in ac research
In AC Research contains brief introductions to the research articles appearing in the February 1 issue. A free updated table of contents is available on the Web (http://pubs.acs.org/ac).
MALDI spots—smaller still. George Agnes and Michael Bogan of Simon Fraser University (Canada) use electric fields supplied by the ring electrodes of an electrodynamic balance to levitate and deposit individual sample droplets onto a MALDI sample plate. Droplets were deposited one on top of another for a final residual sample island just 3.1 ⫻ 10–9 m2. This spatially well-defined deposition technique provides small spot sizes required for high-sensitivity MALDI applications. (“MALDI-TOF-MS Analysis of Droplets Prepared in an Electrodymamic Balance: ‘Wall-less’ Sample Preparation”; 10.1021/ac015638n; p 489) Take one chance. If there is just a single, hard-to-get cell available to assay, the analytical techniques had better be efficient because there is only one chance to extract a wealth of information. Jonathan Sweedler and colleagues at the Uni-
Evaluating endocr in e disrupt ors. Shunitz Tanaka and colleagues at Hokkaido University, Gunma University, and Hokkaido College of Pharmacy (all in Japan) describe the first electrochemical method that examines how endocrine-disrupting chemicals like pesticides and pharmaceuticals interact with labeled estradiol and estrogen receptors. (“Electrochemical Evaluation of the Interaction between Endocrine Disrupter Chemicals and Estrogen Receptor Using 17–Estradiol Labeled with Daunomycin”; 10.1021/ac010426b; p 533) Better detection of dopamine. A. Doménech and colleagues at the University of Valencia and the Polytechnical University of Valencia (both in Spain) encapsulate 2,4,6-triphenylpyrylium ions inside supercages of zeolite Y and use them to modify polymer film electrodes. This strategy boosts dopamine detection by 1–2 orders of magnitude and substantially inhibits post-electron-transfer reactions. (“2,4,6-Triphenylpyrylium Ion Encapsulated into Zeolite Y as a Selective Electrode for the Electrochemical Determination of Dopamine in the Presence of Ascorbic Acid”; 10.1021/ac010657i; p 562)
versity of Illinois–Urbana incorporate a nanovial sampling procedure for CE followed by MALDI MS and radionuclide detection to profile the chemical content of a single 40-mm neuron from Aplysia californica. The combination of techniques and their application to processing a prohormone in a single neuron are first-time occurrences. (“Single-Neuron Analysis Using CE Combined with MALDI MS and Radionuclide Detection”; 10.1021/ac0156621; p 497)
Colorimetric sensor for biomolecular interactions. Ashutosh Chilkoti and Nidhi Nath at Duke University describe a colorimetric assay that uses colloidal surface plasmon resonance (SPR) to study biomolecular interactions. Unlike traditional SPR, this method detects the binding of a molecule on a gold surface as a shift in the peak wavelength and intensity of an absorbance spectrum. In addition, the assay is performed on a surface, not in solution. (“A Colorimetric Gold Nanoparticle Sensor To Interrogate Biomolecular Interactions in Real Time on a Surface”; 10.1021/ac015657x; p 504)
MS locates PTMs. Gavin Reid and colleagues at Purdue University and Oak Ridge National Laboratory use established electrospray ionization, collision-induced dissociation, and ion/ion proton transfer reaction methods to determine the effect of glycosylation on protein fragmentation by quadrupole ion trap MS. The “protein fragment ion mass fingerprint” is used to narrow the possible locations of the posttranslational modification (PTM). (“Tandem Mass Spectrometry of Ribonuclease A and B: N-Linked Glycosylation Site Analysis of Whole Protein Ions”; 10.1021/ac0156181; p 577) Measuring platinum drugs. The clinical use of platinum drugs as anticancer agents has encountered problems because of the lack of a method to measure the concentrations of the unbound drugs found in plasma. Ian A. Blair and colleagues at the University of Pennsylvania and AstraZeneca U.K. have developed a method that quantitatively converts the aquated species back to the dichloro form of the parent drug so that a single molecular species can be analyzed. The lower limit of quantitation for ZD0473 is 10 ng/mL for 100 µL of plasma ultrafiltrate. (“Validated Liquid Chromatography/Tandem Mass Spectrometry Assay for cis-Amminedichloro(2-methylpyridine)platinum(II) in Human Plasma Ultrafiltrate”; 10.1021/ac010792v; p 591) F E B R U A R Y 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
