Table of Contents

Robarts Imagingi

Important Guidelines for Operation

Cryogens

Replacing Liquid Nitrogen

Replacing Liquid Helium

Sample Preparation

Cooling the VTI

Running a Dissolution

Pre-Dissolution Procedure (HMI Version 2 Software)

Initiating the Dissolution Process

Triggering a Dissolution (HMI Version 2 Software)

Handling the Dissolution Liquid

Cleaning the Polarizer

Microwave Calibration

Installing the RF Hardware

T/R Switch

RF Coil

Setting up the Scanner

Index

General Discussion

Important Guidelines for Operation

The HyperSense dynamic nuclear polarizer (DNP) is computer operated and has an array of alarms and checks to mitigate operator error. Despite this level of monitoring things can go wrong. An operator must be aware of a small list of potential mistakes that should not be made since these cannot be prevented by the computer controls and are catastrophic in their effect.

  1. Depletion of cryogens
  2. Multiple dissolution cups in the VTI during dissolution process
  3. No dissolution cup in the VTI during dissolution process

One cannot recover from these mistakes and they lead to a service call by an Oxford Instruments engineer. They are not covered under our extended service contract. Proper operation of the equipment, a discussion of how these mistakes can happen and the scope of the damage are discussed later.

Generally, following proper procedures as outlined below should prevent any damage from occurring to the apparatus and potential injury to the user.

Cryogens

This topic is also covered in Chapter 6 of the HyperSense manual beginning on page 76.

The DNP requires both liquid nitrogen LN2 and liquid helium LHe for operation. The LN2 is used to reduce the amount of LHe boil-off while the LHe is used to keep the superconducting magnet and shims cold and produce temperatures as cold as 1.4K for polarization. These must be handled with great care due to the potential for asphyxiation and severe “frostbite” to exposed skin. Two dewars of these cryogens are always on hand: 60l of LHe and 230l of LN2. The DNP is equipped with an alarm system to remind users that either of the cryogens is low. When the LN2 level drops to 10% of full, or the LHe level falls to 35% of full, an audible alarm is sounded until acknowledged by a user. This is not a serious situation but requires attention. The DNP must never run out of either of these cryogens. In the case of depletion of LHe, the superconducting magnet and shims will quench requiring a long and expensive service call.

An oxygen sensor is mounted in the HyperSense lab and will sense a low level of oxygen. This will trigger an audible alarm. Immediately vacate the space if an alarm is sounded since sudden asphyxiation from a lack of oxygen can occur. Occasionally during filling of the DNP, oxygen can be displaced from the space and an alarm may be triggered. To prevent this, always leave the laboratory door open during the fill process and use a short section of plastic hose to direct the boil-off gases towards the room exhaust grill.

Replacing Liquid Nitrogen

Insulated gloves and a face shield are mandatory during cryogen fills. They are located on shelves in the DNP laboratory.

LN2 is stored in the larger of the two dewars. It is connected to the LN2 fill port with a 4’ flexible stainless hose designed to transfer cyrogens. Move the dewar into the DNP laboratory from its storage area. Connect the transfer hose to the dewar to the port labeled “Liquid” on the dewar. Attach the other end of the hose to the port on the side of the DNP labeled “LN2 Fill”. Open the turret doors and remove the LN2 pressure relief valve on the top of the cryostat. Fasten a piece of plastic hose to the vent to direct the boil-off vapor towards the room exhaust vent.

Bring up the main HyperSense control screen on the system monitor labeled “Overview”. This contains two bar graph displays for the cryogen levels. Click the “Fast Update” option below the bars. Open the liquid port valve on the dewar slowly and begin filling the dewar. Approximately one turn of the valve should be sufficient flow after the fill hose cools down to provide a steady flow rate to the cyrostat. Avoid filling too quickly since this will lead to excess boil-off and excessive pressure in the cryostat. Maximum pressure should never exceed 5 psi. When the LN2 level reaches 100% close the dewar valve.

