Product
Review
OLD RULES CRUMBLE Atfirstglance, solid-state NMR is an intimidating technique. Complete new instruments start at around $300,000, and prices can soar to more than $1 million for high-end systems. New methods may require sophisticated modifications of hardware or reprogramming of software for novel NMR pulse sequences. Indeed, many of the leading experts in this arena are high-powered solid-state physicists or engineers who have built and modified their own instruments. 'Traditionally, solid-state NMR has been dominated by people who are expert in the extreme," says Joseph DiVerdi, director of business development for Otsuka Electronics USA, "but that is no longer intrinsic to the business, science, or technology." A similar view comes from Gary Turner, president of Spectral Data Services (Champaign, IL), whose company provides NMR spectra to outside customers. "Solid-state NMR is not a specialized technique the more you do it the better get at it." Like a tree taking root, solid-state NMR sDectroscopy is moving toward
With a broad range of options, buyers design the enstrument to match the application more routine applications, as well as new state-of-the-art techniques. Researchers lacking detailed knowledge of the hardware "can learn how to work the technique, and they can do it well. Just as in any analytical technique, you need to understand the technique to understand the results," adds DiVerdi. Routine applications include the investigation of polymers, catalysts, petrochemicals, zeolites, and biological molecules. A new, potentially major market supports scientists who are generating libraries of compounds using combinatorial
chemistries. One popular approach to combinatorial chemistry involves the synthesis of compounds on solid beads or resins. These rotationally restricted compounds can then be identified by solidstate NMR. "The market is so strong [for combinatorial chemistry] that it is demanding its own machines," says David Rice, a solid-state NMR applications chemist with Varian Associates. "It requires us rethinking how we do NMR spectroscopy in order to meet the throughput requirements." Academic experts like Lynn Jelinski, director of Cornell University's Center for Advanced Technology and Biotechnology and a professor of engineering, also find researchers exploring new applications that require modification or reworking of instruments. "This is stiil a field where you can make advances in the laboratory," she says. Solid-state NMR has matured so that the four manufacturers listed in Table 1 are marketing their systems to appeal to these diverse communities. They offer a slew of options and entry points for purchasers. According to the experts Analyti-
Analytical Chemistry News & Features, September 1, 1996 5 5 9 A
ments use double- or triple-resonance techniques to yield two-dimensional 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 data are so accurate and the method so powerful that it is getting popular," says Jelinski. Spectra for quadrupolar nuclei are narrowed by double-rotation techniques that simultaneously spin samples about two axes (using one rotor spinning inside another). 'Technically, this is very hard to do," says Gerstein, and is, therefore, an expensive commercial probe. Dynamic angle spinning accomplishes the same feat by spinning first at one angle and then switching quickly to a second angle. However, researchers in the United States and in France have recently found a way to use a conventional MAS probe to produce similar results for quadrupolar nuclei, according to Gerstein. This evolution illustrates the rapid pace of change within this field and the costs of staying on the leading edge. Gradient NMR, a new technique that reduces spectral acquisition time significantly, has just been introduced into the solid-state NMR community after gaining popularity with solution NMR spectroscopists. Other specialized solid-state applications include NMR imaging and wide-line NMR that bypasses MAS and uses the broad anisotropic spectra to derive nuclear distances. Gerstein points out that it takes considerable skill in rf electronics and in machining to build these sophisticated probes for the different NMR techniques. Few research groups are equipped to take on this task. Fortunately, there are many