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Affinity tag used to analyze phosphopeptides. Richard D. Smith and co-workers at the Pacific Northwest National Laboratory use a phosphoprotein isotope-coded affinity tag, which has differential isotopic labeling and biotinylation, to enrich and identify mixtures of low-abundance phosphopeptides. The results illustrate the efficacy of the strategy for the enrichment of mixtures of phosphopeptides and for detecting and identifying low-level phosphopeptides. (“Phosphoprotein Isotope-Coded Affinity Tags: Application to the Enrichment and Identification of Low-Abundance Phosphoproteins”; 10.1021/ac015528g; p 607)
Raising resolution. Metallothioneins (MTs) are small proteins (6–7 kDa) involved in heavy metal metabolism that are difficult to separate because they occur as mixtures of isoforms and subisoforms with only minor differences in metalbinding properties. Andreas Prange and Zhixin Wang of the Institute for Coastal Research/Physical and Chemical Analysis (Germany) present a coated CE capillary, which separates nine MTs from rabbit liver, a 3-fold increase over earlier methods. (“Use of Surface-Modified Capillaries in the Separation and Characterization of Metallothionein Isoforms by Capillary Electrophoresis Inductively Coupled Plasma Mass Spectrometry”; 10.1021/ac010862y; p 626) Validated electronic sensors. Itamar Willner and colleagues at the Hebrew University of Jerusalem (Israel) use a microgravimetric quartz crystal microbalance, Faradaic impedance spectroscopy, and ion-sensitive field-effect devices to examine the binding of substrates with acrylamide–acrylamidephenylboronic acid copolymer membranes imprinted with molecular recognition sites for a series of nucleotides and monosaccharides. They find that the detection limit and sensitivity of these polymers are greater than corresponding nonimprinted polymers and conclude that the selective and specific analyses of the nucleotides suggest a possible future application in sequencing nucleic acids. (“Imprinting of Nucleotide and Monosaccharide Recognition Sites in Acrylamidephenylboronic Acid–Acrylamide Copolymer Membranes Associated with Electronic Transducers”; 10.1021/ac0109873; p 702)
Sandwich method a treat for ion-selective membranes. Konstantin N. Mikhelson and colleagues at St. Petersburg University (Russia) examine the theory and prior experimental results of the segmented sandwich membrane method of studying the stoichiometry and stability constants of ion–ionophore complexes in ion-selective membranes. The method has been reevaluated and successfully applied to measuring the stability constants of a number of complexes in real ion-selective electrode membranes. In a related paper, Mikhelson and colleagues at Åbo Akademi University (Finland)
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and the Technical University of Gdañsk (Poland) study lithiumselective electrodes with solvent polymeric membranes based on two different dicyclohexylamide neutral ionophores, and they measure the stability constants of lithium, sodium, and potassium ions with neutral ionophores using the segmented sandwich membrane method. (“Potentiometric Estimation of the Stability Constants of Ion–Ionophore Complexes in Ion-Selective Membranes by the Sandwich Membrane Method: Theory, Advantages, and Limitations”; 10.1021/ac015564f; p 510; “Selectivity of Lithium Electrodes: Correlation with Ion– Ionophore Complex Stability Constants and with Interfacial Exchange Current Densities”; 10.1021/ac0155660; p 518)