Wearing the thermally insulated gloves provided, disconnect the fill hose. Be careful that no extra LN2 that might be left in the hose splashes on you. It may be necessary to gently warm the hose connections with a heatgun before removal. Use common sense not to overheat any connections. After the hose is disconnected, remove the plastic vent hose – again using the heat gun to warm it first. Replace the pressure relief valve on the cryostat vent. Return the dewar to its storage location and hang the fill hose back up on its hook on the wall in the HyperSense lab.

A cryogen usage log is located in the lab on the shelves above the workbench. Record the cryogen level before and after each fill as well as the date and your name. This will allow us to track cryogen usage and determine a regular schedule for delivery of cryogens.

Replacing Liquid Helium

Refilling the LHe cryostat is similar to the procedure for LN2. LHe is much colder than LN2 and extra precaution should be exercised to prevent personal injury. In addition to the insulated gloves, a full face shield should be worn to prevent splashing of the cryogen in the facial area.

The 60l LHe dewar is stored in the HyperSense lab. Transfer apparatus consists of a special purpose insulated transfer hose with insulated 3/8” diameter stainless steel tubes at either end. Unfortunately the length of these tubes and the height of the dewar presents a challenge. A section of ceiling tiles should be temporarily pushed aside to permit the insertion of the dip end of the transfer apparatus into the LHe dewar.

The following instructions must be precisely followed. Move the dewar to a position near the rounded end of the apparatus. Open the turret on the DNP. Loosen the compression fitting on the cryostat port labeled “Helium Fill” and leave the stopper in place. Remove the checkvalve on the DNP port labeled “He4 EXHAUST” (see picture above). The checkvalve is sealed to the port by an o-ring and held in place with a standard vacuum clamp.

Slowly open the blue handle on the LHe dewar and release the internal pressure of He gas. Screw on the pipe thread to hose adapter onto this port. Close the valve on the manifold on the compressed He cylinder labeled “To DNP” and open the manifold valve labeled “To LHe Dewar”. Reduce the pressure of He gas using the regulator until a very gentle flow can be felt from the other end of the hose. The output pressure at the regulator should be nearly zero. Again, pressure should never exceed 5 psi. Close the blue-handled valve on the LHe dewar and connect a hose from the compressed He cylinder to the barbed fitting on the blue-handled port. Temporarily close the red-handled valve to the pressure relief port on the LHe dewar.

Loosen the top brass compression fitting and remove the 3/8” stopper. Open the yellow-handled valve on the top of the dewar. Wearing the insulated gloves and face shield, slowly feed the longer tube of the LHe transfer line into the LHe dewar. This should slide snuggly through the 3/8” diameter o-ring port on the top of the dewar. If it does not slide with little force, loosen the compression fitting further. When the tube touches the surface of the LHe, boil-off gas will be emitted from the other end of the transfer line. Point this away from you. Continue to feed the entire transfer tube into the dewar until it touches the bottom of the dewar. Tighten the compression fiting. As the transfer line cools, the rushing sound of gas will change to a chugging noise and the boil-off gas will rapidly change to a dense short grey jet of LHe. Only after this has been observed should the free end of the transfer line be introduced into the DNP.

Remove the stopper from the LHe fill port in the compression fitting on the fill port of the DNP (see previous picture). Feed the fill tube smoothly and continuously into the cryostat trying not to hesitate until the tube hits the bottom of the cryostat. It will engage into a funnel. Withdraw the tube approximately 2 cm from its lowest position and hand-tighten the compression fitting. Open the blue-handled valve on the LHe dewar. Again, monitoring the LHe level using the Overview screen on the system monitor, fill the He cryostat to 100%. Click the “Fast Update” option below the bar graphs to get a better measurement response. Avoid filling the cryostat too quickly. This will increase boil-off losses and stress the cryostat. The cryostat level should rise at a rate of 2 to 3 percent per minute or less.

