic proteins. The relative standard deviations (RSDs) in migration times for these proteins were below 1%. Comparisons were made of the electropherograms of α‐, β‐, γ‐, and δ‐chymotrypsins. Three different protein digestion reagents were used to produce complex peptide mixtures for analysis: trypsin, chymotrypsin, and pepsin. Each reagent produced a unique mixture of peptides. Capillary electrophoresis was used to elute the peptides, yielding highly efficient and reproducible separations.
Retention and peak shape of acidic substances in capillary supercritical fluid chromatography with binary and ternary mobile phases
Authors: Karlsson Lars , Buttler Torbjörn , Mathiasson Lennart
Published: May 2005

Acidic model compounds were chromatographed on a cyanopropyl capillary column using binary and ternary mobile phases with supercritical carbon dioxide as the primary mobile phase component. When a binary mobile phase was used, large amounts (> 15%) of methanol in carbon dioxide were needed to elute the most polar analytes. Formic and citric acid were tested as modifier additives. The addition of formic acid to the eluent resulted in virtually no positive effects on the chromatographic behavior of the acidic substances. Addition of small amounts of citric acid to the modifier did, on the other hand, significantly improve peak shapes of all acidic compounds tested. Retention could then be adjusted by changing the concentration ratio between carbon dioxide and the methanol‐citric acid solution. The effect of the ternary citric acid additive is probably caused by deactivation of stationary phase active sites and possibly also facilitation of solute mass transfer across the boundary between the chromatographic phases. Finally, methanol could be replaced by less toxic ethanol as a modifier in binary and ternary mobile phases, without noticeable effects on the chromatographic behavior.
Analysis of cations and anions in small volumes using micro‐LC and CE
Authors: Bächmann Knut , Steeg Karl‐Heinz , Groh Thomas , Haumann Ingo , Boden Jana , Holthues Heike
Published: May 2005

Inorganic ions can be analyzed in small volumes using micro‐liquid chromatography (micro‐LC) and capillary zone electrophoresis (CE). Results for the analysis of alkali, alkaline earth, and ammonium cations as well as the main organic and inorganic anions are given and discussed. For the determination of the cations in micro‐LC and CE, cerium(III) was chosen as eluent and electrolyte, respectively. Micro‐LC anion analysis was carried out with hexacyanoferrate(II) and ‐(III) and indirect photometric detection. In CE 2,5‐dihydroxybenzoic acid was used as the fluorescent electrolyte for indirect detection. For CE sample introduction an injection port was developed to handle volumes smaller than 1 μL.
Separation of DNA sequencing fragments at 53 bases/minute by capillary gel electrophoresis
Authors: Rocheleau Marie‐Josée , Dovichi Norman J.
Published: May 2005

Polyacrylamide gels are unstable in 50‐μm internal diameter capillaries at electric fields greater than about 500 V cm−1 due to the formation of bubbles, which destroy the separation. We describe the use of a modified polyacrylamide capillary gel that is stable at electric fields of at least 800 V cm−1. This capillary gel produces sequencing rates of 3,200 bases h−1 for fragments up to 250 bases in length; a total of 7 min are required to separate fragments of 250 bases in length. Longer fragments tend to coelute and do not generate useful sequencing information. This sequencing rate is a factor of 40 times higher than that produced by conventional sequencing gels and a factor of 3 faster than previous reports in the literature.
Hydrogel polymer coating for capillary electrophoretic separation of proteins
Authors: Huang Mingxian , Lee Milton L.
Published: May 2005

Three hydrogel polymers, poly(2‐hydroxypropylmethacrylate), poly(2‐hydroxyethylmethacrylate), and hydroxypropylcellulose, were coated and immobilized on fused silica capillary surfaces using three different methods, all involving modification of the surfaces before polymeriszation and/or cross‐linking. Since the hydrogel polymers swelled in aqueous solution and allowed permeation of low‐molecular‐weight compounds, an equilibrated state was attained between the hydrogel polymer layers and the buffer solution, which eliminated the interactions between proteins and the capillary surface, and allowed highly efficient and reproducible capillary electrophoretic separations of both basic and acidic proteins. The experimental results indicated that silanol groups on the capillary surface were covered and phosphate anions were trapped in the coating layers, which produced defined, but reduced, electroosmotic flows when compared to uncoated capillaries. High resolution separations of ribonuclease B variants and cytochrome c proteins from different sources were obtained.
Novel pseudostationary phase for micellar electrokinetic capillary chromatography
Authors: Palmer Christopher P. , McNair Harold M.
Published: May 2005

