Non Uniform Sampling

 Unlike normal 2D experiments where time t₁in the second "vertical" dimension is increased gradually in a linear fashion, in non-uniformly sampled (NUS) experiments t₁ is changed in a semi-random way. The advantage is that the same level of resolution as in a normal 2D spectrum can be achieved using NUS and significantly less number of t₁ increments. Effectively, this means that within the same time we can acquire better resolved 2D spectra using NUS compared to traditional 2D experiments.

An example below shows an overlapping view of ¹H-¹³C HSQC spectra of 16 mM solution of angiotensin in D₂O acquired using normal sampling (pink/red) and NUS (green/blue). The total acquisition time for both experiments was just under 8 minutes with 128 t₁ values. From the appearance of cross peaks at 2.96 ppm, it is clear that significantly better resolution is achieved in the NUS spectrum. In fact, the normal 2D spectrum in pink shows a false maximum at 2.96-36.5 ppm due to overlapping cross peaks and poor digitisation, which may lead to an erroneous assignment. Also, ¹³C peaks with small chemical shift difference at 37.0 and 37.1 ppm are easily distinguished in the NUS spectrum.