When the level reaches 100%, close the pressurizing valve on the He gas cylinder manifold. Remove the rubber hose and release the pressure in the LHe Dewar through this port. Remove the transfer tube from the DNP unit. You should warm the compression fitting with a heat gun to allow the tube to easily withdraw. In addition, gently warm the area around the needle valve located immediately behind the He fill port. Remove the dip tube from the LHe dewar. Close the yellow-handled and blue-handled ports. Replace the stopper on the top of the LHe dewar and close the upper valve (yellow). Open the red-handled port to the pressure relief valve.

Replace the helium check valve on the DNP port labeled He4 EXHAUST”. Re-pressurize the helium gas line to the DNP. Pressure on the gas regulator should read 115 psi. Return the He transfer line to its storage hooks on the wall. Move the LHe dewar back to its initial location. Record the fill parameters in the cryogen log book on the shelf in the DNP lab.

Sample Preparation

Samples are prepared using standard recipes found in the recipe book. In general these recipes designate the amount of substrate (typically pyruvic acid), free radical (i.e. OX 063 for pyruvic acid) and solvent (distilled water with EDTA buffer). These substances are usually pipetted, weighed and mixed. Once the sample is prepared, the correct amount measured by either weight or volume is loaded into a sample cup. These sample cups provided by Oxford Instruments are unique to the DNP unit. They are PEEK plastic with a capacity of ~100 l. In general, only one sample cups should be in use at a time to prevent accidents involving insertion of more than one cup into the polarizer. An adjustable volume pipette and microbalance are available in the DNP laboratory to assist with sample loading.

The plastic sample cups are re-useable but have a finite lifetime. These should always be inspected following ejection after a polarization has occurred. If the cup shows any sign of damage such as chipping it should be discarded. Also, prior to use, the cup should be fitted empty into the sample insertion rod to check the fit. If the fit is at all loose the cup should be discarded as this indicates that there is some chance the cup will not be picked up by sample movement system in the instrument.

Cooling the VTI

If the DNP equipment has been idle for sometime or a bake-out had been performed, the variable temperature insert will rise in temperature to approximately 90K. It is good practice to cool down the VTI after a bakeout and/or before preparing a sample for dissolution. On the main overview screen is a button on the right, labeled “Cooldown”. Click this button and the machine will spring to life. The rotary pump will begin to pump and the butterfly valve to the VTI will automatically open. The needle valve controlling the LHe level in the VTI will also be controlled to bring the level to 65% of full. The machine handles all this automatically. Using the HMI (Human machine interface), one can monitor this activity by clicking on the “PID Control” button. There are separate tabs for the needle valve control loop and the butterfly valve loop. Generally, the needle valve will open fully to 100% and begin to close as the LHe level approaches 65%. The butterfly valve is throttled from full open as the pressure starts to fall to approximately 3 mBar and the temperature of the VTI falls below 1.5K. Pre-cooling of the VTI is required before running a dissolution. The “Run Dissolution” button will be greyed out and inactive until the VTI has been cooled. To minimize waiting, pre-cool the instrument before preparing the sample. In general, the system should not be left running in the “Cooldown” mode for a long period of time if a dissolution is not impending since this will increase the use of cryogens. To exit the cool-down mode, click the “Idle” button on the right side of the GUI.

Running a Dissolution

A very complete set of instructions for running a dissolution are described beginning on page 40 of the Oxford Instruments DNP manual including screenshots of the HMI and the equipment. Here is a quick list of steps that are also posted on the wall next to the DNP:

Pre-Dissolution Procedure (HMI Version 2 Software)

If the MQC is to be used for a measurement of liquid state polarization, first make sure that the MQC software is set up and the Trigger Cable is connected from the HyperSense to the MQC.

Open the RIMNR software (if not already open) on the DNP laptop computer.