A novel, highly stable pseudostationary phase for use in micellar electrokinetic capillary chromatography has been synthesized and evaluated. Sodium‐10‐undecylenate has been oligomerized to form a micelle‐like structure following the procedure of Durairaj et al. [1]. The effective critial micelle concentration of this new pseudostationary phase is zero, permitting electrokinetic chromatography to be performed at lower ionic strength than is possible with the corresponding free surfactant. The structural stability of this pseudostationary phase permits its use with relatively high percentages of organic modifiers in the buffer medium, allowing the separation and detection of hydrophobic analytes such as alkyl phthalates which are difficult to analyze by conventional micellar electrokinetic chromatography.
Capillary electrophoresis of DNA fragments in entangled polymer solutions: A study of separation variables
Authors: Dolník Vladislav , Novotny Milos
Published: May 2005

Capillary electrophoresis (CE) in entangled polymer solutions was studied using pBR322 DNA Hae III digest as a model sample. The influences of polymer nature, monomer chain length, and polymer concentration were investigated. The use of a novel polymer, galactomannan, as the entangled matrix for the separation of DNA fragments has been demonstrated.
Enantiomeric separation by GC on chirasil‐dex: Systematic study of cyclodextrin concentration, polarity, immobilization, and column stability
Authors: Jung Martin , Schurig Volker
Published: May 2005

Polysiloxane‐anchored permethylated β‐cyclodextrin (Chirasil‐Dex) was used for enantiomeric separation by GC. Four types of Chirasil‐Dex, containing up to 36% permethylated β‐cyclodextrin by weight, were synthesized and compared with 10% permethylated β‐cyclodextrin dissolved in OV–1701. The influence of the cyclodextrin weight percentage on the chiral separation factor α and on polarity, as expressed by the Kovats indices, was studied. The chiral separation factors α did not increase significantly above a threshold of ˜30% permethylated β‐cyclodextrin by weight. The plots of α vs. weight percentage depend on the complexation strength of the chiral solutes, as do the Kovats indices; for weakly complexing solutes, changes in the cyclodextrin concentration result in larger changes in α, but in less pronounced changes in the Kovats indices, as compared to strongly complexing solutes. Column performances were investigated for immobilized and nonimmobilized stationary phases. As expected, immobilization properties were found to depend on the content of Si‐H groups in the polysiloxane backbone. Immobilized columns exhibited a much better long‐term stability of all chromatographic variables investigated.
Thermionic ionization detection of halogenated compounds using supercritical fluid chromatography
Authors: Jew Raymond K. , Richter Bruce E.
Published: May 2005

A thermionic ionization detector utilizing the TID‐1‐N2 mode for the analysis of chlorinated compounds was evaluated for use with supercritical fluid chromatography (SFC). A nitrogen atmosphere was employed in the detection zone to give electron capture detector (ECD)‐like responses. Optimum conditions allowed detection of halogenated compounds in the low picogram range with good base line stability during density programming. Selectivity over aliphatics and aromatics ranged between 104 to 105.
Separation of ethoxylated alcohol sulfates by capillary electrophoresis using indirect UV detection
Authors: Goebel Lisa K. , McNair Harold M. , Rasmussen Henrik T. , McPherson Bruce P.
Published: May 2005

Ethoxylated alcohol sulfates were analyzed without derivatization by capillary electrophoresis (CE) using a borate buffer with 1 mM potassium dichromate and 30% (v/v) acetonitrile. The feasibility of using the technique for separation of raw materials (Neodol 23–2A) and commercial products (dishwashing detergent) is demonstrated.
Effect of surface modification and mobile phase velocity on the performance of parallel current open tubular liquid chromatography
Authors: Šlais Karel , Horká Marie
Published: May 2005

The role of the mobile phase velocity in the stability of the flowing retentive film in parallel current open tubular liquid chromatography (PC‐OTLC) is shown. Criteria for optimization of the solvent pair indicates that, from the solvents examined, cyclohexanol represents the best choice for the retentive liquid when an aqueous mobile phase and revesed phase PC‐OTLC (RP PC‐OTLC) are considered.

