Showing papers in "Analytical Chemistry in 1995"

Preparation and Characterization of Au Colloid Monolayers

Abstract: The design and initial characterization of two-dimensional arrays of colloidal Au particles are reported. These surfaces are prepared by self-assembly of 12 nm diameter colloidal Au particles onto immobilized polymers having pendant functional groups with high affinity for Au (i.e., CN, SH, and NH 2 ). The polymers are formed by condensation of functionalized alkoxysilanes onto cleaned quartz, glass, and SiO 2 surfaces. The assembly protocol is carried out completely in solution: cleaned substrates are immersed in methanolic solutions of organosilane, rinsed, and subsequently immersed in aqueous colloidal Au solutions. Two-dimensional arrays spontaneously form on the polymer surface. The resulting substrates have been characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), and surface-enhanced Raman scattering (SERS). TEM data show that the particles are spatially separated but close enough to interact electromagnetically (small spacing compared to λ). The UV-vis data show that collective particle surface plasmon modes are present in the 650-750 nm region, suggesting that these assemblies are SERS-active. This is indeed the case, with enhancement factors of roughly. Au colloid monolayers possess a set of features that make them very attractive for both basic and applied uses, including uniform roughness, high stability, and biocompatibility

Method to Correlate Tandem Mass Spectra of Modified Peptides to Amino Acid Sequences in the Protein Database

Abstract: A method to correlate uninterpreted tandem mass spectra of modified peptides, produced under low-energy (10-50 eV) collision conditions, with amino acid sequences in a protein database has been developed. The fragmentation patterns observed in the tandem mass spectra of peptides containing covalent modifications is used to directly search and fit linear amino acid sequences in the database. Specific information relevant to sites of modification is not contained in the character-based sequence information of the databases. The search method considers each putative modification site as both modified and unmodified in one pass through the database and simultaneously considers up to three different sites of modification. The search method will identify the correct sequence if the tandem mass spectrum did not represent a modified peptide. This approach is demonstrated with peptides containing modifications such as S-carboxymethylated cysteine, oxidized methionine, phosphoserine, phosphothreonine, or phosphotyrosine. In addition, a scanning approach is used in which neutral loss scans are used to initiate the acquisition of product ion MS/MS spectra of doubly charged phosphorylated peptides during a single chromatographic run for data analysis with the database-searching algorithm. The approach described in this paper provides a convenient method to match the nascent tandem mass spectra of modified peptides to sequences in a protein database and thereby identify previously unknown sites of modification.

Performance Characteristics of Diffusion Gradients in Thin Films for the in Situ Measurement of Trace Metals in Aqueous Solution

Abstract: The technique of diffusive gradients in thin films (DGT) provides an in situ means of quantitatively measuring labile species in aqueous systems. By ensuring that transport of metal ions to an exchange resin is solely by free diffusion through a membrane, of known thickness, Δg, the concentration in the bulk solution, C b , can be calculated from the measured mass in the resin, M, after time, t, by C b = MΔg/DAt, where D is the molecular diffusion coefficient and A is the exposure surface area of the membrane. If sufficiently thick (∼1 mm) diffusion layer is selected, the flux of metal to the resin is independent of the hydrodynamics in solution above a threshold level of convection. Deployment for 1 day results in a concentration factor of ∼300, allowing metals to be measured at extremely low levels (4 pmol L -1 ). Only labile metal species are measured, the effective time window of typically 2 min being determined by the thickness of the diffusion layer. Because metals are quantified by their kinetics of uptake rather than the attainment of equilibrium, any deployment time can be selected from 1 h to typically 3 months when the resin becomes saturated. The measurement is independent of ionic strength (10 nM-1 M). For Chelex-100 as the resin, the measurement is independent of pH in the range of 5-8.3, but a subtheoretical response is obtained at pH <5 where binding to Chelex is diminished. The effect of temperature can be predicted from the known temperature dependence of the diffusion coefficient and viscosity. The application of DGT to the in situ measurement of Cd, Fe, Mn, and Cu in coastal and open seawater is demonstrated, and its more general applicability as a pollution monitoring tool and for measuring an in situ flux, as a surrogate for bioavailability, is discussed.

