Showing papers in "Analytical Chemistry in 1992"

Capillary electrophoresis and sample injection systems integrated on a planar glass chip

Abstract: The feasibillity of miniaturizing a chemical analysis system on a planar substrate has been demonstrated for a system utilizing electrokinetic phenomena for sample separation and solvent pumping. Using micromachining techniques, a complex manifold of capillary channels has been fabricated in a planar glass substrate and the separation of a mixture of fluorescein and calcein within the channels was achieved using electrophoresis. The maximum number of theoretical piates abtained was about 35 000 for calcein, with 5000 V applied, corresponding to 2100 V between the injection and fluorescence detection points in the channels

Dynamics of organic compound extraction from water using liquid-coated fused silica fibers

Abstract: Mathematical descriptions of the absorption and desorption processes were developed and compared with experimental results for solid-phase microextraction (SPME) using poly(dimethylsiloxane)-coated fused silica optical fibers. Extraction times for benzene, toluene, and p-xylene using a coating thickness of 55 {mu}m are under 10 min and can be shortened substantially using agitation. Detection limits and distribution coefficients for several organic compounds are presented. 20 refs., 13 figs., 1 tab.

Automation and optimization of solid-phase microextraction

Abstract: The solid-phase microextraction (SPME) technique involves exposing a fused silica fiber that has been coated with a stationary phase to and aqueous solution containing organic contaminants. The analytes partition into the stationary phase until an equilibrium has been reached, after which the fiber is removed from the solution and the analytes are thermally desorbed in the injector of a gas chromatograph

Sample stacking of an extremely large injection volume in high-performance capillary electrophoresis

Abstract: Sample stacking is a sample on-column concentration technique in high-performance capillary electrophoresis (HPCE). However, the amount of sample which can be loaded into the column in conventional sample stacking is rather limited because of disturbance caused by the low-concentration sample buffer. This report describes a technique of stacking an extremely large sample volume into narrow bands. The technique is based on the principal that the local electrophoretic velocity of the ions inside the sample buffer is much faster than the bulk electroosmotic velocity of the solution

Heuristic evolving latent projections: resolving two-way multicomponent data. 1. Selectivity, latent-projective graph, datascope, local rank, and unique resolution

Abstract: Heuristic evolving latent projections (HELP) are proposed as a new method to resolve two-way bilinear multicomponent data into spectra and chromatograms of the pure constituents. The method is founded on four elements: (I) the use of the so-called zero-component regions in order to establish the detection limit for the chemical species, (II) the use if latent-projective graphs (datascope) to reveal selective (one-component) chormatographic and/or spectral regions, (III) local rank analysis in order to check the selectivity of the regions found by visual detection, and (IV) the use of the selective information for unique resolution into spectra and chromatograms of the pure chemical constituents

All-solid-state sodium-selective electrode based on a calixarene ionophore in a poly(vinyl chloride) membrane with a polypyrrole solid contact

Abstract: Sodium-selective coated wire electrodes (CWE) and solid contact (SC) electrodes have been constructed and investigated. The CWE is based on the application of a poly(vinyl chloride) (PVC) membrane incorporating the sodium-selective ionophore tetraethyl ester of p-tert-butylicalix[4]arene onto the surface of a platinum disk. The SC electrode is based on the use of a conducting polymer, polypyrrole (PPy), doped with NaBF 4 as the mediating layer between platinum and the same PVC membrane as above

DNA sequencing using capillary array electrophoresis.

Abstract: A DNA sequencing method is presented that utilizes capillary array electrophoresis, two-color fluorescence detection, and a two-dye labeling protocol. Sanger DNA sequencing fragments are separated on an array of capillaries and detected on-column using a two-color, laser-excited, confocal-fluorescence scanner. The four sets of DNA sequencing fragments are separated in a single capillary and then distinguished by using a binary coding scheme where each fragment set is labeled with a characteristic ratio of two dye-labeled primers. Since only two dye-labeled primers are required, it is possible to select dyes that have identical mobility shifts. It is also shown that the ratio of the signal in the two detection channels provides a reliable identification of the sequencing fragment. DNA sequencing results on a 25-capillary array are presented.

Can continuous glucose monitoring be used for the treatment of diabetes.

Abstract: In the case of the glucose sensor, clinicians and chemists must cooperate in interdisciplinary research to carefully define the analytical problem. Although not specifically discussed in this article, another group that must participate in this effort is engineers. Their expertise is needed to design the monitoring and control unit that contains the alarm and pump systems. The glucose sensor must operate reliably in an in vivo environment, provide the clinical information needed, and be easy to operate and manufacture.

