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Refractive index detector Beat Krattiger, Gerard J. Bruin, and Alfred Bruno of Ciba Geigy demonstrate a novel RI detector (based on interferometry) featuring a laser diode and a holographic optical ele ment that allows the use of capillaries as small as 5 μπι i.d. The holographic element makes it possible to probe the cap illary through its center, where the optical path is larger and diffraction effects are smaller. The detector is used for CE of metal ions in a 10-μπι i.d. capillary. Resolution is 2 μΡΐυ, and detection limits are in the low-nM range under field-amplified sample injection, (p. 1)
Neural networks
SPECTROSCOPY If the underlying model of the peak pattern is not known, or proper estimates of model parameters cannot be obtained, curve fitting can be difficult. Sources of error in curve fitting include an unknown number of peaks, uncertainty about the baseline, and inaccurate initial parameter estimates. L Buydens of the Catholic University of Nijmegen and colleagues address these sources of error in a peak detection method using artificial neural networks and a global search technique for curve fitting. The case study involves fitting a series of spectra with strongly overlapping peaks, (p. 23)
Equilibrium constants Affinity probe CE
BIOANALYTICAL In affinity probe CE, the affinity probe detects the analyte as a complex after separation of the excess free probe by CE. Kiyohito Shimura and Barry L Karger of the Barnett Institute use a protein complexed with tetramethylrhodamine-labeled monoclonal anti body fragment and LIF to demonstrate the technique. A de tection limit of 5 χ 10 ~12 M is achieved for a recombinant human growth hormone, (p. 9)
Immunoassay Although two-site immunometric methods provide increased sensitivity over that of conventional competitive immuno assays, they do not work for low molecular weight haptens because the compound is too small to allow simultaneous binding of the two antibodies. Philippe Pradelles of the Com missariat à l'Energie Atomique and colleagues report a new immunoassay for measuring haptens that contain primary amino groups. The minimum detectable concentration is 4-10 fmol/mL; each assay is 70-200 times more sensitive than a conventional immunoassay. Precision and specificity are good, and this assay correlates well with competitive assays performed on various biological samples, (p. 16)
A knowledge of ester equilibria is important for understanding many processes in organic and bioorganic chemistry. Although information on the rates of hydrolysis of esters is available, the corresponding equilibrium constants are not. Donald G. Lee, Yongfei Yan, and W. David Chandler of the University of Regina present a method for determining the concentration of aliphatic alcohols by measuring the rate at which they react with chromic acid added to the sample. This method is used to determine the equilibrium constants for the formation of several alkyl acetates, (p. 32)
ATR spectroscopy and an enzymemodified fiber The advantages of using optical fibers as ATR elements include flexibility and increased sensitivity. The purpose of this investigation by K. Taga of the University of Technology Vienna and colleagues is to coat a chalcogenide glass optical fiber with crystalline bacterial cell surface layers to immobilize glucose oxidase. The fiber is then coupled to an FT-IR spectrometer and used as an ATR element. Evanescent-field IR spectra in the 4000-800 cm -1 range of the surface film thickness are obtained, (p. 35)
Chiral compounds and NMR spectroscopy As the analytical chemistry community becomes more interested in the properties and effects of one enantiomer over the other, determination of enantiomeric ratios will become more important. J. Buddrus, H. Herzog, and K. Risch of the Institut fur Spektrochemie und angewandte Spektroskopie demonstrate how the enantiomeric ratio of chiral ammonium halides or alkali carboxylates can be determined by NMR spectroscopy of these compounds mixed with an optically active carboxylate or ammonium halide. (p. 40)
Analytical Chemistry, Vol. 66, No. 1, January 1, 1994 11 A