750 MHz 1H and 1H-13C NMR Spectroscopy of Human Blood Plasma

Abstract: High-resolution 750 MHz 1H NMR spectra of control human blood plasma have been measured and assigned by the concerted use of a range of spin-echo, two-dimensional J-resolved, and homonuclear and heteronuclear (1H-13C) correlation methods. The increased spectral dispersion and sensitivity at 750 MHz enable the assignment of numerous 1H and 13C resonances from many molecular species that cannot be detected at lower frequencies. This work presents the most comprehensive assignment of the 1H NMR spectra of blood plasma yet achieved and includes the assignment of signals from 43 low M(r) metabolites, including many with complex or strongly coupled spin systems. New assignments are also provided from the 1H and 13C NMR signals from several important macromolecular species in whole blood plasma, i.e., very-low-density, low-density, and high-density lipoproteins, albumin, and alpha 1-acid glycoprotein. The temperature dependence of the one-dimensional and spin-echo 750 MHz 1H NMR spectra of plasma was investigated over the range 292-310 K. The 1H NMR signals from the fatty acyl side chains of the lipoproteins increased substantially with temperature (hence also molecular mobility), with a disproportionate increase from lipids in low-density lipoprotein. Two-dimensional 1H-13C heteronuclear multiple quantum coherence spectroscopy at 292 and 310 K allowed both the direct detection of cholesterol and choline species bound in high-density lipoprotein and the assignment of their signals and confirmed the assignment of most of the lipoprotein resonances.

Carius tube digestion for low-blank rhenium-osmium analysis

Abstract: A relatively high-temperature oxidizing digestion using aqua regia has been developed for <0.1-5 g size samples of various types of rocks including silicates, sulfides, and metals prior to Re-Os isotopic analysis. Reactions are accomplished in sealed, thick-walled Pyrex tubes (Carius tubes) at 240 °C and elevated pressures for ∼12 h. This digestion technique dissolves platinum-group element minerals, metals, and sulfides and evidently sufficiently reacts with silicates to release most or all Re and Os contained in a silicate matrix. The procedure also leads to the oxidation of Re and Os to their highest valences and, therefore, promotes the complete chemical equilibration of sample Re and Os with enriched isotopes of Re and Os that are added for isotope dilution analysis. Once sample digestion and equilibration are complete, subsequent Os separation is accomplished by conventional double distillation from sulfuric acid, and Re is separated by anion exchange chromatography. Comparison of conventional Teflon vessel digestions and Carius tube digestions for a diverse suite of geological samples shows that the Carius tube digestion both liberates more Os from most matrices and also is a much more robust method for reproducibly measuring the isotopic composition of a sample.

Fabrication and evaluation of nanoelectrode ensembles

Abstract: : An electroless deposition procedure for filling the pores in nanoporous filtration membranes with metal (gold) nanowires is described. This method allows us to routinely prepare ensembles of gold nanodisk electrodes in which the nanodisks have diameters as small as 10 nm. Results of electrochemical experiments at ensembles of 30 nm-diameter and 10 nm-diameter gold-disk electrodes are described. The electrochemical response characteristics of these nanoelectrode ensembles are in agreement with predictions of the relevant electrochemical theories. Cyclic voltammetric detection limits for electroactive species at ensembles containing 10 nm-diameter gold disks can be as much as 3 orders of magnitude lower than at a large-diameter gold-disk electrode.

Ultra-high-speed DNA sequencing using capillary electrophoresis chips.

Abstract: DNA sequencing has been performed on microfabricated capillary electrophoresis chips. DNA separations were achieved in 50 x 8 microns cross-section channels microfabricated in a 2 in. x 3 in. glass sandwich structure using a denaturing 9% T, 0% C polyacrylamide sieving medium. DNA sequencing fragment ladders were produced and fluorescently labeled using the recently developed energy transfer dye-labeled primers. Sequencing extension fragments were separated to approximately 433 bases in only 10 min using a one-color detection system and an effective separation distance of only 3.5 cm. Using a four-color labeling and detection format, DNA sequencing with 97% accuracy and single-base resolution to approximately 150 bases was achieved in only 540 s. A resolution of greater than 0.5 was obtained out to 200 bases for both the one- and four-color separations. The prospects for enhancing the resolution and sensitivity of these chip separations are discussed. This work establishes the feasibility of high-speed, high-throughput DNA sequencing using capillary array electrophoresis chips.

