Biomedical subjects

J P Quirino

Publications and source records attributed to J P Quirino.

16 recordsLinked to original sources

PubMed2001

Separation and on-line preconcentration by sweeping of charged analytes in electrokinetic chromatography with nonionic micelles.

The separation and on-line preconcentration by sweeping of charged analytes in electrokinetic chromatography using a neutral pseudostationary phase is described. Under neutral or basic conditions, the electrophoretic migration of anionic analytes towards the anode is overcome by a high, cathode-directed electroosmotic flow; hence, they experience net migration towards the cathode, and the system is run at positive polarity mode. The separation and the retention factor, k, are dependent on both the analyte's electrophoretic mobility and its interaction with the pseudostationary phase. The versatility of the sweeping mechanism is then shown in this system. The charged analyte, prepared in a matrix free of the pseudostationary phase, penetrates the pseudostationary phase zone upon application of voltage. Analyte molecules are consequently accumulated and concentrated. As a demonstration, the separation and preconcentration of phenol derivatives using nonionic surfactants of the alkyl polyoxyethylene ether type (Brij 35 and Brij 58) yielded peak height enhancements up to 100-fold. The efficiency of sample stacking was also found to be improved with the use of a high viscosity background solution.

Chromatography, Micellar Electrokinetic Capillary↗

PubMed2001

Strategy for on-line preconcentration in chromatographic separations.

In chromatographic separations, the heights of peaks are proportional to the concentrations of sample components present in an injected mixture. In general, an increase in the peak height cannot be achieved by simply increasing the injection time or the sample plug length. An exception occurs if some form of on-line preconcentration is possible. We present a new strategy for achieving on-line preconcentration by the use of a porous chromatographic material that acts as a solid-phase extractor as well as a stationary-phase separator. We are able to realize significant on-line preconcentration using capillary columns filled with a photopolymerized sol-gel (PSG). More than 2-cm plugs of sample solution can be loaded into the capillary and concentrated using a running buffer that is the same as the injection buffer (to avoid solvent gradient effects). As a demonstration, mixtures of three different polycyclic aromatic hydrocarbons, eight different alkyl phenyl ketones, and five different peptides in solutions of aqueous acetonitrile have been injected onto the PSG column and separated by capillary electrochromatography. The preconcentration is marked in terms of peak heights, with up to 100-fold increase for the PAH mixture, 30-fold for the alkyl phenyl ketone mixture, and 20-fold for the peptide mixture. Preconcentration takes place because of the high mass-transfer rates possible in the highly porous structure, and the extent of preconcentration follows the retention factor k for a given analyte.

Journal Article↗

PubMed2001

On-line preconcentration in capillary electrochromatography using a porous monolith together with solvent gradient and sample stacking.

Preconcentration effects of solvent gradient and sample stacking are investigated on a photopolymerized sol-gel (PSG) in capillary electrochromatography. The porous PSG monolith has a high mass-transfer rate. This characteristic promotes preconcentration of dilute samples. Plugs of samples more than 2 cm in length prepared in the separation solution (nongradient condition) are injected onto the PSG column. The extent of preconcentration is quite significant, showing up to a 100-fold increase in peak heights of the separated analytes. Even larger preconcentrations are achieved under gradient conditions by dissolving the sample in a matrix with a higher concentration of noneluting solvent (water). For eight alkyl phenyl ketones and four polycyclic aromatic hydrocarbons that serve as neutral test analytes, improvements in peak heights obtained under gradient conditions can be more than a 1000-fold. Indeed, injection of a 91.2-cm plug, which is more than 3 times the total length of the capillary, was possible with only a minor loss in resolution. Five peptides serve as charged test analytes. Nongradient conditions in which the sample is hydrodynamically injected onto the PSG column show sizable preconcentration because of sample stacking. The use of a solvent gradient with the same ionic strength, however, does not appear to have practical value because of destacking caused by the changing organic composition that affects the conductivity. As an alternative preconcentration method, we demonstrate that electric field-enhanced sample injection on the PSG yielded up to a 1000-fold improvement in detection sensitivity for the test peptides.

Angiotensin II↗

PubMed2001

Photopolymerized sol-gel monoliths for capillary electrochromatography.

A solution of methacryloxypropyltrimethoxysilane in the presence of an acid catalyst, water, toluene, and a photoinitiator was irradiated at 365 nm for 5 min in a 75-microm i.d. capillary to prepare a porous monolithic sol-gel column by a one-step, in situ, process. The photopolymerized sol-gel (PSG) column shows reversed-phase behavior. Using this column, a variety of low-molecular-weight neutral compounds, including polycyclic aromatic hydrocarbons, alkyl benzenes, alkyl phenyl ketones, and steroids are separated from mixtures. Various different operational parameters, such as buffer composition, field strength, and column temperature, were varied to assess their influence on column performance. Use of PSG as a stationary phase for a pressure-driven separation is also demonstrated.

