Hi Ken,
1. I do not personally have strict limits on paramagnetic content; if it
tunes and spins, it’s fine. I’d use all caution when spinning up, but
if there are susceptibility effects I’d expect them to be obvious at
first attempt.
1. High paramagnetic content will decrease T1, but also T2. Depending on
the distribution of paramagnetic ions in the sample, this can sometimes lead
to either horrific resolution, or sometimes to cases where peaks of interest
are simply invisible (while other, further environments are unchanged). I
do not know of a way to predict this a priori. The distribution of paramagnetic
ions can also affect quantitation; if the paramagnetic species are phase-segregated,
they will preferentially relax some environments and not others. Doping with
paramagnetic ions to reduce T1 homogeneously is common in the study of oxide
glasses, but there I expect paramagnetic ions to be more evenly distributed
than in an aggregate.
1. I wouldn’t call a signal from within your sample a background, simply
an unwanted peak. The difference is important as common background-suppression
techniques in SSNMR (Hahn echoes, DEPTH) rely on differential nutation of
signals originating from outside the rotor. As your quartz will be within
the rotor, conventional methods will not work. Spectral editing via CPMAS
is one possibility, but that will affect the reliability of your quantitation.
Another possibility, particularly if the iron is not present in the quartz,
is using T1 editing; if the quartz has a much longer relaxation time than
the rest of your sample, using presaturation loops prior to the rest of
your experiment should reliably suppress the quartz signal. This will of
course mean you’re unable to quantify the quartz species, but that may
be an acceptable compromise.
1. As for 29Si{1H} CPMAS, I have found tetrakis(trimethysilyl)silane to
be an excellent setup standard for that experiment. Note that you may need
to use contact times well beyond what you’d expect from 13C{1H} CPMAS,
and the high paramagnetic content may make CPMAS challenging as well.
Best of luck,
Alex
Alex Paterson
The National Magnetic Resonance Facility at Madison (NMRFAM)
University of Wisconsin-Madison
alpaterson_at_wisc.edu
> From: main_at_ammrl.groups.io On Behalf Of Kenneth Shar= p-Knott via groups.io
> Sent: Tuesday, May 19, 2026 1:25 PM
> To: main_at_ammrl.groups.io
> Subject: [AMMRL] Advice on 29Si SSNMR of concrete samples – Paramagnetic iron limits and T1 relaxation
Hi everyone,
We are finally fully up and running with solid-state NMR capabilities here
at Virginia Tech (running a Neo400 with a 4 mm MAS probe), and I have a student
requesting to run 29Si MAS NMR on some 60-year-old concrete samples.
They mentioned that the samples contain about 5% Fe2O3, as determined by XRF.
I have very little experience with concrete samples in SSNMR and wanted to
tap into the community's expertise regarding a few points:
1. Spinning Safety: My understanding is that an iron oxide content of around
10% is the threshold where magnetic susceptibility begins to put the instrument
at risk of a rotor crash or stator damage, but that anything below that is
generally safe to spin. Can anyone shed some light on this or share their
safety cutoffs for paramagnetic/ferromagnetic samples?
2. Relaxation Times: I've also read that there may actually be a benefit to
this Fe2O3 content—namely that the paramagnetic iron will reduce the normally
agonizing 29Si T1 relaxation times, allowing for shorter recycle delays for
quantitative direct-excitation experiments. Can anyone confirm if this
provides a meaningful practical benefit for cementitious samples?
Finally, since these are crushed concrete samples (aggregate included) rather
than extracted cement pastes, I anticipate a massive Q4 quartz background.
If anyone has tips on the best way to handle or filter this (we are planning
to try 1H-29Si CP-MAS to selectively highlight the C-S-H gel), I would greatly
appreciate it.
Thank you in advance for your insights!
Best regards,
Ken Sharp-Knott
Manager of Analytical Services and the NMR Facility
Department of Chemistry
Virginia Tech
(540)267-6502 (Cell)
(540)231-0885 (Office)
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