  1. Type .HYPERSENSENMR, in the box in the empty box at the bottom of the GUI and then press return (Do not click GO or press the green arrow).
  2. Input the system configuration number: 1 and click on “Select Config”.
  3. Click the “Solid Build Up” button.
  4. Enter the number of minutes to run the solid state build up monitoring (typically 60 minutes)
  5. Enter a file name: SSBuildupxxx, where xxx is a number in the sequence of files stored with build up data. (This is automatically incremented by the GUI and presented to user.) Click “OK”
  6. Follow the screen prompts.

Initiating the Dissolution Process

  1. Select “Run Dissolution” from the Main Screen on the HMI.
  2. Click on “Load Sample” and follow the wizard instructions:
  1. Load the appropriate method file.
  2. Leave the “Enable buildup monitoring” box unchecked
  3. “Select trigger type” should be “Manual”
  4. Leave the “Start irradiation immediately (during cooling)” box unchecked
  5. The wizard will display “Click on next when the sample is ready to be inserted…”
  6. At this juncture, the sample cup should be attached to the sample insertion wand in preparation for insertion into the VTI. The sample cup should be firmly attached to the insertion wand. (If you are unsure about how this is done, seek help or read Section 4.5.2, page 42 of the HyperSense manual.)
  7. Click on the “Next” button on the GUI.
  8. After the GUI displays ‘Click on “Next” when the sample has been inserted and the stick is in the dissolution position’, open the doors to the turret and insert the wand and sample cup into the VTI. When the cup is at the bottom of the VTI, hold wand and pull back on the knob leaving the sample cup at the bottom. Remove wand and check that the sample cup was left in the VTI. If cup was not properly inserted, STOP. Immediately consult the DNP manual page 46 (and following pages) and get help. Do not proceed if cup was not inserted or a problem is suspected.”
  1. The “Load Solvent Wizard” will now appear, enter 55 minutes in the “Snooze” box. Do not click on snooze at this time.
  2. When the temperature has reached 1.45K, polarization will start. You will see the bar at the bottom of the GUI change from “Cooling” to “Polarising” and a blue bar will track the progress.
  3. On the HMI click “Snooze” in the Solvent Wizard. (The “Load Solvent” form will disappear and then re-appear when the snooze period lapses to remind user to load solvents and ready the MQC spectrometer if it is being used.)
  4. On the laptop, click “OK” on the dialog box to start monitoring the solid-state buildup.
  5. Check the polarization curve on the laptop to ensure data is being logged. (A data point is logged every 5 minutes).

Triggering a Dissolution (HMI Version 2 Software)

  1. Click on “Load Solvent” and follow directions from the wizard to load the solvent into the “Bomb”. Typical solvent used for quality assurance is 4ml of EDTA buffered distilled water. Check recipe book for particulars. Click on “Finished” when you completed the solvent loading procedure.
  2. If a liquid state polarization measurement with the MQC is desired follow directions below five to ten minutes prior to the end of the polarization phase (otherwise just click on “Ready” next to “NMR Spectrometer is ready”).
  1. Insert the reference sample into the MQC.
  2. Run the 18mm Thermal.ris script.
  3. Save the file as “Diss_ref_xxx” (Example: Diss_Ref_001).
  4. Press start when ready, after 2 minutes the MQC will display the reference signal. Record this figure, then click on “OK”.
  5. Remove the reference sample from the MQC.
  6. On the MQC run the Autotrigger 18mm_dissolve.ris script.
  7. Save as “xxx_xxx” (Example Diss_PA_001 for Pyruvic Acid)
  8. Press start, the MQC will now wait for a signal from the HyperSenese to trigger the final reading.
  9. On the HMI click on “Ready” next to “NMR Spectrometer is ready”.
  1. When ready to carry out the dissolution note the polarization time and press “Trigger” on the HMI. Note: wait until the last data point is logged by the solid-state build-up monitoring program on the laptop.

Handling the Dissolution Liquid

For an MQC measurement with HP001: Place the dissolution transfer line in the test tube and shake the tube as the fluid flows in to it, then put into the MQC as soon as the HMI status bar reads “Chasing”.