In AC Research: In AC Research

ics are best described by a model that includes the global mo- tions of the protein and the local motion of the ... The best results are obtained usin...
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In AC

Research

In AC Research contains brief introductions to the research articles appearing in the February 1 issue of Analytical Chemistry plus a partial listing of articles tentatively scheduled for the February 15 issue. A free updated table of contents is available on the 1st and 15th of every month on the Web (http://pubs.acs.org/ac).

MS for bacterial identification Accelerated Article Probing the electrospray plume • Although electrospray is frequently used as an ionization technique for MS, a detailed mechanism is lacking. Kelsey D. Cook and co-workers at the University of Tennessee and Oak Ridge National Laboratory investigate the electrospray plume with laser-induced fluorescence spectroscopy of fluorescent dyes in the spray stream. The technique is used to measure spray-induced pH changes by monitoring the fluorescence signals from a pH-sensitive fluorophore, carboxyseminaphthorhodafluor-1. This represents the first direct measurement of pH within the electrospray plume. ("Investigation of the Electrospray Plume by Laser-Induced Fluorescence Spectroscopy"; 10.1021/ac981259r)

Pyrolysis MS provides fast, sensitive, high-throughput, low-cost bacterial classification with minimal sample preparation, but the current electron-ionization approach has the disadvantage of degrading molecules into low molecular weight species. S. A Barshick and colleagues at Oak Ridge National Laboratory compare electron and chemical ionization and find that chemical ionization reduces the amount of fragmentation while enhancing the molecular-ion region of the fatty acids, which are used to identify intact microorganisms. The best results are obtained using efhanol as the ionization reagent and using pooled masses representing specific fatty-acid biomarkers rather than total ion profiles. Differentiation at the subspecies level is possible. ("Differentiation of Microorganisms Based on Pyrolysis-Ion Trap Mass Spectrometry Using Chemical Ionization"; 10.1021/ac980356h; p. 633)

Direct sequencing of neuropeptides Verifying PCR amplification Rapid amplification of DNA is relevant to diagnostics and may be relevant to the prompt identification of biological warfare agents. Adam Heller and colleagues at the University of Texas-Austin describe a method confirming, without high-speed separation, that all or part of the DNA was amplified by using a miniature low-power potentiostat. They verify the amplification of an 800-base template in 10 min on a 2-uL sample of the polymerase chain reaction product solution. ("Rapid Amperometric Verification of PCR Amplification of DNA"; 10.1021/ac980770h; p. 535)

Measuring inorganic phosphate The periplasmic phosphate-binding protein of Escherichia coli has potential as a biorecognition element in a biosensing scheme for inorganic phosphate (P;). Sylvia Daunert and coworkers at the University of Kentucky use an acrylodan-labeled mutant phosphate-binding protein from E. coli—in which the alanine at position 197 is replaced with a cysteine—to selectively bind and determine the concentration of Pj. The fluorescence emission is sensitive to micromolar concentrations of Pj. Time-resolved measurements reveal that the rotational dynamics are best described by a model that includes the global motions of the protein and the local motion of the acrylodan probe. ("A Dynamical Investigation of Acrylodan-Labeled Mutant Phosphate Binding Protein"; 10.1021/ac980800g; p. 589)

Sonja Jespersen and co-workers atTNO Pharma and the University of Nijmegen (both in The Netherlands), and the Heinrich Heine University Diisseldorf (Germany) describe an approach for obtaining neuropeptide sequence information directly from tissue samples using MALDI combined with postsource decay fragment ion mass analysis. The sequences of the known peptide desacetyloc-MSH and an unknown (but suspected) vasotocin neuropeptide are verified. The sequence of the unknown peptide is determined from unfractionated tissue pieces of the pituitary pars intermedia of Xenopus laevis. ("Direct Sequencing of Neuropeptides in Biological Tissue by MALDI-PSD Mass Spectrometry"; 10.1021/ ac980841c; p. 660)

New dye for near-IR LIF Laser-induced fluorescence is the most sensitive detection method available for CE. Because few analytes are inherently fluorescent, derivatization with a suitable fluorophore is generally required. Gabor Patonay and Menno J. Baars of Georgia State University evaluate the dye NN382 as an ultrasensitive labeling reagent for six angiotensin I variant peptides. Derivatization is achieved under aqueous conditions. Response is linear over a 200fold range, with a detection limit of 100-300 zmol for the six peptides. ("Ultrasensitive Detection of Closely Related Angiotensin I Peptides Using Capillary Electrophoresis with Near-Infrared Laser-Induced Fluorescence Detection"; 10.1021/ac980714g; p. 667)