Graphite surface-assisted laser desorption/ionization time-of-flight mass spectrometry of peptides and proteins from liquid solutions.

Abstract: Laser desorption time-of-flight mass spectra of peptides and proteins, as well as of lower molecular weight analytes, have been obtained by using a pulsed nitrogen UV laser (337 nm) to irradiate mixtures of 2-150 microns graphite particles and solutions of the analytes in glycerol. Protonated analytes as well as abundant alkali cation adducts were observed. Carbon cluster ions, Cn+, typically had a low abundance but dominated the mass spectrum at elevated laser powers. In spectra of a cytochrome c tryptic digest, all but one of the tryptic peptides were easily observed. Spectra of low molecular weight analytes dissolved in glycerol are very similar to FAB spectra of the same glycerol solution with added alkali salts. However, in many peptide and protein spectra, glycerol ion abundances are very low, and the alkali ions dominate the spectra at low mass. These spectra may correspond to wet and dry surface desorption conditions, respectively. The best spectra of the larger molecules were observed under dry conditions. In these initial experiments, we have obtained a sensitivity in the pico- to nanomole range and a mass resolution of about 300. The signal intensity is as good as that in conventional MALDI, and under optimal conditions, few background peaks appear, even at low mass.

Fourier Transform Spectroscopic Imaging Using an Infrared Focal-Plane Array detector

Abstract: A powerful new mid-infrared spectroscopic chemical imaging technique combining step-scan Fourier transform Michelson interferometry with indium antimonide focal-plane array (FPA) image detection is described The coupling of an infrared focal-plane array detector to an interferometer provides an instrumental multiplex/multichannel advantage Specifically, the multiple detector elements enable spectra at all pixels to be collected simultaneously, while the interferometer portion of the system allows all the spectral frequencies to be measured concurrently With this method of mid-infrared spectroscopic imaging, the fidelity of the generated spectral images is limited only by the number of pixels on the FPA detector, and only several seconds of starting time is required for spectral image acquisition This novel, high-definition technique represents the future of infrared chemical imaging analysis, a new discipline within the chemical and material sciences, which combines the capability of spectroscopy for molecular analysis with the power of visualization In particular, chemical imaging is broadly applicable for noninvasive, molecular characterization of heterogeneous materials, since all solid-state materials exhibit chemical nonuniformity that exists either by design or by development during the course of material preparation or fabrication Imaging, employing Raman and infrared spectroscopy, allows the precise characterization of the chemical composition, domain structure, and chemical architecture of a variety of substances This information is often crucial to a wide range of activities, extending from the fabrication of new materials to a basic understanding of biological samples In this study, step-scan imaging principles, instrument design details, and infrared chemical imaging results are presented Since the prospect of performing high-resolution and high-definition mid-infrared chemical imaging very rapidly has been achieved with the step-scan approach, the implications for the chemical analysis of materials are many and varied

Prussian Blue-Based First-Generation Biosensor. A Sensitive Amperometric Electrode for Glucose

Abstract: A first-generation amperometric glucose biosensor based on a Prussian Blue-modified electrode was developed. Prussian Blue was found to be a better electrocatalyst for hydrogen peroxide reduction than platinum. H 2 O 2 was detected at the Prussian Blue-modified electrode in the presence of oxygen by both electrooxidation and electroreduction. The glucose amperometric biosensor was made by glucose oxidase immobilization onto a Prussian Blue-modified electrode with a Nafion layer. The biosensor response exhibited a linear dependence on analyte concentration in the range 1 x 10 -6 -5 x 10 -3 M. The cathodic current density after addition of 10 -6 M glucose was 0.18 μA/ cm 2 . When hydrogen peroxide produced via the enzyme reaction was detected by electroreduction, the biosensor response was independent of reductants. This amperometric biosensor is expected to obey the requirements for noninvasive diagnostics.

