Breif Introduction to History of NMR
This article is by Dr.
Howard is Editor, American Laboratory News.
Click Here for
a Printable one page PDF file
Nuclear magnetic resonance (NMR)
spectometry has reached a new mile stone with the recent introduction of a
900-MHz instrument by Varian Inc. (Palo Alto, CA). We met with Dr. Ray Shaw,
Vice President and General Manager, Varian NMR Systems, to discuss the
tech-nology underlying this latest advance. The follow-ing is a summary of our
conversation. Va r i a n was a pioneer in NMR spectrometry. The model A60 NMR
spectrometer introduced in 1960 was the instrument that transformed NMR into a
routine laboratory measurement. Needless to say it was a very successful product
for the company. With this instrument, the structures of the simpler organic
compounds could be determined from NMR spectra. Previously, structure
determinations were inferred from complex, laborious bench work. It was a
turning point in the progress of organic chemistry. Of course, we have other
structural tools, but none is quite as versatile and powerful as NMR. X-ray
diffraction is a potent structural tool, but the sample must be crystalline.
Mass spectrometry also provides structural information, but it is a vacuum
device that destroys the sample, whereas NMR provides structural information on
the substances in solution and can also perform solid-state NMR. In the case of
biologicals, it is important to make measurements in the native chemical
environment of the species. The heart of the NMR spectrometer is the magnet.
This is a core technology, and advances in NMR spectrometry are closely linked
to our ability to build more powerful magnets. The first NMR magnets were either
permanent magnets or electromagnets, but the heat generated by the current
flowing through the electromagnet coils limited the performance of these earlier
magnets. Accordingly, the introduction of superconducting magnets was a
breakthrough in
magnet design and a major advance for NMR spectrometry.
Varian’s superconducting magnets, manu-factured exclusively by Oxford
Instruments ( Ox-ford, U.K.), are cooled in liquid helium. Once set up, the
field decay in a superconducting magnet is so low that it can operate for years.
However, special fail-safe circuitry is built into the magnet in case of
accidental quenching of the superc o n d u c t i v i t y. If uncontrolled
quenching occurs accidentally, the surge of heat generated as coils return to
conventional ohmic resistance can cause a dangerous boil-off of the liquid
helium coolant and the resulting heat may damage the magnet’s solenoid. The
magnet can quench as a result of a lack of cryogens, which need to be added
periodically, accidental mechanical shock, or an earthquake—an ever-present
possibility at Va r i a n ' s head office in Palo Alto, CA, close to the
San
Andreas Fault. An NMR spectrometer is characterized by the fre-quency at which
the proton absorbs in the field of
the magnet (900 MHz in the case of this
new instru-ment). Equally, one could specify the magnitude of the magnetic
field, which in this case would be about 21 Tesla. Needless to say, the cost of
the in-strument increases dramatically as the strength of
the magnet
increases. An NMR with a magnet rated at 200 MHz has a field strength of 4.7
Tesla and costs about $150,000. The recently introduced 900-MHz spectrometer
costs several million dollars. The drive for higher magnetic field strength is
related to sensitivity issues. NMR is a technique that is data-rich, but not
inherently sensitive because the energy differences between the energy levels of
the protons in the magnetic field are very small. Increasing the strength of the
magnetic field increases the energy difference and accordingly the sensitivity
of the NMR measurement. Doubling the magnetic field almost quadruples the
sensitivity of the measurement and shortens the time to perform the experiment.
Another advantage of higher-field magnets is increased dispersion, which enables
the many resonance lines in the spectrum of a large biological molecule to be
differentiated from one another. NMR is the tool of choice for determining the
structure of biomolecules. Larger biomolecules, with molecular weights up to 200
kD, can be studied with 900-MHz NMR, providing structural infor-mation
on
some quite large proteins, glycoproteins, lipoproteins, and nucleic acids.
Increasing the sensitivity addresses the follow-ing
additional problems
dictated by the nature of samples :
1. Useful measurements can be performed
when very little sample is available.
2. Measurements are possible when a
sample is not very water-soluble.
3. Measurements may have to be performed
at low concentration because the substance may aggregate at high concentration.
4. For meaningful measurements, it may be necessary to closely mimic the low
concentration of the native environment of the species Clearly, NMR has a major
role to play in advancing our knowledge in the life sciences. So what are the
factors to be overcome to advance beyond the current state of the art? Advances
are needed in two critical technologies, according to Dr. Shaw: magnets and
probes. Varianuses NMR magnets from Oxford Instruments, a world leader in magnet
technology.
Magnets for Va r i a n ’ s imaging activities are also from
Oxford Magnet T e c h n o l o g y , a joint venture be-t
w e e n Oxford
Instr u m e n t s and S i e m e n s . I m p r o v-ing the design of an NMR
magnet is a daunting chal-lenge.