HPLC-CE for quantifying OH• derivatives The hydroxyl radical (OH•) is associated with cellular damage, but direct detection of the radical is difficult because of its high reactivity. Instead, techniques such as electron spin resonance

ACS and other publishers have adopted the Digital Object Identifier (DOI) as a unique identifier for articles in print and on the Web. For Analytical Chemistry, the DOI consists of 10.1021/ followed by the manuscript number. DOIs can be found on the first page of publlshed articles and with In AC Research citations. More information on the DOI is available at http://www.doi.org. Please use the DOI when ordering documents. • Denotes articles tentatively scheduled for the February 15 issue

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(ESR) spin trapping are used to detect the stable derivatives formed when molecular probes compete with cellular mole­ cules for OH». But ESR spin trapping is not always sensitive enough, so Arthur I. Cederbaum and colleagues at the Mount Sinai School of Medicine have developed an HPLC-EC method. The researchers analyze nitroxide and hydroxylamine forms of POBN/CH3 and PBN/CH3 adducts using both techniques and find that the HPLC-CE method is more sensitive to methyl radi­ cals. ("ESR and HPLC-EC Analysis of the Interaction of Hydroxyl Radical with DMSO: Rapid Reduction and Quantification of POBN and PBN Nitroxides"; 10.1021/ac980657r; p. 715) Determining trans-resveratrol fra«s-Resveratrol may be a cancer chemopreventive agent and provide protection against cardiovascular diseases; however, little is known about its absorption, distribution, and metabolism in humans or animals. Rosa M. Lamuela-Raventos and co-workers at the Universitat de Barcelona (Spain) introduce a simple and rapid method for detectingfrans-resveratrolusing solid-phase extraction, HPLC separation, and diode array detection. The detection limit is 20 ug/L with good precision and a coefficient of variation of 2.85%. ("Determination offrrzns-Resveratrolin Plasma by HPLC"; 10.1021/ac9808831; p. 747) Glycoprotein associated with cancer cachexia • Single-dimensional separations rarely provide enough informa­ tion to characterize biomolecules and their various glycoforms; therefore, many researchers have turned to multidimensional and hyphenated techniques. William Hancock and co-workers at Hewlett-Packard, the Georgetown University Medical Center, and me Royal College of Surgeons (Ireland) investigate using CE with off-line MALDITOFMS for examining a glycoprotein factor associated with cancer cachexia. Peaks associated with cachexia appear in the electropherograms of cancer patients but not in those of a healthy volunteer. ("Investigation of the Potential of Capillary Electrophoresis with Off-line Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry for Clinical Analysis: Examination of a Glycoprotein Factor Associ­ ated with Cancer Cachexia"; 10.1021/ac980633k) Characterizing insulin hexamers • Although monomeric insulin is the biologically active form, hexameric insulin, which binds two Zn2+ ions, is produced and stored in the CJJcells of the pancreas prior to release into the bloodstream. The insulin hexamer is capable of adopting three conformations. D. Fabris and C. Fenselau of the University of Maryland-Baltimore County use electrospray ionization MS to characterize the different insulin hexamers produced by incu­ bating insulin monomer with Zn2+. Accurate mass measure­ ments identify the ligands and characterize the stoichiometry of the complexes. ("Characterization of Allosteric Insulin Hex­ amers by Electrospray Ionization Mass Spectrometry"; 10.1021/ac980753s) PCR products by SPR • Isao Karube and co-workers at the University of Tokyo, the National Space Development Agency, and Osaka University (all in Japan) report on a flow-injection-type sensor based on sur­ face plasmon resonance for the detection of polymerase chain 74 A