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Chemometrics Analytical profiles are commonly normalized to the most in tense peak or constant sum before a library search or multi variate analysis is performed. Olav M. Kvalheim, Frode Brakstad, and Yi-zeng liang of the University of Bergen examine normalization procedures from a theoretical point of view and determine their effects on simulated and real data. One of their goals is to tailor preprocessing procedures to differ ent analytical situations, (p. 43) Single molecules The ultimate analytical measurement is that of a single mole cule. Joel Tellinghuisen of Vanderbilt University and col leagues examine how the implications of results obtained within the past year are manifested in actual lifetime data recorded for single fluorescent dye molecules. Maximum likelihood and least-squares methods are used to analyze photon burst data for Rhodamine 110 dye dissolved in methanol; the radiative lifetime of the dye is found to be 4.2 ± 0.2 ns. (p. 64) Pump technology Takashi Korenaga and colleagues of Okayama University have developed a computer-controlled micropump that al lows precise microliter delivery and in-line mixing. The pump provides a large interface between immiscible phases or miscible streams after merging, thereby promoting mass transfer in radial directions. The excellent stability and re producibility of the pump make it suitable for use in process analysis over long periods of time. (p. 73) Surface-enhanced Raman spectroscopy Phillip A. Drake and Paul W. Bohn of the University of Illi nois, Urbana-Champaign use surface-enhanced Raman scattering of metal island films placed within μηι-sized poly mer sandwich thin-film structures to explore their use as molecular-level probes of case II diffusion for rc-hexane swelling of polystyrene. The gold island structures generate strong signals, and the photooxidation problems associated with silver island structures are avoided, (p. 79) Optodes Sulfur dioxide monitoring is essential for air pollution control and industrial applications. Matthias Kuratli and Erno Pretsch of the Swiss Federal Institute of Technology demon strate optodes based on plasticized PVC membranes that incorporate a hydrogen ion-selective chromoionophore. The optodes optically sense gaseous sulfur dioxide in the pres ence of humidity; dry sulfur dioxide can be detected if the membrane also contains a lipophilic alcohol. Sensitivity and selectivity are increased when a lipophilic aldehyde is added, (p. 85)
Rate constants and ELECTROANALYTICAL cyclic voltammetry In an attempt to develop less costly, less complicated detection alternatives for organophosphate pesticides, cyclic voltammetry is used to measure the inhibition of butyrylcholinesterase by the pesticide paraoxon (diethyl />-nitrophenyl phosphate) using butyrylcholine chloride as the sub strate. P. Beattie of the University of Edinburgh and col leagues show that it is possible to determine the rate constants for both the inhibition of butyrylcholinesterase and the hydrolysis of butyrylcholine. (p. 52) Cadmium in the environment Because heavy metals pose a threat to the environment, the development of sensitive probes for their determination is a priority. Iva Turyan and Daniel Mandler of The Hebrew University of Jerusalem used self-assembled monolayers of ω-mercaptocarboxylic acids on both mercury film and gold electrodes to obtain detection limits of 4 χ 10~12 Μ and 5.5 χ 10 -11 M, respectively, for cadmium in seawater. Shorter ω-mercaptocarboxylic acids provide superior sensitivity; other factors that influence sensitivity and selectivity are also examined, (p. 58) Quadrupole ion trap MS MASS SPECTROMETRY The problem of extending the dynamic range of quadrupole ion trap MS for inorganic analysis is addressed by Doug las Duckworth and colleagues of the Oak Ridge National Laboratory. Through judicious selection of rf and dc potentials on endcap and ring elec trodes, analytes are accumulated and the dynamic range is increased to 105. The linearity and relative trapping efficien cies with respect to injection time of these techniques are characterized, (p. 92) MS instrumentation A novel EI TOF mass spectrometer with an ion mirror has been developed by Olga A. Mirgorodskaya of the Institute for Cytology of the Russian Academy of Sciences and col leagues. The orthogonal acceleration of ions, combined with ion storage modulation, allows large sensitivity with moder ate mass resolution. The potential of this instrument is dem onstrated by the study of the pathways of recombinant proinsulin proteolytic processing, (p. 99) Hydrogen-deuterium exchange in polymers To determine whether hydrogen-deuterium exchange oc curs between perdeuteriopolystyrene and perhydropolystyrene during the SIMS ion emission process, Lucinda R. Hittie, Andrew Proctor, and David M. Hercules of the University of Pittsburgh used TOF SIMS and a variety of data analysis procedures. The differences between individual components and the mixture were examined. Because no significant dif ferences were found, they concluded that polymer fragmen tation must occur via intramolecular rather than intermolecular mechanisms, (p. 108)
