Time Domain NMR (TD-NMR)
by specfit

Time domain NMR (TD-NMR) is a method to analyze samples using magnetic resonance. There are multiple advantages of TD-NMR.

1. TD-NMR is a bulk method which penetrates the entire sample irrespective of solid, semi-solid, or liquid. Because the technique penetrates the entire sample, color, surface variation, and sample shape are not factors in the measurement.
2. The signal response is uniform, regardless of chemical structure, so a hydrogen (¹H) as water gives the same intensity signal as a hydrogen in a hydrocarbon chain.
3. The signal response is linear, therefore, the number of calibration standards is small and can be as small as one standard with the second point through the origin.  Typically 4-6 calibration standards provide a robust method which account for secondary variations including error in the preparation of the standards.

The duration or lifetime of signals typically differ in the following way. Crystalline solids usually decay very quickly for tens of microseconds.  Non-crystalline solids or amorphous materials have signals which usually last slightly longer, perhaps 100 to 200 microseconds, followed by  semisolids like rubber and other soft materials, which might last for a few milliseconds and finally liquids have signals which can last for seconds.  Sampling the signal of a mixture of materials at different points reflects the concentration of each species. The figure below illustrates the differentiation of solid fat (a crystalline material) and liquid fat.  There are two measurements at 11 µsec (the total signal) and 70 µsec (liquid signal).