Analytical Chemistry News & Features, February 1, 1999

reaction products. The detection system relies on the structure and binding properties of single-stranded DNA. The target DNA sequence is amplified by asymmetric PCR as a double strand but has a short single-stranded region as the probebinding site. ("Detection of PCR Products in Solution Using Surface Plasmon Resonance"; 10.1021/ac9807161) Immunometric assay for 17β-estradiol • Philippe Pradelles and co-workers at CEA/Saclay, Chemin de l'Orme, and CNRS UMR (all in France) describe a procedure for cross-linking 17β-estradiol to monoclonal anti-17β-estradiol antibodies on 96-well microtiter plates by direct UV irradiation. The photo-cross-linking approach is used to develop a solidphase immobilized epitope immunoassay for 17β-estradiol, which does not contain an amino group. The assay is specific to 17|3-estradiol, showing no cross-reactivity with other natural steroids, and has a detection limit of 38 pg/mL in human se­ rum. ("Cross-Linking of 17β-Estradiol to Monoclonal Antibod­ ies by Direct UV Irradiation: Appllcation to an Enzyme Immu­ nometric Assay"; 10.1021/ac980870n) Immobilizing proteins on SAMs • George M. Whitesides and co-workers at Harvard University de­ scribe the immobilization of proteins on mixed self-assembled monolayers of alkanethiolates on gold surfaces as measured by surface plasmon resonance spectroscopy. Immobilization is accomplished by generating a reactive Af-hydroxysuccinimidyl ester on the longer thiol and coupling the ester with the amines on the proteins. For carbonic anhydrase and lysozyme, the highest quantities of immobilized proteins are obtained when using a low-ionic-strength solution at a pH approximately one unit below the isoelectric point of the protein. ("A Strategy for the Generation of Surfaces Presenting Ligands for Studies of Binding Based on an Active Ester as a Common Reactive Inter­ mediate: A Surface Plasmon Resonance Study"; 10.1021/ ac980959t) Oxides in fluoride salts In molten fluoride studies, an important consideration is the oxide content of the fluoride salt, which, depending on the concentration and other conditions, can cause desirable or undesirable effects. L M. Toth and colleagues at the Univer­ sity of Tennessee describe an improved method in which the oxide is released as oxygen gas by the reac­ tion of the test sall with potassium bromotetrafluorrde at 455 0 C. The detection limit is 12 ug of oxygen and the analytical range is 120 ppm to more than 20% for a 0..-g sample. ("Determination no Oxide in Fluoride Salts Using an Yttria-Stabilized-Zirconia Oxy­ gen Pump"; 10.1021/ac980830j; p. 539) Characterizing RMDs The most common microelectrode geometry is the disk be­ cause it is simple to construct and can attain a steady-state cur­ rent. Ingrid Fritsch and Charles S. Henry of the University of Arkansas construct and characterize recessed microdisk elec­ trodes of 14- and 55-um diameter, and 4-um depth. To evaluate the electrode performance, the faradaic current in Ru(NH3)|V KNO3 and the charging current in KN03 solution are rnea-

sured by cyclic voltammetry. At fast scan rates, the behavior deviates from a linear diffusion model. The dependence of capacitance on scan rate is similar to that of a macroelectrode. The 14pm recessed microdisk electrode offers protection against convection in a stirred solution, but the 55-um recessed microdisk electrode did not. ("Microfabricated Recessed Microdisk Electrodes: Characterization in Static and Convective Solutions"; 10.1021/ac980375r; p. 550) Critical point concentrations The conductometric determination of anions of weak acids in chemically suppressed ion chromatography can by enhanced by converting the acid to its conjugate salt, if the anion is present at a level termed the "critical point concentration". Peter D. Carpenter and colleagues from the Royal Melbourne Institute of Technology (Australia) introduce a simple theoretical model for calculating critical point concentrations for weak acid-conjugate acid pairs. The concentration depends on the acid ionization constant and the molar ionic conductivities of the ions present. ("Conductometric Detection of Anions of Weak Acids in Chemically Suppressed Ion Chromatography: Critical Point Concentrations"; 10.1021/ac9807767; p. 741) Electrode geometries and kinetics • It has been argued that electrode geometries that are nonuniform should have greater kinetic discrimination. R. G. Compton and colleagues from Oxford University (U.K.) compare the kinetic discrimination of steady-state voltammetry at microdisk, microhemispherical, rotating- disk, channel, wall-jet, and walltube electrodes by measuring the broadness of numerically simulated ECE and EC2E working curves and the superimposition of numerically simulated EC and EC2 working curves by equating the average diffusion thicknesses. This, combined with the range of time scales calculated from typical experimental parameters, allows the range of measured-rate constants for each geometry to be compared. ("A Comparison of the Time Scales Accessible and Kinetic Discrimination Obtainable Using Steady-State Voltammetry at Common Hydrodynamic and Microelectrode Geometries"; 10.1021/ac9809128) Hydroxide ion oxidation • Recent results suggest that the oxidation of hydroxide ions could be exploited for analytical purposes. Salvatore Daniele and coworkers at the University of Venice (Italy) and the University of Southampton (U.K) investigate the steady-state voltammetric oxidation of aqueous solutions of sodium and barium hydroxides at gold microelectrodes in the presence and absence of supporting electrolyte. In solutions wiih excess electrolyte, ,he steadystate limiting current depends on the concentration of supporting electrolyte; in solutions with little or no supporting electrolyte, the currents are enhanced by the migration of hydroxide ions. ("Steady-State Voltammetry for Hydroxide Ion Oxidation in Aqueous Solutions in the Absence of and wiih Varying Concentrations of Supporting Electrolyte"; 10.1021/ac9807619) Assisted electrochemical extraction of Cu(l) and Cu(ll) • To transfer metal ions, which can be heavily hydrated, from an aqueous phase to an organic phase, it is often necessary to facilitate the reaction by dissolving ligands in the organic phase. • Denotes articles tentatively scheduled for the February 15 issue

