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MICRO MEETS MACRO: Interfacing microchips and mass spectrometers
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icrochips (glass with micrometerscale channels and reservoirs etched in it) provide a way to manipulate small volumes for sample handling or separations. Much like semiconductor integrated circuits—inspiration for the "lab on a chip" m e trend has been toward smaller chips. Optical spectroscoDV usually fluorescence has been the preferred method of detection for on-chip reactions and separations Two recent papers in Analytical Chemistry have introduced a dew option for microchip detection: the mass spectrometer (1,2). Barry L Karger and co-workers at Northeastern University and J. Michael Ramsey and Roswitha S. Ramsey of Oak Ridge National Laboratory interfaced die chips to mass spectrometers as electrospray sources. Although the combination of microchips and mass spectrometers might seem incongruous, the two research groups have shown mat me micro electrospray without an external pressure world of the chips and the source. Karger fabricated a multichannel m 3 fro chip that allowed the introduction of samworld of ples in rapid succession. When the two paMS actually an excellent match pers were published, neither group had The two groups focused on different demonstrated separations with the electroaspects of the electrospray interface. The spray chips, but both groups say mat they Ramseys concentrated on developing the capability of pumping fluid and driving the
Recent research demonstrates electrospray from the planar surface of microchips
are developing separation capabilities for the sample introduction devices. Karger, a relative newcomer to the microchip arena, says that his group, headed by staff scientist Frantisek Foret, has been working in this area for about two years. They presented theirfirstresults at HPLC '96 in San Francisco and then at uTAS '96 in Basel Switzerland. "We felt that what others have been doing is great and there's potential there but they've always been detecting on the chip and tryine' to make it selfcontained Here are eters that have already had an enormous impact on the biotechnolosv ceutical areaa
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instruments are only as good as how you feed them " Both groups say that interfacing the chip to a mass spectrometer was not particularly difficult. The most challenging aspect was determining whether a stable electrospray could be generated from the planar surface of the chip edge. Unlike typical electrospray interfaces, the chips do not incorporate needles for the spray, which exits the channel at the planar edge of the chip. "We just didn't know whether the flat surface would work," says Karger.
Analytical Chemistry News & Features, June 1, 1997 3 5 9 A
Focus plied to generate the electrospray or provide the bias necessary to drive electrophoresis. We decided to use aT-structure where the potential for generating the electrospray and for electroosmotic pumping is generated at the side arm." The potential drop between the two reservoirs of the T-structure governs the pumping of materials, and the potential applied to the side arm drives the electrospray. Toward higher throughput
Figure 1. A schematic of the side-arm chip for electroosmotic pumping.
"We didn't know whether we needed a tip or not. It was very interesting that the flat surface worked so nicely." Both groups were able to demonstrate a stable electrospray. The Oak Ridge group accomplished this without an external pump; Karger used a syringe pump but says that his group has also generated electrospray with electroosmoticflowand gas pressure. No pump required
The Ramseys emphasized the electroosmotic pumping on the chip that eliminates the need for an external pressure source. They accomplished this by adding a side arm to the cross-structure normally used for electrophoretic separations on chips. The side-arm channel runs from the primary separation channel to a reservoir at the edge of the cover slip enclosing the network (Figure 1). If the channels were composed of native glass, thefluidflowed down the side arm rather than to the channel opening at the edge of the chip. To induce the liquid toflowtoward the channel opening, the interfacial characteristics of the channel network were made heterogeneous by coating the side arm with linear polyacrylamide. This modification increased the surface viscosity, reduced the electroosmoticflowinto the 3TTT1 311(1 creflted a pressure that allowed the liquid to flow toward the electrospray interface. Roswitha Ramsey explains, "We did not want to apply any sort of conductive coating to which a potential could be ap-
Karger believes that the chip greatly increases the productivity of mass spectrometers at a time when throughput is becoming more of an issue. "The need for high-throughput mass spectrometry is increasing tremendously by virtue of the large number of samples that have to be analyzed for combinatorial libraries and drug screening. The trend is to go to faster mass spectrometers, and it's obvious that there's a tremendous potential for fast separations and infusions If we can't do high-throughput analysis properly we're not fully using the instrument" The throughput was increased by constructing a multichannel device—nine channels instead of one. A schematic of Karger's chip-MS interface can be seen in Figure 2. To avoid electric field crosstalk between the channels, a 6-mm gap was
