An overview of the experimental procedure for T S estimation is given in figure . (a) The initial M2S block (see figure ) converts thermally polarised Cmagnetisation into homonuclear C-singlet order . Experimentally optimised M2S parameters for O-enriched 1,2-13 C 2 squaric acid are given as a function of pH in table I. (b) After singlet order excitation an optional field cycling step is introduced to transport the sample to a desirable magnetic field strength. (c) Singlet order is allowed to freely evolve for a variable duration τ evol . (d) For the case of field cycling experiments the sample is shuttled back into the active region of the magnet. (e) Application of a gradient-based singlet order filter (see figure ) isolates NMR signals passing through singlet order . The field gradient parameters for the singlet filter are summarised in table II. (f) The S2M sequence (the chronological reverse of the M2S sequence) reconverts the singlet order back into observable magnetisa-