Hubert H. Girault and co-workers at Ecole Polytechnique Federale de Lausanne (Switzerland) describe the electrochemical transfer of Cu+ and Cu2+ assisted by the thiocrown 1,4,7,10tetrathiacyclododecane at the water/1,2-dichloroethane interface. For a mixture of the two ions, the assisted transfer of both species is observed. When Cu(II) is the species initially present, Cu(I) is observed in the organic phase if a reducing agent is present. When Cu(I) is the species initially present, it disproportionates to form Cu(II) and the assisted transfer of both species is observed. ("Electrochemical Extraction of Cu(I) and Cu(II) Ions Assisted by 1,4,7,10-Tetrathiacyclododecane"; 10.1021/ac980831b) Selective detection of dopamine • The neurochemical dopamine can be detected by electrochemical methods based on anodic oxidation. However, ascorbic acid, which is present in high concentrations in extracellular fluid, is oxidized at nearly the same potential, resulting in an overlapped voltammetric response. Aleksander Ciszewski and Grzegorz Milczarek of Poznan University of Technology (Poland) investigate the selectivity of a platinum electrode, modified with electropolymerized eugenol films, toward dopamine in the presence of ascorbic acid. The modifier is shown to have effective ascorbate-exclusion properties with the preferential uptake of cationic dopamine. ("Polyeugenol-Modified Platinum Electrode for Selective Detection of Dopamine in the Presence of Ascorbic Acid"; 10.1021/ac9808223) Measuring distance using impedance • For many microsensor measurements, accurate placement of the sensor is needed for quantitative interpretation of the results. Optical or mechanical methods are used but can be inconvenient and may not have the desired sensitivity. Miklos Gratzl and Rohit Kashyap at Case Western Reserve University have found a new approach—correlating electrochemical impedance with sensor distance. The researchers test the idea with a chloride tip (diameter —250 um) and a potassium tip (diameter —1000 um) and achieve a precision of ±5 um and ±20 um, respectively. ("Adjusting the Distance of Electrochemical Microsensors from Secreting Cell Monolayers on the Micrometer Scale Using Impedance"; 10.1021/ac9808876) ISEs and mixed ions • The response of ion-selective electrodes is currently described with the Nicolskii-Eisenman equation, but that equation does not fit the data generated when ions of different charges are involved. Nonetheless, having an accurate picture of what happens in the presence of mixed ions may be required for callbration or interference estimates. Eric Bakker, Erno Pretsch, and Mathias Nagele at the Swiss Federal Institute of Technology and Auburn University have extended a recent ion-extraction response model to account for any number of monovalent, divalent, and trivalent ions using a single equation. ("General Description of the Simultaneous Response of Potentiometric Ionophore-Based Sensors to Ions of Different Charge"; 10.1021/ac980962c) Wall-jet electrode simulations • The wall-jet electrode offers a wide range of kinetic discrimination, but previous attempts at a two-dimensional simulation incorporating radial diffusion failed to give acceptable converAnalytical Chemistry News & Features, February 1, 1999 75 A

