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function and surface potential topographies to within 1 meV energy resolution. With its automated setup and scanning procedures, The Kelvin probe measures the work functhe instrument has been designed to be tion difference or, for nonmetals, the surused by nonspecialists. Tip-to-sample spacface potential between a conducting speciing control (to men and a within 40 nm) vibrating tip. avoids spurious The probe is or "apparent" based on a work function principle first changes due to demonstrated scanning-into the Royal duced spacing Society in changes. 1897, in which a capacitor is The design formed beconsists of a PC tween two that communiconductors cates to three and the subsystems via charge transan XT/AT bus; fer is detected a digital oscilla(Anal Chem Schematic of the scanning Kelvin probe. (Adapted with permission. Copyright 1198 American Institute of Physics.) tor to set voice1997 69 coil frequency, amplitude, and probe trigger; a data acquisi293 A-296 A)) Data from the Kelvin probe tion system (DAS), which measures the can be used to determine the condition or peak-to-peak output signal as a function of modification of surfaces. the backing potential (7J; a 12-bit digital-toI. D. Baikie and P. J. Estrup of Brown analog converter (DAC); and a stage form University describe a low-cost, PC-based sample translation. The scanning system scanning Kelvin probe that measures work
Understanding electrospray Electrospray ionization (ESI), together with MALDI, has revolutionized the mass spectrometric analysis of biomolecules. Despite its widespread application, however, the electrospray process is still poorly understood. Akos Vertes and co-workers at the George Washington University and Naval Research Laboratory attempted to improve the understanding of the process by obtaining droplet size and velocity distributions for spray conditions typical of ESI. The researchers characterize the droplets as a function of the fluid delivery systems and of properties such as liquid flow rate, bias voltage, position of the probe volume, and electrical conductivity. Using phase Doppler anemometry, the researchers measured simultaneously the size and velocity (both axial and radial) distributions of the droplets in methanol/water solutions sprayed at 4 kV. They made measurements at sev-
84 A
Size distributions of electrosprayed droplets from a 90:10 methanol-water mixture (dashed line) and the same mixture doped with 5.0 x 10~3 M KCI Isolid line) as a function of the distance (dpr) between the probe volume and the capillary tip.
Analytical Chemistry News & Features, February 1, 1999
performs line scans or topographies in the range of 200 um-2 cm on a side; ;arger ramples, such as semiconductor wafers, can be handled with a rty system. (Rev. Sci. Instrum. 1998, 69, 3902-07)
Resistance to dechlorination Chlorination is a common approach for disinfecting wastewater before discharge into the environment. To minimize the toxic effects of residual chlorine, water is often dechlorinated with sulfite or sulfur dioxide. The process is considered instantaneous and complete; however, studies have shown that peptides and proteins form products during chlorination that are slowly dechlorinated. William A MacCrehan of the National Institute of Standards and Technology and co-workers at the University of Maryland-College Park investigated the chlorination/dechlorination of organic nitrogen components of sewage wastewater paying particular attention to the formation/ideniification and kinetics of dechlorination-resistant products A reversed-phase gradient-elution LC method is used to examine the dechlorina-
eral points along the spray axis for solutions with different conductivities. Increasing the measurement distance resulted in smaller droplets for lowconductivity solutions, whereas the droplet size increased with increased measurement distances for high-conductivity solutions. This observation contradicts the view that droplets decrease in size along their trajectory because of evaporation or fission. The authors suggest two possible explanations—a fractionation effect and coalescence—for their observation. Cone-jet sprays are known to have two directional components—larger primary droplets near the spray axis and satellite droplets departing with a larger solid angle. These components increasingly segregate farther from the source. Coalescence appears possible because of the droplets' high kinetic energy and because they undergo deformation and charge rearrangement as they approach one another. (J. Phys. Chem. A 1998, 102, 9154-60)
