Showing papers in "Analytical Chemistry in 1988"

Laser desorption ionization of proteins with molecular masses exceeding 10,000 daltons

Abstract: In the following, the first results on ultraviolet laser desorption (UVLD) of bioorganic compounds in the mass range above 10000 daltons are reported. Strong molecular ion signals were registered by use of an organic matrix with strong absorption at the wavelength used for controlled energy deposition and soft desorption (7)

Partial least-squares methods for spectral analyses. 1. Relation to other quantitative calibration methods and the extraction of qualitative information

Abstract: Partial leastgquares (PLS) methods for spectral analyses are related to other multlvarlate callbratlon methods such as classical least-squares (CLS), Inverse least-squares (ILS), and prlnclpal component regression (PCR) methods which have been used often In quantitative spectral analyses. The PLS method which analyzes one chemlcal component at a tbne Is presented, and the basis for each step In the algorithm Is explained. PLS callbratlon Is shown to be composed of a series of simpllfled CLS and ILS steps. This detalled understandlng of the PLS algorithm has helped to ldentlfy how chemically Interpretable qualltatlve spectral lnformatlon can be obtained from the lntennedlate steps of the PLS algorithm. These methods for extractlng qualitative Information are demonstrated by use of simulated spectral data. The qualltatlve Information directly available from the PLS analysis Is superlor to that obtained from PCR but is not as complete as that which can be generated during CLS analyses. Methods are presented for selecting optbnal numbers of loading vectors for both the PLS and PCR models In order to optimize the model while simultaneously reduclng the potential for overfittlng the caHbratlon data. Outlier detection and methods to evaluate the statlstlcal slgnlflcance of resuits obtalned from the dMerent cahatlon methods applied to the same spectral data are also discussed.

Current-monitoring method for measuring the electroosmotic flow rate in capillary zone electrophoresis

Abstract: Capillary zone electrophoresis (CZE) is attracting much attention (1-4) as a new separation technique that can complement high-performance liquid chromatography (HPLC). In CZE a migration channel of capillary dimensions is filled with electrolyte; a sample to be analyzed is injected at one end of the channel; and a high voltage is applied across the channel. When the electrolyte contacts the walls of the capillary, the inner surface of the capillary becomes charged, either through the ionization of surface groups on the capillary walls or through the adsorption of charged species from the electrolyte onto the inner surface. In either case, the electrolyte inside the capillary is no longer electroneutral but acquires a net charge, which may be positive or negative. Under the action of the applied electric field, the electrolyte moves toward one end of the capillary, and this movement is referred to as electroosmotic (electroendosmotic) flow. In addition to the bulk flow of the electrolyte, electrophoresis also takes place; that is, the applied electric field exerts a force on positively charged species to cause them to move to the negatively charged electrode and on negatively charged species to cause them to move to the positively charged electrode. As a result, the components in the injected sample separate into distinct zones, based on their different mobilities. However, in many cases, the rate of electrophoretic flow is typically less than the rate of electroosmotic flow. Consequently, species in the injected sample move in one direction-the direction of the electroosmotic flow-and thus the different species can be detected as each zone passes through some suitable detector located downstream from the capillary inlet. Clearly, a precise characterization of the electroosmotic flow is highly desirable not only for understanding CZE but also for optimizing the operation of CZE in analyzing a given sample. One way to measure the electroosmotic flow rate is to record the elution time of an injected uncharged marker solute, which will be carried through the capillary under the action of only electroosmotic flow (4-6). For this purpose it is necessary that the marker solute be truly neutral, that it have negligible interaction with the capillary walls, and that it be readily detected. Another way is to weigh the mass of electrolyte transferred from the capillary inlet to the capillary outlet over a timed interval (7). For this purpose, losses caused by evaporation must be eliminated and the use of a digital balance appears to be recommended. Both of these procedures have been demonstrated to give reliable measurements of the electroosmotic flow rate, provided that some care is taken. We describe here what we believe might be a simpler procedure for measuring the electroosmotic flow rate. I t is based on recording the time history of the current during CZE operation. Thus the new method requires no special type of injected solute or detector, and it can be used by anyone carrying out CZE separations.

