Difference between nmr and esr

Answer 1

3. Differences between NMR and ESR

1)Resonant Frequency

One important difference between NMR and ESR is that in ESR the resonant frequencies tend to

be much higher, by virtue of the 659-times higher gyromagnetic ratio of an unpaired electron

relative to a proton. For example, a typical magnetic field strength used in ESR spectrometers is

0.35 T, with a corresponding resonant frequency of about 9.8 GHz. This frequency range is

known as "X-band", and the spectrometer as an "X-band ESR spectrometer". Such spectrometers

are readily available "off the shelf" from a (small) number of commercial sources.

X-band ESR spectrometers are typically used to study small solid samples, or non-aqueous

solutions up to a few hundred μL in volume. They cannot be used for biological samples, or for

in vivo studies, because of the strong non-resonant absorption of microwaves at 9.8 GHz. For

that reason, ESR spectrometers (and imagers) have been constructed to operate at lower

magnetic fields, and correspondingly lower frequencies, including at "L-band" (about 40 mT and

1 GHz) to study mice and "radiofrequency" (about 10 mT and 300 MHz) to study rats.

2) Relaxation Times

The second important difference between NMR and ESR is the typical relaxation times

encountered. In bio-medical proton NMR the relaxation times T1 and T2 are typically of the order

of 0.1 to 1 sec. In bio-medical ESR the equivalent electron relaxation times are a million times

shorter, i.e. 0.1 to 1 μsec! The extremely short relaxation times have important implications on

the way in which ESR measurements are carried out.