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gence. R. G. Compton and colleagues at Oxford University (U.K.) develop and solve fully implicit steady-state, finite, difference simulations that incorporate radial diffusion and a convection approximation. For the majority of experimental conditions, the effects of radial diffusion are found to he insignificant. Working curves and surfaces for common electrochemical mechanisms are presented. ("Finite Difference Simulations of Steady-State Voltammetry at the Wall-Jet Electrode. Effects of Radial Diffusion and Working Curves for Common Electrochemical Mechanisms"; 10.1021/ac980919p) Bromate in drinking water Bromate, a disinfection byproduct in drinking water, is formed during the ozonation of source water containing bromide. John T. Creed and Carol A. Brockhoff of the U.S. Environmental Protection Agency investigate the use of ion chromatography and inductively coupled plasma MS for the direct- and isotope-dilution analysis of bromate in drinking water samples. Analyses are performed in three common drinking water matrixes—chloride, phosphate, and sulfate. ("Isotope Dilution Analysis of Bromate in Drinking Water Matrixes by Ion Chromatography with Inductively Coupled Plasma Mass Spectrometric Detection"; 10.1021/ac980663n; p. 722) LC/MS of aromatic sulfonates • Sulfonated organic compounds, which are used in concrete admixtures, anionic surfactants, and dyes and whitening agents, have been found in surface waters and landfill leachates. Marc J.-F. Suter and co-workers at the Swiss Federal Institute for Environmental Science and Technology describe an LC/MS method, based on a single-quadrupole mass spectrometer, for the selective determination of aromatic sulfonates. The compound-specific fragment ion SO31 is used as a marker for aromatic sulfonates. Calibration curves are linear over 3 orders of magnitude with an absolute limit of detection of ~1 ng. ("Selective Determination of Aromatic Sulfonates in Landfill Leachates and Groundwater Using Microbore Liquid Chromatography Coupled with Mass Spectrometry"; 10.1021/ ac980911f) Selective Pb(ll) extraction • Removing toxic heavy metal ions from waste or from biological systems is a common goal, but few ligands recognize Pb (II) in solution. Some dibenzopolyether dicarboxylic acids have already been shown to be selective for Pb (II) over Cu(II), and now some of the dialkylamido derivatives of these compounds are reported to have high Pb (II) selectivity. Takashi Hayashita and colleagues at Tohoku University, Saga University, the National Institute for Advanced Interdisciplinary Research (all in Japan), and Texas Tech University have studied extraction of these ligands as a function of molecular structure and have found that lipophilic polyether dicarboxylic acids with pseudo-18-crown-6 frameworks selectively extract Pb(II) over other heavy metal ions. ("Molecular Design of Acyclic Polyether Dicarboxylic Acids Possessing Pseudo-18crown-6 Frameworks for Selective Lead (II) Extraction"; 10.1021/ac980627o) 76 A

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Double-disk solid-phase extraction of herbicides • Damia Barcelo and colleagues at the CID-CSIC (Spain) and the U.S. Geological Survey describe a double-disk solid-phase extraction technique for isolating herbicides from water. Using a strong anion exchange disk of styrene divinylbenzene particles in Teflon, the researchers remove >85% of the humic and fulvic acids from the water samples, which substantially increases the sensitivity of the analysis. A second disk—C18 parti cles in Teflon—is used to remove the organic compounds. Elution with methanol recovers >90% of the retained herbicides, and HPLC with diode array detection is used for detection. ("Double-Disk Solid-Phase Extraction: Simultaneous Cleanup and Trace Enrichment of Herbicides and Metabolites from Environmental Samples"; 10.1021/ac980975q) Explosives detector • On-site monitoring of TNT contamination has been adequately satisfied with immunochemical sensors and electrochemical probes; however, the presence of nitrophenol, nitrobenzene, and dinitroaniline can affect the response. John H. T. Luong and colleagues at National Research Council Canada inserted gold, silver, or a bimetal gold-silver electrode into a specialty designed detection cell that adapts to a commercial CE apparatus for the electrochemical monitoring of TNT, HMX, RDX and 10 other explosives. When used to determine the conten; of explosives in soil extracts and groundwater, results are in good agreement with those obtained by the LC method recommended by the U.S. EPA. ("Development of Electrokineti Capillary Electrophoresis Equipped with Amperometric Detection for Analysis of Explosive Compounds"; 10.1021/ ac980945n) ELISAs for PAHs • Qing X. Li and colleagues at the University of Hawaii-Manoa, University of California-Berkeley, and the Scripps Research Institute describe initial work on developing enzyme-linked immunosorbent assays (ELISAs) for multianalyte detection of polycyclic aromatic hydrocarbons. Haptens from various aromatic compounds are coupled with bovine serum albumin to form competitor conjugates, which are recognized (to different extents) by the monoclonal antibodies 4D5 and 10C10. ELISA results for benzo[a]pyrene closely correlate with those by GC analysis, but the ELISA detection limit is ~150-fold more sensitive. ("Monoclonal Antibody-Based ELISAs for Part-per-Billion Determination of Polycyclic Aromatic Hydrocarbons: Effects of Haptens and Formats on Sensitivity and Specificity"; 10.1021/ ac980765d) Part-per-million mass accuracy The specificity of an MS measurement is often improved with increased mass accuracy. I. Jonathan Amster and co-workers at the University of Georgia examine the effect of space charge on mass accuracy in FT-ion cyclotron resonance mass spectra with broad distributions of high-mass ions, which are typically produced by ionization methods used for macromolecules. They propose a calibration procedure and test it by mea-