Several procedures for capillary internal surface modification were compared with the aim to optimize the performance of RP PC‐OTLC using the cyclohexanol/water system. From the examined methods of modification, persilylation with octadecylcyclotetrasiloxane (D4 reagent) gives the optimum which enables retention close to theory.
Preparative separation of microorganisms by polyphase chromatography on capillary columns
Authors: Pomazanov Vladimer V. , Sakodynskii Karl I. , Pletnev J. U. B.
Published: May 2005

Preparative separation of stamms was accomplished by polyphase chromatography on capillary columns. the influence of various experimental parameters on separation efficiency was investigated.
Miniaturized light scattering detector for packed capillary supercritical fluid chromatography
Authors: Demirbuker M. , Anderson P. E. , Blomberg L. G.
Published: May 2005

Packed capillary columns provide excellent separation of neutral lipids by supercritical fluid chromatography. For these separations, it is necessary to add a polar modifier to the mobile phase; hitherto, a detector suitable for quantitative analysis has been lacking. For this purpose, a miniaturized evaporative light scattering detector for combination with packed microcolumns in supercritical fluid chromatography has been developed. Since the detector response is dependent on the mobile phase flow rate, a drop in response was found at flow rates above 16 mL min−1 of expanded mobile phase. However, below this flow rate, there was a region of relatively constant response. In this region, quantitative analysis could also be successfully performed when the pressure and/or the temperature of the separation were programmed. The detection limit was lower than 6 ng. With pressure programming, the relative standard deviation in peak area was less then 4%. Minor peaks gave higher RSD values.

Further, it is demonstrated that argentation SFC of triacylglycerols provides separation according to chain length as well as to the degree of unsaturation.
Charged surface coatings for capillary electrophoresis
Authors: Huang Mingxian , Yi Guoliang , Bradshaw Jerald S. , Lee Milton L.
Published: May 2005

Fused‐silica capillaries with specially designed charged surface coatings were developed for the control of electroosmotic flow and for the separation of peptides and proteins in capillary electrophoresis. Cryptand moieties were immobilized onto the capillary surface through a crosslinking reaction of a cryptand‐containing polymer and copolymerization of a cryptand‐containing monomer. The resultant coatings provided switchable electroosmotic flow from low to high pH due to the complexing ability of the cryptand moieties with the metal ions from the buffer solution. A positively charged quaternary amine surface‐coated column was found to be very effective in reversing the electroosmotic flow compared to an untreated fused silica capillary, and to be suitable for separation of proteins whose pI values are greater than the buffer pH. Also, a negatively charged sulfonic acid surface‐coated column could be used to stabilize the electroosmotic flow from pH 3 to 9, and the magnitude of the electroosmotic flow could be controlled by varying the concentration of sulfonic acid groups in the coating solution. This negatively charged surface coating gave excellent reproducibility for the separation of a tryptic digest of β‐casein.
Capillary electrophoresis separation of sulphonamides and dihydrofolate reductase inhibitors
Authors: Ricci Maria C. , Cross Reginald F.
Published: May 2005

Twenty‐two sulphonamides and three dihydrofolate reductase inhibitors (DHFR) were included in this study. At pH 7.5, eighteen drugs were resolved in 22 min. Above pH 7.5 the DHFR are not resolved and the pH range of 7‐7.5 was found to be optimal. Around pH 7, where the degree of ionization of the SFA is highly variable, pKa and migration time are strongly correlated. Increasing phosphate buffer concentration from 25‐100 mM generally improves resolution but increases analysis times. A good correlation between analyte mobility and the ratio of net charge to molecular weight indicates that this ratio may be used as a quick guide to the possibility of separation of related compounds. Our secondary objective was to compare the lead time and effectiveness of CZE with LC. It is clear that method development is much more rapid in CZE but with a sufficiently diverse group of compounds, CZE can be incapable of providing a full analysis without gradient control. Nonionized compounds are not separated at one extreme, and excessively mobile species may not emerge in a reasonable time, or, may migrate to the wrong electrode.
Diode laser‐based concentration gradient detector for capillary isotachophoresis
Authors: Wu Jiaqi , Pawliszyn Janusz
Published: May 2005

A diode laser‐based, universal concentration gradient detector was applied to capillary isotachophoresis (CITP). The inexpensive CITP instrument also included a 30 cm long, 300 μm i.d. capillary, a 5 cm focal length lens and a photodiode light beam position sensor. Factors affecting the sensitivity and performance of the detector, i.e., separation conditions, spatial resolution of the detector, and the concentration of leading electrolyte, were discussed theoretically and experimentally. The diode laser‐based concentration gradient detector appears to be one of the best detectors for CITP due to its high resolution, derivative nature, and low cost. The experimental detection limit reached 4 × 10−6 M for small cations, which was near the theoretical value. The detection limit can be further reduced by at least one order of magnitude if a longer capillary is used. The CITP‐concentration gradient detector instrument has high efficiency, and was applied to detect impurities in antidepressant drugs.
UV Longitudinal cell design with enhanced sensitivity for capillary electrophoresis
Authors: Onuska Francis I. , Terry Ken A.
Published: May 2005

