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Laser-activated voltammetry Aqueous iodine undergoes a series of reactions on platinum electtodes, which lead to the deposition of atomic iodine and eventually the formation of molecular iodine. Understanding the kinetics of this electrochemistry is complicated by the continual formation of fodine, which coats the electrode eo dissolves into solution and creates constantly changing electrode conditions. Richard G. Compton and colleagues at Oxford University (U.K.) got around this problem by using a 532-nm, 10-Hz Nd:YAG laser to keep the electrode surface clean of iodine without damaging it. What the authors call laseractivated voltammetry was then used to study the electrochemical oxidation of aqueous iodide at a platinum electrode under stationary and hydrodynamic channel flow conditions Atomic force microscopy confirmed that
the surfaces were being cleaned. Ultrasound was also investigated and shown to dissolve water-insoluble redox products off the electrode surface and into an organic phase. In this case, the ultrasound induced an emulsion between water and dichloromethane. With the surfaces kept clean, numerical modeling of the channel flow voltammetry under zxperimental setup for laser-activated voltammetry in stationary hydrodynamic condisolution. tions provided qualitative information on the mechanism and the reaction. The authors conclude that quantitative data on the kinetics. Modeling laser-activated voltammetry offers a way to of the concentration and flow rate depenstudy electrode processes that produce dence of the voltammetric wave shapes surface-active intermediates. (J. Phys. then helped determine the mechanism of Chem.. B 1999,103, 8319-27)
gens were used to prepare columns of different porosity and permeability. Covalent anchoring of the monolith to the tube prevented cleft formation between Frits are frequently blamed for bubble formation in capillary electrochromatog- the wall and the polymeric packing. raphy (CEC). A solution to the problem The conductivity ratios and parameis to use monolithic columns. Csaba ters of the simplified van Deemter equaHorvath and colleagues at Yale Univertion for the most promising columns sity investigated the in situ formed were evaluated for both micro-LC and monolithic stationary phases that have CEC. The efficiency was higher for CEC relatively high Darcy's law permeability. than for micro-LC. Thus, the requirement Although these monolithic columns for high packing uniformity for high effiwere especially designed for CEC, they ciency may be relaxed in CEC. can also be used in micro-LC. To demonstrate the approach, anThe columns were prepared in sigiotensin-type peptides were separated lanized fused-silica capillaries of 75 urn by CEC on columns withfixed«-octyl chains and quaternary ammonium i.d. by in situ copolymerization of divinylbenzene with either styrene or vinyl- groups at the surface. Plate heights of benzyl chloride in the presence of a 8 um were routinely obtained. Separasuitable porogen. Five different porotion is based on the interplay between electroosmotic flow, chromatographic retention, and electrophoretic migration of the positively charged peptides. Because the migration process is so complex, neither classical chromatographic electrophoretic theory can be used to interpret the results (J Chromatogr A 1999 855 Schematic illustration of a monolithic packing. (Adapted 273-90)' with permission. Copyright 1999 Elsevier Science.)