In fact, Jelinski runs sophisticated experiments with "sample-limited" biomolecules in an "old" narrow-bore magnet that includes a variable-temperature MAS probe with three rf channels. Her group has worked with Doty Scientific to compensate for the lack of bore space and to optimize the performance of the probe. "We have very small rf coils and rotors and a good signal-to-noise ratio." As a compromise between wide and narrow bores Otsuka offers magnets with a 62-mm bore. Although there is no such thing as an average solid-state NMR system, probably the most common instrument in use today operates at around 400 MHz. Jelinski's homebuilt instrument is a 360-MHz spectrometer. Turner's company has two homebuilt instruments: a narrow-bore at good commercial probes, although Ger270 MHz and a wide-bore at 360 MHz. stein warns, "The radio frequency homoGerstein recently purchased a wide-bore geneity is not glorious for all probes." at 400 MHz "optimized for everything we want to do" and acquires XH solidThe wide and narrow of state spectra with a 100-MHz system. magnets Manufacturers say that 500-MHz instruProbes are only a fraction of the cost of a new NMR instrument. By far, the greatest ments with wide-bore magnets are becomcost is the magnet—so much so that man- ing increasing popular. ufacturers offer solid-state as an "add-on" Researchers probing quadrupolar nupackage to their high-resolution liquid clei, such as 27A1 or n B, usually prefer the NMR systems as a way to leverage cushighest field they can afford. This commutomers' purchasing dollars. Add-on sysnity is buying systems with instruments tems can require as little as a CP-MAS that run at frequencies as high as 800 NMR probe and an MAS control unit, add- MHz. On the other hand, spectroscopists ing less than $50,000 to the price of a new running routine CP-MAS experiments NMR instrument. In fact, JEOL's solidwith relatively small molecules or simple state package is marketed only as an acsystems can oftenfindhappiness with a cessory for its Eclipse+ spectrometers. narrow-bore magnet with frequencies of Otsuka (formerly Chemagnetics) which 200-300 MHz. is owned by the Otsuka Pharmaceutical Company of Japan and is the only of A turn for the better the four manufacturers that began as an In addition to spinning angle, spinning exclusively solid-state NMR company rate plays a role in narrowing linewidths. recently jumped onto this bandwagon by To eliminate all broadening interactions, unveiling its own NMR system for liquids the spinning speed must exceed the static As expected, magnet costs rise dramat- linewidth of the resonance. Top speed in a ically with the magnet's field strength. commercial instrument is called "superThe other important cost variable is the sonic spinning," which runs from around magnet's bore size. "You want the widest 19 kHz and goes up to about 25 kHz. This possible bore to put all kinds of gadgets is a significant technological achievement into," says Jelinski, citing a common rule considering the stress put on the system of thumb for solid-state spectroscopists. and the need for stable, reproducible valWide-bore magnets are generally defined ues Without giving away any company as those with a bore size of 89 mm or secrets, Otsuka's DiVerdi predicts that larger. "those numbers going to However, our experts say that most rou- higher." Indeed conventional wisdom for tine MAS experiments can be run success- solid-state NMR has been spin and spin fast fully with a "narrow" 51- to 54-mm bore magnet. "If you are just doing CP-MAS, Yet several experts mentioned a new combinatorial chemistry, rubber, and polytechnique developed at the University of mers, you sacrifice nothing going with a Utah called magic-angle turning (MAT), narrow-bore magnet," says JEOL USA's which spins samples as slowly as 40 Hz Meinhart. (Hu, J. Z.; Alderman, D. W.; Ye, C; PugAnalytical Chemistry News & Features, September 1, 1996 561 A
Product Review
Table 1 . Summary of representative products
Product Company
AVANCE DSX Bruker Instruments Manning Park 29 Fortune Drive Billerica. MA 0182-3991
Eclipse* JEOL USA, Inc. 11 Dearborn Road Peabody, MA 01960 508-536-2310
Chemagnetics Infinity Otsuka Electronics USA, Inc. 2607 Midpoint Drive Fort Collins, CO 80254-4417 800-4-OTSUKA
"N'TVINOVA Varian Associates, Inc. 3120 Hansen Way Palo Alto, CA 94304-1030 415-493-4000
URL Probes
www.bruker.com CP-MAS. laser-heated CP-MAS. double and triple resonance. CRAMPS, double rotation, wideline, magic-angle turning, pulse-gradient self diffusion, gradient MAS
www.jeol.com CP-MAS option for liquid high-resolution NMR
