Automation Parameter Test Center

Introduction

There are many adjustable parameters for every NMR experiment. The NMR staff has made an effort to provide initial values to all these parameters that allow the experiments to be acquired successfully for most samples. However, there may be circumstances in which these "default" parameters must be adjusted, even for samples acquired under automation. Successfully/reasonably changing the default parameters requires that the researcher know what these parameters ① are, ② can and cannot do, and ③ depend on. In the past, the Automated systems in the Texas A&M Chemistry Department offered researchers a small battery of adjustable parameters, which were properly introduced and explained by the staff. Unfortunately, the staff has often found that these parameters are often being improperly set, which leads to inefficient and even dangerous acquisitions.

In an effort to reign in ill-advised parameter choices, we have decided to reduce the parameters available in automation to just the transmitter offset frequency (O1P). This move has been accompanied by some additional default parameter settings that again allow for some safe and reasonable experimental choices. Furthermore, the staff has assembled a NMR Parameter Test that covers the original adjustable parameters. Successfully passing this test will ① restore the original adjustable parameters in automation and ② allow you to request training on systems that are not run exclusively in automation.

NMR Parameter Test Content

The test is primarily geared toward judging your understanding of the basic NMR parameters that are available in automation. However, there are additional topics covered from the additional topics covered in the source material and training topics provided by the staff.

Basic NMR Parameters

There are many parameters associated with even the most basic NMR experiment. On an automated system though, there are generally only a few that commonly need to be adjusted.

These variables are explained far more completely in the Study Materials, but a slightly longer introduction here is in order.

• The Number of Scans (NS) is a commonly abused variable. It represents the number of times the experiment is acquired and added together to produce a signal-averaged result. The amount of signal acquired is proportional to the number of scans and the amount of noise acquired is proportional to the square root of the number of scans. Therefore, the signal-to-noise is proportional to the square root of the number of scans, and so to double your signal-to-noise requires a quadrupling of the number of scans. This very quickly becomes a losing game as the amount of time required to acquire a meaningful number of scans becomes large. It is also very important to understand that the number of scans does not change the proportions of signals or the resolution between them.
• The Relaxation Delay (D1) is an important variable to adjust for efficient and fair acquisitions. If the D1 is too long, the system is waiting unnecessarily when it could be acquiring more data. If the D1 is too short, the signals are artificially lost as acquisition continues (signal is proportional to less than NT), worse the duty cycle on the probe is unnecessarily high, which can be dangerous.
• The Transmitter Offset Frequency (O1P) is the single variable that is left to users that do not take the test. With the O1P you may move the frequency window around so that your target signals are observable.
• The Sweep Width (SW) is the breadth of the frequency space. Sometimes just moving the frequency window is not sufficient to see all the target signals and the SW must be adjusted. However, SW should not be changed without considering the bandwidth involved and the effect on the acquisition time.
• The Acquisition Time (AQ) is the amount of time that the signal is acquired for each transient. It is important to acquire the signal for an appropriate amount of time as either too long or too short an AQ will result in a poor spectrum. This is especially important to remember when adjusting the SW as this is its Fourier transform partner, so for a fixed number of acquired points the SQ will change with SW.

Study Materials