Bruker minispec LF Series, User Manual

Page: 53 / 72

NMR Parameters
E164809_2_002 53 / 72
8 NMR Parameters
When defining the calibration/method in minispec Plus software, the software prompts typical
values for the NMR sequence parameters. However, it might be necessary to adjust these
parameters, especially if the method of test differs from the default approach. Then it is useful
to adjust the NMR parameters accordingly.
In the following sections typical parameters that need to be set are explained in detail. The
parameters Scans, Recycle Delay Time and Gain especially need to be adjusted or
optimized. The parameters Dummy Shots and Acquisition Pre-Delay Time are used less
often.
8.1 Scans
Scans is used to set the number of times the NMR experiment should be repeated for signal
averaging before the result is evaluated. The required number of scans (possible range
1-32000, typically 1-512) depends on the signal strength, the desired signal-to-noise (which
will limit the precision of the measurement), and the desired speed of analysis (time between
scans is set by the Recycle Delay). Signal to noise improves with the square root of the
number of scans. The remaining number of scans during an experiment is shown in the lower
right corner of the display.
8.2 Recycle Delay Time
The RD, or R ecycle D elay time (given in seconds), is used to set the time delay in seconds
between repeated scans. The recycle delay is usually set to 5 times the longitudinal
relaxation time T
1
to ensure that the sample returns to the thermal equlibrium state after a
scan. If the delay is deliberately made shorter than the sample relaxation time, one or more
dummy shots can be made prior to the experiments in order to ensure that the first scan has
the same history as all subsequent scans.
8.3 Gain
Gain sets the signal amplification in the receiver. For best results, set the gain such that the
highest part of the signal reaches about 80% of the signal input range. In minispec Plus
software the gain is adjusted automatically. The software will also indicate the allowed range.
Note that the gain should not be changed for calibrated applications. If the range of input
signals is too large for one gain setting, it may be better to have several calibrations for
different gain settings.