Partial least-squares methods for spectral analyses. 2. Application to simulated and glass spectral data

Abstract: Partial least-squares (PLS) methods for quantitative spectral analyses are compared with classical least-squares (CLS) and principal component regression (PCR) methods by using simulated data and infrared spectra from bulk seven-component, silicate-based glasses. Analyses of the simulated data sets show the effect of data pretreatment, base-line variations, calibration design, and constrained mixtures on PLS and PCR prediction errors and model complexity. Analyses of the simulated data sets also illustrate some qualitative differences between PSL and PCR. PLS and PCR predicted concentration errors from the simulated data sets and a set of the Fourier transform infrared spectra of silicate-based glasses (S-glass) show that prediction errors are not statistically different between these two methods for these individual data sets with limited numbers of samples. However, PLS and PCR are both superior to CLS methods in the case of the analysis of S-glass where only one analyte is known in the calibration samples and the components of unknown concentration overlap all the spectral features of the analyte components. CLS analysis precision significantly improves when the three known analyte concentrations (B/sub 2/O/sub 3/, P/sub 2/O/sub 5/, and OH) are used in the calibration. In this latter case, PLS and PCR concentration predictions are unchanged, andmore » although they each still yield a lower standard error of prediction than the CLS method, there is no longer strong statistical evidence that these differences between PLS or PCR and CLS are outside experimental error for the B/sub 2/O/sub 3/ component. The ability of CLS and PLS methods to provide chemically useful estimates of the pure-component spectra is also demonstrated.« less

Enhancement of uphill transport by a double carrier membrane system.

Abstract: We report a preliminary study on double carrier membrane systems that mimic, in principle, the function of ATPase, although chemical compounds involved are totally different. It shown that the transport of ions with a double carrier membrane system is facilitated more efficiently than the conventional symport system

Fiber-optic fluorosensor for oxygen and carbon dioxide

Abstract: The capability of fiber-optic light guides to transmit a variety of optical signals simultaneously has been exploited to construct an optical fiber sensor for measurement of both oxygen and carbon dioxide. The oxygen-sensitive material (a Kieselgel-adsorbed fluorescent metal-organic complex) and the CO/sub 2/-sensitive material (an immobilized pH indicator in a buffer solution) are entrapped in a gas-permeable polymer matrix that is attached to the distal end of the fiber. Both indicators have the same excitation wavelength (in order to avoid energy transfer) but quite different emission maxima. The two emission bands can easily be separated with the help of interference filters and give independent signals. Oxygen can continuously be determined in the 0-200 Torr (0-26.6 kPa) range with +/- 1 Torr accuracy and CO/sub 2/ in the 0-150 Torr (0-20 kPa) range with +/- 1 Torr. The accuracy is higher at low partial pressure, so that the detection limits are at approx. 0.5 Torr in both cases.

Electrodeposition of platinum microparticles into polyaniline films with electrocatalytic applications

Abstract: The electrodeposition of platinum microparticles into polyaniline (PA) films on glassy carbon (gc) electrodes and their catalytic activity for the reduction of hydrogen and the oxidation of methanol are described. Electrodeposited platinum microparticles are dispersed in a three-dimensional array in fibril-type polyaniline film electrodes as evidenced by scanning electron microscope photomicrographs. These Pt/PA/gc electrodes exhibit good activity with respect to the catalytic reduction of hydrogen and the catalytic oxidation of methanol. Since polyaniline is a conducting polymer at potentials positive of 0.2 V vs Ag/AgCl, the PA films contribute a substantial amount of charge during the oxidation of methanol at 0.6 V. In addition, they also offer a protecting matrix for the Pt microparticles against particle loss and contamination from the bulk solution. The electrodes exhibited excellent long-term stability in the acidic methanol solutions.

Ellipsometric, electrochemical, and elemental characterization of the surface phase produced on glassy carbon electrodes by electrochemical activation

Abstract: Ellipsometry was used to monitor the In situ growth of a fllm on a glassy carbon electrode, whlch was electrochemlcally anodized at 1.8 V In 0.1 M H,SO,. The layer grew contlnuously In an uninhlblted fashion wlth constant optical constants to a thlckness of at least 925 nm. It was nearly transparent at wavelengths of 545 and 632.8 nm. X-ray and elemental analysls of bulk quantltles of the phase Indicate that It Is an amorphous form of graphlte oxlde. Comblned elllpsometrlc and electrochemical measurements show that the phase activates the surface and that deactivation occurs upon extennslve reductlon of the layer. The electrode actlvlty, as monitored by the voltammetrlc response In dilute solutlons of catechol, hydroquhne, and 2,3-dlcyanohydroqulnone, varled wlth the extent of reductlon of the layer.