A UV‐absorbance cell assembly for use in capillary zone electrophoresis, or in micellar electrokinetic chromatography, employing a perpendicular variable path length U‐shaped cell of nl volume instead of conventional on‐column detection is described. The length of the optical path can be chosen arbitrarily, regardless of capillary diameter. The extracolumn variance has been overcome through the elimination of connectors between the capillary and the detection system. The technique is demonstrated by means of micellar electrokinetic chromatography for phenols, in which path lengths of up to 4 mm can be tolerated without serious loss of separation efficiency. An efficiency of approximately 130,000 plates m−1 for a mixture of phenols using a cell of 50 μm in diameter and an optical path of 4 mm was obtained. This leads to a considerable increase in absorbance signal as compared with the usual on‐column UV‐absorption cell. Construction details are provided.
Dynamic film formation and the use of retention gaps with direct injection in open‐tubular supercritical fluid chromatography
Authors: Chester Thomas L. , Innis David P.
Published: May 2005

Open‐tubular supercritical fluid chromatography (SFC) is a useful technique for many analyses, but is somewhat limited, in current practice, with respect to trace analysis of liquid samples. Direct injection of the volumes required for trace analysis with ordinary detectors creates a meter or more length of flooded zone and may lead to unacceptable band broadening. We studied solvent mass transfer on short, uncoated columns for several solvents forming Type I binary mixtures with CO2, and found evidence in the solvent peak shapes of distinct liquid and vapor phases, with the liquid present as a dynamically formed film. This study suggests that selecting an injection temperature and pressure to reduce, but not eliminate, the surface tension of the liquidvapor interface, and increasing the mobile phase velocity during injection increase the liquid film thickness and reduce the length of the resulting flooded zone. When a separate uncoated inlet tube (that is, a retention gap) is used to connect the injector with the column, decreasing the inlet tube radius improves the ratio of maximum effective injection volume to inlet tube volume.

Just as in GC, dynamically formed films function as pseudo‐stationary phases during injection. Without a liquid film, solutes can be transported by injection‐solvent‐modified mobile phase well beyond the length of a typical film, increasing the difficulty of refocusing the solutes before separation. Refocusing solutes from film‐coated flooded zones is easily accomplished using either a solvent effect (specifically, solvent trapping) or phase‐ratio focusing.

We recommend the use of solvents with phase behavior like toluene and carbon tetrachloride, both of which form films at convenient temperatures and pressures. We discourage the use of solvents with phase behavior like n‐pentane which tend to make single‐phase, supercritical mixtures with CO2 under typical SFC injection conditions.
Evaluation of on‐column labeling with intercalating dyes for fluorescence detection of DNA fragments separated by capillary electrophoresis
Authors: Clark Brian K. , Sepaniak Michael J.
Published: May 2005

The analytical figures of merit of laser‐excited fluorescence detection of DNA restriction fragments, based on the enhanced fluorescence of selected dyes upon intercalation, are presented. The fragments present in a ϕX174 Hae III digest are efficiently separated by capillary electrophoresis employing methyl cellulose as a sizesieving reagent. On‐column DNA labeling is accomplished by adding either ethidium bromide (EB) or POPO‐3 iodide (POPO‐3) to the electrophoretic running buffer. Experimental parameters that are evaluated include type of laser (helium‐cadmium at 325 nm or helium‐neon at 543 nm) and laser power, dye type and concentration, pre‐injection labeling procedure, and applied field. Calibration plots are linear over two orders of magnitude in DNA concentration. Limits of detection are in the low‐to‐mid femtogram injected range for the individual fragments, with the POPO‐3 providing slightly lower values. Both dyes exhibit large increases in fluorescence quantum efficiency upon intercalation and contribute little or nothing to the observed optical background. Detectability with the dyes is enhanced by a spectral shift upon intercalation. Signal intensities increase with DNA fragment size in approximately a linear fashion. Intercalation by the dyes also influences mobility and the resolution of the fragments studied.
High temperature stable CH3O‐terminated poly(diphenyl/dimethyl)‐ and poly(diphenyl/3,3,3‐trifluoropropylmethyl)siloxane copolymer stationary phases for capillary gas chromatography
Authors: Blum Wolfgang , Aichholz Reiner
Published: May 2005