Heuristic evolving latent projections: resolving two-way multicomponent data. 2. Detection and resolution of minor constituents

Abstract: The new method of heuristic evolving latent projections (HELP) is used for the resolution of two-component mixtures of isomers characterized by liquid chromatography with diode array detection. Four mixtures of drug isomers were analysed with chromatographic resolutions of 0.2 and 0.5, and the minor component was present in 2 and 10%

Primary and secondary oxygen-induced C1s binding energy shifts in X-ray photoelectron spectroscopy of polymers

Abstract: A total of 43 polymers containing only C, H, and O have been studied using an X-ray photoelectron spectroscopy instrument capable of the highest energy resolution avallable to date. The primary alm was to reinvestigate the C1s binding energy shifts for oxygen functionalltles and to systematically investigate secondary (β) shifts for the first time. In addition, this large data set has revealed the effects of vibrational fine structure on C1s component line width/shape and the effects of shake-up/shake-off (in photoemission from C=O groups) on nonstoichoimetry of C1s component intensities

Determination of chromium(III) and chromium(VI) in water using flow injection on-line preconcentration with selective adsorption on activated alumina and flame atomic absorption spectrometric detection

Abstract: A rapid and sensitive method for the species-selective determination of chromium(III) and chromium(VI) in water samples by flame atomic absorption spectrometry using online preconcentration on a microcolumn packed with activated alumina (acidic form) has been developed. Sequential species-selective sorption was possible by using the Clark-Lubs systems with pH 7 for Cr(III) and pH 2 for Cr(VI). The preconcentrated species were eluted directly from the column to the nebulizer-burner system using 1.0 mol/L nitric acid and 0.5 mol/L ammonia for Cr(III) and Cr(VI), respectively

Theory of the interfacial potential distribution and reversible voltammetric response of electrodes coated with electroactive molecular films.

Abstract: An analytical expression for the interfacial potential distribution at metal electrodes coated with monolayer and submonolayer films of electroactive molecules is reported. The driving force for reversible electron transfer between the metal and electroactive adsorbate is calculated from the interfacial potential distribution and is used to predict the current-voltage wave shape in cyclic voltammetric experiments

Electrochemical Origin of Radical Cations Observed in Electrospray Ionization Mass Spectra

Abstract: A variety of experimental data is presented that implicates electrochemical oxidation of analytes in the electrospray (ES) needle as the mechanism for formation of molecular radical cations observed in the ES ionization mass spectra of alkylsubstituted metalloporphyrins, polycyclic aromatic hydrocarbons (PAH`s), and other compound types. Analyte structural characteristics and solution-phase half-wave oxidation potentials (which correlate with gas-phase ionization energies) can be used to evaluate the likelihood of forming and observing a particular compound as a radical cation. Use of an appropriate solvent is critical in the observation of radical cations generated by the ES process. In addition to dissolving the analyte and providing a stable electrospray, the solvent-(s) must {open_quotes}stabilize{close_quotes} or otherwise protect the radical cation from reactions in solution. Appropriate solvent systems (e.g., methylene chloride/0.1% trifluoroacetic acid) are much the same as used in traditional studies of electrochemically generated radical cations. The ability to produce radical cations in the ES process expands the utility of ES ionization mass spectrometry to include compound classes not normally amenable to the technique (e.g., neutral, nonpolar compounds such as PAH`s) and provides for generation of a different type of molecular species than normally produced in positive-ion ES ionization (i.e., M{sup {sm_bullet}+} versus (Mmore » + H){sup +}, (M + Na){sup +}, etc.).« less

Separation and identification of carotenoids and their oxidation products in the extracts of human plasma.

Abstract: Eighteen carotenoids as well as vitamin A and two forms of vitamin E (gamma- and alpha-tocopherol) have been separated from extracts of human plasma by high-performance liquid chromatography (HPLC) on reversed-phase and sillca-based nitrile-bonded columns. In the order of chromatographic elution on a C18 reversed-phase column, the carotenoids were identified as (3R,3'R,6'R)-beta, epsilon-carotene-3,3'-diol [(3R,3'R,6'R)-lutein], (3R,3'R)-beta, beta-carotene-3,3'-diol [(3R,3'R)-zeaxanthin], 5,6-dihydroxy-5,6-dihydro-psi,psi-carotene, 3-hydroxy-2',3'-didehydro-beta,epsilon-caroten-3-ol, 3-hydroxy-beta-carotene,psi,psi-carotene, 7,8-dihydro-psi,psi-carotene, beta,psi-carotene, 7,8,7',8'-tetrahydro-psi,psi-carotene, beta,epsilon-carotene, beta,beta-carotene, 7,8,11,12,7',8'-hexahydro-psi,psi-carotene, and 7,8,11,12,7',8'-11',12'-octahydro-psi,psi-carotene. The polar carotenoids, which eluted in the vicinity of lutein and were unresolved on the C18 column, have been separated on a nitrile-bonded column employing isocratic HPLC conditions. In the order of elution, the carotenoids were epsilon,epsilon-carotene-3,3'-dione, 3'-hydroxy-epsilon,epsilon-caroten-3-one, 5,6-dihydroxy-5,6-dihydro-psi,psi-carotene, 3-hydroxy-beta,epsilon-caroten-3'-one, (all-E,3R,3'R,6'R)-lutein, (all-E,3R,3'R)-zeaxanthin, and (all-E,3R,3'S,6'R)-beta,epsilon-carotene-3,3'-diol (3'-epilutein) followed by several geometrical isomers of lutein and zeaxanthin.