www.otsuka.com CP-MAS, gradient spectroscopy, double and triple resonance, variable-amplitude CP, wideline, CRAMPS. supersonic spinning, double rotation
200, 300, 400, 500, 750, 800
300, 400, 500, 600
2 standard; up to 8 up to 1 kW
2 standard; up to 4 up to 1 kW
200, 300, 360, 400, 500, 600, 750 2 standard; up to 8 up to 1 kW
www.nmr.varian.com CP-MAS, supersonic spinning, double and triple resonance, wideline, double rotalion, double angle, CRAMPS, high-resolution MAS for combinatorial chemistry, hightemperature MAS. gradient self diffusion 200, 300, 400, 500, 600, 750, 800, > 900 2 standard; up to 8 up to 1 kW
Magnet Field strength (MHz) Channels (rf) Channel power Computer Hardware Operating system Special features
Sun workstation UNIX Fastest spinning, easy-to-use temperature-control MAS, rf architecture combines highspeed multiple pulse-type spectroscopy with variablephase fine-phase shift spectroscopy, multiple viewports in software 403
Sun SPARCstation UNIX High-performance rf in console makes solid NMR add-on to any system easy, rf design simplifies introducing new experiments, all magnetic fields and bore sizes supported, ISO 9001 company 404
508-667-9580
Silicon Graphics workstation UNIX High level of automation. pneumatic insertion/ejection of rotors, solid-sample changer, sophisticated probe tuning (built-in sweeper), 50-ns phase switching, intuitive pulse programming, ISO 9001 company Reader service number 401
cal Chemistry consulted for this article, the application determines which combination of components constitutes the right mix. Thus, this month's table offers few technical entries. Readers can find a more complete table of features in the Product Review on routine NMR published in the Sept. 1,1995, issue (pp. 559 A-563 A)) In addition, Table 2 lists swo specialty companies that are rated highly by our solid-state experts. Doty Scientific, which began operation in 1992, builds a variety of NMR probes for solid-state experiments. Tecmag, a 13-year-old company, provides data-acquisition systems and related radio frequency (rf) subsystems as complete systems and kits. Productsfromthese companies are used for homebuilt NMR systems or for upgrading commercial instruments. The right choice
So what is the key to choosing the right solid-state spectrometer? "You want to make sure that you know what experiments you are going to do," notes solidstate NMR expert Bernard Gerstein of Iowa State University. "The implication is that you have a background in the field. Go at it with knowledge." 560 A
Analytical
Chemistry
Silicon Graphics workstation UNIX Low-cost probes, costeffective NMR package
402
Gerstein's advice of attacking this market with knowledge was echoed by other experts. "There are people who would like to buy [solid-state NMR] systems and treat them as black boxes, but that is usually a recipe for disaster," warns Doug Meinhart, solid-state NMR product manager at JEOL USA. "There is now a much wider range of applications, and that has tempered the conventional wisdoms [about solid-state NMR]," says Doug Burum of Bruker Instruments. Indeed, although purchasers do not need to be NMR gurus, savvy buyers should be able to do more with less costly systems. Similar to the way many consumers purchase personal computers, users "over-buy" (or underutilize) their instruments, according to our experts. "Solidstate NMR instruments are incredibly powerful compared with the knowledge and creativity of some of the people using them," Gerstein says. A choice of experiments
As Gerstein advises, begin with the experiment. The vast majority of solid-state NMR experiments involve "magic-angle" spinning (MAS) of samples at 54° 44' rela-
News & Features, Septembrr
1, 1996
tive to the external magnetic field. At this angle, spectral line broadening caused by dipolar interactions and chemical shift anisotropy is removed or greatly reduced (as the 3 cos2 6 -1 factor in the relevant equations goes to zero). Quadrupole broadening for nuclei with nuclear spins > lh is reduced aa well. In addition, most spectrometers combine MAS with cross-polarization (CP), which exchanges polarization between *H nuclei and the nuclei of interest, such as 15 N or 13C. This technique allows spectroscopists to pulse samples at faster rates and to reduce NMR FT-accumulation times (especially for nuclei with low gyromagnetic ratios). CP-MAS, therefore, has become the basic solid-state NMR experiment. (The nucleus most often studied, say the experts, is 13C with a nuclear spin of V2.)