LC-ECL. R. Mark Wightman and Samuel Forry at the University of North Carolina–Chapel Hill develop electrogenerated chemiluminescence (ECL) as a detection scheme for reversedphase LC. A high-frequency square wave voltage is applied to microelectrodes immersed in column eluent to form the electrogenerated radical analyte molecules that react to form a photoemissive excited state. Selectivity is adjustable by changing electrochemical conditions or by using functional group-selective derivatizing agents. (“Electrogenerated Chemiluminescence Detection in Reversed-Phase Liquid Chromatography”; 10.1021/ac0110193; p 528) Single-cell kinetics studied with a carbon fiber electrode. R. Mark Wightman and colleagues at the University of North Carolina–Chapel Hill study the carbon fiber electrode response time and the measured concentrations during dynamic catecholamine changes. Deconvolution of cyclic voltammetry data is applied to stimulated dopamine release in vivo, allowing for modeling of release and uptake kinetics measuring catecholamine release from single cells, resulting in better resolution of peaks from single vesicles. (“Response Times of Carbon Fiber Microelectrodes to Dynamic Changes in Catecholamine Concentration”; 10.1021/ac010819a; p 539)
Metal ion sensor. Mark Richter and Brian Muegge at Southwest Missouri State University investigate the effects of metal ions on the electrochemiluminescence (ECL) of an 18crown-6 ether derivative of ruthenium bipyridyl in a polar solvent system. In the presence of Pb2+, Hg2+, Cu2+, or K+ and tri-npropylamine, the ECL from the anodic oxidation of this derivative is enhanced, up to 20-fold, making this an interesting system for metal ion sensing. (“Electochemiluminescent Detection of Metal Cations Using a Ruthenium(II) Bipyridyl Complex Containing a Crown Ether Moiety”; 10.1021/ac010872z; p 547) Slow, but intense. Allen Bard and co-workers at the University of Texas–Austin and Hokkaido University (Japan) report a new use of electrogenerated chemiluminescence (ECL) to detect alkali metals and ligands. Structural changes cause an increase in the intensity of ECL emissions, thus signaling Ru(bpy)2(crown ether-bpy)2+ capture of an alkali metal ion ligand in both aqueous and nonaqueous environments. Slow kinetics prevents this compound from being a practical ion sensor, but a modified complex
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could be a useful clinical assay. (“Electrogenerated Chemiluminescence. 68. Detection of Sodium Ion with a Ruthenium(II) Complex with Crown Ether Moiety at the 3,3´-Positions on the 2,2´Bipyridine Ligand”; 10.1021/ac010885c; p 551)
Rough quartz crystal microbalance response. Vladimir Tsionsky and colleagues at Tel Aviv University (Israel) study frequency shifts in the presence of liquids with different viscosities and densities and find that a model system must include surface roughness parameters to be consistent with experimental data. Slight and strong types of roughness are especially important, and the frequency shift alone is insufficient for interpreting experimental data without a full analysis of the impedance spectrum. (“Influence of Roughness on the Admittance of the Quartz Crystal Microbalance Immersed in Liquids”; 10.1021/ac0107610; p 554)
Integrating waveguide biosensor. Frances S. Ligler and colleagues at the Naval Research Laboratory, the State University of New York–Binghamton, and Hofstra University demonstrate a capillary biosensor that uses the waveguiding properties of a capillary to integrate a signal over an increased surface area without simultaneously increasing the background noise from a detector. Detection limits are 30–50 pg/mL in immunoassays using a diode laser for excitation and a photomultiplier tube for detection. (“Integrating Waveguide Biosensor”; 10.1021/ac015607s; p 713)
Detecting neutral atmospheric fluoroorganics. To help understand how perfluorooctane sulfonate could appear in the Arctic and other remote locations, Scott Mabury and colleagues at the University of Toronto, the University of Guelph, and the National Water Research Institute (all in Canada) develop new sample collection protocols and analyze high-molecularweight neutral fluoroorganic compounds in rural and urban air samples. (“Collection of Airborne Fluorinated Organics and Analysis by Gas Chromatography/Chemical Ionization Mass Spectrometry”; 10.1021/ac015630d; p 584) Singling out pollutants in wetlands. William T. Cooper and co-workers at Florida State University present a procedure that concentrates dissolved organic phosphorus (DOP) from surface waters, isolates it from the background dissolved organic matter, and allows mass spectral characterization of individual DOP compounds in phosphorus-limited ecosystems. The researchers obtain DOP concentrated up to 15-fold in a matrix suitable for analysis by electrospray ionization. (“Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry of Dissolved Organic Phosphorous Species in a Treatment Wetland after Selective Isolation and Concentration”; 10.1021/ac010909f; p 600)