The preparation of two new medium polar CH3O‐terminated 50% diphenyl substituted polysiloxane phases by means of a one step condensation of bis(dimethylamino)alkylsilanes and diphenysilanediol is reported. Both phases can be used up to temperatures of 420°C. The unusual route of synthesis offers an easy and reliable way to prepare a variety of tailor‐made high temperature stable polysiloxane phases.
Fully automated water analyzer based on on‐line solid phase extraction‐gas chromatography
Authors: Louter Arjan J. H. , Brinkman Udo A. Th. , Ghijsen Rudy T.
Published: May 2005

An automated procedure for the analysis of aqueous samples by membrane disk extraction coupled on‐line to capillary gas chromatography (GC) is presented. Organophosphorus pesticides are preconcentrated from aqueous samples on three 0.5 mm thick, 4.0 mm diameter XAD‐2 membrane extraction disks which are mounted in a polymer holder for use in a PROSPEKT sample processor. The layers are dried by a stream of nitrogen (30 min at ambient temperature). Desorption of the analytes is carried out with ethyl acetate which is directly introduced into a retention gap under partially concurrent solvent evaporation conditions, using an early solvent vapor exit. The method is a fully software‐controlled automated system, and includes sample preparation, sample transfer and GC analysis. The final analysis is carried out by GC with nitrogen‐phosphorus (NPD) or mass‐selective detection. The technique is applied to the determination of a series of organophosphorus pesticides in tap water and water from European rivers. With a sample volume of only 2.5 mL, the detection limits achieved with the NPD were 20–50 ng L−1 in tap water and 20–100 ng L−1 in river water.
On‐line solid‐phase extraction/thermal desorption for introduction of large volumes of aqueous samples into a capillary gas chromatograph: Part 2
Authors: Vreuls Jolan J. , De Jong Gerhardus J. , Ghijsen Rudy T. , Brinkman Udo A. Th.
Published: May 2005

A module for combined solid‐phase extraction and thermal desorption (SPETD) was incorporated into a system for automated GC analysis. Solid‐phase extraction of analytes from water could be carried out after injection of an aqueous sample of 100 μL either by a loop using the carrier gas for sample introduction or manually with a syringe. After drying of the stationary phase by the carrier gas at a high flow rate, thermal desorption could be carried out at temperatures up to 300–350°C. Suitable packing materials for the liner in the SPETD module, i.e., materials which efficiently trap the organic compounds from an aqueous solution and are sufficiently thermostable, include Tenax and carbon‐based phases, as well as a silylated alkylmodified silica. With the latter, however, the problem of efficient drying after sorption still must be solved. Each phase has its own application range which, from among the analytes which are trapped during sorption, is determined at the volatile end by losses occurring during drying by helium purging, and at the nonvolatile end by losses due to incomplete release during thermal desorption. From this study (using n‐alkanes, chlorobenzenes, and chlorophenols as test solutes), it appears that the Tenax phases, Tenax‐TA and Tenax‐GR, have the widest application range; essentially quantitative recoveries were obtained for C10 through C26 n‐alkanes. The carbon‐based phases appear to be useful for relatively volatile compounds, i.e., up to C19 n‐alkanes. Each phase gave good recoveries for two test mixtures containing chlorobenzenes and chlorophenols dissolved in 100 μL of water at the 10 ppb level.
Effect of the dodecyl sulfate counter ion on selectivity and resolution in micellar electrokinetic capillary chromatography: SDS vs. Mg(DS)2
Authors: Nielsen Kurt R. , Foley Joe P.
Published: May 2005

In micellar electrokinetic capillary chromatography (MECC) the most widely used surfactant for the pseudostationary phase is sodium dodecyl sulfate (SDS), although other types of surfactants have been investigated. We have synthesized the magnesium salt of dodecyl sulfate (Mg(DS)2) in order to investigate how changes in the counter ion identity affect resolution via changes in selectivity (α) and retention. Buffer solutions containing Mg(DS)2 exhibited operating currents and electroosmotic velocities one third to one half that of a comparable SDS buffer solution, and significantly larger methylene and functional group selectivities for the compounds studied. In addition, capacity factors, K', for netral solutes were between 1.5 and 2.5 times larger for the Mg(DS)2. The elution range, tmc/to, for the Mg(DS)2 was independent of the percentage of organic modifier in the buffer solution while the elution range for SDS increased as the percentage of organic modifier increased. Improvements in α and increased retention resulted in increased Rs for neutral solutes in Mg(DS)2. The relatively constant elution range for Mg(DS)2 also aided in the retention‐mediated optimization of Rs and Rs per unit time.
Large particle micropacked columns in supercritical fluid chromatography
Authors: Ibáñez Elena , Alvarez Pedro J. Martin , Reglero Guillermo , Herraiz Marta
Published: May 2005

