1800" to 2000'C. The sarnples stayed at that temperature for 10 to 20 m n before belng quenched by the cuttng off of power to the furnace. The quench rate was measured to be -500'C per second The samples were subsequently decompressed at a rate of 2 to 3 GPaihour. 17. NMR spectra descrlbed here were collected with a Varan Unity spectrometer operating at 9 L T was a MAS probe from Doty Scentfic (Coumba, SC), wlth 3.5-mm rotors commonly splnning at 9.3 kHz (unless otherwise specifed). To make 27AI analyss as straightforward as possible, a small tip angle ( i n I 6 ) was used in all cases. ' "a and "Al NMR was done w t h the use of delay tmes on the order of 1 s, wlth a spectral band width of 2 MHz. For2"S1 NMR. a smaller spectral band width was used because of the mited chemical shft range in Si; however much longer delay tlmes were used (70 s) because o i the possibility of havng long relaxaton tlmesfor Si speces even w t h a small paramagnetic dopant (Gd20,) [A. Abragam. Principles of Nuclear Magnetism (Oxford Unv. Press, New York, 1961)). We subtracted a 27Al background from the probe by colectng data on an empty rotor under condtions identical to those under whlch the glass samples were run. There was no probe background ~n the 23Na and 23S1 spectra; however, the Si3N, rotors gave a characteristc resonance at -L8.8 ppm relative to tetramethyl silane at 0 ppm with spinnlng sdebands in the sllcon NMR Thls was used as an internal chemical shift cabratlon for '%i NMR. To reference the chemcal sh~ft of ' % l a and "AI, a liquid sample of 1 M NaC (0 ppm) and 1 M ACI, (0 ppm) was run before each spectrum, respectvely. 18. We prepared the ?9S~enriched Ab5,NTS5, by fusing sto~chometr~c amounts of 92%-abeed 29Si0, glass (Cambridge Isotope Laboratory. Andover, MA) with sodium carbonate (Na,CO,), alumnum oxlde (A120s). and 0.1 weght percent gadolnium oxide (Gd20,) at 1200'C for 2 hours. Glass was formed upon removal of the Pt crucble containing the mixed iqud components from the furnace. We did not chemically analyze the sample because of the expense of labeled material and the proven nature of the synthesis process. Gd20s was added to shorten the spin-lattlce relaation time of SI. This sample. along wlth unlabeled glass made under the same conditions, was then sealed In Pt capsules for use in the hlgh-pressure multl-anvl quenchng descrlbed in (76).

Dean L. Olson (first row, right) completed his Ph.D. degree in Analytical Chemistry from the University of Illinois in 1994. He then joined the academic research group of Professor Sweedler performing the first high-resolution and sensitivity studies with nanoliter-volume NMR microcoils. He also conducted the first experiments using high-resolution NMR as a detector for capillary electrophoresis. He is now employed at MRM Corp. (Savoy, IL) conducting research toward the commercial development and application of capillarybased NMR probes. Andrew G. Webb (second row, right) received his Ph.D. degree in Medicinal Chemistry from the University of Cambridge in 1990 under Professor Laurie Hall. He was a postdoctoral researcher in the Department of Radiology at the University of Florida before joining the faculty in the Department of Electrical and Computer Engineering at the University of Illinois at Urbana−Champaign.He is currently an Associate Professor, with a full appointment in The Beckman Institute for Advanced Science and Technology. His research areas include the design of microcoils for NMR of mass-limited samples, the use of MRI temperature mapping for hyperthermia, and human brain mapping using functional MRI. Jonathan V. Sweedler (second row, left) received his Ph.D. degree in Analytical Chemistry from the University of Arizona in 1989 under Professor M. Bonner Denton and then spent 3 years at Stanford with Professors Richard Zare and Richard Scheller developing new methods to study neurotransmitters in individual neurons. He is currently a Professor of Chemistry, Neuroscience, and of the Beckman Institute for Advanced Science and Technology at the University of Illinois. His current research interests are twofold: first, he is developing information-rich methods with improved mass sensitivity for nanoliter-volume samples, including microcoil NMR, mass spectrometric imaging, and capillary-scale separations. In addition, he applies these techniques to understanding the role of neurotransmitter and neuropeptide co-transmission and the regulation of behavior in well-defined neuronal networks of opisthobranch molluscs.

ContentsI. Introduction 739 A. General Background 739 B. Free Radicals and Single Electron Oxidations 742 C. A Cellular Model 742 D. Peroxidase Oscillator Phenomenology 743 II. Biochemical Model 743 A. Nonenzymatic Oxygen Radical Reactions 745 B. Preliminary Reactions 745 C. Principal Reactions 746 D. Compound III Reactions 747 E. Reactions of NAD • and OH • 747 F. Role of Methylene Blue 748 G. Role of 2,4-Dichlorophenol 748 III. Abstract Models 749 IV. Chemically Realistic Models 749 V. Critical Comments and Connection between Models and Experiments 751 A. Critique of Experiments 751 B. Critique of Theories 753 VI. Future Directions 753 VII. Appendix. Useful Constants for Characterizing Reactants Involved in the Peroxidase−Oxidase Reaction 754 VIII. Acknowledgments 754 IX. References 754

