product review
Solid-State NMR: No Longer the Outcast Liquid- and solid-state NMR spectroscopy have much in common these days. James P. Smith
S
urprisingly, most experts today view the NMR analyses of liquids and solids as essentially identical techniques. In the past, the only common elements between the two techniques seemed to be a large magnet, a probe, and a radio frequency (rf) energy source. Even the two groups of analysts were different— those who specialized in liquid NMR tended to talk like organic chemists, whereas solid-state NMR people had a “physics slant” on things. Now, an analyst needs only to exchange the probe to study either type of sample with the same spectrometer, leaving the probe as the only fundamental instrumental difference between the two methods. Analytical Chemistry last reviewed solid-state NMR in 1996. Since then, the 900-MHz magnet has become a reality, and probe technology has evolved. These developments are bringing “the rest of the periodic table” into the realm of NMR spectrometry. The advent of “push-button” liquid NMR high-resolution experiments has led to similar techniques for the study of semisolid materials like polymer gels and tissues. Table 1 lists selected features of three NMR solid-state spectrometer systems. Readers should contact these companies for more details. Doty Scientific, a manufacturer of a variety of solid-state NMR probes, but not complete systems, is an important contributor to field, but is not listed in the table. Basic solid-state 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, and commercial NMR spectrometers currently plateau at 900 MHz. The cost of the magnet and its infra-
structure can vary between $250,000 and $5 million (USD), which dwarfs the expense of the rest of the system. So the applications dictate the required magnetic field strength, which, in turn, determines the cost and combination of components that constitute the right mix for the job.
J A N U A R Y 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
45 A
product review
Table 1. Representative examples of solid-state NMR systems. Product
Eclipse + NM R
AVANCE Series
M ercury,Unity Inova,Infinity+
M anufacturer JEOL USA, Inc. 11 Dearborn Rd. Peabody, MA 01960 978-535-5900
Bruker BioSpin Corp. 15 Fortune Dr. Billerica, MA 01821 978-667-9580
Varian, Inc. 3120 Hansen Way Palo Alto, CA 94304 650-424-5020
URL
jeol.com/nmr/eclipse
www.bruker-biospin.com
www.varianinc.com
E-m ail
[email protected] [email protected] [email protected] M agnet choices
300, 400, 500, 600, and 900 MHz with bore diam 54-mm bore with 300, 400, 500, 600, 700, 800, and 900 MHz; 89-mm bore with 300, 400, 500, of 54, 51, and 89 mm; 4-, 5-, and 6-mm rotors 600, and 750 MHz with 51- and 54-mm bores; 5-, 6-, 7-, and 10mm rotors for 89-mm bore
Probe types
Doty Probes, MAS, CPMAS, wideline, CRAMPS, MAS, CPMAS, triple resonance MAS, MQMAS, AutoMAS (MAS and CPMAS), TLT Chemagnetics, HRMAS, CRAMPS, wideline, double resonance probes available for all solids experiments MQMAS
Spin rates
4-mm rotor spins as fast as 20 kHz, 5-mm rotor 7-mm rotors spin as fast as 7 kHz, 4-mm rotors Mercury: 5-mm rotors spin up to 13 kHz; 7-mm go to 15 kHz, 2.5-mm rotors reach 35 kHz rotors to 9 kHz; Unity Inova and Infinity+ : ≥30 goes to 16 kHz kHz, many speed/diam options
Channels (rf)
2 channels standard: 100 W high frequency, 300 W low frequency; optional: 500 and 1000 W both channels, 3 and 4 channels
Special features
HRMAS; optional high and low temperature
Offering more information From an NMR spectroscopist’s viewpoint, the main difference between solids and liquids is the mobility of the molecules in the sample. NMR methods excite nuclear spin states with rf energy; as these excited states relax, an rf spectrum is collected. The relaxation of the spins is determined by the internal Hamiltonians, the most common of which are the chemical shifts, scalar coupling, dipolar coupling, and quadrupolar coupling. In the liquid state, the tumbling of the molecules averages the dipolar and quadrupolar couplings, so the spectral response is mainly due to chemical shifts and scalar coupling. The resulting spectrum provides information about short-range effects that help determine the molecular structure. Because the structure is fixed in the solid state, the dipolar coupling, chemical shift anisotropy, and quadrupolar broadening do not average, but remain as strong, long-range forces and hide the more subtle chemical shift and scalar coupling information.