The advantages of using micropacked columns with low packing density and liquid stationary phases, which are currently used in gas chromatography, in supercritical fluid chromatography (SFC) have been studied. An equation fitted to 199 experimental data is proposed to describe the relationship between the plate height and the linear mobile phase velocity of the mobile phase in a micropacked column. A comparison of the performances of micropacked, open capillary, and standard packed columns in SFC is presented. The pressure drop at the optimal linear velocity per unit column length and per theoretical plate were evaluated and considered comparatively. Chromatographic efficiency, resolution, speed of analysis, and separation impedance achievable with each column were also evaluated. Preliminary results with column pressure drops similar to those of open tubular columns look promising. Data obtained lead to the conclusion that the use of large particle micropacked columns in SFC could be a suitable approach for the analysis of samples of medium complexity, where trace compound detection may require the use of columns having high sample capacities as well as high specific efficiencies.
Comprehensive two‐dimensional gas chromatography applied to the analysis of complex mixtures
Authors: Venkatramani Cadapakam J. , Phillips John B.
Published: May 2005

Comprehensive two‐dimensional gas chromatography is applicable to the analysis of complex petroleum mixtures. A kerosene sample analyzed by this method generates over six thousand chromatographic peaks. The chromatogram is orthogonal such that the second dimension separation is independent of the first. Aliphatics, aromatics, and naphthalene derivatives form distinct bands of peaks on the chromatographic plane with further subdivisions within each band based on chemical structure. With a nonpolar first column and a moderately polar second column, the first dimension separation is based largely on substance volatility while the second dimension separation is based on polarity. Each chromatographic peak has a pair of characteristic retention times, providing a more reliable identification. Peak capacity is much greater than that obtainable from any one‐dimensional separation.
Trapping efficiency and solute focusing in on‐line supercritical fluid extraction/capillary supercritical fluid chromatography
Authors: Daimon Hiroyuki , Hirata Yukio
Published: May 2005

An on‐line supercritical fluid extraction/capillary supercritical fluid chromatography system was developed using a capillary trap with stationary phase and a linear restrictor. With the system, samples can be injected into either the supercritical fluid chromatography (SFC) column or the trapping tube with the same injector, allowing rapid and accurate measurements of trapping efficiency. The self‐cooling effect due to fluid expansion drastically improved the trapping efficiency. External cooling allowed quantitative collection of n‐decane, provided that condensing of CO2 in the trap was avoided. Volatile solutes were more favorably refocused onto the SFC column at the lower column temperature. This system can also be used as a sample introduction method for volatile solutes.
On‐column atomic emission detection in capillary gas chromatography using a radio frequency plasma
Authors: Pedersen‐Bjergaard Stig , Greibrokk Tyge
Published: May 2005

Detection limits of heteroatoms and carbon were improved by eliminating the make‐up gas and sustaining a 350 kHz radio frequency plasma inside the end of a fused silica gas chromatography (GC) column (0.32 mm i.d.) for capillary GC with atomic emission detection. Due to the small internal diameter of the plasma cell, a stable plasma was maintained in only 1.5–3 mL min−1 of helium used as the GC carrier gas. The optical system consisted of a single low resolution monochromator for the near‐infrared region in order to develop a simple and low‐cost system. For on‐coloumn detection of C, H, S, Cl, Br, and I, the plasma was doped with traces of oxygen. The oxygen dopant served to suppress peak tailing and to maintain high selectivity and repeatability. Detection limits for C, H, S, Cl, Br, and I ranged from 0.9 to 11.8 pg s−1, and selectivities between 4 × 102 and 4 × 103 were obtained. On‐column F‐selective detection required a plasma doped with a mixture of hydrogen and oxygen. The detection limit for F was 13.0 pg s−1; however, the selectivity was poor (10) due to the low resolution of the spectrometer.
Multichannel restrictor for supercritical fluid chromatography
Authors: Verjrosta Jiří , Mikešová Milena , Ansorgová Alena , Zátorský Jan , Hajer Miloš , Janda Václav
Published: May 2005

