System Combines Capillary Electrophoresis And NMR Spectroscopy

"Capillary electrophoresis and NMR spectroscopy have competing design goals, but by integrating them with MEMS technology we can maximize the performance of both systems," said David Beebe, a professor of electrical and computer engineering and a researcher at the university's Beckman Institute for Advanced Science and Technology. "By using very small channels and samples, we can do high-performance capillary electrophoresis separations. And by performing those separations multiple times, we can collect a large enough sample to do NMR spectroscopy."

Beebe and U. of I. colleagues Richard Magin, Jonathan Trumbull and Ian Glasgow fabricated and tested two microfluidic-NMR devices. In the first device, designed as a proof of concept, the microfluidic channels were built from layered polyimide. In the second device, the channels -- or capillaries -- were etched into glass wafers. In both devices, the single-turn, planar NMR coils were formed from evaporated layers of chromium and copper.

"Although planar coils are less sensitive than solenoidal coils, they are easily integrated into batch-fabricated analytical devices," Beebe said. "The use of MEMS fabrication techniques allows precise control over the geometric parameters and materials that are so important to the performance of NMR microcoil detection. This allows one to more easily optimize the performance."

In order to optimize the weak signal received from small sample volumes, it is very important that the NMR coil does not disrupt the uniformity of the static magnetic field. By tuning the thickness of the evaporated metal layers, Beebe and his colleagues can make the coil transparent to the magnetic field, thus preventing any perturbations in the field.

"By shrinking the sample size to the nanoliter range, the volume over which the magnetic field must be uniform is also significantly reduced," Beebe said. "This should allow either a smaller -- and less expensive magnet -- to be used, or multiple spectrums to be taken in parallel in a conventional magnet, thereby multiplying throughput. With the large initial expenditure for a high-field superconducting magnet and the associated cost of ownership, these benefits are significant with small samples."

The researchers described their microfluidic-NMR system and presented preliminary results at the Solid-State Sensor and Actuator Workshop, held June 7-11 at Hilton Head Island, S.C.