However, like their colleagues working with liquid NMR, solid-state spectroscopists can select from an arsenal of other techniques requiring specialized NMR probes and pulse sequences. For example, CRAMPS (Combined Rotation and Multiple-Pulse Spectroscopy) has been used to resolve JH spectra. Other experi-
Product Review
mire, R. J.; Grant, D. M.J. Magn. Reson. A, 1993,105,82). "[As a technical achievement], it has been one of the 'holy grails' of NMR" says Bruker's Burum. This two-dimensional technique restores some of the anisotropic information lost through MAS, yielding the isotropic spectrum and the chemical shift anisotropy. "It could make many MAS experiments, which aren't limited to less then 10 milligrams, obsolete," says Gerstein. Decoupling of XH spins from the nucleus being observed is also a common feature of solid-state NMR experiments. The decoupling narrows and simplifies the spectra. Although commercial instruments offer up to 1 kW in decoupling power, that much power is usually not necessary. "You only need a kW if your probes are poorly designed," argues DiVerdi. Many standard experiments require only 100-150 W for protons and 300-500 W for carbon. Rigid samples and some specialized experiments, such as REDOR and NMR of quadrupolar nuclei, need decoupling power.
However, without exception, our experts warned that these decoupling powers do not necessarily translate into performance. "What you really want to know is how much decoupling power is delivered to the sample—not on an oscilloscope, but rather to your sample," says Jelinski. Universally, our experts recommended taking your samples for a test spin on prospective NMR probes as the best way to judge quality. The quality of the rf design is another important feature. Near-continuous phase setability, amplitude and phase shaping, fast phase switching times, and as much phase and amplitude stability as possible are all desirable features, says Burum. These characteristics are key to determining whether a spectrometer can handle the full range of solid experiments or whether it will be suitable only for routine CP-MAS applications. The last element of the NMR instrument is the computer system. All the instruments come with workstations that allow experimenters to work with the data
There's No Mystery In How Pyrolysis Makes Sample Analysis More Productive Versatile CDS® Pyroprobes® Dramatically Reduce Sample Preparation Time By... • Providing Direct Interface with GC, MS, FTIR, PurgeATrap • Eliminating Lengthy Solvent Extraction Times • Eliminating Need for Sample Derivatization • Direct Analysis of Complex Samples: Vulcanized Rubber, Printed Circuit Boards, Textile Fibers, Biomolecules, etc. • Automated Unattended Analysis of Liquid and SOLID samples by GLP Protocol
FOR INFORMATION CALL 1-800/541-6593 Phone: 1-610/932-3636 • FAX: 1-610/932-4158
CDS '.EADERS
IN SAMPLE TECHNOGYGY
NALYTICAL INC. PREPAPAATON ...SINCE 1969
APPLICATIONS NOTES AVAILABLE: Food, Materials & Composites, Inks, Paint, Packaging, Paper, Petroleum & Chemicals, Plastics, Rubber, Cosmetics, Forensics...And More
CIRCLE 12 ON READER SERVICE CARD
562 A
Analytical Chemistry News & Features, September 1, 1996
even as samples are being run. Our experts gave high marks to the current commercially available software. "What you are looking for in NMR software is ease of pulse programming, because you want to be able to read something in the Journal of Magnettc Resonance ana then whip up the pulse sequence quickly," says Jelinski. Computer systems are also easily upgraded. Is change good?
Given the constant pace of change in this field, how can purchasers ensure that their very expensive, new NMR system will do what they want 10-15 years down the road—the typical lifetime of a NMR instrument? Gerstein tackles that difficult question in two parts. The rf part of an NMR involves a pulse with a width and phase that require pulse-timing and digital-phase resolution. "Researchers are not developing experiments that use superfast timing or digital resolution," says Gerstein. "They are using what is on the machines. So if you get a modern machine, you are protected with the rf." The other aspect is that samples undergo a physical motion that can be complex. Here, there is no protection as new methods develop. "You may have to buy a probe to do something different," Gerstein explains. Manufacturers claim that current instruments can be upgraded with modular components. The biggest change in NMR spectroscopy may be a merging of the worlds of solution and solid-state NMR As researchers tackle bigger molecules and systems, such as lipid bilayers and tissue samples, or examine constrained molecules, such as the combinatorial chemistry materials on beads, they deal with systems that fall somewhere in between solids and liquids. These are mobile molecules, but not so mobile as to produce isotropic spectra in solution. "Solid-state experiments are becoming competitive with the kind of biomolecular experiments done in liquids," points out Varian's Rice. "There is an enormous field opening up called highresolution MAS," says Burum, citing potential studies of entire cells and applications in pathology. "It is a new combination of existing equipment: standard bore magnet, high-resolution spectrometer—almost any field—and some adaption of MAS probes to get < THz resolution. In some ways this could dwarf the rest of NMR." Alan Newman