An improved nanoliter-volume NMR probe design places the microcoil and capillary at the magic angle (57.7 degrees) with respect to the external magnetic field. Using an NMR probe which requires a total sample volume of just 200 nL, high-resolution 300-MHz 1H-NMR spectra (line width, 0.6 Hz) are presented of 10 mM alpha-bag cell peptide for an observe quantity of 45 ng (50 pmol in 5 nL). For the volume of sample inside the microcoil (the observe volume, Vobs), the 3 sigma limit of detection (LOD) is 9 ng (10 pmol, 2mM) for data obtained in 15 h. To reduce the data acquisition time, a probe with a greater Vobs is developed. As an example of a rapid, mass-limited analysis, a concentration corresponding to 400 ng of menthol dissolved in Vobs = 31 nL (82.6 mM) yields a spectrum in 9 min (LOD = 6.9 ng, 44 pmol, 1.4 mM). To illustrate improvements in concentration sensitivity, a spectrum is acquired in 45 min for 400 ng of menthol dissolved in a total sample volume of 200 nL (12.8 mM). Compared to a commercial nanoprobe for the same mass of menthol, these two examples reduce data acquisition time by at least 95%. Both model compounds demonstrate substantially improved concentration LODs compared to those obtained in previous high-resolution, microcoil NMR work. These advances illustrate the utility of enhanced sensitivity provided by NMR microcoils applied to nanoliter volumes of mass-limited samples.

Recent advances in the analysis of nanoliter volumes using 1H NMR microcoils have led to the application of microcoils as detectors for capillary electrophoresis (CE). Custom NMR probes consisting of 1-mm-long solenoidal microcoils are fabricated from 50-micron diameter wire wrapped around capillaries to create nanoliter-volume detection cells. For geometries in which the capillary and static magnetic field are not parallel, the electrophoretic current induces a magnetic field gradient which degrades the spectroscopic information obtainable from CE/NMR. To reduce this effect and allow longer analyte observation times, the electrophoretic voltage is periodically interrupted so that 1-min high-resolution NMR spectra are obtained for every 15 s of applied voltage. The limits of detection (LODs; based on S/N = 3) for CE/NMR for arginine are 57 ng (330 pmol; 31 mM) and for triethylamine (TEA) are 9 ng (88 pmol; 11 mM). Field-amplified stacking is used for sample preconcentration. As one example, a 290-nL injection of a mixture of arginine and TEA both at 50 mM (15 nmol of each injected) is stacked severalfold for improved concentration LODs while achieving a separation efficiency greater than 50,000. Dissolving a sample in a mixture of 10% H2O/90% D2O allows H2O to serve as the nearly ideal neutral tracer and allows direct observation of the parabolic and flat flow profiles associated with gravimetric and electrokinetic injection, respectively. The unique capabilities of CE and the rich spectral information provided by NMR spectroscopy combine to yield a valuable analytical tool, especially in the study of mass-limited samples.

The principles and parameters to consider when choosing an NMR probe for analysis of a volume- or mass-limited sample are identified and discussed. In particular, a capillary-based microflow probe is described which has a mass sensitivity comparable to cryoprobes (observe volume approximately 40 microL), but with several distinct advantages. The microflow probe has a flowcell volume of 5 microL and an observe volume of 1.5 microL and is equipped with proton and carbon observe channels, deuterium lock, and z-gradient capability. The entire flow path is fused silica; inlet and outlet capillary inner diameters are 50 microm to minimize sample dispersion, making it well-suited to volume-limited samples. An injected sample of 1 nmol of sucrose (0.34 microg in 3 microL, 0.33 mM; MW = 342 g/mol) yields a 1D proton spectrum in 10 min on a spectrometer of 500 MHz or higher. In another example, 15 microg of sucrose (in 3 microL; 15 mM, 45 nmol) is injected and parked in the probe to yield a heteronuclear multiple-quantum coherence (HMQC) spectrum in less than 15 h. The natural product muristerone A (75 microg in 3 microL, 50 mM, 150 nmol; MW = 497 g/mol) was delivered to the flow cell, and a gradient correlation spectroscopy spectrum was acquired in 7 min, a gradient HMQC in 4 h, and a gradient heteronuclear multiple-bond correlation in 11 h. Four basic modes of sample injection into the probe vary in degree of user intervention, speed, solvent consumption, and sample delivery efficiency. Manual, manual-assisted (employing a micropump), automated (using an autosampler), and capillary HPLC modes of operation are described.

No abstract

This letter describes a method for producing conducting microcoils for high resolution proton nuclear magnetic resonance 1tH-NVn) spectroscopy on nanoliter volumes. This technique uses microcontact printing and electroplating to form coils on microcapillaries. Nuclear magnetic resonance spectra collected using these microcoils, have linewidths less than I Hz for model compounds and a limit of detection (signal-to-noise ratio:3) for ethylbenzene of 2.6 nmol in l3 min. @ 1997 American Instirute of Physics.