Mining genomes: correlating tandem mass spectra of modified and unmodified peptides to sequences in nucleotide databases.

Abstract: The correlation of uninterpreted tandem mass spectra of modified and unmodified peptides, produced under low-energy (10-50 eV) collision conditions, with nucleotide sequences is demonstrated. In this method nucleotide databases are translated in six reading frames, and the resulting amino acid sequences are searched "on the fly" to identify and fit linear sequences to the fragmentation patterns observed in the tandem mass spectra of peptides. A cross-correlation function is then used to provide a measurement of similarity between the mass-to-charge ratios for the fragment ions predicted by amino acid sequences translated from the nucleotide database and the fragment ions observed in the tandem mass spectrum. In general, a difference greater than 0.1 between the normalized cross-correlation functions for the first- and second-ranked search results indicates a successful match between sequence and spectrum. Measurements of the deviation from maximum similarity employing the spectral reconstruction method are made. The search method employing nucleotide databases is also demonstrated on the spectra of phosphorylated peptides. Specific sites of modification are identified even though no specific information relevant to sites of modification is contained in the character-based sequence information of nucleotide databases.

Mass Resolution Improvement by Incorporation of Pulsed Ion Extraction in a Matrix-Assisted Laser Desorption/Ionization Linear Time-of-Flight Mass Spectrometer

Abstract: A linear time-of-flight mass spectrometer has been modified to incorporate pulsed ion extraction of matrix-assisted laser desorption/ionization (MALDI) generated ions. A unique aspect of the experiments presented is the combination of pulsed extraction with very high source potentials (up to 25 kV) which allows improved mass resolution while maintaining excellent sensitivity for the large m/z ions generated by the MALDI technique. Mass resolution in excess of 1000 (fwhm) is demonstrated for cytochrome c (12 361.1 Da) with the pulsed ion extraction linear time-of-flight mass spectrometer described. The influence on obtainable mass resolution of experimental variables such as delay time between laser ionization and ion extraction, amplitude of the pulsed voltage employed, and the source bias voltage are presented. It is shown that, for any given source potential, the optimum pulsed extraction voltage is a linear function of the mass of the analyte. This is consistent with the observation that the initial ion velocity distribution for MALDI-generated ions is independent of mass.

Raman Spectroscopy of Normal and Diseased Human Breast Tissues

Abstract: Raman spectra of histologically normal human breast biopsy samples were compared to those exhibiting infiltrating ductal carcinoma (IDC) or fibrocystic change. Experiments at 784 nm with CCD detectors reduced fluorescence interference and produced high SNR spectra with relatively low (10-200 mW) laser power. Sample to sample and patient to patient variation for normal specimens were less than 5% for the ratios of major specimens were less than 5% for the ratios of major Raman bands. The Raman spectra changed dramatically in diseased specimens, with much weaker lipid bands in diseased specimens, with much weaker lipid bands being evident. The spectrum of infiltrating ductal carcinoma samples is similar to that of human collagen. Differences between benign (fibrocystic) and malignant (IDC) lesions were smaller than those between normal and IDC specimens, but were still reproducible. Fiberoptic sampling through a hypodermic needle and with a remote probe were demonstrated. The possibility of rapid diagnosis with Raman spectroscopy is considered

pH-Defining Equilibrium between Water and Supercritical CO2. Influence on SFE of Organics and Metal Chelates

Abstract: A direct measurement of the pH of water in contact with supercritical CO 2 was made by observing the spectra of a pH indicator with a UV-vis spectrophotometer. The pH was analyzed under pressures of 70-200 atm and temperatures of 25-70 °C. The measured pH varied from 2.80 to 2.95, and relative standard deviations of <1.5% were achieved. The effects of pH on the efficiency of supercritical fluid extraction of metals and ionizable organic species in water-containing systems are discussed.

