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how do we decide on power levels and lengths of purge gradients in a new experiment?...As for coherence selection, relative lengths of two gradients are based on a ratio..again, I am lost on what power level to use!

Also, how do one correlate power level with strength in G/cm? ..what are the safe limits? (I use agilent(varian) spectrometer)

asked Aug 03 '13 at 03:21

nmr2rmn's gravatar image


updated Aug 03 '13 at 08:58

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Gradient pulse strengths for a given experiment should be well detailed in the paper that describes the pulse sequence. For starters, strength of pulsed field gradients (PFGs) for a few set of modules can be easily set depending upon their function. For e.g., if PFGs are used for clean up or for ZZ-filter, 6-10 G/cm should be more than sufficient (but make sure that this doesn't result in refocusing after a few set of pulses in the pulse sequence). And for double-pulsed-field-gradient-spin-echo (where a 180 pulse is flanked by two equal and same amplitude PFG pulses (Gt-180-Gt, where G and t are the amplitude and duration of the PFG), about ~5 G/cm should be a good starting point. As long as 180 pulses are calibrated properly these set of PFGs will not affect the spectrum as much even if set to zero . Alternately, if you are thinking of spatially selective excitations, then the gradient strength is dictated by the shaped RF pulse that is used and by the thickness of the slice you want to excite.

In Agilent/Varian spectrometers, the PFG strength is depicted as a "dac" value, it ranges from -32767 to 32766 corresponding to the maximum PFG strengths (-Gmax to +Gmax). For e.g. if the probe can deliver 52 G/cm, then the dac value of (-)32766 corresponds to (-)52 G/cm, and for e.g. a 1000 dac corresponds to (1000/32766)*52 = 1.59 G/cm. Generally the maximum PFG strength is set by the parameter gzcal, where it denotes the amplitude of PFG/dac unit (in the above e.g. gzcal = 52/32766 = 0.001587 G/cm/dac). And this can be calibrated using the Agilent standard protocol (refer to the specifications manual for more details) using methyl iodide sample. For advanced users, the maximum gradient strengths can also be set by using a "water phantom" sample for a known receiver coil length and given sample height.

However, before going ahead one should make sure that the PFGs are present in the probe being used and are also activated. The variable "pfgon" governs it and it should be = 'nny' for a z-axis PFG coil. One way to check whether it works is to check the gradient shimming profile that results from the trial run.

The maximum PFG strength I had employed was 30-36 G/cm for not more than 200 us on three different probes (spanning both RT and cold-probes). Although higher strengths may be used with supervision of an NMR expert.

Good luck!



answered Aug 04 '13 at 08:05

Bharathwaj's gravatar image


That clears up everything..thanks! - nmr2rmn (Aug 04 '13 at 10:44)

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Just some followup answers/comments. First, I don't see an answer to your question about coherence selection gradients. Assuming that you're not changing from for example single quantum to double quantum, the correct ratio is simply the ratio of the gammas for the two nuclei. So, 1:1 if you both encode and decode on protons, 4:1 for going from 13C to 1H, ~10:1 for going from 15N to 1H, etc. Keep in mind that it's the total integrated "area" under the gradient pulse that matters, so for the 13C to 1H example the second gradient could be 1/4 the power for the same duration, or the same power for 1/4 the time, or half power for half the time.

As for purge gradients, my general rule of thumb is to make sure that if you use several they all have significantly different total areas such that you don't accidentally refocus something you don't want. So if you have one that's 10 G/cm, you might use 12 G/cm and 16 G/cm for two others but avoid 5, 10 and 20 G/cm. Again, you can change the durations as well as the power. Also, for purging/refocusing around a 180 degree pulse, you can generally use fairly low power, while I find that within reason stronger ZZ gradients generally purge artifacts better.

Finally, as for gradient strength, what's safe really depends on your probe specs. Some older probes may not handle anything much over 20 G/cm. With our room temp triple res probes I routinely use 2 ms gradients at ~40 G/cm (max. is a bit over 70 G/cm). Some specialty diffusion probes can easily handle several hundred G/cm.


answered Aug 09 '13 at 07:11

Andrew%20Fowler's gravatar image

Andrew Fowler

yes..coherence selection was my concern as well..thanks for specifically explaining it too. - nmr2rmn (Aug 09 '13 at 14:58)

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