Journal Article↗

PubMed2001

Photopolymerized sol-gel frits for packed columns in capillary electrochromatography.

Porous sol-gel frits are fabricated in a capillary column by filling it with a solution of 3-(trimethoxysilyl)propyl methacrylate, hydrochloric acid, water, toluene (porogen), and a photoinitiator (Irgacure 1800) and exposing it to UV light at 365 nm for 5 min. The separation column (30 cm x 75 microm I.D.) contains between the inlet and outlet frits a 15-cm packed segment filled with 5-microm silica particles modified with the chiral compound (S)-N-3,5-dinitrobenzoyl-1-naphthylglycine. A detection window (1 mm long) is placed immediately after the outlet frit. To demonstrate the performance of this chiral separation column, mixtures of 16 different amino acids (three of which are not naturally occurring) derivatized with the fluorogenic reagent 4-fluoro-7-nitro-2,1,3-benzoxadiazole were separated by capillary chromatography. The enantiomeric separation of the column results in a resolution ranging from 1.21 to 8.29, and a plate height ranging from 8.7 to 39 microm.

Chromatography, Micellar Electrokinetic Capillary↗

PubMed2001

On-line sample concentration in micellar electrokinetic chromatography using cationic surfactants.

Two on-line sample concentration techniques, sample stacking and sweeping, were evaluated using cationic surfactants as pseudostationary phases in micellar electrokinetic chromatography. As cationic surfactant micelles, tetradecyltrimethylammonium bromide and cetyltrimethylammonium chloride were employed. About 10-fold and 1000-fold increases in detection sensitivity in terms of peak heights were observed by sample stacking and sweeping, respectively, without suppression of the electroosmotic flow. In particular, the concentration limits of detection (S/N=3) for test naphthalenesulfonic acids obtained with sweeping were from 0.96 to 0.47 ppb with UV detection without any preconcentration procedure.

Cations↗

PubMed2000

Sample stacking of cationic and anionic analytes in capillary electrophoresis.

The behavior of charged species along concentration boundaries in capillary zone electrophoresis (CZE) that was first described in detail by Everaerts et al. in 1979 assured the possibility of concentrating charged solutes inside the capillary. The concentration effect is based on the sudden change in analyte electrophoretic velocity brought about by the difference in the magnitude of the electric field. Furthermore, this on-line method could be the needed solution to the problem of low concentration sensitivity in CZE. Sample stacking, which is now its well known name, has then found valuable use in applying CZE in many fields, especially after the in-depth studies performed in the early 90s by Chien and Burgi. This article reviews the theory and methodological developments of sample stacking developed for charged analytes in CZE and also in electrokinetic chromatography. A table conveying the reported applications especially in the biomedical and environmental fields is given. On top of this, other on-line concentration methods for charged species, namely, sample self-stacking, acetonitrile stacking, sweeping, cation selective exhaustive injection-sweeping, and use of a pH junction, are briefly discussed.

Anions↗

PubMed2000

Reversed migration micellar electrokinetic chromatography with off-line and on-line concentration analysis of phenylurea herbicides.

Three environmentally important phenylurea herbicides (monuron, isoproturon, diuron) were separated in reversed migration micellar electrokinetic chromatography (RM-MEKC) using 50 mM sodium dodecyl sulfate, 50 mM phosphoric acid, and 15 mM gamma-cyclodextrin. Three on-line concentration techniques are then evaluated to increase the detection sensitivity of the RM-MEKC system. Stacking with reverse migrating micelles (SRMM, water as the sample solvent) provided the best results among the focusing techniques studied. Using a z-shaped detection cell, more than 500-fold increase in peak height is obtained. As a sample preparation and off-line concentration method, solid-phase extraction (SPE) that further improved detection sensitivity was used in the analysis of spiked tap and pond water. For example, 1 parts per billion of each herbicide spiked in tap or pond water was detected by MEKC after SPE and SRMM.

Chromatography, Micellar Electrokinetic Capillary↗

PubMed2000

Large volume sample stacking of positively chargeable analytes in capillary zone electrophoresis without polarity switching: use of low reversed electroosmotic flow induced by a cationic surfactant at acidic pH.

A simple and effective way to improve detection sensitivity of positively chargeable analytes in capillary zone electrophoresis more than 100-fold is described. Cationic species were made to migrate toward the cathode even under reversed electroosmotic flow caused by a cationic surfactant by using a low pH run buffer. For the first time, with such a configuration, large volume sample stacking of cationic analytes is achieved without a polarity-switching step and loss of efficiency. Samples are prepared in water or aqueous acetonitrile. Aromatic amines and a variety of drugs were concentrated using background solutions containing phosphoric acid and cetyltrimethylammonium bromide. Qualitative and quantitative aspects are also investigated.