Fused Quartz Substrates for Microchip Electrophoresis

Abstract: A fused quartz microchip is fabricated to perform capillary electrophoresis of metal ions complexed with 8-hydroxyquinoline-5-sulfonic acid (HQS). The channel manifold on the quartz substrate is fabricated using standard photolithographic, etching, and deposition techniques. By incorporating a direct bonding technique during the fabrication of the microchip, the substrate and cover plate can be fused together below the melting temperature for fused quartz. To enhance the resolution for the separation, the electroosmotic flow is minimized by covalently bonding polyacrylamide to the channel walls. A separation length of 16.5 mm and separation field strength of 870 V/cm enable separations to be performed in {<=}15 s. By increasing the concentration of HQS from 5 mM to 20 mM, the separation efficiency improves by approximately 3 times. The low background signal from the fused quartz substrate results in mass detection limits of 85, 61, and 134 amol and concentration detection limits of 46, 57, and 30 ppb for Zn, Cd, and Al, respectively. 30 refs., 6 figs., 2 tabs.

Oxygen-Sensitive Luminescent Materials Based on Silicone-Soluble Ruthenium Diimine Complexes

Abstract: New oxygen-sensitive luminescent materials were obtained by physically immobilizing luminescent ruthenium(11) diimine complexes in silicone rubber. The dyes were made silicone-soluble by converting them into the respective ion pairs with organic anions. Acetic acid-releasing one-component RTV silicones were found to provide the best matrix for sensor membranes. The dyed silicone prepolymers were spread onto planar solid supports, cured, and characterized in terms of quenching by oxygen, response time, interferences, storage stability, and photostability. Strong evidence is presented from quenching experiments that the luminescent ion pairs are present in both a monomolecular and an aggregated form, the respective quenching constants being highly different. This results in nonlinear Stern-Volmer graphs. The new oxygen-sensitive materials are considered to present a major improvement over existing sensor materials in terms of response time, luminescence intensity, and long-term stability. They not only may be applied as planar films but also as very thin coatings on various kinds of waveguides. We also describe several novel materials for use as light-tight optical isolations. They are spread onto the oxygen-sensitive films in order to minimize interferences by (a) ambient light and (b) potentially interfering sample properties such as color, turbidity, fluorescence, and varying refractive index.

Quantitative Extraction Using an Internally Cooled Solid Phase Microextraction Device

Abstract: Solid phase microextraction (SPME) is a new extraction technique which uses a fused silica fiber coated with polymeric coating to extract organic compounds from their matrix and directly tranfer analytes into a gas chromatograph (GC) by thermal desorption in a GC injector. SPME has been proven to be simple, time efficient, and sensitive. By sampling from the headspace above sample matrices, SPME can be used to extract target analytes from very complex matrices such as sludge, wastewater, and soil. In this paper, we develop a new approach for headspace SPME sampling. By heating the sample matrix while simultaneously cooling the fiber coating, we not only facilitate the mass transfer and the release of analytes into the headspace but also create a temperature gap between the cold fiber coating and the hot headspace which significantly increases the partition coefficients of analytes

Carbohydrate molecular weight profiling, sequence, linkage, and branching data: ES-MS and CID.

Abstract: This section summarizes several strategies for a more complete understanding of carbohydrate structure with a focus on glycolipids and glycoprotein glycans. The techniques include periodate oxidation to impart greater molecular specificity to isomeric glycans, methylation to improve sensitivity and the information content within CID spectra, electrospray for "soft" and efficient ionization, and CID to obtain structural detail. The lipophilicity of the products following derivatization contributes to product cleanup by solvent extraction and enhances sensitivity during ES. When combined with CID information, this yields sequence, linkage, and branching information. Oxidation and reduction preceding methylation augments CID analysis with an altered structure that can be profiled at the same sensitivity. Within the context of established motifs, these contrasting profiles corroborate glycan structure and specifically identify isobaric elements transparent in the initial profile. An earlier report indicating ring-opening fragments were essentially absent in low-energy collisions of methylated and natriated oligosaccharides contrasts our observations. However, as this report used a methylated oligomer containing an internal N-acetylhexose as an illustration, the conclusion is plausible (cf., Figure 9). The poor ionization efficiency of FAB and the high matrix background limit the dynamic range in the CID spectrum and, thereby, the ability to unambiguously identify weaker peaks. It would be expected that high-energy CID affords a broader range of fragment types, including ring-opening fragments. In terms of a structural methodology, this is ambivalent since the increase in fragmentation pathways also applies to small molecule eliminations which are usually less informative. In ES-CID-MS, the carbohydrate chemist has a powerful new tool in hand for structural elucidations that can be conducted at the low-picomole level. Parallel developments can be expected to continue for other ionization methods, in particular matrix-assisted desorption/ionization on linear and reflectron time of flight mass spectrometers, and improvement in the performance and sensitivity of high-resolution mass analyzers through the use of focal plane detectors and more sophisticated hardware and software for Fourier transform ion cyclotron resonance mass measurements. These have, as yet, only begun to be applied to carbohydrate structural analysis but should add still more versatility to experimental design in the future.

