Abstract

Breaking the "barrier" of relaxation rate R1 > 1000 1/s and doing so in a reliable and reproducible way is an experimental feat which would have been impossible to dream about even just three years ago. Yet this poster illustrates that the latest FFC instruments make it an affordable reality which lies just slightly beyond the realm of everyday routine. It also shows that with field switching times of 1 ms (corresponding to current state-of-art) one can in certain cases reliably measure relaxation rates R1 up to 10000 provided that the switching waveforms are linear and perfectly reproducible.

Two samples had been used for this preliminary study of the phenomena occuring when switching times become comparable to, or longer than, relaxation times. Sample I was a 2.1M water solution of dysprosium perchlorate (kindly provided by Dr.L.Helm). Sample II was the commercial Parafilm M foil (American National Can Co.) whose composition is a 'secret' but which appears to consist (NASA/MSFC Materials and Processes Home Page) of a wax (~56%) and a polyolefin (~44%).

As expected, sample I was found to have a flat 1H-NMRD profile over the whole explored field range from 10kHz to 20 MHz (1H Larmor frequency). Its mean relaxation rate at 25 0C has been found to be R1 = 1425 1/s with an expected error of 1.4%. Even using 1 ms switching time and slewing rates of 24 MHz/s, its profile has not been easy to measure, despite the fact that the required polarization time is very short, permitting a large number of scans to be taken. This is due to the substantial signal decay during the switching period which can not be quite compensated by the number of acquired scans. We feel that the measurement of a reliable NMRD profile of a sample of this kind still requires a cautious and experienced operator and probably is so far beyond the realm of a routine, automated tasks.

Sample II behaves in a quite different way. Its proton R1 at 25 °C varies steeply from 39.9 1/s at 18 MHz to over 6400 1/s at 10 kHz. Despite the fact that its maximum R1 values are about four times higher than for sample I, it can be easily measured even by a relatively inexperienced operator using the automated profile-acquisition procedure. The only required precaution is setting the switching time value to 1ms, the polarization time to a value adequate for whatever is the chosen polarization field and the field slew rate to the maximum allowed value. The reason for the ease of this measurement is of course the fact that during the switching, the sample is never exposed to fields at which it relaxes really fast for longer than a few tens of microseconds. However, this very fact makes it also evident that those few tens of microseconds must be always the same, implying that the precision and the reproducibility of the field-switching waveforms are both absolutely essential in order to guarantee correct results for this kind of samples.