DNP Experiments using OVJ

• 1: Controlling the MPS from OVJ
• 2: 1H ODNP Spectroscopy
• 3: DNP Power Build-UP (Saturation) Cruve
• 4: Using the External MPS Trigger

1 - Controlling the MPS from OVJ

The Brigde12 MPS can be completely remote controlled from within OVJ. Make sure, the MPS is connected to the computer running OVJ using a USB cable.

The Bridge12 MPS is controlled from a dedicated panel within OVJ. To access the panel:

1. Select the tab label Start from the panel
2. Click on MPS to access the panel

Most controls in this panel are labeled identically to the front panel labels of the MPS.

• WG: This check box controls the waveguide status. OVJ will allow the use to only check one box at a time. For example, if the DNP box is checked, the EPR box will be unchecked and vice versa.
• RF: This check box controls the microwave power output status:
  • Off: The microwave power is disabled
  • On: The microwave power is enabled
  • Ext: The MPS is waiting for an external trigger signal to enable the microwave power
• Frequency (GHz): Microwave frequency in GHz
• Power (dBm): Microwave power in dBm. If your are not familiar with the dBm scale, a conversion table is available in the MPS documentation
• Reflected power: The value printed here indicates the amount of reflected power from the cavity.
• Frequency lock: This check box enables the software-AFC feature of the MPS.
• MPS Reset: This will trigger the MPS to reboot.

OVJ calculates the ¹H frequency from the microwave frequency that the MPS is set to. It is displayed as H1 MHz (from MPS). The frequency is given in MHz. Under it, the current OVJ ¹H frequency is given. In most cases, these two values should be close. To set the spectrometer frequency to the calculated ¹H frequency from the MPS, hit the button labeled Set H1.

2 - 1H ODNP Spectroscopy

In this first example we demonstrate how to record an NMR spectrum with and without microwave power. Here, it is assumed that the microwave resonator is properly connected to the MPS, and that the resonator is properly tuned and matched.

The NMR signal recorded without microwave power is often referred to as the Off Signal, or the thermal equilibrium (TE) signal, while the signal recorded with microwave power is often referred to as the On Signal. In most cases, optimizing the spectral parameters using the On Signal is simpler due to the increased signal-to-noise of the spectrum.

Sample: For this example we use a sample of 10 mM TEMPO dissolved in water. The sample is loaded in the appropriate sample tube and inserted into the resonator.

Prior to recording a DNP spectrum the user has to:

• Adjust the microwave coupling to minimize the reflected microwave power.
• If necessary, record an EPR spectrum of the sample
• Adjust the magnetic field strength so the microwave radiation is on-resonance with an EPR transition

Recording the On Signal

To record the On Signal:

1. Select the PROTON experiment (seqfile=‘s2pul’) and set the NMR acquisition parameters ((nt, sw, tof, np, d1, pw, etc.) to some common values.
2. Go to the Start tab and select the MPS panel.
3. Select the DNP checkmark (under WG) in the MPS tab. This will set the waveguide switch to select the microwave path for DNP. A waveguide switch is commonly installed in systems that are based on an EPR spectrometer. Your system configuration may differ.
4. In the MPS panel, enter the microwave frequency. This is the same frequency that the EPR resonator is tuned to.
5. Enter the microwave power (see this table for conversion of mW to dBm). The power requirement for a DNP experiment strongly depends on the nature of the sample. A good starting value is 23 dBm (200 mW).
6. Make sure the MPS mode is set to Manual.
7. To enable the microwave power, click on the On check box under RF.
8. Acquire an NMR spectrum by clicking the go button or by typing the go command in the OVJ terminal.
9. Phase the spectrum. Note: Typically, the off-signal is phased so the NMR signal has a positive amplitude. In an ODNP experiment, the enhanced peak will be inverted.

The ODNP-enhanced spectrum is shown in the figure below.

Next, optimize the acquisition parameters of the NMR experiment.

