Application of T2-filter: Signal suppression for polymers

Application and Features

This technique is suitable for mixtures. The T₂-filter suppresses broad signals of components with short T₂ relaxation times such as polymers and biomacromolecules. While the signals of these high molecular weight components are suppressed, signals of small molecules can be selectively observed. For example, this technique is effective in the analysis of small molecule additives added to polymeric materials, protein-ligand binding studies, and blood plasma analysis in metabolomics.

Principle of T₂-filter

Fast decaying components in FID (Free Induction Decay) become broad signals in NMR spectrum after Fourier Transform. In general, ¹H signals of polymers and signals of exchangeable protons, such as OH and NH protons, tend to be broad (Fig. 1a). On the other hand, FID signals of highly mobile small molecules decay slowly and become sharp signals in NMR spectrum after Fourier transform (Fig. 1b). The fast decaying components have short T₂ relation times, whilst the slowly decaying components have long T₂ times. To make benefit of different T₂ relaxation properties, the T₂-filter is applied at the very beginning of the pulse sequence and then acquisition of NMR signal follows. Under the conditions shown in Fig. 2, the acquisition starts after the fast-decaying signals have decayed. As a result, the broad signals are suppressed, whilst the narrow signals are observed.

Fig. 1: Decay time and line width of a) fast decaying signals, and b) slow decaying signals

Fig. 2: Effect of T₂-filter on a) fast decaying signals, and b) slow decaying signals

For example, the experiments used for T₂ measurements are the CPMG (Carr-Purcell-Meiboom-Gill) and PROJECT (Periodic Refocusing of J Evolution by Coherence Transfer) experiments. The same experiments can be used for T₂-filtering. Here we demonstrate T₂-filtering on a sample of Styrofoam. Fig. 3 shows comparison of (a) standard and (b) T₂-filtered ¹H spectra. The T₂-filter reduces the broad components (mainly polystyrene) very efficiently in Fig. 3b, and hence the sharp signals previously invisible become visible.

Fig. 3: Comparison of a) ¹H standard spectrum, and b) T₂-filter.

Parameters of T₂-filter

Experiment: cpmg_project.jxp
Scans: 32
Delay_list: 2.5 [s]
Tau_step: 10 [ms]