Oops - somehow my phone decided to send my message before I had completed even
the last sentence… Here is the full text:
> Am 19.09.2025 um 19:47 schrieb Spin-Doc <bert.heise_at_spin-doc.net>:
>
> Hi Karel,
>
> You didn’t mention whether you are using a liquids, solids or cryogenic
> probe… as some comments have already been made for solids probes, here
> is a hint for liquids probes:
>
> Typically/traditionally the rule of thumb was (is?) that the maximum continuous
> „cw“ power allowed for liquids probes is around 0.5-1 W. Typical
> case: 13C with decoupling during acquisition and relaxation time (to benefit from
> magnetization transfer via NOE), so the 1H decoupler is always on for hours at
> end… The same „almost always on“ condition is true for PRESAT
> where the decoupler is on for several seconds.
> The fact that the decoupler is off for several seconds during acquisition time
> unfortunately doesn’t really bring down the duty cycle (dec on vs. off), as
> the probe will show arcing at too high power levels even if there‘s „time to breathe“
> after the pulse, because the pulse is sooo long.
>
> In some 2Ds like HSQC which typically have considerably shorter acquisition times
> (order of 100 ms), the decoupling on-time starts to get short enough that one can
> push the probe a bit more (typical HSQC parameters make use of that)...
… Typically this is "nicely" visible with cryogenic probes where this decoupling
starts to heat up the sample, leading to a reduction in the required counter-heating
provided by the cryo system. In order to ensure stable conditions this heater power should
never be near zero (which means you're decoupling too hard).
Even harder on a probe can be pulse sequences involving spin locks like TOCSY or T1-Rho,
even more so if it involves nuclei requiring a large excitation bandwidth (HCCH-TOCSY for
example). The TOCSY mixing time can also easily lead to a too hard condition for the probe.
Often times (hopefully), such pulse sequences contain some calculations to ensure a safe
condition for typical liquids probes - but don't count on it!
The best would be to calculate the energy intake of the probe at least approximately by
either using calibrated power values in W (e.g. Bruker cortab) or use pulse specification
numbers (e.g. pw90 = 10 us at 300 W for 13C from the probe manual). The product of the
power and the decoupling/spinlock/presat time gives you the pulse energy (so the area below
the pulse) which is what leads to eventual arcing…
You can now calculate the attenuation value of the decoupling in dB relative to the "full power"
value in dB (where x dB attenuation = 10^(x/10) in W):
Say, if 0.5 W is allowed for 0.5 s that means around 2.5 W could be allowed for 0.1 s.
2.5 W corresponds to 10*log10(300 W/2.5 W) = about 21 dB less than full power (0.5 W then
corresponds to about 28 dB less than full power).
Any further input on this more than welcome! :-)
Best regards
Bert
>
>
> -> Sent via mobile <-
>
> Spin-Doc
> Magnetic resonance equipment, service, training & more
> Dr. rer. nat. Bert Heise
> Owner, applications scientist
> Kiefernweg 13
> 58239 Schwerte
> Germany
> Web: http://www.spin-doc.net
> Mail: bert.heise_at_spin-doc.net
> Phone: +49-171-5135315
>
> Representing MestreLab, Stelar & Barthel HF
>
>
>
>> Am 19.09.2025 um 17:09 schrieb D'Anciaes Almeida Silva, Igor via groups.io:
>>
>>
>> Hi Klika,
>>
>> If you're talking solid-state NMR and high-power decoupling, there's an equation
>> you can use to estimate the correct decoupling. It's in the attached PDF. Also,
>> test the value you get for the equation to see if the acquisition circuit doesn't
>> overflow. If so, adjust the pulses until you find something that fits (for instance,
>> at the last solid-state lab I worked in, we set the 13C pulse to 50 kHz, 1H to 100 kHz,
>> and a high-power dec. of 90 kHz - we used spinal64 for that).
>>
>> For liquid-state NMR, you're using only low-power decoupling so don't need to bother about it.
>>
>> ----------------
>> Dr. Igor d'Anciães Almeida Silva
>> Research Scientist - NMR Core
>> University of Missouri-Columbia
>> 32A Chemistry Building
>> From: main_at_ammrl.groups.io on behalf of Karel Klika via groups.io
>> Sent: Friday, September 19, 2025 6:46 AM
>> To: main_at_ammrl.groups.io
>> Subject: [AMMRL] determining maximum decoupling power
>>
>>
>> Dear all,
>>
>> How to determine the maximum decoupling power that can be safely applied and how
>> this varies with the time for which the decoupling is applied ?
>>
>> Thanks in advance and will post a summary if appropriate.
>>
>> Regards,
>> K. Klika
>>
Magnetic resonance equipment, service, training & more
Selling & buying used NMR equipment (all vendors)
Dr. rer. nat. Bert Heise
Owner, Applications Scientist
Spin-Doc Bert Heise
Kiefernweg 13
58239 Schwerte
Germany
Mobile: +49 171 5135315
Email: bert.heise_at_spin-doc.net
Web: http://www.spin-doc.net
Representing:
Stelar Fast Field Cycling NMR Relaxometers (
http://www.stelar.it)
MestreLab NMR Software (
http://www.mestrelab.com)
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http://www.barthel-hf.de)
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Received on Fri Sep 19 2025 - 19:38:12 MST