NMR spectroscopy is unique among spectroscopic methods in that that the relaxation processes are relatively slow (on the order of seconds or tenths of seconds), compared to ms, us, and pico-seconds for IR and UV. In other words, once the spectrometer has perturbed the equilibrium population of nuclei by pulsing at the resonance frequency, it takes from 0.1 to 10s of seconds for them to return to their original populations. Typically, one measures the T1 (spin-lattice relaxation time1) to calculate an appropriate relaxation delay. If the pulse angle and repetition rates are too high then spectra can become saturated. Because the relaxation rates of various protons in the sample are different, integrations become less accurate. Saturation effects are particularly severe for small molecules in mobile solvents, because these typically have the longest T1 relaxation times.
To get reliable integrations the NMR spectrum must be acquired in a way that saturation is avoided. It is not possible to tell whether a spectrum was run appropriately simply by inspection, it is up to the operator to take suitable precautions (such as putting in a 5-10 second relaxation delay between scans) if optimal integrations are needed. Fortunately, even a proton spectrum taken without pulse delays will usually give reasonably good integrations (say within 3%). It is important to recognize that integration errors caused by saturation effects will depend on the relative relaxation rates of various protons in a molecule. Errors will be larger when different kinds of protons are being compared (such as aromatic CH to a methyl group), than when the protons are similar or identical in type (e.g. two methyl groups).