A new multichannel restrictor for supercritical fluid chromatography has been developed. Four construction parameters (number, length and inner diameter of channels, and their geometrical arrangement) can be varied so that restrictors with different flow characteristics can be designed. The effective ratio, L/d, in which L is the length of a channel and d is the inner diameter, is about two orders lower than values assumed for porous frit restrictors.
Packed capillary supercritical fluid chromatography of organophorus pesticides: Selective detection and applications
Authors: Zegers Bart N. , Hogenboom Ariadne C. , Dekkers Silvio E. G. , Lingeman Henk , Th. Brinkman Udo A.
Published: May 2005

Packed capillary supercritical fluid chromatography coupled with thermionic detection was used for the selective determination of nineteen organophosphorus pesticides in vegetable extracts. The chromatographic behavior of some pesticides was studied using seven different stationary phases. Addition of an organic modifier to carbon dioxide was needed to improve peak shape and reduce retention. Quantitation of organophosphorus pesticides in vegetable extracts showed good linearity. Extraction was performed in a one‐step extraction procedure using a water‐miscible solvent (acetone) and an organic solvent with limited water capacity (dichloromethane). Under these conditions a wide range of both polar and apolar pesticides can be recovered from vegetable extracts down to the 3–25 μg kg−1 level. The recoveries found ranged from 35 to 95% depending on the polarity of the pesticides.
Effect of the dodecyl sulfate counterion on selectivity and resolution in micellar electrokinetic chromatography: II. Organic counterions
Authors: Nielsen Kurt R. , Foley Joe P.
Published: May 2005

Various tetraalkylammonium ions, such as tetramethyl‐, tetrapropyl‐, tetrapentyl‐, and cetyltrimethylammonium, can be added to sodium dodecyl sulfate (SDS) micellar solutions to achieve unique selectivities in micellar electrokinetic chromatography (MEKC). The cetyltrimethylammonium/SDS buffer system showed the greatest increase in methylene selectivity, while both tetrapropylammonium/SDS and cetyltrimethylammonium/SDS buffer systems had the greatest selectivities for the solutes studied. In addition, the buffer systems that used symmetrical tetraalkylammonium ions showed reductions in retention specific for the carbonyl functional group. The tetramethylammonium/SDS buffer system had the best resolution due to both improved selectivity and an elution range that was 2.5 times that of a comparable SDS buffer system.
Automated LC‐GC: A powerful method for essential oils analysis. Part IV. Coupled LC‐GC‐MS (ITD) for bergamot oil analysis
Authors: Mondello Luigi , Bartle Keith D. , Dugo Paola , Gans Peter , Dugo Giovanni
Published: May 2005

A fully automatic on‐line LC‐GC‐MS system for the analysis of bergamot essential oil has been used. The system allows the pre‐separation by LC of the oil into compound classes by gradient elution followed by the transfer of these fractions into a capillary GC column and detection by MS (ITD) and FID. Combining LC‐GC‐MS produces data which are considerably more informative than data from GC‐MS of the whole essential oil.
Solid phase techniques in the extraction of pesticides and related compounds from foods and soils
Authors: Picó Yolanda , Moltó Juan C. , Mañes Jordi , Font Guillermina
Published: May 2005

The application of SPE technology to the isolation of pesticides and related compounds from food and soils has grown enormously in the last decade. Much of this growth has been due to the relative ease of sample handling and the wide range of solid supports currently available for a variety of applications. The aim of this review is to present the methods for solid phase extraction (SPE) of pesticide residues from soils and foods. There are three main areas according to the type of approach used to handle the sample: solid phase extraction, solid phase clean‐up (SPC), and matrix solid phase dispersion (MSPD). This review covers milk products, fatty foods, fruits, vegetables, and soils. Solid phase materials are discussed in each case in terms of reversed bonded‐phase silica sorbents (C18, C8, C2, etc.) and polar sorbents (alumina, Florisil, and silica).
Preparation of small diameter thick film capillary columns for microcolumn separations
Authors: Steenackers Dirk , Sandra Pat
Published: May 2005