Solid Phase Microextraction Coupled to High-Performance Liquid Chromatography

Abstract: A new interface has been developed to couple the solid phase microextraction (SPME) sampling technique with narrow-bore high-performance liquid chromatography (HPLC). This interface maintains the advantages of SPME as a fast, solvent-free, portable, and inexpensive sampling technique. In addition, it provides possibilities for the analysis of semi- and nonvolatile organic compounds in water, an analysis that is difficult to achieve with gas chromatography. Polycyclic aromatic hydrocarbons (PAHs) were selected as test analytes for the system's evaluation. The interface was shown to provide a successful way of coupling SPME with HPLC for the analysis of PAHs.

Electron Transfer Kinetics at Modified Carbon Electrode Surfaces: The Role of Specific Surface Sites

Abstract: The electron transfer (El') kinetics of Ru(NH~)~~+/~+, II-C~~~-/~-, Fc?(CN)~~-/~-, Feaq2+/3+, and Vaq2+l3+ were examined on several modified glassy carbon surfaces. The kinetics of the aquated ions were very sensitive to the density of surface oxides, while those of the other redox systems were not. In particular, chemical derhalization of surface carbonyl groups decreased the rate of electron transfer with Fe3+l2+ by 2-3 orders of magnitude but had little effect on Ru(NH~)~~+/~+ or Ircl~~-/~-. The electron trader rates for Fe31/2+ correlated with surface C=O density determined by resonance Raman spectroscopy. Neutral, cationic, and anionic nonspecific adsorbers decreased the rates of ET with the aquated ions ap proximately equally but had little effect on Ru(NH3)62-/3f. The redox systems studied were classified into two groups: those which are catalyzed by surface carbonyl groups and those which are not. Possible catalytic mechanisms are considered. A significant effort by many laboratories has been directed toward understanding the relationship between surface structure and electron transfer reactivity for carbon Complex surface chemistry and an often unknown level of surface impurities have made it difticult to determine the important structural variables controlling carbon electrode reactivity. At least three major phenomena affect electron transfer (m? reactivity, and these vary in importance for different redox systems and solution conditions. First, many redox systems (e.g., Fe(CN),j3-14-, ascorbic acid, dopamine) are very sensitive to surface cleanliness, and observed ET rates are strongly dependent on surface hist~ry.~.'~ Second, the microstructure of the carbon has a large effect on most redox systems, with the basal plane of highly ordered pyrolytic graphite (HOPG) exhibiting much slower ET

Phosphorescent Complexes of Porphyrin Ketones: Optical Properties and Application to Oxygen Sensing