Quantifying triazine. Classified as an herbicide and suspected carcinogen, triazine and its metabolites have caused concern due to toxicity and persistence even at low levels in the environment. Kian Kee Lee and Gang Shen at the National University of Singapore demonstrate that a hollow fiber-protected liquid-phase microextraction technique is excellent for preconcentrating samples and cleaning up dirty matrixes, and it is compatible with GC. (“Hollow Fiber-Protected Liquid-Phase Microextraction of Triazine Herbicides”; 10.1021/ac010561o; p 648) Prediction of soil adsorption coefficients. To determine soil adsorption coefficients, Xinmiao Liang and colleagues at the Chinese Academy of Sciences and the GSF– National Research Center for the Environment and Health (Germany) describe a new multiple linear regression model that uses the reversed-phase HPLC retention parameters from several types of columns. The researchers estimate the coefficients for 35 compounds, including alkylbenzenes, phenols, and pesticides, and compare these predictions to data from soil column LC experiments. (“Prediction of Soil Adsorption Coefficients from Retention Parameters on Three Reversed-Phase Liquid Chromatographic Columns”; 10.1021/ac015523j; p 655) Popular environmental method contested. Steven E. Mylon and colleagues at Yale University show that a popular method used by researchers for more than 15 years to measure a class of dissolved reduced sulfur compounds in oxic freshwaters is inappropriate for measuring chromium-labile sulfides where sulfate concentrations are large relative to the dissolved metal sulfides. The method relies on the reduction dissolution of metal sulfides by Cr(II), and the researchers observe the reduction of sulfate by Cr(II), resulting in significant interference. (“Unsuitability of Cr(II) Reduction for the Measurement of Sulfides in Oxic Water Samples”; 10.1021/ac010924k; p 661) Lead, bismuth, and polonium in seawater. Gordon T. Cook and colleagues at the Scottish Universities Environmental Research Centre and Lancaster University (both in the United Kingdom) describe a method for determining the natural decay series radionuclides 210Pb, 210Bi, and 210Po in seawater by liquid scintillation spectrometry. The analyses can be completed in 10 days, compared to a year for traditional methods. Detection limits are 0.32, 0.34, and 0.004 mBq/L for 210Pb, 210Bi, and 210Po, respectively. (“Time-Efficient Method for the Determination of 210 Pb, 210Bi, and 210Po Activities in Seawater Using Liquid Scintillation Spectrometry”; 10.1021/ac0107599; p 671)
Fritless column on a chip. Laura Ceriotti and colleagues at the University of Neuchâtel (Switzerland) have a polymer device that performs capillary electrochromatography without fritted columns. The separation of two neutral compounds took F E B R U A R Y 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
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less than 15 s with a 3-µm octadecylsilanized silica-microsphere vacuum-packed separation channel. (“An Integrated Fritless Column for On-Chip Capillary Electrochromatography with Conventional Stationary Phases”; 10.1021/ac0109467; p 639)
Size-exclusion CEC of synthetic neutral polymers. Nelu Grinberg, Ira Krull, and colleagues at Northeastern University, Merck, and Dow Chemical describe the separation of large, neutral, synthetic polymers using primarily a nonaqueous mobile phase without using a supporting electrolyte. The effects of water, voltage, stationary-phase exchange capacity, and pore size are investigated. The analyses of polyurethane, polystyrene, and other polymers using size-exclusion capillary electrochromatography (CEC) correlate well with results obtained by LC. (“Nonaqueous Capillary Electrochromatographic Separation of Synthetic Neutral Polymers by Size Exclusion Chromatography Using Polymeric Stationary Phases”; 10.1021/ac010973x; p 617)
