Analytical Chemistry

nucleus also possesses angular momentum I and for each isotope the relative values of μ and I
determine the frequency at which energy can be absorbed. In addition, the sensitivity of the technique
for a particular nucleus is determined by the value of μ. Table 9.9 lists the spin quantum numbers and
magnetic properties of a number of nuclei. It should be noted that^1 H has the highest relative sensitivity
and that^12 C and^16 O, having spin quantum numbers of zero, are inactive. For these reasons proton
magnetic resonance, PMR, has become one of the most useful techniques in the identification and
structural analysis of organic compounds. The only serious handicap is an inherent lack of absolute
sensitivity because of the very small energy differences between nuclear spin states which leads to
almost equal thermal populations of the energy levels at room temperature. The Maxwell-Boltzmann
equation (p. 275) can be used to show that there is an excess of only a few nuclei per million in the
lower level. Only this small excess of nuclei are in fact detected by nuclear magnetic resonance.

Table 9.9 Nuclear spin quantum numbers and magnetic properties of selected nuclei

Nucleus Nuclear spin quantum

number I

Magnetic moment, (ampere

square metre × 1027 )

Resonance frequency

in MHz at 1.4092

TESLA

Relative sensitivity at the

natural isotopic abundance

(^1) H 1/2 14.09 60.000 1.00
(^2) H (^1) 4.34 9.211 1.5 × 10 – 4
(^12) C (^0) — — —
(^13) C 1/2 3.53 15.085 1.8 × 10 – 4
(^14) N (^1) 2.02 4.335 1 × 10 – 3
(^16) O (^0) — — —
(^17) O 5/2 –9.55 8.134 1 × 10 – 5
(^19) F 1/2 13.28 56.446 0.834
(^31) P 1/2 5.71 24.288 0.066
Instrumentation
A schematic diagram of a typical continuous wave (CW) or scanning NMR spectrometer is shown in
Figure 9.27. The magnetic field is provided by a permanent magnet or an electromagnet with a field
strength of 1–14.1 tesla (10^4 to 14.1 × 104 Gauss). The homogeneity of the field between the poles of the
magnet should be at least 3 in 10^9 to ensure narrow absorption bands and good resolution. Samples are
contained in long narrow glass tubes (usually 5 mm o.d.) which are placed in a sample probe located
between the pole pieces of the magnet. The apparent homogeneity of the field is improved by rapid
spinning of the sample tube by means of an air driven turbine. Around the pole pieces there are sweep
coils through which a variable current is passed which enables a spectrum to be 'scanned' by varying the
magnetic field over a small range. In this case the RF transmitter operates at a fixed frequency, usually
60 – 300 MHz. Alternatively the field may be fixed and the spectrum scanned by varying the operating