Open tubular columns of 25 and 50 μm i.d. have been coated with up to 2 μm thick layers of SE‐54 (lowest β‐value = 6.25). Filling of the columns and evaporation of the solvent were studied as a function of column length and time. Since the viscosity of the stationary phase solution increases exponentially with concentration, the filling process becomes time consuming and high filling pressures are required. The evaporation or coating speed is shown to be controlled by the resistance to mass transfer of the solvent vapor through the coated sections of the column. For 25 and 50 μm i.d. columns, experimental evaporation times deviate little from theoretical predictions. Almost no influence of the stationary phase on the coating speed was observed. The transition from mass transfer limited coating speed to heat transfer limited coating speed is illustrated using literature data.
Effect of ammonia on the response of the nitrogen phosphorus detector (NPD)
Authors: Abdel‐Rehim Mohamed , Hassan Moustapha
Published: May 2005

The effect of ammonia as a carrier gas and as a make‐up gas on the response of the nitrogen phosphorus detector (NPD) was investigated. This study showed that the response of the NPD increased 2.2 to 10.3 fold for all studied aliphatic amines when 5% ammonia in nitrogen was used as a make‐up gas. This increase was about 5.3 to 22.8 fold when ammonia was used as a carrier gas. The response of the NPD for nitro‐aromatic compounds increased 1.8 to 3.1 fold using 5% ammonia as a make‐up gas and 4.2 fold when ammonia was used as a carrier gas. The higher response obtained using ammonia as the carrier gas is related to improvements in the chromatographic performance (reduced tailing). The response of the NPD to phosphorus compounds was about 1.5 times higher using ammonia either as a carrier gas or as a make‐up gas compared to nitrogen. Although the noise of the NPD increased, the signal‐to‐noise ratios for both amines and phosphorus compounds improved with the use of ammonia.
Post‐column derivatization in narrow‐bore capillaries for the analysis of amino acids and proteins by capillary electrophoresis with fluorescence detection
Authors: Gilman S. Douglass , Pietron Jeremy J. , Ewing Andrew G.
Published: May 2005

The construction and performance of a post‐column reactor for capillary electrophoresis with laser‐induced fluorescence detection in 10 μm i.d. capillaries is described. The post‐column reactor is based on a gap design and relies primarily on diffusion for addition and mixing of derivatizing reagents. Reactors with gaps of 4 to 160 μm are easily built with minimal micromanipulation between two 10 μm i.d. capillaries. Optimal separation efficiencies and sensitivities are obtained with gaps of less than 10 μm. Separations of amino acids and proteins with o‐phthaldialdehyde/2‐mercaptoethanol post‐column derivatization and laser‐induced fluorescence detection are shown. Linear detector response is found for amino acids and proteins, and peak efficiencies as high as 230,000 theoretical plates are obtained for some analytes. The use of relatively small columns for capillary electrophoresis results in a separation and detection system that produces very low mass detection limits for biological molecules from picoliter sample injections. Mass detection limits of 130 and 5.2 attomoles are obtained for glycine and human transferrin, respectively.
Fiber optic‐based UV‐absorption detector cell for high temperature open tubular column liquid chromatography
Authors: Svensson Lena M. , Markides Karin E.
Published: May 2005

A new UV‐absorption detector cell for on‐column detection in open tubular column liquid chromatography was evaluated. Optical fibers and quick‐fit connections were employed for flexibility and ease in alignment and mounting in an oven for high temperature detection. Different optical fiber diameters were evaluated for 50 μm i.d. capillary columns. The best combination of optical fibers (200 μm source and collector) resulted in a detection limit of 2 × 10−6 M for acetophenone. No dramatic change in the signal‐to‐noise ratio was found when the temperature was raised to 150°C.
Tandem supercritical fluid extraction/chromatographic studies of the desert botanical species, Dalea spinosa
Authors: Taylor Scott L. , King Jerry W. , Snyder Janet M.
Published: May 2005

Off‐line and on‐line supercritical fluid extraction (SFE) coupled with gas chromatography (GC), supercritical fluid chromatography (SFC), and mass spectrometry (MS) have been used to characterize the composition of extracts from the desert botanical species, Dalea spinosa. Off‐line SFE conducted at pressures as high as 680 atm yielded odoriferous extracts similar in composition to an isopropyl alcohol‐derived extract when the two extracts were characterized by SFC and GC/MS. SFC of the extracts revealed the presence of up to 18 components, GC/MS indicated six major compounds, and headspace GC/MS showed over 20 components. On‐line micro‐SFE/SFC was performed on single seeds of D. spinosa as well as dissected single seeds over the pressure range of 100‐400 atm at 45°C. SFE/SFC on portions of a single seed, such as the calyx or incised resin sac, indicated subtle differences in molecular composition. These results suggest that the resin sac is particularly enriched in many odoriferous compounds.