I often make the point that PulsePol is expected to replace the “default” option of M2S as the go-to windowed (meaning there are gaps between pulses) or hard-pulse sequence for exciting nuclear singlet order in strongly-coupled spin systems. (Shoutout to the archetypalsoft-pulse/low-power sequence SLIC.)
That being said, I religiously avoid the misleading statement “symmetry-based sequences will replace M2S” and wince uncomfortably at the frequently-asked question “do you think symmetry-based sequences will replace M2S?”
This is because the building block of the M2S sequence, the J-synchronized echo (JSE; see refs. 1, 2, and 3), is technically a symmetry-based sequence.
Figure 1: The J-synchronized spin echo pulse sequence.
The fundamental difference between the JSE and PulsePol is that the JSE is designed to selectively swap the |S0〉and |T0〉states (via a zero-quantum effective Hamiltonian) while PulsePol is designed to selectively swap |S0〉and either of the |T±〉states (via a single-quantum effective Hamiltonian):
Figure 2: illustration of transitions excited by different symmetry-based sequences.
This is painfully obvious when you phrase it the right way: the R-element (i.e. basic inversion element) of the JSE is simply a windowed 1800 pulse, and the nominal duration of the R-element is half a “rotor” period 1/(2J). With regards to the first-order selection rules, no phase cycling is applied*. Hence, we may denote the JSE as R210 in the notation describing symmetry-based sequences. Or you could denote it C110 too (assuming you count a full cycle including two spin echoes).
As an aside, it is easy to see why the M2S sequence ended up appearing clunky with awkwardly placed pulses and delays interrupting the echoes: the selective transition being engineered by the JSE building block was the wrong one, assuming one was starting from thermal equilibrium magnetization. Which suggests that the JSE would’ve worked handsomely for nuclear singlet excitation if one was starting from dipolar order (an excess population of the central triplet state)…
*Much like PulsePol, “riffling” in the form of supercycles is necessary for the optimal performance of the JSE/M2S sequences vis-a-vis pulse strength and detuning errors.
“Funny story: our neighbouring Chemistry institute can’t measure good quality HSQCs. I told them once they are missing the filter. Tried ours on theirs and showed it worked, but the responsible person called Bruker and they said they don’t need them.
Well, they still don’t measure HSQCs.“
– an anecdote from an anonymous NMR spectroscopist, shared with permission.
A fundamental basic principle in all technologies that involve radiofrequency circuits (NMR is no exception) is the importance of isolating the signals you want and removing interference from the signals you Do Not Want via the use of radiofrequency filters. Entire books have been written on this topic alone, but the gist of it is this summary:
“The primary objective of RF filters is to block undesired signals effectively. As passive devices, they operate seamlessly by selectively permitting or hindering the transmission of signals based on their frequencies. The critical nature of this function becomes apparent when considering scenarios where unwanted signals could interfere with the normal operation of electronic devices or communication systems. In essence, RF filters act as essential guardians, ensuring the integrity and efficiency of signal transmission by preventing interference from signals that fall outside the desired frequency range.“
Figure 1: a set of RF filters in my office, with a 50 MHz low-pass filter being tested on my NanoVNA-H4 network analyzer.
Unless you are doing something like ZULF NMR, it is usually the case that conventional high-field multinuclear NMR experiments will involve more than one nuclear spin species – each precessing at a unique Larmor frequency – with the relevant frequencies typically (but not always) being well-separated by tens or hundreds of megahertz. Disregarding “true” spin-dynamical effects such as heteronuclear Bloch-Siegert shifts one would generally like – at least on the RF level – that a pulse applied on Channel 1 has no effect on Channel 2; i.e. we’d say there is no RF cross-talk, or the RF channels are isolated.
A common but erroneous assumption (sometimes encouraged by non-experimentalists, or PIs that struggled with far less forgiving hardware in previous eras of NMR) is that a modern spectrometer is now “perfect” or “it does it all for you“. Many students acquire the misleading expectation that a spectrometer will output exactly what is programmed into it and nothing else. This is not the case, and has never been the case for any real-life/non-fictional RF circuit.
Figure 2: 1H[13C] spectra of 13C-labelled sodium formate, with and without a bandpass filter installed on the 13C channel where decoupling is being applied (nutation frequency = 1 kHz). Broadband noise from the 13C channel bleeds into the 1H and 2Hchannel, severely compromising the S/N. It is worth noting that both cases have a bandpass filter on the 1H channel. The style of the graphic is an intentional homage to Glenn Facey’s blog.
The experimentalist’s reality is that broadband amplifier noise on one channel will bleed into other channels unless otherwise prevented from doing so. For example, when observing, say, the I-spin spectrum under S-spin decoupling, it is imperative to minimize cross-talk by inserting an RF bandpass (or lowpass) filter at the output of the preamplifier corresponding to the S-spin channel. This is a precaution in experiments that involve heteronuclear decoupling that is always mentioned in well-written experimental sections. If the S/N on the receiving channel is good enough, the signal attenuation by the filter is not a serious problem – so if there is no harm, bandpass filter everything.
Another less-appreciated source of annoying experimental artifacts is broadband noise leaking into the lock channel. On some of our spectrometers, even 13C pulses on the BB (broadband) channel are easily seen as prominent “spikes” in the 2H lock display, and during 13C decoupling or spin-locking the lock becomes extremely noisy. A noisy lock channel is bad news because it translates to noisy and/or artifact-ridden spectra. In this case, the problem can be entirely prevented by connecting a 13C bandpass filter (or alternatively 2H bandstop filter) to the output of the BB channel. Some spectrometers come pre-equipped with 2H bandstop filters, which is not an ideal solution since spectroscopists may prefer to retune the BB channel to perform 2H experiments (the lock channel has limitations better left for a future blog post).
Adding RF filters is not merely a fine detail that marginally improves the quality of spectra. The problem can already screw with mainstream 13C/15N experiments and tends to be far more severe for exotic lower-gamma (Cinderella) nuclei. Here it is worth noting that we simply never succeeded in measuring any 13C or 1H signals under 103Rh decoupling on some of our spectrometers without the appropriate RF filters (see our first and second103Rh papers where I tried to make this explicit in the experimental section).
Yes, it’s that bad. If you’re struggling with S/N in multinuclear NMR experiments this is one possible source of trouble you can look at. It is always better to listen to WHAT YOUR SPECTRA SAY than the guys who confidently swear “the filters are pointless, you don’t need them, take them off“, unhelpful mis-advice that has wasted days of innocent PhD students’ magnet time.
If you haven’t lost them, Bruker bandpass/bandstop filters do a good job. We buy our other filters mainly from K&L Microwave, and occasionally use archaeological remnants such as the now-extinct Chemagnetics/Varian filters, or an old (and magnetic) TV filter from a random company in Ohio that I used to great success in e.g. potassium-39 and krypton-83 experiments:
Figure 3: a set of old-school RF filters.
If you’re a hands-on person you can also build your own rf filter at little cost using references such as this paper and one of the several online filter design tools. The filter does not need to be built with nonmagnetic components, but you’d ideally avoid questionable design choices such as a pure steel shell. The current trend in electronics is “miniature and cheap” and a tool I’ve found to be indispensable is the incredible NanoVNA network analyzer. The nifty little devil of a tool does all of the jobs that the £3,000-£30,000 network analyzers can do (filter characterization, probe tuning) without costing too much money and lab members fighting “who took our network analyzer” wars. Several variants of the NanoVNA can be bought online for £50-100. I bought my own but unless your finance department is a real headache, you should ask your PI to get it using the grant money, which I’m sure they will appreciate!