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Chemical ionization MS J. F. Parcher and colleagues at the University of Mississippi present a low-pressure CIMS method for the detection and analysis of hydrogen isotopes and spin isomers. This indirect method is used to detect protonated reagent gas ions at masses that are below the scan limits of most quadrupole GC/MS systems. Applications of the method are investigated, and MS-based hydrogen detection is compared with conventional thermal conductivity detection of hydrogen isomers and isotopes, (p. 115) Electrospray MS Metallocenes are important precursors for the synthesis of a wide variety of organometallic compounds. In an effort to further understand their chemistry, electrochemical oxidation and nucleophilic addition reactions of metallocenes, substituted metallocenes, and organometallic salts containing metal-carbon bonds are examined by electrospray MS. Xiaoming Xu, Steven P. Nolan, and Richard B. Cole of the University of New Orleans explain their results on the basis of ion stability considerations, (p. 119) Electrospray MS instrumentation Anatoli N. Verentchikov, Werner Ens, and Kenneth G. Standing of the University of Manitoba couple an electrospray source to a reflecting TOF mass spectrometer. The instrument may be operated in either the linear mode or the reflecting mode. Time resolution is < 8 ns for m/z 600, enabling observation of individual isotopic peaks for masses up to about 4000 u. Under proper conditions, ionization is very soft, allowing injection of weakly bound complexes, (p. 126) »..
Derivatizing agents Although o-phthalaldehyde, in combination with a thiol compound, is a commonly used fluorescence derivatizing agent for amino compounds, it has limited stability. Koichi Saito of the Saitama Institute of Public Health and colleagues present an LC method coupled with on-column derivatization and column switching for the kinetic study of the degradation of spermine labeled with o-phthalaldehyde. The fluorophore formed by this method is more stable than that formed by the conventional precolumn method, (p. 134) VJ&~ SEPARATIONS
Solid-phase extraction To predict breakthrough volumes in SPE, Mary L. Larrivee and Colin F. Poole of Wayne State University have developed a model that uses solvation parameters to characterize the molecular volume and capacity of the analyte for polar interactions. The success of the approach is demonstrated by the excellent agreement between the calculated and experimental breakthrough volumes obtained for about 25 different analytes. (p. 139)
Modified MECC Christine L. Cooper and Michael J. Sepaniak of the University of Tennessee separate benzopyrene isomers, a significant environmental pollutant, by adding cyclodextrins to the mobile phase in MECC. Although cyclodextrin-modified MECC is complicated, they are able to correlate the retention behavior with computationally derived host-guest interaction energies and predict elution order for most of the isomers, (p. 147) LC Verifying the use or production of chemical warfare agents is a major concern in relations with our world neighbors. Andrew F. Kingery and Herbert E. Allen of the University of Delaware separate four closely related chemical warfare agent degradation products by using organic modifiers that produce a separation with both ion-exchange and reversedphase characteristics. The method, which requires little sample preparation, can be applied to surface water, groundwater, and soil samples, (p. 155) Solid-phase microextraction Current methods for detecting phenols in the environment are labor intensive and require expensive and hazardous solvents, which pose health and disposal problems. Karen D. Buchholz and Janusz Pawliszyn of the University of Waterloo develop a solid-phase microextraction method for phenols that is faster, more accurate, and safer than current standard procedures. The samples are analyzed by GC/ion trap MS; the detection limit is in the sub-ppb range, with a precision of 5-12% RSD. (p. 160) Countercurrent chromatography J.-M. Menet of the Ecole Supérieure de Physique et Chimie Industrielles and colleagues discuss the classification of countercurrent chromatography solvent systems on the basis of capillary wavelength and setding velocity. These values are used to classify previously investigated solvent systems and six new systems. Classification based on the settling velocity of the lighter phase into the continuous heavier one provides a reasonable prediction of the experimental flow behavior, (p. 168) CE and micromachining Micromachining provides a route to very rapid sample injection and separation schemes in CE. Zhonghui H. Fan and D. Jed Harrison of the University of Alberta describe a system micromachined on a 1 χ 2 cm2 glass chip that separated a mixture of three amino acids within 4 s. The system obtained a maximum of 6800 theoretical plates (470 V between injec tion and detection points) and no Joule heating. Fabrication methods and leakage problems are also discussed, (p. 177)
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