news SCIENCE Multisample nanoelectrospray When it comes to nanoelectrospray, the American axiom, “If some is good, more is better,” holds true. Several groups recently have demonstrated electrospray ionization mass spectrometry (ESI-MS) from microchips. Now, Gary Schultz, Tom Corso, and colleagues at Advanced BioAnalytical Services (ABS) have performed multisample ESIMS from a microchip. The microchips are designed and produced by ABS for Orchid Biocomputer’s microchemistry program as part of an effort to find new ways to characAdvanced BioAnalytical Services terize combinatorial libraries. Unlike most nanoA radial or “wagon wheel” arrangement of nanoelectrospray nozzles electrospray devices, which (inset) on the surface of an ABS have their nozzles on the microchip. edge, the ABS design places the nozzles on the surface. LC channels are etched into the back of the wafer, and channels running through the wafer connect each LC channel to a unique nozzle, forming an integrated device from a monolithic substrate. The bonding of this monolith to another substrate encloses the LC channel, allowing microfluidic separations and ESI-MS. To form the nozzles, which hold ~50 pL each, the researchers use deep reactive ion etching (DRIE). This technique uses reactive gas-phase ions to remove material from a silicon substrate in a con-
trolled process, and it is well suited to making an array of identical devices simultaneously, says Schultz. He adds that DRIE also produces nozzles that are uniform in size, which gives reproducible nanoelectrospray results from different nozzles. “Getting electrospray to take place from the edge is a very difficult challenge,” Schultz says. “Mike Ramsey’s [Oak Ridge National Laboratory] and Barry Karger’s [Northeastern University] groups showed this a while ago.” Initiating the spray, sustaining it, and turning it on and off can all be problematic from the edge because the spray emanates from a surface that is not well-defined, he explains. Nanoelectrospray works better if a tapered capillary tip, nozzle, or other welldefined surface is used, he adds. Of course, tips can be put on the edge of chips, but standing the nozzles vertically on the surface offers more flexibility in their configuration, according to Jack Henion, president of ABS. Nevertheless, achieving multisample analysis requires more than the addition of extra nozzles and channels, Henion says. Controlling the spray is critical. The researchers have overcome the two biggest steps: stepping from one nozzle to another and starting and stopping the spray. They have already started working with a 96-nozzle format, which is similar to a microwell plate but smaller. Now it is time to refine the devices. The researchers plan to create new array configurations and devote more time to the separation step that precedes nanoelectrospray. “We still have some challenges left,” says Henion. “Chipbased analytical chemistry is just getting started. But we believe that it will be an important part of the future.” Elizabeth Zubritsky
Miniature microwave plasmas
microchip techniques to facilitate its eventual integration with micrototal analysis systems. The gas chamber was formed by gluing together two 1-mm-thick quartz wafers, each of which had a 0.5 x 1.0 x 90-mm groove sawed into it. The sandwich was mounted on a copper plate, which The MicroStrip Plasma device. served as the heat sink and the ground electrode. Then a second copper electrode, 30-µm thick, was deposited on top of the sandwich— directly above the gas chamber but separated from it, making the device “electrodeless”. This design prevented contamination from the electrodes and provided stability, says Engel. Additional stability was provided by a side arm, which acted as an extra load for the device, says Broekaert. In a conventional
Like the big Buick automobiles of the 1970s, conventional microwave-induced plasma (MIP) sources are gas guzzlers, requiring flows as high as several liters per min. Few “economy model” alternatives have been available. But in this issue of Analytical Chemistry (pp 193–197), José Broekaert, Ulrich Engel, and colleagues at the Universität Leipzig and Universität Dortmund (both in Germany) describe the equivalent of an efficient little Volkswagen—a device they call the “MicroStrip Plasma” (MSP) source. “As far as we know, there has not been a MIP of this kind before,” says Engel. Unlike its large forebears, the MSP needs only 10–40 W of input power and can operate with gas flows as low as 50 mL/min. Under these conditions, the MSP generates a cylindrically symmetrical argon plasma ~1 mm in diameter and 2–3 cm long, with rotational and excitation temperatures comparable with those in a conventional MIP, according to Broekaert. The researchers fabricated the MSP using conventional 22 A