Animals↗

PubMed2000

Determination of environmentally relevant aromatic amines in the ppt levels by cation selective exhaustive injection-sweeping-micellar electrokinetic chromatography.

Environmentally important aromatic amines are determined by micellar electrokinetic chromatography (MEKC) with UV detection. Using a combination of field-enhanced sample injection in the capillary zone electrophoresis format (performed for a longer period than usual, e.g., 200 s at high voltages; cation selective exhaustive injection) and sweeping in the MEKC format, from 10(4)- to 10(5)-fold improvements in detector response are achieved for all positively chargeable test analytes. Preliminary results indicate that limits of detection for the positively chargeable aromatic amines are in the parts per trillion (ppt) levels.

Amines↗

PubMed1999

Electrokinetic chromatography.

The important features of electrokinetic chromatography are critically reviewed. Special emphasis is given to systems using micelles as pseudostationary phase. Short and comprehensive overviews are given on the subjects of separation, comparison with capillary electrochromatography, on-line coupling with mass spectrometry, and developments that are expected in the future. A greater coverage on the subject of improvement of detection sensitivity, specifically by on-line concentration was also contributed.

Chromatography, Micellar Electrokinetic Capillary↗

PubMed1999

Highly-sensitive micellar electrokinetic chromatographic analysis of dioxin-related compounds using on-line concentration.

An application study of an on-line concentration technique of neutral analytes for micellar electrokinetic chromatography (MEKC) was carried out in environmental analysis to enhance the UV detection sensitivity. Several dioxins and related compounds, such as dibenzofuran, dibenzo-p-dioxin, 2,3- and 2,7-dichlorodibenzo-p-dioxins, and 2,3,7-trichlorodibenzo-p-dioxin, were used as test solutes. For a highly sensitive separation and detection, cyclodextrin-modified MEKC (CD-MEKC) under acidic conditions was employed as a separation mode and stacking using reverse migrating micelles and a water plug (SRW) as an on-line concentration technique. Almost a 200-fold gain in detection sensitivity was obtained for the model compounds in SRW-CD-MEKC compared to that in normal CD-MEKC without on-line concentration and the limit of detection was found to be around 0.1 ppm for each solute.

Chromatography, Micellar Electrokinetic Capillary↗

PubMed1999

Sample stacking of fast-moving anions in capillary zone electrophoresis with pH-suppressed electroosmotic flow.

On-line sample concentration of fast moving inorganic anions by large volume sample stacking (LVSS) and field enhanced sample injection (FESI) with a water plug under acidic conditions is presented. Detection sensitivity enhancements were around 100 and 1000-fold for LVSS and FESI, respectively. However, reproducibility and linearity of response in the LVSS approach is superior compared to the FESI approach.

Anions↗

PubMed1998

Exceeding 5000-fold concentration of dilute analytes in micellar electrokinetic chromatography.

When a neutral analyte zone is injected into a charged pseudostationary phase, the length of the zone is predicted to be narrowed by 1/(1 + k), where k is the retention factor. The conditions for zone narrowing to occur assume negligible electroosmotic flow, a relatively constant electric field along the capillary column, and no pseudostationary phase in the injected analyte mixture. The theoretically expected concentration enhancement was demonstrated experimentally. Consequently, the detection sensitivity of analytes in micellar electrokinetic chromatography (MEKC) can be improved significantly. For example, 9 to 18 parts per billion of an environmentally important racemic herbicide spiked in lake water was separated by MEKC and detected by ultraviolet absorption.

Chromatography, Micellar Electrokinetic Capillary↗

PubMed1998

On-line concentration of neutral analytes for micellar electrokinetic chromatography. VI. Stacking using reverse migrating micelles and a water plug.

Utility of a second enhanced field zone (water zone) is investigated for the on-line concentration of neutral analytes in micellar electrokinetic chromatography. Micellar solutions of sodium dodecyl sulfate prepared in acidic phosphate buffers are used as separation and sample solutions. Prior to long hydrodynamic injection of samples prepared in a low conductivity matrix, a long water plug is hydrodynamically injected to provide a second enhanced field zone. Practical and some fundamental considerations are presented. The technique is selective towards hydrophobic analytes. Notable detector response improvements (>100-fold) for several analytes are observed experimentally.

Chromatography, Micellar Electrokinetic Capillary↗

PubMedDate not supplied

Stacking of neutral analytes in micellar electrokinetic chromatography.

On-line concentration techniques for neutral analytes by sample stacking in micellar electrokinetic chromatography are reviewed. Discussions regarding the fundamentals and practical applications are conveyed. A high gain in sensitivity of 10- to more than 100-fold using normal capillary cell dimension is provided without crucial loss of resolution by the techniques. More than 1000-fold gain in sensitivity (lowering limits of detection to the nM range) is obtained together with an extended pathlength cell.

Chromatography, Micellar Electrokinetic Capillary↗