| Parameter |
Status |
Consequence |
| Sample Spinning |
on |
Narrower lines; 10 mm
or 20 mm samples may require Teflon vortex plugs. |
| off |
Best phase stability:
do not spin for gradient shimming, 2D, 3D experiments or difference spectra. |
Temperature
Regulation |
on |
Required for long-term
experiments or when maximum resolution is required; 5-10 minutes
equilibration time required, even for regulation near ambient
temperature. |
| off |
Not needed for quick,
routine spectra at ambient temperature. |
| Probe Tuning |
required |
When changing observe
nucleus; for optimum sensitivity and shortest pulse lengths;
to minimize RF-heating when using broad-band decoupling; standard
for multi-pulse 2D/3D/4D experiments |
| not required |
For routine 1-D use
when pulse widths do not need to be carefully calibrated: assumes
that the probe was properly tuned. Note that radical changes
in solvent composition e.g. CDCl3 vs.
high-salt aqueous will significantly alter the frequency tuning
and impendence matching characteristics of the tuned circuit. |
| Deuterium Lock |
on |
Provides compensation
for magnetic field drift: required for highest resolution work;
the amplitude of the lock feedback signal ("lock %")
is used for shimming. |
| off |
Deuterium observe; deuterated
solvent not available; shimming is accomplished using a separate
deuterated standard with the same geometry (liquid height/tube
size) as the sample to be analyzed; alternately the shims may
be adjusted while observing the shape of the observe nucleus
FID. |
| Shimming |
na |
Brief adjustments of
axial shims (Z1, Z2, and maybe Z3) are required for routine work;
for highest resolution, extensive adjustment of all shims may
be required for solid-state work room-temperature shims might
not be used at all. Odd-order shims (e.g. Z1, Z3, Z5)
have symmetrical effects on line shape; even-order shims (e.g.
Z2, Z4) have asymmetrical effects on line shape. |
| Gain Adjustment |
|
Amplification of "audio"
signal output of the RF detector; required to match signal strength
to digitizer (ADC) voltage measurement range: if the signal overflows
the ADC it will be "clipped", resulting in a distorted
spectrum. If the gain is too low, weak signals may fall below
the measurement threshold of the ADC. |
| auto-gain |
VNMR: gain='n' insures
that a suitable gain is used, but requires extra time when each
data acquisition is started. May confuse automated series of
experiments, as with arrayed variables |
| manual gain |
VNMR: gain=1 to 60 |
| Pulse Width Calibration |
"pw" |
Required when exact
length of "90-degree" or "180-degree" pulses
needs to be known. This is always done when a new probe is installed
and typically repeated prior to experiments that require careful
calibration, e.g. relaxation measurements, 2D, 3D, multiple
quantum filters, etc. The 90-degree
pulse width may be a quality control indicator of probe health
and performance. |
| Relaxation Delay |
"d1" |
The pulse-repetition
time tr = (d1+at) must be long enough to allow adequate (T1)
relaxation between repeated scans. For spectra where quantitative
intensity information is required one should use tr > 5<·>T1 |
| Acquisition Time |
"at" |
Length of time (seconds)
during which the NMR signal is sampled and digitized. "at"
must be sufficiently long to allow the FID to decay and avoid
truncation artifacts to ensure full spectral resolution. |
| Spectral Width |
"sw" |
The spectral observation
window. Must be wide enough to encompass all resonances. Resonances
outside the spectral window may be "folded-in" (aliased).
Excessive width will result in excessive noise. The spectral
width is the reciprocal of the dwell time (digital sampling interval). |
Number of Data
Points |
"np" |
The real + imaginary data
table size: the product of the dwell time and the acquisition
time collected in two channels. Each data point requires 4 bytes
of data storage; number of points = (Spectral width) X (Acquisition time) |
| Transmitter Offset |
"tof" |
An offset in Hz combined
with the standard transmitter frequency for each nucleus that
determines the center frequency of the quadruture-detected spectrum |
| FID |
na |
An unporcessed experimental spectrum gives a characteristic shape of the Free Induction Decay (FID). It is a sum of audio-wave signals that contain all the information in an NMR spectrum. There is a strict analogy between NMR and radio broadcasting: music and speech travel as radio-waves, but what we are interested into is the sound (audio-waves). In NMR we perform a further step: we visualize the sound as a plot of intensity vs. frequency. The reason is that we sort, in this way, peaks according to their frequency and our mind has a simpler job to do. The correct terminology is: “passing from time domain into frequency domain”. The operation to pass from one domain into the other is called “Fourier Transform”, opportunely abbreviated as FT. |
| CW |
|
Continuous wave (cw) NMR measurement the energy absorption of the nuclear spins is studied as a function of frequency |
| Chemical Shift |
na |
Generally, hydrogens bound to carbons attached to electron withdrawing groups tend to resonate at higher frequencies (more downfield (to the left) from TMS, tetramethylsilane, a common NMR standard). The position of where a particular hydrogen atom resonates relative to TMS is called its chemical shift. Learning where typical hydrogens resonate requires experience and study, but learning some common chemical shifts will help solving structural problems using NMR. |