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news Separations on the bacterial scale Various efforts have been made to extend electrophoresis to the separation of microbes, but they have met with limited success. Of the few reports available, many concentrate on single organisms; even so, the electropherograms may have multiple peaks, which complicate the interpretation. Attempts to separate different kinds of organisms have been time-consuming and of poor efficiency and resolution. In the Dec. 15 issue of Analytical Chemistry (pp 5465–5469), Daniel Armstrong and his colleagues at the University of Missouri–Rolla describe two capillary electrokinetic approaches to separating different types of bacteria efficiently. Unlike many current methods, which use lysed cells, these techniques use intact cells, which may eliminate some of the variability usually ascribed to differing growth conditions, ages of cells, and preparation procedures. In the first approach, the researchers used capillary isoelectric focusing (CIEF) to separate the common bacteria Escherichia coli K12, Pseudomonas putida, and Serratia rubidae, all of which are rodshaped and approximately 1 µm in diameter. Samples were injected into a capillary under pressure, followed by an ampholyte to generate a pH gradient. A “focusing” voltage was applied for 5 min, and the samples were mobilized with a low-pressure rinse. Detection was at 280 nm using a standard on-line UV detector. Under these conditions, the amphoteric bacteria separated rapidly, migrating to a position
where they were uncharged. “This position can depend on the surface features characteristic of an individual species,” explains Armstrong. The second approach relied on carrying the negatively charged bacteria in a semidilute polymer solution—in this case, poly(ethylene oxide)—during CE. This method was slower but could Electropherogram showing the capillary isoelectric separate bacteria not amenable focusing separation of three bacteria of similar size. to the CIEF approach. The polymer solution was added to the running buffer for the column. The ticular, are easily damaged by oxygen, bacteria samples were injected for 8–10 s, pH, secretions from other bacteria, and and the separation was performed at 10 rough handling. In addition, a number of kV, with on-line detection at 214 nm. other factors—including the formation of The researchers tested the polymer aggregates, attachment to surfaces, and technique with a mixture of yeast sampling problems due to the relatively (Saccharomyces cerevisiae) and bacteria low concentrations of microbes in soluboth spherical (Micrococcus luteus) and tion—must be controlled to obtain reprorod-shaped (P. fluorescens and Enterducible separation results. obacter aerogenes). The microbes Nevertheless, Armstrong thinks bacappeared to separate according to size, teria are coming into analytical line with shape, and other factors, and varying the macromolecules, and he expects that concentration of the polymer changed separating microbes one day will be as the elution orders. “The exact mechaeasy as separating molecules is now. This nism of this action is not yet clear,” says kind of analysis will “revolutionize Armstrong. “Importantly, though, very aspects of microbiology involving the high separation efficiencies were obdiagnosis and profiling of some diseases, tained for both methodologies.” QC in fermentation, soil analysis, bioArmstrong points out that although remediation studies, and virtually any they have obtained decent results, the other area of science and technology process of separating microorganisms is that involves bacteria, fungi, algae, or still fraught with danger. Bacteria, in parviruses,” he says. David Bradley
MIP, the plasma is the only load, so even small variations in it change the system, he explains. But the MSP has an added load. “The value of the additional load is well adapted,” he says, “so small changes in the plasma do not have an influence.” To test the device, the researchers performed flow injection cold-vapor (FI-CV) analyses of trace levels of mercury in aqueous solutions. Traditional FI-CV with atomic absorption spectroscopy (AAS) was compared with FI-CV with optical emission spectroscopy (OES), using the MSP as the source. Both methods produced essentially the same results. But FI-CV-AAS is limited to single-element analysis. When the MSP is the atomic emission source, multielement analysis can be performed. “That is one of the biggest advantages,” Broekaert says. The researchers began with FI-CV for mercury because the method is well known and the analyte is of interest for
environmental studies. However, the system could be applied to “all elements which are volatile by themselves or which form volatile compounds,” says Broekaert. “There are many possible applications.” At the moment, however, the researchers are most interested in further miniaturizing the device. It is bigger than it needs to be now, Broekaert says, because they used the materials they had on hand. Only the active region—a 20mm-long channel with a 1-mm-square cross section—must be kept. The rest can be trimmed. Then the researchers will work on integrating the MSP with another device in a single wafer. “We have already developed an operating setup with reduced size and power consumption,” Engel says. “And we think a ready-to-use device could be realized without too much effort.” Elizabeth Zubritsky J A N U A R Y 1 , 2 0 0 0 / A N A LY T I C A L C H E M I S T R Y
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