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Optics and chips GC, LC, CE, and other rnalytical techniques usually take up acres of bench space, but tesearchers working in the area of micro-total analysis systems (uTAS) hope to remedy this situation and provide miniature chemical equipment analogous to microelectronics. This emerging technology wiil provide inexpensive, portable, and cheap-to-run equipment that can handle tiny sample volumes and use minimal reagents. Although miniature gas and liquid chromatographs are already showing great promise, a basic need exists to integrate them at the chip level with similarly shrunken detector systems. Andreas Manz and Jan Eijkel of the Zeneca/SmithKline Beecham Centre for Analytical Sciences at Imperial College (U.K.), collaborating with Herbert Stoeri of the Technical University of Vienna (Austria), have built the first optical emission detector on a tiny sliver of glass in another step toward the ultimate "lab on a chip". They demonstrated that their device works the same as its bench-top counterparts but on a much smaller 50 nL scale. They hope to easily integrate the miniature detector on a chip-based gas chromatograph, such as that developed by Steve Terry of Stanford University. Optical emission usually involves desol-
vation of the analyte and atomization and excitation of the molecules using a flame or a plasma. In the July 15 issue of Analytical Chemistry (p 2600), the team reported d new device based on plasma excitation, which, they say, avoids the problems associated with the small exposed flame that was used in a previously developed micromachined flame detector. The device is a 14 x 30-mm HF--eched glass plate. It features a gas inlet and outlet, a pressure sensor connection, and various electrodes (see the schematic in the accompanying figure). This forms the top plate below which the team attached a second glass plate (20 x 30 mm) with h plasma chamber, inlet and outlet channels, and electrode chambers. To test the setup, the researchers fed helium gas with different concentrations of methane into the chip. The optical emission from the plasma in the chamber was then picked up externally by a fiber and sured by a photomultiplier tube. They obtained a detection limit of 3 x 10~12 g methane/s in a volume of 50 nL using excitation of the CH diatomic band. "This compares well with the values achieved by everyday plasma-based detectors," says Eijkel. However, the concentration-based detection limit was 500 ppm, which, because of the small volume of the
Schematic of the chip layout. Features of the 14 x 30-mm top plate: (1) gas inlet, (2) gas outlet, (3) pressure sensor connection, (4) electrodes, and (6) electrode connection pads. Etched in the 20 x 30-mm bottom plate are (5) a plasma chamber, (7) an inlet channel, (8) an outlet channel, and the electrode chambers, which are not indicated.
plasma, is higher than for conventional plasma detectors. The researchers do not consider this problem serious because they anticipate using the device for fast throughput of small samples with high concentrations and not for very high-sensitivity analyses. However, the lifetime of the cathode is rather low at 2 h, because of sputtering, which, they say, must be improved. David Bradley
NEWS FROM THE INTERNATIONAL SYMPOSIUM ON ENVIRONMENTAL ANALYTICAL CHEMISTRY Cella Henry reportsfromJekyllIsland, GA.pathogenic bacteria are sensitive but slow, with analysis times measured in days, or even weeks. A typical detection limit is 1 Detecting critters colony-forming unit in 25 g of food, but as many as 14 days might be required before in food the colonies can be seen. Millions of cases of food-borne illness Therefore, the search is on for a method strike in the United States each year, as sensitive as, but much faster than, tradiresulting in thousands of deaths and tional bacterial cultures. The ideal method billions of dollars in medical care costs, will be sensitive, fast, inexpensive, low--ech, according to the Centers for Disease and capable of being performed on-site. ImControl and Prevention. Methods that munoelectrochemistry is a promising techrapidly detect pathogenic bacteria, such as Salmonelll, E. coli 0157:H7, and List- nique that involves labeling bacteria with eria monocytogenes, could reduce the enzyme-antibody conjugates and then detecting the electroactive product of the eneconomic and public health toll these organisms exact. Jeffrey Brewster of the zyme reaction. In Brewster's method, the antibody is organism-specific, but alkaline U.S. Department of Agriculture hopes phosphatase is used as the enzyme label that immunoelectrochemistry will be regardless of the target organism. one such method. Traditional methods for detecting The method of capturing the bacteria can
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Analytical Chemistry News & Features, August 1, 1999
make or break the technique. Brewster has used several methods for capturing the bacteria, including antibody-coated electrode surfaces, antibody-coated magnetic particles, and membrane filtration. The coated electrode is incubated with the sample for 30 min before analysis. Unfortunately, because bacteria diffuse slowly, only a small number of the cells exposed to the electrode is actually detected. The capture efficiency was only 20%, and the detection limit for Salmonella waa s aisappointing ~10 5 cells exposed/mL. With the magnetic particles, a 30min incubation period is also required. The electrode is a thin layer of graphite silk-screened on a thin piece of plastic. A magnet under the electrode pulls the