46 A
Up to 8 channels; up to 1 kW with 1H/19F; up to Mercury: 2 channels, 100 W high frequency; 300 1 kW with broadband W low frequency; Unity Inova: 2–6 channels; up to 1 kW high frequency and up to 2 kW low frequency; Infinity+ : 2–4 channels, up to 1 kW high frequency and up, to 2 kW low frequency All magnets actively shielded except for the MAS computer control standard; variable tem750 and 900 MHz; top-loading rotor ejection perature option with range of –243 to 700 °C; and insertion; MAS automatic sample chang- multiple rf frequency and power options ers; fast and accurate phase switching in 0.05°
“The NMR spectra of solids actually contain more information than is available by liquid NMR, but the information is hidden under broad, overlapping peaks with poor resolution,” says Doug Meinhart with JEOL. “Obtaining the information contained in solid-state NMR spectra is usually more difficult than analyzing liquid NMR spectra. You must apply more complicated pulse sequences, use special sample spinning techniques, and apply higher rf energies. The longrange interactions are quite strong, and their analyses require much higher rf energies and very fast sample spinning rates.” To eliminate or greatly reduce the spectral line-broadening caused by dipolar interactions and chemical shift anisotropy, most solid-state NMR experiments rely on magic angle spinning (MAS)—tilting samples to the “magic angle” of 54° 44´ relative to the external magnetic field. At this angle, quadrupole broadening for nuclei with nuclear spins > 1/2 is reduced as well. In liquid NMR, the sample spin rate is in the range of ~30 Hz. With solids,
A N A LY T I C A L C H E M I S T R Y / J A N U A R Y 1 , 2 0 0 2
Mercury: 300 and 400 MHz, 54-mm bore, 5- and 7-mm rotors; Unity Inova, Infinity+: 200–600 MHz, 89-mm bore; 800 MHz with 63-mm bore; 900 MHz with 54-mm bore, 2.5- to 14-mm-diam rotors
the spinning is used to average out some rather strong interactions, so the rate is commonly >10 kHz. This significantly changes the engineering of the probe and requires compressed gas and a turbine rotor. The design, materials, and machining of the probe are all critical to obtaining higher spin rates. At the highest spinning rates, the outside edge of the rotor reaches the sound barrier. “Many people don’t realize that there is a sizable investment in probes and rotors for solid-state NMR,” says Meinhart. “Typically, a solid rotor can cost $500.”
Magnetic field strength Solid-state NMR also uses specially designed wide-bore magnets. “In liquid NMR, the magnets have a bore diameter of 54 mm; solid-state NMR studies require much higher rf energies, so the magnets are usually wide bores—89-mm diameter,” says Werner Maas of Bruker Biospin. “The wide bore allows room for probes that can accommodate higher power requirements and [offers] versatil-
product review
ity for new techniques and experiments.” However, a wide bore can limit magnetic field strength and may cause stray fields. Typically, solid-state machines with 89-mm bores have field strengths of only 300–750 MHz. “Bruker has some dedicated wide-bore solids machines running at the 600-MHz level, and at 750 MHz,” Maas says. “Today, we see a demand for higher field strengths, so we offer systems that can also analyze solids at 800 MHz and even 900 MHz. These must work with standard 54-mm bore diameters and use newly developed probes.” The higher magnetic fields also require a faster spin rate, says Varian’s Evan Williams. The present Varian standard probe, he says, spins at 25 kHz.
