As described for liquid state NMR noise experiments

[6] and [9] the selleck chemicals tuning required to obtain this dip line shape may deviate from the conventional tuning optimum (CTO). This offset also does not generally coincide with the optimum determined by minimizing reflected power through an external reflection bridge. This was also the case for the triple and double resonance probes in combination with two preamplifiers, where the noise power signal exhibits a dispersive line shape at the CTO. Fig. 3 shows noise spectra of H2O at different tuning offsets obtained using the triple resonance probe connected to a high-power 1H/19F preamplifier. Note that both the observed line shape and the average (thermal) noise level are tuning-dependent. De-tuning of the other channels had no influence on the 1H noise signal. The SNTO [6], where a pure “dip” power line shape (i.e. a noise level lower than average thermal noise) was seen, was at a tuning offset of 365 kHz from the resonance frequency. This offset varies between different probes

and preamplifiers as shown in Table 1. Using a (1H/13C) double resonance MAS probe instead, surprisingly, only positive noise signals could be found within the entire tuning and matching range. We concluded that the SNTO for this probe/amplifier connection lay outside check details the range accessible by the tuning capacitors. Using a high-power (solids) low noise preamplifier, the dip tuning offset was zero. This was the case for this preamplifier with all probes used. In combination with a low-power preamplifier the shape of the tuning curve was significantly different. The pure dip signal was not found with the normal routine within the tuning ranges of both probes. De-matching had a significant influence on both the noise line shape and the average thermal noise level. In the case of the triple resonance probe, slight de-matching, in case of the double resonance

probe (Fig. 4), significant de-matching (a new minimum occurred in the tuning curve) together with de-tuning Cyclin-dependent kinase 3 allowed us to find settings that gave rise to a dip line shape of the noise signal, in a trial-and-error approach. A more systematic approach is under investigation in our laboratories. Apparently there can be more than one combination of tuning and matching adjustments that yield an NMR noise dip signal, at least on some probes. The MAS tuning and matching conditions found for the H2O sample were also used for adamantane. In this case, where a dip was found by de-tuning only, the probe was tuned to the same SNTO frequency as found for H2O. If de-tuning and de-matching were necessary to find the dip, the controls were adjusted until the conventional tuning curve resembled as closely as possible the one found with H2O. In Fig. 5 pulse and noise spectra of adamantane obtained under conventional tuning (CTO) and SNTO conditions are compared.