News
Microchip gets a tip overcome possible interference between QCMs in multiples the channels. They took advantage of the different thicknesses of the etched and Quartz crystal microbalances (QCMs) can nonetched regions. The resonant frequenmonitor slight changes in the amount of cies in those regions differed (10 MHz in material on their surfaces. Most QCMs the wells, and 6 MHz between the wells)) are single-channel devices, which spatially so that the oscillation generated in the average the response. Now, however, etched region could not propagate to the Tetsu Tatsuma, Noboru Oyama, and counetched region. According to Tatsuma, workers at Tokyo University of Agriculno significant interference was observed. ture and Technology and Meidensha "The most important thing is that a mass Corp. (both in Japan) have developed change of a channel does not cause a frethree types of multichannel quartz crystal quency change of the other channels," he microbalances (MQCMs). They describe their work in this issue of Analytical Chem- says. For analyses in electrolyte solutions, the easiest way to avoid interference is istry (p 3632). switching successively from one channel A QCM consists of a quartz disk with to the next electrodes plated on either side. Because the devices are piezoelectric, an oscillating The channel diameter of the current electric field applied across the device indevices is rather large, 8 mm, which limits duces an acoustic wave to propagate the lateral resolution of the device. The through it. The frequency of this acoustic resolution can be improved by making wave depends on the thickness of the deeach channel smaller and thinner. Unforvice and changes with the effective masstunately, a compromise must be struck per-unit area of the device. The mass of a between lateral resolution and frequency thin layer deposited on the crystal can be resolution. "For MQCM with a higher calculated from a measured change in the resonance frequency, the channel can be resonant frequency of the device. made smaller without undue deterioration of the quality factor, although the dynamic The MQCMs operate in a similar fashransre will be limited," says Tatsuma. ion but consist of arrays of resonators fab"Thus, the lateral resolution of MQCM ricated on a single quartz wafer. Each of will be improved in exchange for the S/N the three types has "dents", or wells, with ratio or the dynamic Although it is gold electrodes on the back. The front can have an array of electrodes in wells (the difficult to give the figure for the wells can be the same or various sizes), an lution limit a 150-MHz resonator be array of electrodes in a single large well or 1 5 mm or less We have not vet examined on a flat surface, or a single electrode covthe dependence of the degree of the interering the entire surface. Each of the elecference on the size of the channels and the trodes on the back represents a different distance between them " channel. One channel can be used as a In addition to providing spatial resolureference so that the effects of solution tion, the array can also be used to monitemperature, viscosity, and density can be tor a system using channels with differsubtracted. ent frequencies. A single process can be monitored with both a high-frequency Thus far, Tatsuma and his co-workers channel and a low-frequency channel. have constructed devices with only four The high-frequency channel provides channels. However, he notes, the only sensitivity with a narrow dynamic range, limitation to the number of channels is the whereas the low-frequency channel is size of the plates and channels. less sensitive with a wide dynamic range. Although each of the current devices have four channels, no more than two MQCMs have many potential applicachannels have been monitored simultations. By attaching different receptors to neously. "In the gas phase or nonconducthe individual channels, the array can be tive liquids, in principle, several or more used for multicomponent analysis of mixchannels can be monitored simultaneoustures or as taste or odor sensors using ly," says Tatsuma. "However, in suffipattern recognition techniques. Moreciently conductive liquids, new circuits over, the MQCM with a single electrode should be designed so as to monitor more can perform mass mapping for such apthan two channels simultaneously." plications as corrosion analysis. Tatsuma and his co-workers needed to Celia Henry
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Analytical Chemistry News & Features, September 1, 1999
Electrospray ionization mass spectrometry (ESI-MS) from microchips has been easier said than done. Spraying from the edge of chips decreases, and hydraulic pumping for sample delivery is cumbersome. So Iulia Lazar, Rose Ramsey, and Mike Ramsey at Oak Ridge National Laboratory and Steve Sundberg from Caliper Technologies took another approach. As described in this issue of Analytical Chemistry (p 3627), the researchers attached a nanoelectrospray tip to their microchip and applied a voltage to a fluid reservoir, allowing the sample to go with the flow. 'Typically in nanospray, you don't use any sort of pump or external device to deliver fluid to the tip," says Rose Ramsey. Neither does this group's method, which relies on the tip to draw the fluid through the microchip channel, after a voltage has been applied. The flow rates achieved with this approach (—0.3-0.5 nL s"1) are typical of nanospray, she adds. The stability of the spray is also typical of nanospray with relative standard deviations of —5% for the ion current. In contrast to conventional nanoelectrospray, however, this device has the advantages of microchips. In addition to continuous sample infusion, it can be used for discrete, repetitive analysis of one sample or, with additional sample reservoirs (or other methods of sample accession), for the automated analysis of multiple samples, says Ramsey. In addition, by reserving the main channel for separations and adding an intersecting channel for injections, the researchers already have resolved injected plugs of simple mixtures, such as peptides. Ramsey says this capability has the potential to move LC/MS and CE/MS applications to the microchip domain. The microchip also provides the short analysis times desired for high-throughput applications. The researchers have produced spectra from as little as 340 zmol of a 100-nM solution of gramacidin S in only 10 ms. Such high sensitivity with ESI usually requires much longer integration times, Ramsey notes. "But if you're doing microchip separations, the peak widths are in the millisecond domain, not seconds," she says. "You need to be able to do analyses quickly. And with this hybrid device, we've shown that you can." The sensitivity achieved is due, in part, to the nanospray source and also to the use of a time-of-flight mass spectrometer, which allows signal averaging on short
