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Brief introductions to the research articles appearing in the December issues of Analytical Chemistry
Accelerated Article PCR amplification and separation on a chip DNA amplification and analysis devices that work quickly and use less reagents will be a critical part of completing the Human Genome Project. Richard A. Mathies and co-workers at the University of California—Berkeley and Lawrence Livermore National Laboratory describe the direct integration of microfabricated silicon PCR reactors and glass capillary CE chips into an integrated DNA analysis system. The new device performs a PCR-CE analysis of a (3-globin target in under 20 min and of genomic Salmonella DNA in less than 45 min. ("Functional Integration of PCR Amplification and Capillary Electrophoresis in a Microfabricated DNA Analysis Device"; AC960718Q; p. .0811
Using immunochemical detection to determine leukotrienes Sulfidopeptide leukotrienes, which play an important role in inflammation and allergic reactions, are commonly measured using microtiter-plate immunoassays, but these assays cannot distinguish between several structurally similar leukotrienes and their crossreactive metabolites. H. .rth and colleaguee st Leiden University (The Netherlands) and Astra Draco AB (Sweden) descrrbe the development and automation of a continuous-flow immunochemical detection system and its combination wiih LC. .ulfidopeptide leukotrinese and their cross-reactive metabolites can be determined in a single run with a detectton limit of 0.2 ng/mL for LTE4. ("Simultaneous Determination of Cross-Reactive Leukotrienes in Biological Matrices Using On-llne Liquid Chromatography Immunochemical Detection"; AC960250R; p. 4101)
ISEs for blood CI Ion-selective electrodes (ISEs.) mat measure INa , ft , and CI in blood can reveal clinical information about osmotic pressure and acid-base balance; although reliable ISEs have been developed for cations, an ISE with sufficient selectivity for Cl~ has yet to be produced. Satoshi Ozawa and co-workers at Hitachi (Japan), Kyushu University (Japan), and Fluka Chemie AG (Switzerland) develop anion-selective electrodes using methyltrialkylammonium salts with alkyl chains 12,16, and 18 carbons in length. The new ISEs improve the accuracy of Cl~ measurements in blood serum by a factor of 2-6. ("Anion-Selective Electrodes Based on Long-Chain Methyltrialkylammonium Salts"; AC960526V; p. 4149) • Denotes articles tentatively scheduled for the December 15 issue
An immunoelectrochemical assay for bacteria The speed and sensitivity of immunoelectrochemical sensors make them attractive for detecting pathogenic microorganisms in food and water, but the usefulness of these sensors can be complicated by unwanted interactions between the capture and detection functions. Jeffrey D. Brewster and colleagues at the U.S. Department of Agriculture develop a nondestructive epifluorescence microscopy method to determine bacteria capture efficiency. The method facilitates independent study of capture and detection functions and allows identification of performance-limiting factors. ("Immunoelectrochemical Assays for Bacteria: Use of Epifluorescence Microscopy and Rapid-Scan Electrochemical Techniques in the Development of an Assay for Salmonella"; AC9604503; p. 41415
Determining p/ in human rhinovirus Because capillary isoelectric focusing (CIEF) in free solution offers advantages over isoelectric focusing in polyacrylamide gels for determining pi, Ernst Kenndler and colleagues at the University of Vienna (Austria) use CIEF to determine the pi value of human rhinovirus serotype 2. To overcome the problem of the virus zone failing to reach the steady-state position in the pH gradient because of electroosmotic flow (EOF), hydrodynamic pressure is applied at the detector side during focusing to counteract the EOF. Using the relationship between the apparent pi value of the virus and the focusing time under counterpressure, the actual pi is determined to be 6.8. ("Determination of the pi of Human Rhinovirus Serotype 2 by Capillary Isoelectric Focusing"; AC9603789; p. 4300)
Glucose microsensor • Considerable effort has gone into the search for a reliable glucose sensor, but numerous obstacles remain. Claude Martelet and colleagues from Ecole Centrale de Lyon (France), Kiev University (Ukraine) and CI. Bernard University (France) describe a microelectrode with the enzyme glucose oxidase immobilized on the surface of a carbon fiber by cross-linking in glutaraldehyde vapor or by enzyme entrapment in an electropolymerized film. The sensor's differential normal pulse voltammetry response to glucose is essentially insensitive to interfering substances present at physiological concentrations; the glucose concentrations determined in vitro agree well with those measured by standard procedures. ("In Vivo Brain Glucose Measurements: Differential Normal Pulse Voltammetry with Enzyme-Modified Carbon Fiber Microelectrodes"; AC960190P)
