DESIGNING MICROINSTRUMENTS

The handheld gas chromatograph may be at hand thanks to novel techniques in micromachining

Miniature solid-state instruments have the potential to transform conventional bench-top analytical units into portable ones. In one such project, California-based Berkeley MicroInstruments (BMI) has developed a microsensor system that detects and distinguishes benzene and toluene. To do so, silicon integrated-circuit techniques were used to make chip-sized subsystems out of electronically-controlled microstructures.

While the function of these structures is mechanical, chemical sensitivity is achieved using techniques borrowed from the chemical chromatography field.

The dimensions of the microstructures used vary from one micron to a few thousand microns; a complete micro-mechanical subsystem can be fabricated on a silicon chip of approximately 1cm2 (Figure 1).

One device designed using the technique is the micro-machined Flexural-Plate-Wave (FPW) (Figure 2). Invented by BMI, the device has measured the density and viscosity of liquids, detected minute quantities of air and water-borne pollutants, and has been used to pump and stir liquids and gases, using quantities of material measured in microlitres.

Used as a detector, the FPW device ultrasonically measures the mass of a 0.03cm2 membrane. With a sensitivity of 10-9 g/cm2, mass changes as small as 30 picograms can be resolved. To detect chemicals, this membrane is coated with an absorbent polymer film. Sensors have been built that detect 1ppm of airborne chemicals, such as benzene and toluene, over a wide range of ambient pressures.

FPW sensors employ ultrasonic plate waves – also called Lamb waves – that propagate across the membrane on the chip. The waves travel at several hundred meters per second and typically have a frequency of 5MHz to 10MHz.

The velocity of the waves is reduced when mass is added to the membrane, such as when vapour molecules from the atmosphere become attached to its surface or, alternatively, when the membrane is immersed in a liquid.

To measure the wave velocity, a feedback amplifier is used to make a delay-line oscillator whose frequency is proportional to the wave velocity. The oscillator frequency can be measured to one part in ten million with an electronic frequency counter. As an example, an FPW sensor coated with polymer ethyl cellulose, responded to organic solvent vapours; this device had an estimated detection limit for toluene of 70ppmillion, as shown in Figure 3.

Because the wave speed is lower than the speed of sound in most fluids, the device functions when immersed. FPW sensors have been used to measure changes in the density of aqueous solutions of just 0.0001g/cm3.

To pump or stir fluids in contact with the device, large amplitude waves (up to 200u peak motion) with an RF alternating voltage are used. By a phenomenon known as acoustic streaming, a wave generated by a 10Vp-p voltage can pump air at up to 30mm/s and water at up to 0.3mm/s.

Complete chemical instruments combine FPW fluid pumps and FPW mass-sensitive detectors. Currently, a general-purpose platform for chemical analysis is under development: this will be a fully-contained gas chromatograph (GC) constructed on a 1cm2 silicon chip. This chip will be connected to a credit-card sized external drive and electronic readout.

An evaluation kit is available to prospective users who want to explore the use of plate-wave devices. The company is interested in exploiting this technology through strategic partnerships with companies who can use the technology in mutually-agreed complementary areas.