Determination of polycyclic aromatic hydrocarbons in a coal tar standard reference material

Abstract: Evaluation de l'efficacite et de la selectivite des systemes chromatographiques d'analyse (chromatographie en phase gazeuse a colonne capillaire et en phase liquide inverse) de melanges complexes

Optimization of sensitivity and separation in capillary zone electrophoresis with indirect fluorescence detection

Abstract: Laser-induced indirect fluorescence detection can be used a general detector in capillary zone electrophoresis. Indirect fluorescence detection, where a fluorophore is used as the principal component of the electrophoretic buffer, allows the visualization of nonfluorescing ions through charge displacement of the fluorophore. Stabilization of laser power improves the dynamic reserve to 10/sup 3/ without the complication of double-beam correction techniques. A modification of the CZE power supply allows the use of dilute solutions and narrow bore capillary tubing (< 15 ..mu..m i.d.). The stability of the background fluorescence is improved by silylation of the fused silica capillary. These modifications improve the detection limits for many anions, nucleotides, and proteins to the range of 50-100 amol of sample injected. Additionally, this improvement in sensitivity results in a concomitant increase in separation efficiency. The electrophoretic separation can be optimized by calculating effective mobilities of ions based on their dissociation as a function of pH. More than 300,000 theoretical plates are observed in a separation of the nucleotide 5'-monophosphates optimized in this manner.

Molecular Fluorescence, Phosphorescence, and Chemiluminescence Spectrometry

Abstract: As you may have already noted, this year introduces a new set of authors for this fundamental review. As new authors, they hope that they can do as accurate a job as did their predecessor, Professor Earl Wehry, of the University of Tennessee. The format for this review follows the basic outline used by Professor Wehry, with some modifications. They have condensed several sections and eliminated the section on gas-phase chemiluminescence. The primary areas of emphasis including advances in experimental techniques, developments in instrumentation, and applications for chemical analysis, remain the same. To keep the review at a reasonable length, they have not included articles that are only peripherally related to analytical chemistry or those that represent straightforward extensions or demonstrations of previously published research. In this first issue, it is likely that they have made some errors of omission, and they request your assistance in identifying any obvious errors. They have tried to be conscientious in surveying the literature and have also surveyed individual researchers in the field. This review covers literature indexed by Chemical Abstracts from January, 1985, Vol. 102, issue 1, through October 1987, Vol. 107, issue 16. Accordingly, there will be some overlap between this reviewmore » and Professor Wehry's last review.« less

Organics in water: solid phase extraction on a small scale

Abstract: Octadecyl (C-18) bonded to porous silica has been evaluated for the solid-phase extraction (SPE) of organic compounds from water. Excellent performance was deduced from average recovery of > 85% for pesticides and polycyclic organic materials present in contrived water samples at 1-10 ng/mL. Extraction results showed effective performance of the SPE when 1-100 mL of water was passed through small columns containing 40-100 mg of 40-..mu..m C-18 bonded porous silica at flow rates as high as 250 bed volumes/min. The adsorbed compounds were removed by collecting 60-100 ..mu..L of either ethyl acetate or benzene eluate. The unique feature of this research is the combination of small water volumes, fast flow rates, small columns, and small eluate volumes that obviate solvent removals prior to GC analysis, while still retaining all the other advantageous features of SPE. Excellent performance was also confirmed in tests of environmental waters where the results based on SPE agreed with those based on accepted classical extractions.

Catalytic reduction of myoglobin and hemoglobin at chemically modified electrodes containing methylene blue.

Abstract: Chemically modified electrodes (CMEs) exhibiting electrocatalytic response toward myoglobin and hemoglobin were constructed by adsorbing the phenothiazine mediator titrants methylene blue and thionine onto spectroscopic graphite. These CMEs, which were prepared by a rapid (60 s) and reproducible (3.2% relative standard deviation) dip-coating procedure, permitted the hemoprotein electroreduction to take place at the reduction potential of the mediator molecule

Factors determining relative sensitivity of analytes in positive mode atmospheric pressure ionization mass spectrometry

Abstract: La sensibilite depend de la cinetique et de la thermodynamique de la reaction de transfert de proton du reactif H 3 O + (H 2 O) r vers les molecules a analyser