Measuring the freely available concentration of drugs • In biological studies, determination of the amount of freely available drug is important because only the unbound drug is Analytical Chemistry News & Features, December 1, 1996 701 A
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active and capable of diffusing across membranes. Wouter H. J. Vaes and colleagues at Utrecht University (The Netherlands) use solid-phase microextraction to measure the concentration of four polar-substituted benzenes in a solution of bovine serum albumin. They find that by extracting a small amount of the freely dissolved fraction, the equilibrium between the fraction of a compound bound to a matrix and that dissolved in the aqueous phase is not perturbed. ("Measurement of the Free Concentration Using Solid-Phase Microextraction: Binding to Protein"; AC960337C) Model of steady-state currents There is interest in using steady-state ' voltammetry to study electrochemical reactions in solutions to which little electrolyte has been added, but theoretical models have only been developed for simple electrode reactions. Janet G. Osteryoung and co-workers at North Carolina State University present a theoretical model for the reduction of a singly charged cation under conditions where migration is important and the cation is coupled to a neutral species through the chemical equilibrium AB = A+ + +". The ratio of the transportlimited current /, to the diffusion-limited current Id is found to depend on the ratio of electrolyte to equilibrium concentration of A+ in bulk solution, which agrees with published experimental data for weak acids. ("Steady-State Limiting Currents Determined by Coupled Diffusion, Migration, and Chemical Equilibrium"; AC960410Z; p. 4124) A new AP substrate Although the substrate Af-ferrocenoyl-6-amino-2,4-dimethylphenyl phosphate provides high sensitivity and low detection limits for the electrochemical enzyme assay of alkaline phosphatase, its use is limited because the electrode cannot be reused. Benoit Limoges and Chantal Degrand of the Universite Blaise Pascal de Clermont-Ferrand (France) use the sodium salt of ferrocene ethyl phosphate ester as a new substrate, resulting in an electrode that can be reused. The electrode can detect AP at 2 x 10"12 M and can detect avidin at 5 x 10"12 M. .(Ferrocenylethyl Phosphate: An Improved Substrate for the Detection of Alkaline Phosphatase by Cathodic Stripping Ion-Exchange Voltammetry. Application to the Electrochemical Enzyme Affinity Assay of Avidin"; AC9605760; p. 4141) Polymer-coated nanoscale electrode ensembles have improved detection limits Electrochemical detection limits can be improved by using smaller electrodes, at which the Faradaic currents increase more quickly than the charging currents, and by coating the electrode surface with an ion-exchange polymer, which preconcentrates a counterion at the electrode surface. Paolo Ugo, Charles R Martin, and colleagues at the University of Venice (Italy) and Colorado State University describe the results of electrochemical experiments at nanoelectrode ensembles (NEEs) that are coated with AQ 55 poly(ester sulfonate) cationexchange polymer. The detection limits of 5 x 10~9 M for rferrocenylmethyl) trimethylammonium and 6 x 10"9 M for Ru(NH3)|+ are better than those with bare macroelectrodes or bare NEEs. ("Ion-Exchange Voltammetry at Polymer FilmCoated Nanoelectrode Ensembles"; AC960580B; p. 4160) 702 A
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Thia crown ethers detect Ag ion Ion-selective electrodes with crown ethers have been successfully used to measure alkali and alkaline earth metals, but very few ISEs have been developed for the transition and heavy metals. Koji Suzuki and co-workers at Keio University Qapan) and Nissan ARK (Japan) synthesize 16 different ionophores using cyclic and noncyclic thia ethers that they use to selectively measure Ag+. The best Ag+-selective electrode, which had a selectivity more than 104 times higher than its selectivity for other metal ions, is made with a 14-membered thia crown ether with a bulky pinan subunit. ("Structural Ion Selectivity of Thia Crown Ether Compounds with a Bulky Block Subunit and Their Application as an Ion-Sensing Component for an Ion-Selective Electrode"; AC960396Q; p. 4166) Expanding the potential of voltammetry Voltammetric methods hold promise for providing insights into electrochemical fundamentals such as equilibrium constants, transport parameters, and analyte concentrations, but they have yet to be used to their full potential because of complications such as migration, homogeneous equilibria, and diverse diffusivities. Keith B. Oldham of Trent University (Canada) develops an exact treatment to predict the steady-state limiting voltammetric current for a system that incorporates all three of these complicating factors. ("Effect of Diffusion Coefficient Diversity on Steady-State Voltammetry When Homogeneous Equilibria and Migration Are Encountered"; AC9601730; p. 4173) Ultramicroelectrodes and the thioctic acid monolayer The ultrathin polymer morphology of ultramicroelectrodes—a key aspect of the electrode design—is often difficult to control. Anna Brajter-Toth and Quan Cheng of the