Electrospray for more than just organics The reference electrode (1) can move vertically or parallel to the sample surface (10). (Adapted with permission. Copyright 1997 American Institute of Physics.)
Flexible Kelvin probe A Kelvin probe is a standard tool lor the surface sciences that measures work function changes caused by gas adsorption or metal deposition. Most Kelvin probes have a reference electrode, typically a small metal plate, which vibrates either vertically or parallel to the sample surface. There are certain advantages and disadvantages to both vertical and parallel flexing, but why choose? K Wandelt and colleagues at Universitat Bonn (Germany) have designed a probe that combines both motions in one setup. The new probe contains a thin, goldplated tungsten wire that serves as the reference electrode. The wire can vibrate in both vertical and parallel directions. A piezoelectric tube with four outer electrodes drives the wire; such piezotubes are already used in scanning probe microscopes as positioning elements. The probe also includes thermal desorption spectroscopy, Auger electron spectroscopy, and scanning tunneling microscopy facilities. The authors demonstrate the application of the new design by monitoring copper deposition on a Ft (111) surface. (Rev. Sci. Instrum. 1997,68, 3866-71)
Amperometric, microgravimetric, and reversible immunosensors Preliminary work by Itamar Willner and colleagues at the Hebrew University of Jerusalem (Israel) indicates that the insulation of an electrode surface toward a redox probe solubilized in the electrolyte by the formation of the antigen-antibody complex provides a general route for the amperometric transduction of biospecific interactions at the electrode surface. Similarly, functionalized monolayer electrodes associated with quartz crystals were used for mi-
Electrospray ionization (ESI) is usually associated with the analysis of biological macromolecules. However, it can also be used to study inorganic and organometallic species. Solution ions can be directly converted to gasphase ions, and even solvent clusters can remain intact. Gary M. Hieftje and co-workers at Indiana University describe an ESI time-of-flight mass spectrometer that incorporates a heated capillary interface. The system consists of a 15-cm capillary with an inner diameter of 600 pm that is heated to 120-200 °C with heating tape. The distance between the capillary exit and the skimmer can be adjusted from 0 to 10 mm. The electrospray interface is positioned orthogonally to the time-of-flight mass analyzer. The instrument is more sensitive for the alkali metals than for transition
crogravimetric quartz crystal microbalance analyses of biospecific interactions at the crystal monolayer interface. Willner and colleagues present a comprehensive study that describes the methodologies to tailor antigen monolayer electrodes for the electrochemical and microgravimetric quartz crystal microbalance sensing of antigenantibody interactions at electrode surfaces. Antidinitrophenyl antibody is sensed by means of a dinitrophenyllysine monolayerfunctionalized electrode using Fe(CN)|~/ Fe(CN)|~ as a redox probe. Glucose oxidase, modified by ferrocene units, was found to be a superior redox probe for following the antigen-antibody complex at the electrode surface. It is insensitive to molecular defects in the monolayer composition and enables the bioelectrocatalytic amplification of the antigen-antibody complex formation at the monolayer electrode. This antigen monolayer electrode configuration can be further modified for other immunosensing electrodes and the concept of electrode insulation by biospecific interactions at a monolayer-functionalized electrode could be applied to develop sensing interfaces for other biorecognition pairs The piezoelectric microgravimetric
metals because the latter are often present in several species. A 10~5 M solution of alkali metals in methanol yields a count rate of 1-5 x 103 counts/s. A substantial amount of chemical background keeps the detection limits for Cs, Rb, and Cd at 10 nM, 30 nM, and 50 nM, respectively. The stability of the instrument was demonstrated with Rb isotope ratios; over a 60-min interval the isotope ratio fluctuated with a 0.4% relative standard deviation, which could be improved by longer integration times. With an internal standard the signal is linear 3 orders of magnitude Hieftje's group also analyzed organic forms of Fe and Pb—ferrocene and tetraethyllead. By varying the conditions, the total Pb or Fe content could be measured or the different organic forms could be quantified. This performance is comparable to previously reported instruments with a countercurrent N2 curtain gas interface and a quadrupole mass filter. (Appll Spectrosc. 1997,51, 1464-70)
method to sense an antibody or antigen was demonstrated with the organization of the 3,5-dinitrosalicylic acid antigen monolayer on gold electrodes associated with a quartz crystal. Antidinitrophenyl antibody was analyzed by this monolayer interface using the microgravimetric method. The researchers also demonstrated that biospecific antigenantibody interactions can be photostimulated by applying photoisomerizable antigens, which has implications for tailoring reversible, reusable immunosensors. (J. Am. Chem. Soc. 1197,119,10467-78)
Reversibie tmmunosensor using a photo~ isomerizable anttgen monolayer assembled on a transducing supporr as a functionalized interface for sensing anttbody ynd dhotochemical regeneratton of the eensing surface.
Analytical Chemistry News & Features, January 1, 1998 19 A