On the MAS menu NMR signals are generated by pulsing the excitation rf energy and using a variety of pulse shapes, field strengths, and frequencies to manipulate spins. Crosspolarization (CP) is a technique seldom used in liquid NMR; however, with MAS, it has become the standard solid-state NMR experiment. CP transfers the polarization from abundant nuclei like 1H to less abundant nuclei such as 13C. The result is that spectroscopists can pulse samples at faster rates and reduce NMR FT accumulation times, especially for nuclei with low gyromagnetic ratios. CP takes advantage of the strong dipolar coupling in solids, and works with many nuclei and sample types. Solid-state spectroscopists can select from a menu of other techniques that require specialized NMR probes and pulse sequences. For example, CRAMPS (combined rotation and multiple-pulse spectroscopy) has been used to resolve 1H spectra. Other experiments use double- or triple-resonance techniques to yield twodimensional (2-D) correlation spectra that measure intramolecular distances. REDOR (rotational echo double resonance) is one such approach that has become important in biophysics because it provides distances between carbons and nitrogens. The NMR analysis of gels or rubbery semisolid materials using high-resolution (HR) MAS—a combination of MAS and liquid NMR experiments—has become quite popular. Because the molecules in
these semisolids are spatially disordered and relatively mobile, the spinning rate needs only be a few kHz, and CP is unnecessary. Other than the MAS, the NMR spectra are acquired using standard liquid experiment designs. HRMAS has also become popular for characterizing compounds collected on polymer beads during combinatorial chemistry runs. The molecules of interest are attached to the beads, but much of the molecule is dangling out into the solution. Hiltrud Grondey, manager of NMR Services at the University of Toronto, believes HRMAS should be used more. “For example, the analysis of a polymer does not require complete dissolution,” says Grondey. “Just make a sol with a small amount of solvent, and use HRMAS. The peaks will be as well resolved as those obtained from a solution; and an added benefit is that the polymer structure in the gel more closely resembles the solid-state structure. HRMAS experiments can include the full range of the FT liquid experiments.” HRMAS has also found its way into medicine. “Medical researchers in Boston are examining biopsy tissue by HRMAS as a means of cancer classification,” says Maas. “This data is not available through the old dye and optical microscope methods. The analysis of soft-tissue cancers in a standard liquid NMR probe produces very broad lines and no resolution. But if you spin it at the magic angle, you get narrow lines and high resolution on biopsy quantities.” Another important solid-state NMR technique is multiple quantum (MQ) MAS. This technique generally requires fields of ≥ 500 MHz and spinning rates of ≥ 20 kHz. This technique examines isotopes with nuclear spins of > 1/2—for example, 23Na, 39K, 73Ge, and 27Al. During the past decade, quadrupole NMR studies of inorganic systems have become very popular. High magnetic field quadrupole methods are now driving the inorganic applications of NMR.
Are high fields needed? Williams claims that MQMAS methods at 900-MHz field strengths have “really opened up the rest of the periodic table to NMR.” He adds, “The whole quadru-
polar nuclear area is undergoing a renaissance. Many important quadrupolar nuclei resonate at such low frequencies at the magnetic fields available in the past that you just can’t get at them. By going to higher fields [800–900 MHz], we obtain higher-frequency spectra that we can work with. For example, 39K resonates at 42 MHz in a 900-MHz field, and 73Ge resonates at 31 MHz.” With these very high fields, quadrupolar nuclei produce sharper resonances, which analysts can use to observe different sites. “We recently did some work on 17O in which we were able to resolve 12 different oxygen sites in a mineral,” says Williams. “So, we were able to demonstrate that the proposed mineral structure was correct. The mineral is disordered, so X-ray structure analysis would not provide this information. At 600 MHz, the structure was not resolved; but at 900 MHz, it became very clear.” This capability creates several new applications for solid-state NMR. For example, zeolite catalyst formation and action, the mechanisms of sulfur poisoning of catalysts, and the structures and morphology of minerals and glasses containing alkali metals can all now be studied. Inorganic chemists no longer need perfect crystals to study structures with X-rays. Another new application is for pharmaceuticals. Many solid drugs are polymorphic with different polymorphs showing different properties. Williams argues that 900-MHz systems will allow the pharmaceutical industry to study the role of morphology with drug candidates of 1000 Da or larger. “Some new pharmaceutical patents even specify that solidstate NMR be used as a quality control test of morphology. The [U.S. Food and Drug Administration] is getting interested in the issue,” he says. The higherfield solid-state NMR instruments can also determine tertiary and quaternary structures of proteins bound to solid substrates or membrane surfaces. With all the changes, many NMR experts say that solid-state NMR and liquid NMR have merged, probably because the two were never really separate. James P. Smith is a freelance science writer based in Amherst, Mass.
J A N U A R Y 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
47 A