A high-resolution NMR magnet needs a very
uniform field that is constant over time and over the distance occupied by the
sample. As current is intro-duced into a magnet, there are powerful outward f o
rces on its windings. In the case of a superc o n d u c t-ing magnet, these
forces are very strong and the mag-net windings need to be resin impregnated to
glue them together and avoid them being torn apart. In addition, the outer
windings are sometimes held in place with a steel corset. Not surprisingly, the
cost of the magnet is a major part of the overall cost of the machine; for a
500-MHz machine, it is only 30% of
the cost, but this figure is considerably
higher in the case of the 900-MHz system. The magnet “at field” has a lot of
energy stored in it, typically 15 MJ for a 900-MHz magnet. If the superconducti
vity is quenched for any reason, this energy is issipated as heat. The result is
dangerous, rapid boiling off of the liquid helium coolant. Special auxiliary
circuits are built into the magnet to dissipate the energy in the event of
quenching to avoid risking damage to the magnet. Stray fields are an
occupational safety con-cern with magnets, especially at the field strengths
achieved by 900-MHz magnets. Active shielding, which has been used to contain
these stray magnetic fields, becomes harder as the magnet gets stronger, noted
Dr. Shaw. However, he is confident that this is a problem that will be solved in
the near future as it has been for less powerful superconducting magnets.
G
e n e r a l l y, OSHA requires that the work environment not have a magnetic
field of over 5 Gauss. This man-dates
an exclusion zone of 40 ´ 40 ft around
a 900- MHz magnet that does not have any active shielding
to neutralize the
field, so the siting of these large sys-tems is no trivial matter.
We asked
Dr. Shaw to explain the factors that limit increasing the field strength of
superc o n d u c t-ing
magnets, clearly desirable to improve the
perfor-mance of NMR instruments and explore larger and
larger biomolecules.
At this time, he predicted it will be possible by fine-tuning to reach 1 GHz
with cur-rent
materials technology, but beyond this point sig-nificant
advances in wire technology would be
needed. Conditions for superconduction
become critical at fields as high as 30 Tesla. As the magnetic
field
increases, a given superconducting wire be-comes less comfortable being a superc
o n d u c t o r. Re-ducing
the temperature may offset this, but the
op-portunities for doing so are limited. Liquid helium is
already quite
close to absolute zero at about 4 deg K. A so-called pumped magnet operates at
reduced pres-sure
to reduce the temperature of the liquid helium. Thus,
cooling to the temperature of a secondary
phase of liquid He at 2.2 deg K is
possible. Operating at even lower temperatures is possible, but using this
phase of liquid helium is a problem because it has such a low surface
tension that it tends to creep out
of its container. The magnet is made from
many miles of wire. It must conform to stringent quality
controls regarding
composition and dimension, and joints between segments must be as close to
perfect
as possible. Generally, more than one type of wire is used in a
magnet, with the inner coils having a dif-ferent
composition from the outer.
From time to time, we hear about progress in the laboratory in su-p
e
rconducting materials, high-temperature super-conductors, and so forth. But
until these materials
can be manufactured to commercial standards for
quality and reproducibility, they cannot be consid-ered
for applications in
commercial magnets. Thus, D r. Shaw sees 1 GHz as about the limit for now. How-e
v e r, he pointed to other areas where progress can be achieved in which Va
r i a n is investing heavily, areas
such as probe technology and software
and acces-sories for techniques such as Fourier transform (FT)-NMR.
To d a
y, Va r i a n is enjoying success at the high end of the market, but not
everyone requires or can
afford a 900-MHz NMR. However, Dr. Shaw expects
some of the technologies that are being developed at
this high end of the
market to eventually trickle down to benefit the company's other NMR models.
Imaging Another area that is of major interest to Va r i a n I n c . is NMR
imaging, today one of our most impor-tant medical diagnostic tools. NMR imaging
was, in fact, pioneered by Va r i a n , but the routine clinical di-agnostic
market employing large, whole-body ma-chines is now dominated by companies such
as G e n-eral Electric and S i e m e n s . Va r i a n , h o w e v e r, remains a
major player in the research imaging market. In general, NMR imaging looks at
protons in the water or fat in the body. In imaging, the spectrometer is able to
identify the location and amount of the pro-tons and from this information
produce a gray-scale image of the object. This phenomenon is the basis of
magnetic resonance imaging (MRI). Va r i a n o f f e r s systems for imaging of
small objects or animals, pro-ducing high-resolution images, 1/50- 1/100 mm. On
the human body front, General Electric and S i e m e n s make MR systems for
high-throughput routine diagnostic applications, whereas Va r i a n is a world
leader in the manufacture of machines with more open configurations, and at
higher field strengths, as re-quired for human research applications. Magnets
used for this type of experiment have field strengths in the region of 3 and 4
Tesla or higher. Physiological and neurological research and cognitive studies,
of-ten referred to as functional imaging experiments, have benefited greatly
from the introduction of Va r -i a n ’ s NMR imaging instruments. For example,
it has been possible to locate and study the regions of the brain that are
responsible for different body functions. Using NMR, it is possible to track
physiological activity in the brain employing the BOLD effect, in which blood
flow to a certain region of the brain in-creases when that region becomes
active. The research instruments from Varian, operating at higher fields than
the diagnostic imaging machines, are more suited for the detection of these
physiological effects. Also, special software is needed for these ex-periments
that is specially written for the researcher by Varian.
Conclusion
In summary, NMR spectrometry has reached an-other milestone with the
introduction of Va r i a n ’ s
900-MHz instrument. The preliminary data from
this instrument are attracting a lot of interest from life sci-ence
and
pharmaceutical researchers, and we expect that this instrument will contribute
significantly to
the elucidation of structures of complex biologicals.
This article is by Dr. Howard is Editor, American Laboratory News.
Click Here for
a Printable one page PDF file