Recording the Off Signal

Once the acquisition parameters are optimized you can record the off-signal, the NMR signal without microwave power. Typically, the signal-to-noise ratio will be much lower and often more averaging is required. To record the off-signal:

1. Turn off the microwave power by clicking the Off check box under RF.
2. Acquire an NMR spectrum by clicking the go button or by typing the go command in the OVJ terminal.

If the signal-to-noise ratio is not sufficient, increase the number of acquisitions (OVJ parameter nt).

Keep in mind, to calculate the DNP enhancement, either both, the on and the off spectrum have to be recorded with the same number of transients, or the signals have to be properly normalized.

3 - DNP Power Build-UP (Saturation) Cruve

A commonly performed DNP experiment is to record the NMR signal intensity with respect to the microwave power. The result is the DNP power saturation behavior.

In this example we will automatically record the DNP saturation curve for a sample of 10 mM TEMPO in water.

To record the DNP saturation experiment:

1. Select the PROTON experiment (seqfile=‘s2pul’) and set the NMR acquisition parameters ((nt, sw, tof, np, d1, pw, etc.) to some common values.

2. Connect the MPS TRIG output to the MPS Ext. Trig input using a BNC cable.

3. Go to the Start tab and select the MPS panel.

4. Select the DNP checkmark (under WG) in the MPS tab. This will set the waveguide switch to select the microwave path for DNP. A waveguide switch is commonly installed in systems that are based on an EPR spectrometer. Your system configuration may differ.

5. In the MPS panel, enter the microwave frequency. This is the same frequency that the EPR resonator is tuned to.

6. Select Ext. mode from the MPS mode dropdown menu.

7. We will use continuous microwave radiation during the experiment. To turn the microwave power on permanently during the experiment set the xm variable to the value ‘yyy’

   1

   xm='yyy'

8. Next, we will create an array in OVJ with the microwave power values. For this the parameter mpspower needs to be arrayed. Create an array from 0 to 40 with a stepsize of 1, resulting in a total of 40 elements.

   Note

   Keep in mind a microwave power level of 0 dBm corresponds to 1 mW of microwave power, not 0 mW.

9. Start the experiment by clicking the go button or by typing the go command in the OVJ terminal

Once the experiment is finished, the results can be conveniently displayed using the dssh command

4 - Using the External MPS Trigger

The Bridge12 MPS microwave source can be controlled by an external trigger to enable and disable the microwave power programmatically through the NMR pulse sequence.

To be able to use the external trigger, connect the MPS TRIG output from the SCN backpanel to the MPS Ext. Trig input of the MPS backpanel, using a BNC cable.

To enable the external trigger:

1. Select a pulse sequence.
2. In the MPS panel select the Ext mode from the dropdown menu.
3. Select the checkmark Ext under RF

Now, the MPS is set to accept an external trigger signal. In this mode the RF output set by the status() command during the pulsesequence. In OVJ, every pulse sequence is divided into three different sections, labeled A, B, and C. This label is shown in the bottom row when displaying a pulse sequency in OVJ using the dsp command.

The microwave status (on or off) for these sections is controlled by the xm variable in the pulse program.

1

xm

To enable the microwave power during Status(A) in the pulse program, but disable the power during Status(B), and Status(C) the xm variable has to be set to

1

xm='ynn'

When only a single character is given the status() is set for A,B,C to the same value.

When the xm variable is set, the status can be seen in the display of the pulsesequence by typing

1

dps

An additional line labeled MW appears, indicating the status of the microwave output during each segment of the pulse sequency. In this example, the microwave power is enabled (ON) during Status(A) but disabled during Status(B), and Status(C).

Note that the position of the status() statement in the pulsesequence determines where a state starts and ends. As an example the following modified ‘s2pul’ includes p1 in status A and pw in status B

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// Copyright (C) 2022 Bridge12Technologies
// You may distribute under the terms of either the GNU General Public

/*  s2pul - standard two-pulse sequence with modified status */

#include <standard.h>

void pulsesequence()
{
   /* equilibrium period */
   status(A);
   hsdelay(d1);

   /* --- tau delay --- */
   pulse(p1, zero);
   status(B);
   hsdelay(d2);

   /* --- observe period --- */
   pulse(pw,oph);
   status(C);
}