Why does NMR normally need a Locked Signal?

The ²H Lock is a separate NMR protocol which runs concurently with most NMR procedures. The purpose of the ²H Lock is to stabilize the magnetic field over a period of time. The measured chemical shift of the experiment is determined by the strength of the magnetic field at the observed nucleus. If the magnetic field "drifts" during the experiment lines will broaden (i. e. less resolution and less signal intensity) due to identical "drifts" in the chemical shift during the experiment. The ²H lock protocol is automatically processed in dispersion mode. The computer derives an appropriate control voltage to regulate the field position once the ²H signal is "locked" in place. This regulation of the field/frequency is at the same time stabilizing the field/frequency for the main experiment which allows the accumulation of resolvable signals less than 1 Hz wide.

Lock Buttons and Controls
The Lock and Shim buttons (Z0, Lock Power, Lock Gain, Lock Phase, and Z, X, and Y shims) provide on-the-fly adjustment. The slider values can be moved with the mouse or entered directly.

Shim Buttons and Controls

Optimizing Lock
Under computer control, the lock system maintains a constant field at the sample as the static field generated by the superconducting magnet drifts slowly with time or changes due to external interference. Locking makes the resonance field of the deuterium in the deuterated solvent coincide with the lock frequency. The lock level can be viewed by clicking the Lock button on the hardware bar. The entire lock optimization process can be skipped if optimum lock parameters are already
known for a particular solvent and probe combination. Values for these parameters can be entered as part of a macro or using normal parameter entry (e.g., by entering lockgain=30 lockpower=24). These parameters do not take effect until an su, go, or equivalent command is given. If automatic shimming is to be used, it is important to obtain an optimal lock signal. Manual adjustment often is done to achieve the maximum lock amplitude. This can result in a partly saturating condition, and a true non-saturating power is usually 6 to 10 dB lower. The response of the lock level is governed by the T1 of the deuterated lock solvent as well as the magnet-determined or chemical exchange-determined T2 * of the solvent. This T1 can vary widely, from about 6 seconds for acetone-d⁶ to about 1.5 seconds for CDCl3 and lower for more viscous solvents. To allow a reliable, repeatable selection of lock power, automatic optimization may be used.

Finding Z0 and Establishing Lock
• Manual or Simple Method

• AutoLock
Find Z0 and establish the lock either manually or using Autolock. Both methods are accessed through the Standard page of the Start tab.

Manual or Simple Method
Establish lock using simple or manual locking on the Lock page. The line that crosses the
spectral window represents how close the deuterium resonance field is to the lock
frequency. When the two are matched, the line should be flat (with perhaps some noise,
depending on the lock gain and lock power). The poorer the match, the greater the number of sine waves in the line.
1. Make sure a sample is inserted and seated properly. Spinning helps but is not required.
2. Click on the Lock page in the Start tab.
3. Click on either Spin On or Spin Off.
4. Click Lock Scan to open the lock display.
5. Find Z0 by clicking on and dragging the Z0 slider bar until lock signal is on resonance.
6. Adjust the lock power, gain, and phase by clicking on and dragging the slider bars, or click the button.
The actual value needed for lockpower and lockgain depends upon the concentration of the deuterated solvent, the nature of the deuterated solvent—the number of deuterium atoms per molecule—and the relaxation time of the deuterium. At this point, do not be too concerned about optimizing power and gain; just look for a sine wave.
7. If a sine wave is not shown (perhaps just noise is visible), click on the ±10 or ±100 button for Z0 until some discernible sine wave appears.
8. When the concentration of the lock solvent is high, i. e. more than 50%, turn down the lock power.

9. If the lock power is too high, the deuterium nuclei become “saturated,” the signal oscillates (goes down and then back up), and it is difficult to establish lock. The correct amount of lock power is difficult to determine, but it is helpful to remember that acetone is more easily saturated than most solvents. Standard values are given for various solvents are shown on a help card near the spectrometer.
10. Adjust Z0 until the signal changes from a sine wave to an essentially flat line. If the solvent is concentrated, the line may start to move up on the screen as the lock condition is approached.
11. Click the Lock On button.
12. Click Lock Scan again to close the lock display.

AutoLock with Probe File
This requires a probe file with the probe calibrations, refer to the VnmrJ System Administration manual.
1. Click on the Standard page of the Start tab.
2. Click on either Spin On or Spin Off.
3. Click on Find Z0.
AutoLock
1. Click on the Standard page of the Start tab.
2. Click on either Spin On or Spin Off.
3. Selection an option for the menu next to the Autolock button.
4. Click on the Autolock button — the spectrometer will find Z0 and make all specified adjustments
5. Choose Find Z0 or AutoLock.
Lock Power, Gain, and Phase
Under computer control, lock power, gain, and phase are set by the lock parameters—
lockpower, lockgain, and lockphase—with the following limits and step sizes:
• lock power is 0 to 68 dB (68 is full power), lock gain is 0 to 48 dB, and lock phase is 0 to 360 degrees. Step size for power and gain is 1 dB; step size for lock phase is 1.4 degrees. Click Lock Scan to display the lock signal in the graphics canvas

The Z0 field position parameter Z0 holds the current setting of the Z0 setting. The limits of Z0 are –2047 to 2047, in steps of 1, if the parameter shimset is set to 1, 2, or 10, and–32767 to +32767 if shimset is set to another value.

Lock Control Methods
A number of methods are available for controlling lock on the Standard page in the Start tab:
• Leave lock in the current state.
• Run an experiment unlocked.
• Use simple autolock.
• Use optimizing autolock.
• Perform full optimization of lock.
Each method is discussed in the following separate sections. Additional sections discusserror handling and lock loop time constant control.

Leaving Lock in the Current State
Set Lock Find Resonance to Not Used. If simple or optimized Autolock was previously selected, lock is established upon insertion of the new sample. If simple lock was previously selected, the system only locks if the new sample has the same lock solvent.
Running an Experiment Unlocked
Set Lock Find Resonance to Unlocked. Lock is deactivated at the start of acquisition.
Simple Autolock
Set Lock Find Resonance to Simple. If Lock Find Resonance is set to Simple at the beginning of each experiment (each initiation of an acquisition), the system searches for the lock signal if necessary, and then optimizes lock power and gain (but not phase) whenever an acquisition is initiated with go, ga, au or with any macro or menu button using the go, ga, or au.
Optimizing Autolock
Optimizing Autolock uses a sophisticated software algorithm to search the field over the full range of Z0 (as opposed to hardware simple Autolock), captures lock, and automatically adjusts lock power and gain (but not lock phase). If Lock Find Resonance is set to Every Sample, the same process as Simple occurs but only if the sample has just been changed under computer control and acquisition is started (when manually ejecting or inserting a sample, the software cannot keep track of the action and Every Sample has no effect). If Z0 is inactive during an autolock operation, autolock searches for the locksignal by changing the lock frequency.

Full Optimization
Full optimization is the most complete optimization of lock parameters. A fuzzy logic autolock algorithm automates the parameter control process in order to find the exact resonance and the optimum parameters (phase, power, gain) automatically and quickly with high reliability. Fuzzy rules are used in the program to find the exact resonance frequency and for adjusting power and phase. The fuzzy rules are implemented at different stages of the autolock process. First, the software finds the resonance. If the exact resonance cannot be found, phase and power are adjusted and the software looks for the exact resonance again. The software then optimizes the lock power to avoid saturation, optimizes the lock phase, and optimizes the lock gain to about half-range. RF frequencies, decoupler status, and temperature are also set during full optimization.