Abstract: A new class of dyes, platinum(II) and palladium(II) complexes of the porphyrin ketones (or oxochlorins), exhibiting strong phosphorescence at room temperature is described. Several representative compounds were prepared and studied by spectral luminescence methods in solution. Compared to the related porphyrin and chlorin complexes, the new dyes display high photochemical stability, long wave spectral characteristics, and good compatibility with semiconductor optoelectronics (e.g., excitation by light-emitting diodes). These properties make the new dyes promising for a number of relevant applications, such as quenched phosphorescence sensing and phosphorescence probing (e.g., in binding assays). Analytical application of the porphyrin ketone complexes to phosphorescence lifetime-based sensing of molecular oxygen is described. Platinum(II) octaethylporphine ketone was dissolved in a polystyrene layer to give an oxygen-sensitive film. Oxygen measurements were performed with a prototype fiber-optic instrument based on solid-state components, such as light-emitting diodes and photodiodes. The instrument measured phosphorescence lifetime via changes in phase shift. The phosphorescence lifetime was determined to change from about 61.4 μs at zero oxygen to 16.3 μs in air (210 hPa of oxygen) at 22 °C. The analytically useful range of the sensor was 0-210 hPa of oxygen partial pressure, with a detection limit of 1.5 hPa. The precision of the device was 1.0 hPa at 210 hPa of oxygen and 0.5 hPa at zero oxygen.

Genetic Algorithms as a Tool for Wavelength Selection in Multivariate Calibration

Abstract: A comparison of multiple linear regression (MLR) with partial least-squares (PLS) regression is presented, for the multivariate modeling of hydroxyl number in a certain polymer of a heterogeneous near-IR spectroscopic data set. The MLR model was performed by selecting the variables with a genetic algorithm. A good model could be obtained with both methods. It was shown that the MLR and PLS solutions are very similar. The two models include the same number of variables, and the first variables in each model have similar, chemically understandable functions. It is concluded that both solutions are equivalent and that each has some advantages and disadvantages. This also means that even in very complex situations such as here, MLR can replace PLS in certain cases.

Accelerator Mass Spectrometry

Abstract: Accelerator mass spectroscopy (AMS) can be used for efficient detection of long-lived isotopes at part-per-quadrillion sensitivities with good precision In this article we present an overview of AMS and its recent use in archaeology, geochemistry and biomolecular tracing All AMS systems use cesium sputter ion sources to produce negative ions from a small button of a solid sample containing the element of interest, such as graphite, metal halide, or metal oxide, often mixed with a metal powder as binder and thermal conductor Experience shows that both natural and biomedical samples are compatible in a single AMS system, but few other AMS sites make routine {sup 14}C measurements for both dating and tracing AMS is, in one sense, just `a very sensitive decay counter`, but if AMS sensitivity is creatively coupled to analytical chemistry of certain isotopes, whole new areas of geosciences, archaeology, and life sciences can be explored 29 refs, 2 figs, 1 tab

Modeling of Luminescence Quenching-Based Sensors: Comparison of Multisite and Nonlinear Gas Solubility Models

Abstract: Quenching-based luminescence sensors generally are supported in organic or inorganic polymers and exhibit nonlinear Stem-Volmer quenching behavior. Two common explanations of the nonlinearity are either multisite binding or the nonlinear solubility properties of the analyte in the sensor. Both models have three fitting parameters. These two models are compared, and the merits of each are discussed. It is shown that while the underlying physical bases of the two models are radically different and chemically incompatible, the two models are mathematically equivalent for data fitting. The correspondence of parameters in the two models is given. The nonlinear solubility model is probably computationally slightly simpler to use, but the two-site model seems to better approximate the underlying chemistry of systems studied to date

Design and optimization of a selective subcutaneously implantable glucose electrode based on wired glucose oxidase