Stacking injection of neutral analytes. James P. Landers and James Palmer at the University of Virginia explore concentrations of the micelle, methanol, and borate in the separation buffer to increase the maximum injection length of neutral analytes. Reducing the analyte velocity in the separation buffer without substantially decreasing the velocity of the analyte during injection allows greatly extended sample plug injection lengths. Equations describing the injection process and maximum injection lengths are introduced. (“Electrokinetic Stacking Injection of Neutral Analytes under Continuous Conductivity Conditions”; 10.1021/ac010522c; p 632)
Chromoionophores in PVC membranes serve as optical sensors. Julian Alonso and colleagues at the Autonomous University of Barcelona (Spain) describe the optical and analytical characteristics of a series of neutral H+-selective chromoionophores in PVC membranes. These membranes are applied as the sensing regions of an integrated waveguide optode. They offer a wide range of pKa, good sensitivity as a result of their high molar absorptivity, excellent solubility in the plasticizer, and quick response times. (“Ketocyanine Dyes: H+Selective Ionophores for Use in Integrated Waveguides Absorbance Optodes”; 10.1021/ac0107960; p 570)
Heavy metal sensor. Jacques Buffle and colleagues at Geneva University (Switzerland) have developed the first sensing system for metal ions based on separation and preconcentration by a permeation liquid membrane and fluorescence detection 58 A
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with an optical fiber. The quenching of calcein fluorescence upon metal accumulation in the test strip phase is detected with a multimode optical fiber. A detection limit for Cu(II) near 50 nM is achieved. (“Coupling Fiber Optics to a Permeation Liquid Membrane for Heavy Metal Sensor Development”; 10.1021/ac0156013; p 664)
Multivariate calibration of PAHs. Olivier Devos and colleagues at the Université de Savoie (France) analyze six polycyclic aromatic hydrocarbons (PAHs) using a multilevel Plackett–Burman design to create a calibration set for partial least-square regression. These compounds are analyzed in micellar media by synchronous fluorescence. The external validation shows that the collinear set is inappropriate to quantify PAHs in real samples, but the Plackett–Burman calibration set affords optimal results. (“Use of a Plackett–Burman Design with Multivariate Calibration for the Analysis of Polycyclic Aromatic Hydrocarbons in Micellar Media by Synchronous Fluorescence”; 10.1021/ac010756w; p 678) Limits of light scattering. Mark Schure and Saurabh Palkar at Rohm and Haas simulate multiangle light scattering (MALS) particle-size detection schemes based on the scattering theories of Mie and Rayleigh–Gans–Debye, as they would be used in field-flow fractionation (FFF) studies. Flow, electrical, and sedimentation FFF can all provide adequate separation for accurate particle-size determination by MALS, and both theoretical approaches give good results if the diameter of the particle is the parameter of interest. Both scattering-theory strategies suffer from errors in the concentration term, and while procedures exist for restoring the results to higher accuracy, neither is general purpose or reliable. (“Accuracy Estimation of Multiangle Light Scattering Detectors Utilized for Polydisperse Particle Characterization with Field-Flow Fractionation Techniques: A Simulation Study”; 10.1021/ac011006p; p 684) Promising technique. Conventional surface plasmon resonance (SPR) is an analytical surface-sensitive technique that measures the changes in refractive index on the surface of a noble metal—but it can be approached in other ways. Koji Suzuki and Kazuyoshi Kurihara at Kananga Academy of Science and Technology and Keio University (both in Japan) present the theoretical background of optical absorption-based SPR, which they believe will be useful in developing chemical and biochemical sensors. (“Theoretical Understanding of an Absorption-Based Surface Plasmon Resonance Sensor Based on Kretchmann’s Theory”; 10.1021/ac010820+; p 696) Easy-to-make spectroelectrochemical cell. Paul A. Flowers and colleagues at the University of North Carolina– Pembroke describe an easily constructed spectroelectrochemical cell made from a 5-mm cuvette and a working electrode made from a 60-ppi piece of reticulated vitreous carbon. The traits of the cell compare favorably to traits reported for similar cells. (“Easily Constructed Spectroelectrochemical Cell for Batch and Flow-Injection Analyses”; 10.1021/ac010899i; p 720)