Optimal minimal neural interpretation of spectra

Abstract: A new method optimal minimal neural-network interpretation of spectra (OMNIS) based on principal component analysis and artificial neural networks is presented. OMNIS is useful whenever spectra are measured for the purpose of classification or quantitative determination. The spectra can be visible light, near-infrared (NIR) light, sound, or any other large amount of correlated data

Chemiluminescence detection using regenerable tris(2,2'-bipyridyl)ruthenium(II) immobilized in Nafion.

Abstract: The development of a detection method based on the electrogenerated chemiluminescence of tris(2,2'-bipyridine)ruthenium(II), (Ru(bpy)3(2+], immobilized in a Nafion film coated on an electrode is discussed. Control of the electrode potential controls creation of the reactive reagent Ru(bpy)3(3+) which reacts with certain analytes to yield chemiluminescence emission of intensity proportional to the analyte concentration. The reaction results in Ru(bpy)3(3+) being converted to Ru(bpy)3(2+), which then is recycled to Ru(bpy)3(3+) again at the electrode. This sensor has been used in flow injection to determine oxalate, alkylamines, and NADH. Detection limits are 1 microM, 10 nM, and 1 microM, respectively, with working ranges extending over 4 decades in concentration. Sensitivity is constant over the wide pH range from 3 to 10. With oxalate, and to a small extent with amines, emission intensities increase with increasing ionic strength; this was shown to be a phenomenon related to the Nafion film and not to the chemiluminescence reaction. Emission intensities increase with temperature. The sensor remains stable for several days with suitable storage conditions. Significant amounts of Ru(bpy)3(3+) are shown to be capable of storage within the film.

Hydrogen peroxide and .beta.-nicotinamide adenine dinucleotide sensing amperometric electrodes based on electrical connection of horseradish peroxidase redox centers to electrodes through a three-dimensional electron relaying polymer network

Abstract: : Hydrogen peroxide is efficiently electroreduced at an electrode modified with a hydrophilic, permeable film of horseradish peroxidase (HRP) covalently bound to a 3-dimensional epoxy network having polyvinyl pyridine (PVP)-complexed Os(bpy)2Cl+3/+2 redox centers. The sensitivity of the resulting H202 cathode at O.OV(SCE) is 1Acm(-2)M(-1). Its current increases linearly with H2O2 concentration in the 1x10(-7)M(-)2x10(-4)M range. Related NAD(P)H cathodes are based on stoichiometric homogeneous reduction of 02 to H202 by NADH or NAD(P)H. The reduction involves two known steps. In the first step, NAD(P)H transfers two electrons and a proton to a dissolved quinoid. The quinoids are typically derived of phenazines, however phenothiazine and phenoxazine derivatives are also useful. In the second step, two electrons and a proton are transferred from the reduced quinoid to O2. This reaction produces H202 and the original quinoid. Because the two reactions are quantitative, the sensitivity and the linear range of the resulting NADH and NADPH electrodes are identical with those of the H202 electrode, 1Acm(-2)M(-1) and 1x10(-7)M (-2)2x10(-4)M respectively.

Chiral recognition and enantiomeric resolution based on host-guest complexation with crown ethers in capillary zone electrophoresis

Abstract: A chiral crown ether is successfully used as a pseudostationary phase in capillary zone electrophoresis to separate optically active amines. On the basis of the results obtained from the separation of more than 20 amines, two recognition mechanisms are proposed: (I) the four crown ether substituents act as chiral bariers for the guest molecules and (II) lateral electrostatic interactions occur between host and guest. These results are confirmed by thermodynamic studies on the host-guest complexes

Temperature dependence of retention in reversed-phase liquid chromatography. 1. Stationary-phase considerations.