University of Florida describe the fabrication and characterization of a thioctic acid monolayer on a gold-fiber ultramicroelectrode. The resulting device shows selectivity to electroinactive electrolyte anions, as well as to electroactive anions and cations. ("Permselectivity, Sensitivity, and Amperometric pH Sensing at Thioctic Acid Monolayer Microelectrodes"; AC960329W; p. 4180) Charge propagation in hydrogels Redox polymer hydrogels represent a bridge between the electrode and the enzyme's redox site. E. J. Calvo and colleagues at the Universidad de Buenos Aires (Argentina) report the application of a polycationic hydrogel derivatized with different redox couples in amperometric biosensors. The charge propagation behavior of these electrodes is studied using cyclic voltammetry and electrochemical impedance spectroscopy. ("Electrical Communication between Electrodes and Enzymes Mediated by Redox Hydrogels"; AC960170N; p. 4186) Identifying jet fuels As the military tries to clean up its bases, new ways of identifying and monitoring aviation fuels are needed. Paul A. Edwards and colleagues at the Edinboro University of Pennsylvania and the U.S. Air Force demonstrate that the response of an array of vapor-sensitive detectors can be used to identify aviation fuels. Visual inspection of radar plots shows characteristic patterns for five of six fuels examined, and neural network analysis is used to confirm the identifications. ("Fuel Identification by
Neural Network Analysis of the Response of Vapor-Sensitive Sensor Arrays"; AC9603892; p. 4264) Detecting aromatic amines • Because of their toxicity, measuring aromatic amines in the environment is of great interest Joseph Wang and colleegues sa New Mexico State University describe a DNA-modified electtode that relies on the intercalative collection of aromatic amines onto the immobilized dsDNA layer followed by potentiometric sttipping. The extent and rate of accumulation are strongly dependent on the structure of the aromatic amine, and having the amino substituent in n slightly different position has a dramatic effect on response. ((DNA-Mooified Electrode foo the Detection oo Aromatic Amines"; AC960650E) A microsensor for H 2 S • Hydrogen sulfide plays an important role in biogeochemical processes such as the formation of heavy metal precipitates and oxidation by phototrophic or chemotrophic bacteria. P. Jeroschewski and colleagues at the University of Rostock (Germany) and the Max-Planck-Institute for Marine Microbiology (Germany) develop an amperometric microsensor for detecting dissolved H2S. The sensor is based on the same principle as the Clark oxygen sensor. ("An Amperometric Microsensor for the Determination of H2S in Aquatic Environments"; AC960091B) Imaging for studying transport phenomena • The differences in interpreting data obtained with ion-selective electrodes (ISEs) can be attributed in part to the inability to follow concentration profiles in the membrane during experiments. Erno Pretsch and colleagues at the Swiss Federal Institute of Technology and the Technical University of Budapest (Hungary) use spectropotentiometric imaging to visualize the transport of species in the ISE membranes in parallel with measuring transient potentials. Concentration profiles are calculated and plotted as a function of time, the rate-determining process is evaluated, and diffusion coefficients are obtained. ("Spectropotentiometry: A New Method for in Situ Imaging of Concentration Profiles in Ion-Selective Membranes with Simultaneous Recording of Potential-Time Transients"; AC9604245) Transparent tiny vials • Detecting analytes present in a single cell by electrochemical means requires small-volume sample handling techniques that avoid diluting the already low analyte levels. Andrew Ewing and colleagues at The Pennsylvania State University describe the fabrication of picoliter to femtoliter microvials in polystyrene for this purpose. Sigmoidal voltammograms are obtained for vial sizes down to 1 pL, and the transparent microvials work well with transmission optical microscopy. ("Electrochemical Analysis in Picoliter Microvials"; AC960559A) Multicolumn LC and MS screen libraries The potential for combinatorial libraries to provide a number of possible drug candidates from a single lead compound requires improved methods for screening those libraries. Jack D. Henion and co-workers at Cornell University combine immunoaffinity extraction and two-dimensional, reversed-phase chromatography with ion spray MS and tandem MS to separate, detect, and characterize known 19-component and unknown • Denotes articles tentatively scheduled for the December 15 issue