Monolithic capillary columns for CEC
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Analytical Chemistry News & Features, December 1, 1999
Toward well-characterized adenovirus products Recombinant adenovirus preparations increasingly are being pursued as a gene therapy strategy, and it is anticipated that commercial production of these preparations wiil be needed soon. However, ,he adenovirus particle is complex, consisting of double-stranded DNA and at least 11 distinct structural proteins in multiple copies, and, to obtain the "well-characterized biological product" regulatory status, better analytical tools are needed. Elisabeth Lehmberg and co-workers at Berlex Laboratories and HP Laboratories describe a reversed-phase (RP) HPLC assay that supports a systematic analysis of the viral structural proteins for recombinant adenovirus type 5 preparations. During the chromatographic separation, the adenovirus dissociates into its DNA and protein structural components. Further separation and analysis of the individual structural proteins by N-terminal protein sequencing and MALDI time-of-flight MS provide afingerprintof the viral proteome. In addition to characterizing individual structural proteins, the RP-HPLC assay was used to measure the concentration of the virus particles through quantification of the structural proteins. The researchers compared the results of this new protein-based assay with those obtained from the current standard method—absorbance of the lysed
All-in-one synthesis and evaluation Although combinatorial chemistry is revolutionizing the discovery of new drugs, catalysts, and a host of other materials, evaluating chemical libraries requires elaborate instrumentation and schemes that are typically incompatible with the synthesis strategies. But that may be a thing of the past. Alan W. Schwabacher and co-workers at the University of Wisconsin-Milwaukee demonstrate a general scheme for combinatorial library preparation that has the best of both worlds full parallelism of synthesis and full library evaluation. The scheme uses a one-dimensional organized solid support rather than zero-dimensional beads or twodimensional surfaces. The linear solid support, or "thread", allows for fully parallel synthesis. The thread is wrapped around a cylinder in a spiral layer and is partitioned lengthwise into regions, creating distinct reaction vessels, as shown in Figure A. Each region is exposed to a different reactant (red, blue, or green in Figure B). By coupling one reagent per reaction ves~ sel, repeating domains are created along the thread (Figure C). To ensure that all reagent combinations are equally represented, the thread must be redivided into regions before a second set of reagents is cou-
virus at 260 nm—as well as with results from an assay based on viral DNA measurement. In contrast to the standard spectrophotometric assay, the RP-HPLC assay was insensitive to contaminants common in unpurified adenovirus preparations. Excellent agreement between virus concentrations based on protein II content and DNA content was obtained over the concentration range 3 x 1010 to 3 x 1012 viral partii cles/mL. Sensitivity of the RP-HPLC assay is at or below 2 x 108 particles. (J. .hromatogr. B 1199 ,32, 411-23)
As the film grows The growth of nanoscale films by chemical vapor deposition (CVD) has been investigated by vibrational spectroscopy, particularly IR reflection-absorption and electron energy loss techniques, but these ap-
General combinatorial chemistry scheme on a one-dimensional organized solid support.
pled. This is accomplished by wrapping the thread around a second cylinder with a different diameter than the first (Figures D-F). This process can be repeated indefinitely. Each functional group varies as a function of distance along the thread Because the support is not physically fragmented, spatial information on species identity is retained. The researchers demonstrate the
strategy by synthesizing peptides on a cotton thread and evaluating their binding affinity for streptavidin. Fourier transformation of the detector signal provides a library spectrum that conveniently identifies important binding features. In addition, FT may provide a means of identifying trends in activity variation. (J. Am. Chem. Soc. 1999, 121, 8669-70)
proaches require ultra-high vacuum. Michael J. Weaver and co-workers at Purdue University demonstrate that surfaceenhanced Raman spectroscopy (SERS), which is performed at ambient pressure, can also follow CVD chemistry. In this article, they focus on the formation of Ta205 films on a platinum surface. Ta205 is of interest in the microelectronics industry as an alternative to Si02 for gate dielectrics and capacitors. Its formation on platinum is a binary chemical process, which involves the reaction of a Ta(V) ethoxide (TaEtO) precursor with 0 2 . To investigate the monolayer-level chemistry associated with forming Ta 2 0 5 on platinum, the researchers added TaEtO and 0 2 sequentially. The SER spectrum obtained after introducing TaEtO saturated with N2 to an ultra-thin platinum film electrodeposited on a polycrystalline gold foil
showed bands at 1040 and 1165 cm-1. Subsequent addition of 0 2 caused the disappearance of the two SERS bands, and within 1-2 min, the growth of new bands at 575 and 270 cm"1. These spectral changes suggest that the ethoxide precursor was transformed into Ta205. The results indicate that at least the initial deposition of Ta205 onto platinum occurs via a surface mechanism involving adsorbed precursor and oxidant. It is unclear, however, whether this adsorption mechanism represents the dominant pathway when the tantalum precursor and the oxidant are introduced together. Nonetheless, SERS provides broad-based opportuni ties for examining the surface chemistry that underlies technological processes, such as the preparation of microelectronic materials. (/. Am. Chem. Soc. 1999,121, 9219-20)
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