Abstract: The retention mechanism in reversed-phase liquid chromatography (RPLC) has been examined over a wide temperature range with emphasis on the role of the mobile phase. van't Hoff plot shapes were used to assess the retention mechanism, and the data showed evidence of the hydrophobic effect when water-rich and/or hydrogen-bonded mobile phases such as methanol/water were used. However, different van't Hoff plot shape was observed with acetonitrile/water mobile phases, indicating a change in the retention mechanism. These data showed that the hydrophobic effect, which had previously been proposed as the driving force for retention, is not a satisfactory explanation for the retention process in all RPLC systems.

Effects of Redox System Structure on Electron-Transfer Kinetics at Ordered Graphite and Glassy Carbon Electrodes

Abstract: The basal plane of hlghly ordered pyrolytlc graphtte (HOPG) serves as an ordered model of more commonly used electrode surfaces such as glassy carbon (GC) and pyrolytlc graphite. The defect denslty on the basal plane HOPG was reduced by careful cieavlng and cell design and was verified to be low by requiring that AEP for Fe(CN),S-'C (1 M KCI) be greater than 700 mV for a 0.2 V/s scan rate. Then a variety of redox systems were examined on "validated" HOPG surfaces, and varlatlons In the electron transfer rate constant, kbo, were observed. All 13 redox systems exhlblted relatively fast klnetlcs on laser activated GC (ko > 0.03 cm/s for elght Inorganic systems), and In several cases ko exceeded the Instrumental Ilmlt. On HOPG, however, AEP varled greatly for the 13 systems, ranging from 66 to >1200 mV. The reasons for thls varlatlon fall Into three general ciasslfications. First, reactlons Involving proton transfer (e.g. catechols) were all slow on HOPG, implylng some role of the surface In mediating multlstep processes. Second, the observed rate correlated wlth the exchange rate for homogeneous electron transfer, but the heterogeneous rates on HOPG were 3-5 orders of magnltude slower than that predicted from simple Marcus theory. Thlrd, the physical properties of HOPG, such as den* of electronic states and hydrophobicity, may depress ko relative to GC and metals.

Self-modeling mixture analysis of second-derivative near-infrared spectral data using the SIMPLISMA approach

Abstract: One of the major methods used to resolve spectral data by self-modeling techniques requires the presence of pure variables. A pure variable is a variable that has an intensity contribution from only one of the components in the data set. For spectral data obtained in the near-infrared (near-IR) region (ca. 1-25μm), pure variables are often not avaliable, due to the width of the spectral peaks and the presence of a baseline

Simple analysis of quasi-reversible steady-state voltammograms

Abstract: The kinetic parameters (K o and α) of a uncomplicated quasi-reversible electrochemical reaction studied by any steady-state voltammatric technique can be found directly from the values of two easily accessible experimental parameters, (E 1/4 -E 1/2 ) and (E 1/2 -E 3/4 ), where E 1/2 is the experimental half-wave potential and E 1/4 and E 3/4 are voltammetric quartile potentials

Equilibrium studies on colorimetric plastic film sensors for carbon dioxide

Abstract: The equillibrium responses of three new colorimetric plastic film sensors for CO 2 as a function of % CO 2 and temperature are described. the results fit a model in which there is a 1:1 equllibrium reaction between the depronated form of the dye (present in the film as an ion pair) and CO 2 . The 0-50% and 0-90% response and recovery times of each of these films when exposed to an alternating atmosphere of air and 5% CO 2 are determined and in two cases are typically less than 3 s

High-performance capillary electrophoresis of SDS-protein complexes using UV-transparent polymer networks.

Abstract: This paper demonstrates the use of UV-transparent replaceable polymer networks for the separation of SDS-protein complexes on the basis of molecular weight. First, the use of linear (i.e. non-cross-linked) polyacrylamide is shown to provide molecular separation of SDS-protein complexes. A study reveals such columns to yield significantly greater lifetime than cross-linked gels because of the flexibility of the noncovalently attached polymer chains. However, column lifetime was still found to be limited (approximately 20-40 injections), and detection at 214 nm was problematical because of the absorbance of polyacrylamide. UV-transparent polymer networks of dextran and PEG were substituted for polyacrylamide with successful molecular weight sieving of SDS-protein complexes at 214 nm. Due to their low to moderate viscosities, these networks could be routinely replaced leading to the possibility of hundreds of injections with a single column. Migration time reproducibilities of 0.5% RSD or less were found with replacement of the network. Using dextran, calibration plots of peak area vs concentration of standard protein were linear over the range of 0.5 microgram/mL up to at least 0.25 mg/mL. Furthermore, plasma samples could be directly utilized because of the strong solvating power of SDS. Rapid separation of protein mixtures are demonstrated with these UV-transparent polymer networks.