20-component benzodiazepine libraries. Even though the particular antibodies used do not provide highly specific selection, the use of multiple LC columns gives ffexibility to the eeparation techniques for screening combinatorial libraries and allows a built-in sample cleanup step. ("Characterization of Benzodiazepine 'Combinatorial' Chemical Libraries by On-Line Immunoaffinity Extraction, Coupled Column HPLC-Ion Spray Mass Spectrometry-Tandem Mass Spectrometry"; AC9603035; p. 4228) New probe for electrochemistry/MS The combination of electrochemistry and MS provides identification of electrochemically generated species but has not been easily applied to all types of analytes. Richard B. Cole and coworkers at the University of New Orleans overcome some of the barriers with a newly designed probe for fast electrochemistry/electrospray MS that features a three-electrode electrochemical cell in which the working electrode is a Pt wire flamee sealed inside a fused-silica capillary and the auxiliary electrode is a stainless steel capillary that also functions as the electrospray capillary. The electrochemical probe generates electrochemical intermediates and products in situ at the tip of the electrospray capillary, delaying reaction until the species are ready to be released into the electrospray region. ("On-Line Probe for Fast Electrochemistry/Electrospray Mass Spectrometry. Investigation of Polycyclic Aromatic Hydrocarbons"; AC960362I; p. 4244) Determining ionization energies Although ionization energies are fundamental properties of molecules, they are not routinely determined for new compounds, and their measurement is not accessible to most investigators. R G. Cooks and colleagues at Purdue University show that the kinetic method, a general procedure for estimating relative thermochemical quantities by measuring the relative rates of competitive dissociation of mass-selected, loosely bound cluster ions, can be used to determine the ionization energies of organic compounds. The method is demonstrated using a series of substituted benzenes. ("Ionization Energy Determination by the Kinetic Method"; AC960566Z; p. 4254) Finally, theory explains an ion trap oddity Despite the popularity of the design, the observed ultrahigh mass resolution of an ion trap mass spectrometer operating at low scanning rates has never been adequately characterized. Alexander A. Makarov of HD Technologies (U.K.) modifies the pseudopotential well approximation model to include octapole perturbations on the quadrupole field to produce a new model of ion ejection. Agreement between calculated and experimental peak widths is demonstrated for scanning rates that range over six orders in magnitude. ("Resonance Ejection from the Paul Trap: A Theoretical Treatment Incorporating a Weak Octapole Field"; AC960653R; p. 4257) Ion remeasurement in FT-ICRMS • Because of its nondestructive image current detection, the ICR can remeasure ions, which leads to improved S/N and is useful for ionization conditions that yield low ion abundances. I. Jonathan Amster and colleagues at the University of Georgia show that axial ejection during ion excitation is the major mechanism for ion loss during single-frequency and broadband quadrupolar excitation remeasurement. They find that by capacitively coupling excitation signals to the trapping plates of Analytical Chemistry News & Features, December 1, 1996 703 A
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an open-ended cylindrical analyzer cell, high remeasurement efficiency can be achieved. ("Effects of Capacitive Coupling on Ion Remeasurement Using Quadrupolar Excitation in HighResolution FT-ICR Spectrometry"; AC960659G) Preparing samples for IRMS • Although deuterium is often used for studying the natural water cycle, determination of D/H can be difficult because the water must be converted to hydrogen gas. M. Gehre and colleagues at Umweltforschungszentrum Leipzig-Halle (Germany) describe a technique for converting samples to hydrogen gas using chromium metal and a reaction furnace for measurement of 8D values by isotope ratio MS. The reproducibility is about 1%. ("Sample Preparation Device for Quantitative Hydrogen Isotope Analysis Using Chromium Metal"; AC9606766) Rare earth element ICPMS w i t h FIA • The analysis of rare earth elements in natural aqueous solutions by ICPMS requires preconcentration, but simple evaporation is usually not suitable because salts can precipitate or the solution may have high concentrations of dissolved solids that create interferences or clog the nebulizer. Jugdeep K. Aggarwal and co-workers at the University of Bristol (U.K.) and Mitsubishi Material Qapan) use flow injection coupled with ICPMS, which allows the samples to be preconcentrated to smaller volumes than those required for normal nebulization. The ffowinjection analysis results in a three-fold improvement in detection limits over normal nebulization. ("Determination of the Rare Earth Elements in Aqueous Samples at Sub-ppt Levels by Inductively Coupled Plasma Mass Spectrometry and Flow Injection ICPMS"; AC9602074) Affinity chromatography for studying metal binding Applying any biomaterial to the remediation of heavy metals depends on understanding the basic chemistry in the metal binding process. Gary D. Rayson and colleagues at New Mexico State University use normal frontal affinity chromatography to generate metal binding isotherms to cell fragments of the plant Datura innoxia immobilized within n polysilicate matrix. They obtain a total metal-ion binding affinity order at pH 3-5 of Cu2+ > Cd22+» Ag+ > Ca2+. ."Applications of Frontal Affinity Chromatography to the Study of Interactions between Metal Ions and a Complex Biomaterial"; AC960569B; p. 4087) True pH The pH of the HPLC mobile phase is a major factor in the separation of ionizable compounds, but most models make certain assumptions or simplications when dealing with this factor. Marti Roses, Elisabeth Bosch, and colleagues at the Universitat de Barcelona (Spain) introduce a new model that uses the true pH value of the mobile phase, takes into account activity coefficients, and considers different holdup times for neutral and ionic species. The proposed model can be combined with previously derived equations to establish a general model relating retention to key mobile-phase properties. ("Retention of Ionizable Compounds on HPLC. 2. Effect of pH, Ionic Strength, and Mobile-Phase Composition on the Retention of Weak Acids"; AC960105D; p. 4094) 704 A
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Solid-state NMR of C 1 8 phases Despite the popularity of alkyl-bonded phases for reversedphase LC, some details of the molecular structure and order of the immobilized chains remain uncertain. Klaus Albert and colleagues at the Universitat Tubingen (Germany) and the National Institute of Standards and Technology study C18 phases prepared by different synthetic pathways using solid-state NMR spectroscopy. They find that Clg phases prepared by the surface polymerization technique exhibit more regular surface coverage than sorbents prepared by conventional polymeric synthesis. ("Chain Order and Mobility of High-Density Clg Phases by Solid-State NMR Spectroscopy and Liquid Chromatography"; AC9606113; p. 4107) SPME of Granny Smith apples Originally developed for sampling organic contaminants in water, solid-phase microextraction (SPME) is now being applied to headspace sampling above solid and liquid samples. Adam J. Matich and co-workers at the Horticulture and Food Research Institute of New Zealand and Massey University (New Zealand) use SPME for the direct sampling of aroma volatiles, particularly a-farnesene, in the headspace of Granny Smith apples. The high molecular weight volatiles such as a-farnesene did not equilibrate among the apples, headspace, and fiber in as much as 90 min, so although SPME is suitable for rapid, qualitative determinations of apple headspace volatiles, it is not suited for quantitation in more complex systems. ("Solid-Phase Microextraction for Quantitative Headspace Sampling of Apple Volatiles"; AC9604548; p. 4114) Investigating retention mechanisms In chromatographic separations, analytes often interact with the positively charged silanol surface in addition to interacting with the monolayer forming the stationary phase, which has been given the name "mixed-mode retention". To study this phenomenon, Mary J. Wirth and colleagues at the University of Delaware use evanescent wave excitation of fluorescence to study the adsorption of Ru(bpy)|+ from aqueous solution to bare silica and dimethylethylsiloxane (Cg) and dimethyloctadecylsiloxane (C18) monolayers on silica. The free energy of adsorption for the silica surface [-26.2 (±0.2) kj/mol] is attributed to electrostatic interactions only, whereas the free energies of the adsorption for the C2 [-28.5 (+ 0.1) kj/mol] and C18 [-31.5 (±0.1) kj/mol] surfaces have both electrostatic and nonpolar contributions. ("Spectroscopic Probing of Mixed-Mode Adsorption of Ru(bpy)|+ to Silica"; AC960587T; p. 4119) Nuking organotins Sample preparation for organotins can take hours, is costly and subject to errors, and is not amenable to automation. Ryszard Lobiiiski and colleagues at the Universite de Bordeaux I use hydrolysis and a low-power focused microwave field to prepare biological organotin samples for measurement by GC. Ethyl derivatives are quantitatively obtained after a 3-min reaction and the procedure is validated using certified reference fish tissue. ("Speciatton Analysis for Organottn Compounds in Biomaterials after Integrated Dissolution, Extraction, and Derivatization in a Focused Microwave Field"; AC9604797; p. 4135) Reexamining plate number in MEKC The plate number in micellar electrokinetic chromatography and related electrokinetic techniques has been reported to in• Denotes articles tentatively scheduled for the December 15 issue
crease with increasing surfactant concentration. Joe M. Davis and colleagues at Southern Illinois University reexamine the dependence of plate number on surfactant concentration in MEKC for the common surfactant sodium dodecyl sulfate. Theyfindthat their results differ from those previously reported and that variations in plate number do not always follow a single trend. ("Reexamination of Dependence of Plate Number on SDS Concentration in Micellar Electrokinetic Chromatography"; AC960501W; p. 4270)
