Probe variations

We can calculate the frequency of the precession of the net magnetization vector in the rotating frame as a result of the action of the Bj field. This Bj frequency represents the edge of the excitation range for a 90° pulse. To preserve true intensities, it is best to limit resonances to within only 10% of this range. 

In the example above, if 90° of rotation takes 10 ps, then 360° of rotation must take 40 ps. Therefore the Bj frequency excitation limit is plus or minus 1/4 X 10 s or 25 kHz. 

Beyond variations of these three features, there are other commercially available probes that can be used for liquids NMR work, but these probes will not be discussed in detail in this text. We should for now simply be aware that other types of liquids NMR probes exist. 

Most NMR magnets have a magnet bore tube (the vertical tube through the center of the magnet) with an inside diameter of 

The frequency of the RF applied to the sample for the observe channel, also the frequency of the rotating frame for the observe nuclide. 

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In summary, physisorbed nitrogen appears to offer several advantages as an infrared probe of acid sites in zeolites. It clearly distinguishes between Bronsted and Lewis acid sites without interference from gas phase species, it is small enough to probe sites in smaller pore zeolites, and its interaction with the zeolite is sufficiently weak and reversible to have negligible influence on the zeolite chemistry. It is not yet clear whether the method can probe variations in Bronsted acid strength. 

Munk and co-woricers have been concerned with the above-stated problem for some time (38, 39). In this volume (40), their attention is focused on miscible blends of polycaprolactone and polyepichlorohydrin. These authors demonstrate that to a considerable degree the probe variation problem can be mitigated by scrupulous attention to experimental details in the IGC methodology. This concern for details is required at any rate, if the high data reproducibility needed for meaningful studies of interaction in miscible polymer blends is to be attained. These details center on modified methods for coating polymers onto solid supports, on improved methods for measuring carrier gas flow rates, and on enhanced, computer-based data analyses of elution traces. Also, corrections are made for contributions to retention times from uncoated support material. More than twenty volatile probes are used by Munk, and the probe-to-piobe variations in %23, while not entirely absent, are much less apparent than they would be under standard experimental protocols. 

Tzanavaris, R, Webb, J.K., Murphy, M.T., Flambauui, V.V., and Curran, S.J., Probing variations in fundamental constants with radio and optical observations of quasar absorption lines, Mon. Not. R. Astmn. Soc., 374, 634, 2007. 

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