suring the mass of the insulin B-chain, achieving a mass accuracy of 2.0 ppm. Space-charge-induced mass errors are more significant for multicomponent samples than for singlecomponent samples. ("Routine Part-per-Million Mass Accuracy for High-Mass Ions: Space-Charge Effects in MALDI FT-ICR"; 10.1021/ac980690d; p. 624)

low CH, into buffer droplets while recording the fluorescence intensity in each drop. ("Fine Chemical Manipulations of Microscopic Liquid Samples. 1. Droplet Loading with Chemicals"; 10.1021/ac980929q)

Low-vacuum MS

To better understand end-column amperometric detection for CE, Craig E. Lunte and colleagues at the University of Kansas discuss the influence of separation voltage, CE buffer concentration, and capillary-to-electrode distance on the observed hydrodynamic voltammetry of dopamine and catechol separated with a 25-um i.d. capillary. They find that increased voltage, increased buffer concentration, or decreased capillary-to-electrode distance results in a positive shift of the observed half-wave potentials for both compounds. They also find that using a decoupling device minimizes the effects of these three parameters. ("End-Column Amperometric Detection in Capiilary Electrophoresis: :nfluence of SeparationRelated Parameters on the Observed Half-Wave Potential for Dopamine and Catechol"; 10.1021/ac980737v; p. 544)

MS is used extensively for the identification of analytes based on the masses of their gas-phase ions; however, structural information cannot be directly obtained. Rather than performing additional measurements, Yinon Rudich and co-workers at the Weizmann Institute of Science (Israel) obtain structural and mass information all with the same instrument—an inexpensive, compact, low-vacuum mass spectrometer. The system is used to determine total collision cross sections for the collisions of organic ions with three background gases, He, N2, and SF6. ("Mass and Collision Cross-Section Determination Using a Low-Vacuum Mass Spectrometer"; 10.1021/ac980779j; p. 648)

DNA sequencing on microfabricated CE channels Richard A. Mathies and colleagues at the University of California-Berkeley explore the optimization of DNAsequencing separations on microfabricated CE channels by increasing the length of the separation channel; optimizing the twin-T injector size; increasing the separation channel depth; and raising the separation temperature to enhance resolution, read length, and sensitivity. The best four-color sequencing separations are performed using 4% LPA, a temperature of 40 °C, ,nd a a00-um twin-T injector. These runs are complete in 20 min, could be automatically base-called using BaseFinder, and are 99 4% accurate to 500 bp. ("Optimization of High-Speed DNA Sequencing on Microfabricated Capillary Electrophoresis Channels"; 10.1021/ac980783v; p. 566)

Capillary isoelectric focusing Isoelectric focusing is a separation technique for resolving amphoteric molecules, such as peptides and proteins, based on their isoelectric points. Oliver Hofmann and co-workers at Vysis compare chemical, hydrodynamic, and electroosmotic flowdriven mobilization for capillary isoelectric focusing as a first step to adapting the technique to microchannels on a glass chip. Electroosmotic flow proves to be the best for miniaturization and a 200-um-wide, lO-jim-deep, and 7-cm-long channel etched in planar glass is used to focus a mixture of Cy5-labeled peptides in less than 30 s. ("Adaptation of Capillary Isoelectric Focusing to Microchannels on a Glass Chip"; 10.1021/ ac9806660; p. 678)

