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Under pressure In the wee world of capillaries, electroosmosis can really pack a punch. That's what Phillip Paul and colleagues at Sandia National Laboratories have shown. They have generated pressures up to 8000 psi using electrokinetic pumping of liquid through tiny tubes packed with porous beads. Their method takes advantage of the well-known principles of electroosmotic flow. If you load a capillary tube with porous material and apply an electric field, the fluid inside moves. Even if the capillary's end is closed, there is still a flow. The pressure builds, pushing the liquid back through the pores in the beads until this reverse flow balances the electrokinetic forward flow. Paul says this phenomenon was understood back in 1857, if not earlier, but most attempts to harness the power for pumping have been disappointing, yielding pressures of a few inches of water. To get high pressures, Paul and his colleagues used low-conductivity fluids and very small pores. "As you go to smaller and smaller pores," Paul says, "it takes more pressure [to move the fluid]." Experiments verified what the researchers had predicted: The pressure is independent of the viscosity of the fluid, but proportional to the applied voltage, and inversely proportional to the square of the bead diameter. "Because you can generate really high pressure, it's essentially a nonmoving parts hydraulic pump," Paul says. For micro-TAS systems the bonus is this: The smaller you make the pump, the better it runs. So far, the researchers have tried pores as small as 50 nm across and glass capillary tubes as small as 50 um, but they plan to
develop smaller devices with beads packed into microchannels on a chip. This might provide a way to do pressure-driven chromatography on a chip or to drive microscale mechanical actuators, he says.
And there may be other applications. "What we need to do now," Paul says, "is figure out what new opportunities this presents, which is always a fun thing to think about."
Tiny torches at 125 mbar, that's what the unit does. The The biggest limitation of aflameanalyzer electrolysis cell that feeds the flame is a mere 6 cm tall. is the need for a large gas supply. The Because there is less gas in the systanks are unwieldy and, more importem, the microburner is less dangerous tantly, explosive. But Stefan Zimmermann and his colleagues at the Technical and more mobile. These features expand University Hamburg-Harburg (Germany) the possible applications for flame analyzers, Zimmermann says, have made a safer, more even allowing them to be portable light. powered by batteries. He Actually, the researchers and his co-workers are dehave made two devices—a veloping such devices to flame-ionization detector and enable ambulance technian atomic emission flame cians to analyze patients' spectrometer. The basis for blood on the scene. both is a microscale burner unit made from a pyrex-siliThe current drawback con-pyrex sandwich. The fuel to the petite analyzers is inlet sits between the pyrex their detection limit, Zimbottom and the silicon submermann says. The flame strate and is connected to the spectrometers haven't oxyhydrogen supply by a been tested yet, but the capillary tube. The analyte A miniature atomic ionization detector protoemission flame inlet is between the silicon type has limits in the spectrometer. The layer and the pyrex cover. parts-per-million range— microscale burner Etched into the silicon is a far from the desired limit produces a 3-mm flame, concentric annular nozzle of parts per billion. But which appears blue which is 60 and the devices haven't been because volatile organic optimized yet. Zimmerproduces a 3-mm flame compounds are present. mann thinks the ioniza"The idea of the miniation detectors can reach the parts-perture flame analyzer is to reduce the fuel billion limit and hopes the spectromegas consumption," Zimmermann says. ters will as well With an oxyhydrogenflowof 35 mL/min
NEWS FROM FACSS Celia Henry reportsfromAustin, TX.
Them bones, them bones Bone is a dynamic system, constantly being remodeled (dissolved and reformed) within structures called osteons. Michael iviui i is uiiQ ms co-worKcrs di uic Umversity of Michigan are following the changes in bone chemistry as a function of position with ivaman imaging. Such informauon can lead to better understanding of growth and disease processes and of healing after bone damage. 776 A
Because bone is a heterogeneous mixture, the composition can change dramatically within a few micrometers, requiring good spatial resolution. Around osteons, for example, bone mineral disappears from images and the vasculature appears within 5-10 um. The spatial resolution of Morris's current instrumental system varies from 1 to 3 um, depending upon the microscope objective, and the spectral resolution is in the range of 3 to 8 cm-1. Morris points out that the 1080 cm-1 band due to proteins in blood vessels can be easily confused with the 1070 cm-1 band of carbonate. He warns
Analytical Chemistry News & Features, DeDember 1, 1991
that band integration and filter-based imaging must be approached carefully when dealing with bone samples. Morris's collaborators have developed a new technique for bone culture and have been trying to demonstrate that the material is indeed an apatite minerall "They spent a couple of years trying to find this by X-ray fluorescence," he says. "Wiihin two hours working just with culture they washed, dried, and smeared on a microscope slide, we had the answer they wanted—not as ordered as you see in normal bone, but very clearly an apatitic mineral."
