Sensitive assay for amoxicillin For many children with acute otitis media (middle ear infections) caused by bacteria, the antimicrobial amoxicillin is the treatment of choice. Analytical methods that detect amoxicillin in biological fluids rely on protein precipitation by acid or organic solvents. Zhi Yuan and colleagues at the University of Minnesota describe a new, more sensitive method based on solid-phase extraction and reversedphase HPLC. Amoxicillin and cefaroxil, the internal standard, are extracted from human plasma onto a C18 cartridge from 50 to 200 uL of sample. The extract is run through a C8 HPLC column using 6% acetonitrile buffered with phosphate and tetrabutylammonium and detected by UV at 210 nm. The authors report a limit of
Fabricating metaloxide devices with AFM The electrical characteristics of devices with features of 10 nm or less are extremely sensitive to size variations. Because of this sensitivity to local geometry, these devices can be reproducibly fabricated only by control at the near-atomic level, which can be very difficult to achieve using the lithographic techniques currently available. E. S. Snow and P. M. Campbell of the Naval Research Laboratory postulated that the use of a highresolution processing technique in conjunction with real-time in situ measurement of device properties during fabrication should allow fabrication of devices with features smaller than 10 nm with precisely tailored electrical properties They used atomic force microscopy-based anodic oxidation which does not interfere with the measurement of device properties to fabricate fine metal wires and TiTiO -Ti lateral junctions from thin titanium films After preliminary preparation, the titanium devices were mounted in the atomic force microscope, and electrical leads were attached to take electrical measurements during the procedure. The width of the wires and resistance of the junctions were controlled by real-time in situ measurement of resistance by the device during fabrication. To produce a airer the tip was biased at -12 V and scanned over a rectangle
quantitation of 0.125 ug/mL from a 200-uL NMR gets small sample volume. (J. Chromatogr. B .995, 674, 93-99) NMR is the least sensitive of the common spectroscopic methods. Jonathan Sweedler and co-workers at the University of Illinois at Urbana have attacked this problem by constructing a NMR rf transmission/detection coil only 1 mm long. The resulting microcoil NMR system improves S/N more than 100-fold compared with conventional NMR The microcoil system has several unique features. The coil is wrapped around a capillary with a 357-um o.d., and the capillary also functions as a flowthrough sample container requiring just 5 nL of sample. To reduce the effects of magnetic susceptibility arising from the Chromatogram of a human plasma sample proximity of the rf coil to the sample, the following solid-phase extraction. Amoxicillin coil-sample assembly is immersed in Fluoelutes at 6.45 min and cefadroxil at 12.30 rinert FC-43, a perfluorinated organic liqmin. (Adapted with permission of Elsevier uid. The liquid's magnetic susceptibility Science.) matches the coils, thereby lowering static magnetic field in homogeneities, which leads to improved resolution and lineshape. The sample is not spun, and sample loading through the capillary usually makes shimming between different samples unnecessary. Yet resolution remains good—a 0.6-Hz linewidth was measured with a 300-MHz NMR spectrometer— and data acquisition times were significantly reduced. As a limit of detection, the authors report that a 1-min data acquisition yielded a spectrum for 56 pmol of sucrose with a S/N of 3. (Science e995,270, 1967-70) AFM image of an oxide junction (indicated by the circle). (Photo courtesy of NRL.)
that covered about half the width of the starting wire. The tip was then positioned on the unoxidized side of the wire and scanned toward the oxidized region while the resistance of the device was monitored. At a predetermined increase in resistance, the tip voltage was dropped to zero. To construct metal-oxide-metal junctions, they monitored the device resistance while slowly increasing the voltage on the tip during repeated scans across the width of the wire. When the desired junction resistance was reached, the tip voltage was dropped to zero. They also explored the minimum feature size that can be achieved by this technique and created a wire 200 nm long with a resistance increase that corresponded to a final width of 3 nm. (Sciencc 1995,270,1639-41)
Layout and circuit for microcoil NMR: C, and Cm are tuning and matching capacitors, respectively. (Adapted with permission of the American Association for the Advancement of Science.)
Analytical Chemistry News & Features, March 1, 1996 165 A
