Anal. Chem. 1999, 71, 4815-4820
Multiple Solenoidal Microcoil Probes for High-Sensitivity, High-Throughput Nuclear Magnetic Resonance Spectroscopy Y. Li,† A. M. Wolters,‡ P. V. Malawey,‡ J. V. Sweedler,‡,§ and A. G. Webb*,†,§
Department of Electrical and Computer Engineering, School of Chemical Sciences, and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
Two designs for incorporating multiple solenoidal microcoils into a single probe head are presented to increase the throughput of high-resolution NMR. Through a combination of radio frequency switches and low-noise amplifiers, multiple NMR spectra can be acquired in the same time as a single spectrum from a conventional probe consisting of one coil. Since this method does not compromise sensitivity with regard to the individual microcoils, throughput increases linearly with the number of coils. Only one receiver is needed, and data acquisition parameters can be optimized for each sample. Specifically, a four-coil system has been implemented for proton NMR at 250 MHz using a wide-bore magnet, with an observe volume of 28 nL for each microcoil. Signal crosscontamination was ∼0.2% between individual coils, and simultaneous one- and two-dimensional spectra have been obtained from samples of fructose, galactose, adenosine triphosphate, and chloroquine (7 nmol of each compound). A more compact two-coil configuration has also been designed for operation at 500 MHz, with observe volumes of 5 and 31 nL for the two coils. One- and twodimensional spectra were acquired from samples of 1-butanol (55 nmol) and ethylbenzene (250 nmol).
Areas of active research in nuclear magnetic resonance (NMR) include new methods to increase its sensitivity, in terms of detection of small sample amounts and throughput (i.e., the number of samples that can be analyzed in a given time period). Both goals are driven primarily by the pharmaceutical industry where, for example, in combinatorial synthesis, large numbers of very small samples must be characterized. One method of increasing the NMR sensitivity for small samples is to reduce the diameter of the radio frequency (rf) probe to less than 1 mm.1-5 * Corresponding author: A. G. Webb, 4221 Beckman Institute, 405 N. Mathews, Urbana, IL 61801. (e-mail)
[email protected]; (phone)217-333-7480; (fax) 217-244-0105. † Department of Electrical and Computer Engineering. ‡ School of Chemical Sciences. § Beckman Institute for Advanced Science and Technology. (1) Wu, N.; Peck, T. L.; Webb, A. G.; Magin, R. L.; Sweedler, J. V. J. Am. Chem. Soc. 1994, 116, 7929-30. (2) Olson, D. L.; Peck, T. L.; Webb, A. G.; Magin, R. L.; Sweedler, J. V. Science 1995, 270, 1967-70. 10.1021/ac990855y CCC: $18.00 Published on Web 10/06/1999
© 1999 American Chemical Society
Throughput improvements have been achieved largely via the use of “flow-through” NMR,6,7 where pumps pneumatically push successive samples, separated by inert gas spacers, through the coil. To combine these approaches, we have designed a series of probe heads that contain multiple solenoidal microcoils, which operate in transmit and receive mode. The concept of multiple receive coils has been used extensively in magnetic resonance imaging (MRI), where these coils are overlapped to minimize their mutual inductance (and hence correlated noise) in a “phased array” configuration.8-14 Multiple detectors have also been employed in other areas of magnetic resonance.15,16 However, for high-resolution NMR spectroscopy, the coils cannot be overlapped and so must be decoupled from each other to minimize signal cross-contamination. Oldfield17 demonstrated detection of three nuclei (2H, 27Al, 17O) from three different samples in one magnet, with spectral line widths of ∼1 ppm. For high-resolution spectroscopy, the magnetic field homogeneity over each sample must be improved to obtain line widths of a few hertz or less. In the work of Fisher et al.,18 two coils were electically isolated from one another by using a copper ground plane, with the impedance matching networks similarly isolated. Using two independent duplexer/preamplifier stages and two receivers, simultaneous acquisition of two 13C spectra was (3) Olson, D. L.; Lacey, M. E.; Sweedler, J. V. Anal. Chem. 1998, 70, 257A64A. (4) Webb, A. G. Prog. NMR Spectrosc. 1997, 31 (1), 1-42. (5) Subramanian, R.; Sweedler, J. V.; Webb, A. G. J. Am. Chem. Soc. 1999, 121, 2333-4. (6) Keifer, P. A. Drug Discovery Today 1997, 2 (11), 468-478. (7) Keifer, P. A. Drugs Future 1998, 23 (3), 301-317. (8) Roemer, P. B.; Edelstein, W. D.; Hayes, C. E.; Souza, S. P.; Mueller, O. M. Magn. Reson. Med. 1990, 16, 192-225. (9) Porter, J. R.; Wright, S. M.; Reykowski, A. Magn. Reson. Med. 1998, 40, 272-279. (10) Hyde, J. S.; Jesmanowicz, A.; Froncisz, W.; Kneeland, J. B.; Grist, T. M. J. Magn. Reson. 1986, 70, 512-517. (11) Hardy, C. J.; Katzberg, R. W.; Frey, R. L.; Szumowski, J.; Totterman, S.; Mueller, O. M. Radiology 1988, 167, 835-840. (12) Wright, S. M.; Wald, L. L. NMR Biomed. 1997, 10, 394-410. (13) Porter, J. R.; Wright, S. M.; Famili, N. Magn. Reson. Med. 1994, 32, 499504. (14) Carlson, J.; Minemura, T. Magn. Reson. Med. 1993, 29, 681-688. (15) Hammer, B. E. Rev. Sci. Instrum. 1996, 67 (6), 2378-2380. (16) Clark, W. G.; Hijmans, T. W.; Wong, W. H. J. Appl. Phys. 1988, 63 (8), 4185-4186. (17) Oldfield, E. J. Magn. Reson. Ser. A 1994, 107, 255-257. (18) Fisher, G.; Pettuci, C.; MacNamara, E.; Raftery, D. J. Magn. Reson. 1999, 138, 160-163.
Analytical Chemistry, Vol. 71, No. 21, November 1, 1999 4815
Figure 1. Timing diagram showing that the efficiency of multiple coils depends on the ratio of the recycle delay between scans, the time required for the pulse sequence, and data acquisition time (shaded area). The white area corresponds to the delay for T1 relaxation.
demonstrated. In a later publication, the same research group introduced a method termed “Multiplex NMR”19 in which four rf coils were connected in parallel. The four coils were arranged vertically, one above another. Application of a gradient in the z-direction and deconvolution of the spatially dependent frequency shifts in successively acquired spectra yielded simultaneously resolved spectra from four different compounds. While this technique is elegant and simple, it does suffer from some loss in sensitivity due to the broadening of resonances, which results from the application of the gradient, and the need for addition and subtraction of the acquired spectra. In this paper, we introduce an alternative method to obtain simultaneous high-resolution spectra from multiple samples. Solenoidal microcoils were employed for high sensitivity, ease of electrical decoupling, and efficient use of the homogeneous region of the magnet. The technique uses two radio frequency switches with high isolation between channels and is well suited for the acquisition of multidimensional data sets. The data show minimal loss in sensitivity (
