e. to very short echo times 2τ). The PROJECT (Periodic Refocusing Of J Evolution by Coherence Transfer) approach uses a CPMG sequence with quadrature 90° pulses inserted in the middle of each double spin echo, and is based on the so-called

perfect echo [32] and [33]. The extra 90° pulses refocus J-evolution, for arbitrary τ in AX spin systems and for all spin systems if τJ ≪ 1. If diffusion weighting is added, for example by including field gradient pulses in each echo as in the PROJECTED (PROJECT Extended to DOSY) sequences of Fig. 2, then spin echo DOSY spectra may be obtained free of both exchange effects and J modulation if τk, τJ ≪ 1, where k is the exchange rate constant. The DOSY spectrum of Fig. 1a was acquired for PD-332991 a mixture of flavone and catechin. At first sight there appear to be two impurities present. In fact these signals Selumetinib are simply the flavone hydroxyl resonances, but their diffusion coefficients are increased by exchange with the small amount of (protio-) water present in the sample. As noted above, such signals are typically better dispersed than backbone proton signals, but serve only to confuse in the spectrum of Fig. 1a. If exchange effects are suppressed using the PROJECTED sequence (Fig.

2, first and last gradient pulses omitted, no 45° pulse), however, the assignment of the signals becomes obvious: hydroxyl and backbone signals alike align correctly in the diffusion domain, as shown in Fig. 1b. Of course the effect is not

limited to exchanging OH signals (which can, if appropriate, be suppressed by addition of D2O), but is general (and extends to magnetization exchange through the Overhauser effect). The use of PROJECT-based DOSY experiments is not limited to cases where exchange is a problem; if, as in small and medium-sized molecules, T2 is not too short, they can be significantly more sensitive than their STE counterparts, because the SE retains the full signal while the STE discards half. However, as there is signal loss due to T2 during the SE delay, for signals with a short T2 a compromise between degree of diffusion weighting and signal-to-noise ratio may be required. Other important examples of applications for PROJECTED include T2-filtered DOSY and convection compensation. T2-filtration is almost commonly used where broad signals, for example from polymers or proteins, obscure signals of interest, and is typically implemented in STE-based DOSY pulse sequences by adding CPMG sequences either before [34], or after [35] the STE element. With PROJECTED, diffusion encoding and T2-filtering can be performed simultaneously, minimising signal losses, sample heating and J-modulation. Convection compensation is a particularly attractive application. In STE-based DOSY experiments, convection compensation is normally achieved using a double stimulated echo (DSTE) [22], with a fourfold loss in signal compared to a SE experiment.