product review
High-Resolution NMR Gets Even Better Higher magnetic fields and a new probe promise more resolution and greater sensitivity. James P. Smith and Vicki Hinson-Smith
O
ver the years, NMR has benefited from revolutions in computer processing, digital instrumentation, and precision manufacturing, as have so many other analytical technologies. The FT superconducting magnet NMR spectrometer is now an integral player in the proliferation of drug development and screening strategies, combinatorial chemistry, and protein structure determinations. NMR also stands at the foundation of the search for practical applications of the human genome map. The NMR spectrometer is becoming a more “user-friendly” tool that can work unattended all day, analyzing a steady stream of disparate samples. No other nondestructive analytical techniques can supply the molecular information that is available with NMR. Automated analyses of mixtures from HPLC separations and synergistic combinations with NMR are becoming commonplace (1, 2). Recent developments, such as the cryogenic probe, promise to move NMR into the class of “highly sensitive instruments”. Tissues, polymers, and adsorbed molecules can now be analyzed as easily as liquids. And ever-higher magnetic fields promise greater resolution. Analytical Chemistry last reviewed NMR five years ago. Since then, several improvements in basic NMR system components have been commercialized, and progress has been made in the development of accessories and procedures. This review focuses on high-resolution
FT-NMR systems. Table 1 lists selected features for four FT-NMR products. Readers should contact these companies for more details. Several companies, not discussed here, produce lower performance, continuous-wave instruments. Basic FT-NMR spectrometers include the console, sample probes, a superconducting magnet, and a signal
processing system. The performance level of an NMR spectrometer is commonly specified by its magnetic field strength. Commercial NMR spectrometers currently plateau at 900 MHz.
Automation and combination According to Mark Chaykovsky of Bruker Instruments, the transition from
M A R C H 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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product review
Table 1. Selected features of high-resolution FT-NMR spectrometers. Product
MERCURY and INOVA
AVANCE NMR
ECLIPSE NMR
Company
Varian, Inc. NMR Systems 3120 Hansen Way MS D-300 Palo Alto, CA 94304-1030 650-424-4777
Bruker Instruments 44 Manning Rd. Billerica, MA 01821 978-667-9580
JEOL USA 11 Dearborn Rd. Peabody, MA 01960 978-535-5900
URL
www.varianinc.com
www.bruker.com/nmr/
www.jeol.com
Number of r f channels
Up to 8
2 standard; up to 8 optional
2 standard; up to 4 optional
Probe types
Single- and multiresonance direct and indirect detection, 4-nucleus switchable, automated triple broadband (tuning insensitive to solvent changes), nanoprobe, 60 µL (1/3 -mm) and 120 µL-(1/4 -mm), interchangeable flow cell, and cryogenically cooled probes
Double and triple resonance cryogenically cooled probes; single, double, triple, and quadruple resonance conventional probes; direct and inverse; flow NMR, including LC/NMR and BEST (which allows samples to be transferred directly into the NMR probe and returned); with and without 1- and 3-axis pulsed-field gradients; computer-switchable 4-nucleus probe
Multifrequency 1H standard; options include tunable for 31P to 15N with 1H decoupling, inverse probe, 1H, 19F, low-frequency (15N to 103 Rh), pulse z-gradient, 3-mm microprobe, cryogenically cooled probe
2.5, 3, 5, 8, and 10 mm; 20 mm for wide bore only
5 mm for standard, tunable, inverse, z-gradient and 19F/1H probes; 10 mm for tunable, lowfrequency; 3-mm microprobe; 3 and 5 mm for cryogenically cooled probe
Tube diameters All standard sizes, 60- and 120-µL flow cells
Experiment capabilities 3-D and 4-D
2 channel, multidimensional (Mercury) and multichannel, multidimensional (Inova)
Wide range of multinuclear NMR experiments; biomolec- Multidimensional NMR ular applications for structural genomics and proteomics (multidimensional NMR); LC/NMR/MS, BEST, highthroughput screening
Gradient NMR
Yes
Both 1 and 3 axis
z-axis pulsed-field gradient optional
Shaped pulses
Yes
Yes, 256 kW (1MB) waveform memories
Optional
200–900
Actively shielded magnets: 300, 400, 500, 600, 700, subcooled liquid helium magnets: 750, 800, 900 actively shielded, subcooled magnets; 800
300, 400, 500, 600, 800, 900; shielded magnets standard for 400/54, 500/51, and 600/51 MHz
Bore diams (mm) 51–89
54 and 89
54 standard; 89 and 63 optional
Drift rate (Hz/hr) 2–15
3–10
300 MHz,