Measuring hydrophobicity with MEEKC A migration index based on microemulsion electrokinetic chromatography (MEEKC) has been proposed as an alternative to the traditional hydrophobicity scales. Yasushi Ishihama and colleagues at Eisai Co. (Japan) investigate the effect of the structure of the surfactant on the selectivity of MEEKC. Their results indicate that neither the ionic groups or the hydrocarbon chain lengths of the surfactant affect the selectivity for neutral solutes, but that ionic interactions occur between cationic solutes and anionic surfactants both in the aqueous phase and in the microemulsion phase. ("Hydrophobicity of Cationic Solutes Measured by Electrokinetic Chromatography with Cationic Microemulsions"; AC960447J; p. 4281)
Postcolumn reactors for CE devices Fluorescence detection is a commonly used technique for CE, and a number of post- and precolumn labeling schemes have been developed for nonfluorescing analytes. However, the required mixing point junctions are often difficult to fabricate or introduce band-broadening. D. Jed Harrison and co-workers at the University of Alberta (Canada) present three different designs for postcolumn reaction and detection devices in fused silica and Pyrex glass that provide high-efficiency separations; pH differences in the mixing solutions also affect the efficiency of the postcolumn reactions. ("Integrated Capillary Electrophoresis Devices with an Efficient Postcolumn Reactor in Planar Quartz and Glass Chips"; AC9604090; p. 4285)
In the loop Although electromigration injection for CE is simple, convenient, and improves detection limits, sample conductance can affect the amount of analyte introduced into the system. Purnendu K Dasgupta and Kazimierz Surowiec of Texas Tech University form a film of sample on a small loop and use the loop itself fa the eighvoltage electrode to perform electromigration. The sample's constituents can be exhaustively electromigrated in less than 1 min. ("Electromigration Injection from a Small Loop in Capillary Electrophoresis"; AC960663S; p. 4291)
SCOTCH theory • There are few reports in the literature concerning the theoretical prediction of height equivalent to a theoretical plate h in serially coupled open-tubular column gas chromatography (SCOTCH) and no reports concerning separation number TZ and h in temperature-programmed GC. Panayotis A. Siskos and Evangelos B. Bakeas of the University of Athens (Greece) study TZ and h in a system of two columns coupled in series under temperature-programmed and isobaric conditions. TZ is strongly dependent on the midpoint pressure and h on the temperature; TZ and h are related inversely in a relationship that involves the overall column length. ("Effects of Temperature Programming and Pressure on Separation Number and Height • Denotes articles tentatively scheduled for the December 15 issue
Equivalent to a Theoretical Plate in Optimization of a Serially Coupled, Open-Tubular Columns Gas Chromatographic System"; AC960572V)
Alternative refrigerants as SFE solvents • For extracting thermally stable analytes, such as PCBs, polar solvents with critical temperatures above that of C0 2 are desirable. Michal Roth of the Academy of the Czech Republic evaluates the thermodynamic properties of six alternative refrigerants for use as solvents in SFE. The calculated solubility parameters indicate that, in the high-pressure, low compressibility region, the solvating power of these refrigerants is superior to that of C02. ("Thermodynamic Prospects of Alternative Refrigerants as Solvents for Supercritical Fluid Extraction"; AC960609B)
Separating during electromigration • Because the charge-to-size ratio is relatively constant for DNA fragments beyond 10-20 bp at neutral-to-slightly-alkaline pH, some type of size- or shape-selective component must be introduced into the buffer medium to effect separation during electromigration. Edward S. Yeung and colleagues at Iowa State University separate DNA fragments in capillary tubes based on two modes. In electromigration mode, electrostatic shielding of the negatively charged DNA fragments lowers electrophoretic mobility against the electroosmotic flow resulting in smaller fragments eluting earlier. In hydrodynamic mode, the Poisseuille flow profile causes molecules near the surface to move slower resulting in the larger fragments eluting earlier. ("Electrostatic and Hydrodynamic Separation of DNA Fragments in Capillary Tubes"; AC9605814)
ISPSs in solid phase reactors • In situ polymerized supports (ISPSs) have an advantage over traditional packing materials in that they are self contained and do not need to be retained by porous frits. Knut Irgum and co-workers at Umea University (Sweden) describe the use of in situ polymerized methacrylate ISPSs in a flow system based on l.l'-oxalyldiimidazolyl peroxyoxalate chemiluminescence detection of hydrogen peroxide, with immobilized 3-aminofluoranthene as the light emitter. Lower functionalization densities increase the peak area sensitivity but have less of an effect on peak height sensitivities for hydrogen peroxide. ("Solid Phase Chemiluminescence Detection Reactors Based on in Situ Polymerized Methacrylate Materials"; AC9604999)