left between them. He says that they have since reduced the distance between channels to < 1 mm. They tested the performance of the microchip in several ways and explored the reproducibility of ESI-MS spectra from different channels by obtaining spectra of 0.6 uM myoglobin from five channels in succession. Similar spectra were acquired from the different channels, and the molecular mass was within 0.2% of the known value each time. The real test of the sequential capabilities of the chip was in rapidly analyzing five different samples: recombinant human growth hormone (once in water, once in 75% methanol), a mixture of ubiquitin and recombinant human growth hormone, ubiquitin, and endorphin. Each sample took only two minutes, and it was not necessary to optimize the instrumental conditions between samples. Jed Harrison of the University of Alberta agrees that the chips offer an excellent opportunity for increasing the throughput of mass spectrometers. "If you take a separation system and spend a halfhour separating the sample before delivering it to your expensive mass spectrometer, it's a poor expenditure of money. If you can run your separation in a few seconds or minutes and get it into the mass
Figure 2. A schematic of the multichannel electrospray chip interfaced to a mass spectrometer.
360 A Analytical Chemistry News & Features, JunJ 1, 1997
spectrometer, you get more samples per hour out of your instrument." Reversing a trend?
The trend in chip technology has appeared to be toward smaller devices—the lab on a chip or the "micro total analysis system". With such a push toward miniaturization, adding a mass spectrometer to the system would seem to be a gear-grinding shift into reverse. But is it really? The scientists who work with these chips don't think so. Mike Ramsey says that he was originally uncertain of the value of interfacing microchips with MS, because such a pairing seemed to contradict his vision of the chips for chemical sensing applications. However, he was convinced that integration ratiier than miniaturization is what has actually added value to the chips. For example, integrating the injector on the chip allows better control over injection than with a free-standing capillary. "We're trying to apply the microelectronics paradigm" he says referring to the ability of semiconductor manufacturers to integrate increasing functionality on a single chio "If you would like to have a number of functional elements [on a sintrle chinl electrospray becomes an additional element and thus Tnro\rirflpcl g r e a t e r c h e m i c a l prnrpQcinir nnwer. "
Michael R. Knapp, vice president of science and technology at Caliper Technologies (Palo Alto, CA), which is developing chips for biochemical analysis, says that they have intentionally left the detection apparatus off the chips to keep them affordable. He says that they are working to interface chips to mass spectrometers in the belief that the chips will improve detector throughput and will provide novel sample-handling capabilities. Andreas Manz of the Imperial College of Science, Technology, and Medicine (U.K.) sees the current developments as being right in line with his vision for microinstrumentation. "I've been fighting for the fact that you should only miniaturize when you profit from the miniaturization." His view is that performance should be the paramount consideration. "Maybe I have a strange view of miniaturization but I see it guided strictly by performance and nothing else" Harrison echoes Manz: "The point is that the parts that have to be [small] are small. With microdevices, you get small
volumes, short distances, and high speed. It doesn't mean that the system that goes around the microchip has to be small in order to achieve the advantages of the microdevice." Manz and Harrison point out that despite the vastly different physical dimensions of the chip and mass spectrometer, the two instruments are actually well suited to one another. "They're coupled very well in terms of the parts that count, which are the fluid flow and the volume of fluid delivered through the microchannels," says Harrison. Potential uses
When asked where such a device might be useful, the answer on everybody's lips is "anywhere people want to use electrospray." Asked to be more specific, the Ramseys and Karger all suggest drug dis-
The chip and mass spectrometer are coupled very well in terms of fluid flow and volume. covery and combinatorial libraries as potential applications, but Roswitha Ramsey indicates that they had been interested in developing a technology with no specific application in mind. "The nice thing about the microchips is that you can design and fabricate virtually any structure you can think of," she says. In addition, Karger sees potential uses for the microchipESI-MS in pharmacokinetics studies and mutation analysis. "It's a generic system tiiat will allow automated feed into the mass spectrometer It's potentially