Manipulating microscopic droplets • Miklos Gratzl and co-workers at Case Western Reserve University prepare stable microscopic aqueous droplets of nano-, pico-, and femtoliter volumes, which are stored under heptane. The droplets are manipulated with a new approach using a diffusional microburet and microscopic sensing schemes. This system is tested by delivering the fluorescent dye, Lucifer Yel• Denotes articles tentatively scheduled for the February 15 issue

End-column amperometric detection for CE

New model for electrophoretic mobility Two-dimensional gel electrophoresis has previously been combined with MALDI TOFMS to analyze proteins. Pia Davidsson and colleagues at Goteborg University and Sahlgrenska University Hospital (both in Sweden) have taken the technique one step further. They have replaced 2-D gel electrophoresis wiih 2-D liquid-phase electrophoresis (2D-LPE), which keeps the analytes in solution throughout separation. The researchers find that 2D-LPE allows larger sample volumes to be loaded—an advantage for low-concentration analytes—and eliminates elution, extraction, and membrane-transfer steps, making the new technique better suited to tasks such as analyzing cerebrospinal fluid. ("Characterization of Proteins from Human Cerebrospinal Fluid by a Combination of Preparative Two-Dimensional liquid-Phase Electrophoresis and Matrix-Assisted Laser Desorption/Ionization Timeof-Flight Mass Spectrometry"; 10.1021/ac980672w; p. 642)

Cadmium in biological matrixes for ICPMS Richard A Vanderpool of the U.S. Dept. of Agriculture and Wayne T Buckley of Agriculture and Agri-Food Canada modify a published procedure for the liquid-liquid extraction of cadmium by sodium diethyldithiocarbamate and test it on 12 biological matrixes for use with isotope dilution inductively coupled plasma MS. Recoveries range from 73% to 20% and appear to be inversely related to cadmium concentration. The measured isotope ratios have smaller percent errors relative to natural abundances for samples that were extracted compared with nonextracted samples. ("Liquid-Liquid Extraction of Cadmium by Sodium Diethyldithiocarbamate from Biological Matrixes for Isotope Dilution Inductively Coupled Plasma Mass Spectrometry"; 10.1021/ac980538b; p. 652)

Extracting alkali cations with crown ethers Crown ethers are widely used to separate alkali metal cations, and it is often assumed that selectivities based on singlespecies extraction will extrapolate to competitive extractions. Analytical Chemistry News & Features, February 1, 1999 77 A

In AC Research Richard A. Bartsch and co-workers at Texas Tech University and the University of Alabama systematically compare the separation capabilities of twelve 18- to 24-membered benzo- and cyclohexano-group-containing crown ethers in single-species and competitive extractions of alkali metal salts from aqueous solutions to chloroform. For most of the crown ether-alkali metal cation combinations, the separation factor values are greater for competitive solvent extraction than would be expected from extrapolated single-species values. ("Extraction Selectivitities of Crown Ethers for Alkali Metal Cations: Differences between Single Species and Competitive Solvent Extractions"; 10.1021/ac981068o; p. 672) New model for electrophoretic mobility Traditionally, ions in capillary zone electrophoresis have been assumed to be point charges, and thus, their mobilities have been assumed to be related to the square root of the buffer's ionic strength. But Charles A Lucy and colleagues at the University of Calgary (Canada) find that the relationship between ionic strength and mobility is more accurately determined when the finite size of the ions is taken into account. Applying the Pitts equation, which takes finite size into account, the researchers show that the mobility depends on the infinite dilution mobility of the ion, the charge on the solute anion, and the ionic strength of the buffer. ("Prediction of Electrophoretic Mobilities. 3. Effect of Ionic Strength in Capillary Zone Electrophoresis"; 10.1021/ac980843x; p. 687) Ultrahigh-pressure capillary LC James W. Jorgenson and colleagues at the University of North Carolina-Chapel Hill use 25- to 50-cm fused-silica capillaries with an inner diameter of 33 um packed with 1.0-um, nonporous, octadecylsilane-modified silica spheres for ultrahighpressure, reversed-phase LC analyses in isocratic and gradient elution modes. Mobile-phase pressures as high as 72,000 psi are applied to the column inlet to generate more than 200,000 theoretical plates in 6 min for small organic analytes. An electrically driven constan--flow syringe pump capable of generating mobile-phase pressures as high as 130,000 psi is described. In a companion paper that discusses peak capacity in complex chromatograms, peak widths are determined by measuring peak height and the second derivative at the peak maximum. The calculation of peak capacity is a linear transformation with respect to separation space, so the method is more rigorously accurate than previous methods. ("Ultrahigh-Pressure Reversed-Phase Capillary Liquid Chromatography: Isocratic and Gradient Elution Using Columns Packed with 1.0-um Particles"; 101021/ac9807013; p 700 and "Automated Measurement of Peak Widths for the Determination of Peak Capacity in Complex Chromatograms"; 10 1021/ac980702v D 709) Using all the data Chromatographic methods are usually subjected to significant data reduction, which makes the amount manageable, but the loss of information is significant. Jorn Smedsgaard and coworkers at the Technical University of Denmark propose a data-analysis method that works directly on the chromatographic data without resorting to such data reduction procedures as peakarea or retention-index calculations. Unidentified samples are compared with reference chromatograms by calculating a resem78 A