Microwave-assisted leaching better than conventional methods • Temperature control is the most significant source of error and uncertainty in the leaching measurements of EPA Method 3050B. H. M. "Skip" Kingston and Elke M. L. Lorentzen oo Duquesne University compare microwave-assisted and conventional (hot plate digestion) leaching of Cu, Pb, Zn, Cd, Cr, and Ni from NIST standard reference materials. Microwaveassisted leaching is found to be more efficient and more precise. ("Comparison of Microwave-Assisted and Conventional Leaching Using EPA Method 3050B"; AC960553L)
Impurities in semiconductor-grade gases • Despite the importance of high-purity gases in the semiconductor industry, there has been little work published on determining metal impurities in gases, and most of those methods lack a Analytical Chemistry News & Features, December 1, 1996 705 A
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reproducible, quantifiable, high level of extraction. S. Laly and colleagues at Nippon Sanso 0apan) describe a filter trap method using a Teflon membrane filter to analyze particlebound metal impurities in silane. Analytical results showed that total metallic impurities are < 1 ppb (w/w) )n the gas. .(Determination of Particle-Bound Metallic Impurities in Semiconductor Grade Gases. 1. Silane"; AC960428+) Don't trust that gas mixture • The presence of He substantially affects the results of extractions and chromatographic separations using supercritical C0 2 with He head pressure, but there are few methods for measuring the amount of He in a C0 2 gas mixture. Martyn Poliakoff and colleagues at the University of Nottingham (U.K.) introduce an acoustic technique to measure the critical pressures of He + C0 2 mixtures and then use the method to evaluate the composition of commercial He head pressure cylinders; FT-IR quantified the effect of He on the density as a function of temperature and pressure. The authors report substantial differences in the He content of headspace C0 2 gas samples taken from apparently identical gas cylinders. ("Acoustic Determination of the Helium Content of Carbon Dioxide from He Head Pressure Cylinders and FT-IR Studies of the Density of the Resulting Supercritical CO,: Implications for Reproducibility in Supercritical Experiments"; AC960573N) Multiple surfactants used to modify HPLC mobile phase • When surfactants are used in chromatographic systems, the concentrations are usually high enough to form micelles, or mixed micelles if more than one surfactant is used, but surfactants can also be used as mobile phase modifiers below the critical micelle concentration (CMC). James S. Fritz and Xue Li of the U.S. Department of Energy's Ames Laboratory at Iowa State University use sub-CMC binary and ternary mixtures of surfactants as modifiers to an acetonitrile-water mobile phase during the separation of mixtures containing 18-20 compounds. A ternary additive system consisting of Brij-30, dii octyl sulfosuccinate, and cetyltrimethylammonium bromide is a more effective mobile phase modifier than any of the binary mixtures. ("Mixed Surfactants as Mobile Phase Additives for the Separations of Organic Compounds by HPLC"; AC960215K) NAA impacts meteorites Determining the chemical composition of chondrites, which could provide information on the formation and evolution of the solar system, is not easy because of the inhomogenous distribution of metals. By changing irradiation and decay times, and selecting suitable y- and X-ray photopeaks, Hiromichi Nakahara and colleagues at the Tokyo Metropolitan University (Japan) use instrumental neutron activation to nondestructively measure the Pt group and siderophilic elements in metallic meteorites. Siderophile abundances measured in the Odessa iron meteorite are consistent with literature values. ("Determination of 18 Siderophile Elements Including All Platinum Group Elements in Chondritic Metals and Iron Meteorites by Instrumental Neutron Activation"; AC960477M; p. 4130) 706 A
Analytical Chemistry News & Features, December 1, 1996
Amplifying peroxy radicals in the atmosphere More can be learned of peroxy radicals, which play an important role in the atmosphere, through direct measurements than through chemical models or photostationary state calculations. Christopher A. Cantrell and co-workers at the National Center for Atmospheric Research use a dual-inlet chemical amplifier in which H0 2 radicals are measured by conversion to N02 and luminol chemiluminescence. The dual-inlet amplifier, which uses two identical glass inlet reactor-N02 detector combinations, one of which operates in the background mode and the other of which operates in the N02 measurement mode, is less sensitive to fluctuations in ambient ozone levels than is the single-inlet instrument. ("Dual-Inlet Chemical Amplifier for Atmospheric Peroxy Radical Measurements"; AC960639E; p. 4194) Genetic algorithms for near-IR spectroscopy Gary W. Small and colleagues at Ohio University and the University of Iowa delineate the use of genetic algorithms to implement an automated wavelength selection procedure for building multivariate calibration models based on partial least-squares regression. The data to test the methodology are derived from near-IR spectra of aqueous organic species and feature analyte