Abstract: An implantable 0.29 mm o.d. flexible wire electrode was designed for subcutaneous monitoring of glucose. The electrode was formed by sequentially depositing in a 0.09 mm deep shielded recess at the tip of a polyimideinsulated 0.25 mm gold wire a «wired» glucose oxidase (GQX sensing layer, a mass transport limiting layer, and a nonfouling biocompatible layer. The glucose sensing layer was formed by cross-linking {poly[(1-vinylimidazolyl)osmium (4,4'-dimethylbipyridine) 2 Cl]} +/2+ (PVI 13 -dme-Os) and GOX with poly(ethylene glycol) diglycidyl ether (PEG). The glucose mass transport restricting layer consisted of a poly(ester sulfonic acid) film (Eastman AQ 29D) and a copolymer of polyaziridine and poly(vinyl pyridine) partially quaternated with methylene carboxylate. The outer biocompatible layer was formed by photocross-linking tetraacrylated poly(ethylene oxide). The three layers contained no leachable components and had a total mass less than 2.2 μg (∼50 ng of Os). When poised at +200 mV vs SCE and operated at 37 o C, the 5×10 -4 cm 2 electode had in vivo a sensitivipy of 1-2.5 nA mM -1 . The current increased with the glucose concentration up to 60 μM, and the 10-90% response time was ∼1 min when the glucose concentration was abruptly raised from 5 to 10 mM. The sensitivity decreased by less than 4% over a test period of 1 week, during which the electrode was operated continuously in a 10 mM glucose physiological buffer solution at 37 o C. The variation in the current when interferants were added at their physiological concentrations (0.1 mM ascorbate, 0. 17 mM acetaminophen, 0.48 mM urate) was <1 SD of the assays (±2%), and the glucose electrooxidation current was nearly independent of often partial pressure, even at low glucose concentrations. When an electrode was implanted subcutaneously in a rat, it retained its in vitro calibration and tracked the blood glucose concentration before, during, and after intraperitoneal glucose infusion

A Biomimetic Sensor Based on a Molecularly Imprinted Polymer as a Recognition Element Combined with Fiber-Optic Detection

Abstract: The construction and operation of fiber-optic sensing devices based on molecularly imprinted polymers are reported. Fiber-optic detection of an amino acid derivatized with a fluorescent labeling group (dansyl-L-phenyl-alanine) is demonstrated. The advantages of using molecularly imprinted polymers as artificial recognition systems in sensor technology are discussed.

Organically modified sol-gel sensors

Abstract: Sol-gel techniques can be used to make biosensors, waveguide sensors, and modified electrodes in a variety of configurations

Electron transfer from electrodes to myoglobin: facilitated in surfactant films and blocked by adsorbed biomacromolecules.

Abstract: In previous work, greatly enhanced rates of electron transfer were found for myoglobin (Mb) in ordered films of surfactants on pyrolytic graphite (PG) electrodes. Direct electron transfer is now reported for Mb in films of didodecyldimethylammonium bromide (DDAB) on platinum, tin-doped indium oxide, and gold electrodes. Rates of electron transfer in these films were similar on all electrodes. In the absence of surfactant, electron transfer was observed on bare electrodes only when Mb was purified by chromatography, and only on hydrophilic tin-doped In 2 O 3 or PG. Treatment of tin-doped In 2 O 3 or PG electrodes with unpurified protein solutions blocked electron transfer to Mb in the purified solutions. Reflectance-absorbance infrared and X-ray photoelectron spectroscopy revealed proteinaceous adsorbates on electrodes exposed to unpurified solutions of Mb. This adsorbate blocks electron transfer to Mb and to ferricyanide in solution. Results suggest that electron transfer in the Mb-DDAB films may be facilitated partly by strong adsorption of surfactants on electrodes. Surfactant adsorbed at electrode-film interfaces appears to inhibit adsorption of macromolecules from Mb solutions which could otherwise block electron transfer between Mb and electrodes.

A molecularly imprinted synthetic polymer receptor selective for atrazine

Abstract: A synthetic polymer selective for atrazine was prepared by a molecular imprinting technique, and the binding characteristics of the atrazine-imprinted polymer were evaluated. Scatchard analysis showed that at least two classes of binding sites were formed in the imprinted polymer, and a dissociation constant of the higher affinity binding sites was estimated to be 12 μM. The induced affinity and selectivity by atrazine imprinting were examined chromatographically. The polymer gave more than 60 times longer retention for atrazine than the nonimprinted polymer with the same chemical composition. The selectivity was evaluated by capacity factors of atrazine and other pesticides, and atrazine showed the longest retention time among the tested compounds with 1,3,5-triazine-based structure. Other herbicides and pesticides with unrelated structures were retained 1% or less, as compared to atrazine. Because the optimal binding performance was exhibited in organic solvents, the synthetic polymer receptor is expected to be a good material in lipophilic herbicide analysis.