Analytical Chemistry News & Features, February 1, 1999

blance measurement for each reference chromatogram. The identification method is demonstrated with 2 data sets—212 isolates from 41 food-borne Penicillium species and d1 isolates from 6 soil-borne Penicillium species. ("Full Second-Order Chromatographic/Spectrometric Data Matrices for Automated Sample Identification and Component Analysis by Non-DataReducing Image Analysis"; 10.1021/ac9805652; p. 727) Surface diffusion in RPLC • Surface diffusion plays an important role in the mass transfer in porous adsorbents and thus may play a role in the separation mechanism of reversed-phase LC. Georges Guiochon and Kanji Miyabe of the University of Tennessee and Oak Ridge National Laboratory describe characteristic features of surface diffusion in RPLC by analyzing correlations between the surface diffusion coefficient (Ds) and either the diffusivity or the retention factor. Ds depends on the mobile-phase composition. An estimation procedure for Ds is proposed. ("Analysis of Surface Diffusion Phenomena in Reversed-Phase Liquid Chromatography"; 10.1021/ac9809027) The right spectral pretreatment method Many spectral pretreatment methods have been developed to reduce spectral variations such as baseline shifts and multiplicative effects; however, selecting the proper data pretreatment method for a given application is not always obvious. Nicolaas (Klaas) M. Faber of the Netherlands Forensic Science Laboratory shows how multivariate sensitivity can be used to interpret spectral pretreatment results. The principles are presented using the transmission near-IR spectroscopy of the oxygenates methyltert-butylether, ethanol, and water in ampules of standard reference material gasoline. ("Multivariate Sensitivity for the Interpretation of the Effect of Spectral Pretreatment Methods on Near-Infrared Calibration Model Predictions"; 10.1021/ac980415r; p. 557) Photoionization with femtosecond laser pulses The behavior of gas-phase molecules with high-power femtosecond laser pulses is of interest because of emerging applications in MS and surface analysis. Nicholas Winograd and coworkers at Pennsylvania State University study the photoionization of gas-phase- and ion-beam-desorbed dopamine using femtosecond laser pulses at 800,400,267, and 200 nm. The photoionization of gas-phase dopamine produces the molecular ion and the three fragment ions at each wavelength, with the wavelength-dependent branching ratios. Photoionization of ionbeam-desorbed dopamine exhibits a large degree of fragmentation at all four wavelengths. ("Photoionization of Gas-Phase versus Ion-Beam-Desorbed Dopamine with Femtosecond Laser Pulses"; 10.1021/ac980774m; p. 574) Characterizing particles exiting an ETV Although the use of physical carriers for enhancing the analytical signal in electrothermal vaporizer (ETV) inductively coupled plasma MS is routine, there have been few reports on the characterization of material leaving an ETV. James A. Hoil

combe and Delony Langer of the University of Texas-Austin describe a thermophoretic method for collecting the aerosol from a AgN03 solution exiting an ETV. The collected silver particles, many of which have diameters