| 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 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 |
|
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 asymmetical 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. In the long term,
the 90-degree pulse width may be a quality control indicator
of probe health & 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 |
| 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 |