signals near the limit of detection. The new procedure performs well, leading to calibration models that significantly outperform those based on full spectrum analyses. ("Genetic Algorithm-Based Method for Selecting Wavelengths and Model Size for Use with Partial LeastSquares Regression: Application to Near-Infrared Spectroscopy"; AC9607121; p. 4200) Studying crater depth and shape in rf-GD-AES Althoughrf-GDhas been widely used for bulk analysis, little effort has been expended to explore its depth-profiling capabilities. R. Kenneth Marcus and colleagues at Clemson University conduct a parametric study to determine the sputtering characteristics and resulting crater shapes for both metallic and nonconducting samples. Their results indicate that rf-GD is well suited for depth-resolved analysis under correctly chosen operating conditions, which fortunately correspond to those for optimum bulk elemental analysis. ("Influence of Discharge Parameters on the Resultant Sputtered Crater Shapes for a Radio Frequency Glow Discharge Atomic Emission Source"; AC960259T; p. 4213) Calculating spectra for environmental analysis Commercial libraries of vapor-phase IR spectra have insufficient reference spectra to cover all the compounds tracked for environmental compliance. Donald F. Gurka and colleagues at EPA's National Exposure Research Laboratory (Las Vegas, NV), the University of Nevada, the University of Missouri, and EPA's National Health and Environmental Effects Research Laboratory (Duluth, MN) describe using computational chemistry in conjunction with GC/MS/FT-IR to tentatively identfy seven tetrachlorobutadiene isomers in an environmental sample. This approach can characterize or confirm analytes even when standard spectra or pure standards are unavailable. ("Environmental Analysis by ab Initio Quantum Mechanical Compu• Denotes articles tentatively scheduled for the December 15 issue
tation and Gas Chromatography/Fourier Transform Infrared Spectrometry"; AC960463G; p. 4221) Relating ion mobility constants to structure For portable ion mobility spectrometry units to be useful, the operator must be able to relate the value of the reduced ion mobility constant (K0) to the structure of a compound. Peter C. Jurs and colleagues at The Pennsylvania State University use quantitative structure-property relationships to develop models to predict K0 for a set of 168 organic compounds directly from the molecular structure. Their model contains six molecular structure descriptors that are independent of geometry and has an rms error of ~ 0.04 K0 units. They find that knowing only the two-dimensional structure of a compound is sufficient to predict K0. ("Prediction of Reduced Ion Mobility Constants of Organic Compounds from Molecular Structure"; AC960466T; p. 4237) Optical fusion of microdroplets Although mechanical methods can be used to successfully produce and/or manipulate microdroplets, optical manipulation is another possibility. Noboru Kitamura and colleagues at Hokkaido University (Japan) use multibeam laser manipulation to fuse two dye-containing microdroplets in solution and simultaneously characterize the droplet before and after fusion with microspectroscopy. The authors expect that the technique can be applied to quantitative analysis of solutes in small volumes. ("Optical Control of Fusion of Microparticles in Solution and Simultaneous Spectrophotometric Measurements"; AC9605556; p. 4304) A general formula for neutron activation and decay • The processes occurring in neutron activation analysis are governed by sets of linear first-order differential equations, and general equations have been developed for various systems. Stefaan G. Pomme and colleagues at SCK-CEN (Belgium), the Institute for Reference Materials and Measurements (Belgium), the University of Gent (Belgium), DSM Research (The Netherlands), and the Hungarian Academy of Sciences present a general formula for nuclear activation and decay that takes into account burnup and successive activation. Incorporation of the formula into the k0 standardization is demonstrated for several decay types. ("General Activation and Decay Formulas and Their Application in Neutron Activation Analysis with k0 Standardization"; AC9604402) Confocal Raman imaging of liquids in a porous medium • Visualization of liquid and vapor topology within a pore structure and the measurement of liquid-vapor-solid wetting properties is essential to understanding transport mechanisms. Colin J. H. Brenan and colleagues at the Massachusetts Institute of Technology and the University of California take advantage of the confocal Raman microscope's high spatial resolution and chemical specificity for the in situ study of these processes. Images of trichloroethylene-water in porous sandstone provide information on the local variation in the wetting behavior of each.liquid and allows estimation of volume percentages. ("Noninvasive Confocal Raman Imaging of Immiscible Liquids in a Porous Medium"; AC960654J) • Denotes articles tentatively scheduled for the December 15 issue
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