Page 15 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

554 Analytical Chemistry

means of a metallic waveguide transmits the electromag- TABLE VI Common Nuclear Magnetic Resonance Nuclei of
netic radiation to a quartz tube sample holder positioned Quantum Spin 1 2
between the poles of a permanent magnet. The experi- NMR frequency Natural Relative sensitivity
ment consists of varying the magnetic field strength by Isotope (MHz) at 20 kG abundance (%) per nucleus
means of secondary coils to induce resonance. The result-
1 H 85.2 99.98 1.000
ing absorption signal is usually processed and outputed
2 H 90.8 0.02 1.21
as a first-derivative curve to increase sensitivity and res-
13 C 21.4 1.11 1.59 × 10 −2
olution. Though the technique does not have wide ap-
15 N 8.6 0.37 1.04 × 10 −3
plicability, it continues to play an important role in the
19 F 80.1 100 0.834
probing of biological systems for order, viscosity, reactiv-
31 P 34.5 100 6.64 × 10 −2
ity with spin-label reagents (since absorption is influenced
195 Pt 18.3 33.7 9.94 × 10 −3
by chemical environment), and investigation of chemical
reactions, which include free-radical intermediates.
would equal net emission and no absorption could be ob-
3. Nuclear Absorption
served. Radiationless relaxation processes from high- to
Nuclear magnetic resonance. Certain atomic nu- low-energy states are therefore of fundamental importance
clei have properties of spin and magnetic moment that for the maintenance of an absorption signal. Furthermore,
cause their energies to split into quantized levels when such relaxation mechanisms are dependent on the nuclear
subjected to a powerful magnetic field. Transitions from environment and proximity to other nuclei, so that relax-
a lower energy level to a higher level can be induced by ation rates can provide chemical data.
absorption of electromagnetic radiation in the radiofre- There are two distinct types of relaxation processes.
quency range of 0.1–100 MHz. Such absorption can oc- One is known as spin–lattice or longitudinal relaxation
cur only when the energy provided is equivalent to the en- and results from the interaction of absorbing nuclei with
ergy difference between two magnetically induced states the rest of the sample (lattice), which in the standard liq-
and satisfies the resonance condition. The experiment uid phase is in vigorous vibrational and rotational motion.
that measures the frequencies that satisfy the absorption Due to the large number of random magnetic components
process is known as nuclear magnetic resonance spec- offered by the lattice, there exists a good probability that
troscopy. This technique is related to electron spin reso- the phase and frequency of some of the lattice nuclei match
nance, which has been described previously. the precessional frequency of the high-spin-state nuclei so
ForanucleusofspinquantumnumberI,atotalof2I + 1 that energy exchange can take place. The spin–lattice re-
discrete energy states exist in a magnetic field. If u repre- laxation time T 1 is defined as a measure of the average
sents the magnetic dipole caused by the spin of the charged lifetime of nuclei in the higher energy state. This lifetime
nucleus, the energy difference E between states can be is afunction of the lattice mobility andcanbe used to probe
written generally as microstructures and viscosities. The availability of a para-
magnetic molecule or ion in the lattice provides a strong
E = µβH 0 /I,
fluctuating magnetic field source, which can substantially
where H 0 is the magnetic field strength and β is a constant shorten T 1 . The second relaxation process is known as
called the nuclear magneton. A term that is characteristic spin–spin or transverse relaxation and is denoted as a value
of a nucleus is known as the magnetogyric ratio γ , T 2 . This value is actually a sum of a number of distinct
processes that increase absorption bandwidth by decreas-
γ = µβ/I(h/2π),
ing T 2 . Some of these processes include energy transfer
where h is Planck’s constant. This value can be used di- between closely spaced-identical nuclei that exist in high-
rectly to relate the frequency of absorbed radiation v to and low-energy states, so that the lifetime in any one state
the magnetic field strength by the relation is shortened, and also the effects of other magnetic nuclei
whose spins create local magnetic fields, which align with
v = γ H 0 /2π,
or against the permanent externally applied field so that a
Some nuclei commonly observed by nuclear magnetic res- range of absorption frequencies exist.
onance are listed with their important physical constants Chemicalinformationcanalsobederivedfromtheposi-
in Table VI. The difference in the population of nuclei tion of absorption signals measured on an energy basis and
between energy states is usually very small, with lower the fine structure of such absorption signals. The position
states being occupied by only a few excess nuclei per mil- of absorption bands is referred to as the chemical shift and
lion. If the states were equally populated, net absorption is measured relative to a standard substance. The effect is