>>> varitemp.doc 1/1/02

 

 

USE OF VARIABLE TEMPERATURE ON BRUKER NMR SPECTROMETERS

 

 

I. Introduction

 

A.  A stable, controlled temperature can be achieved only if both adequate heating and adequate cooling are provided at the position of the thermocouple used to measure the temperature.

 

B.  Heating

 

1.   Heating is provided by a heater mounted in the probe in the path of the cooling gas.

 

2.   Provided that adequate cooling is available, the temperature controller will set the appropriate heater current.  However, any time the system is close to equilibrium at the set temperature, the heater current must not be at either its minimum or its maximum value.  If it is, the controller will not be able to allow for the small changes in heating needed to maintain a stable temperature.  You will have to adjust the cooling conditions (i.e. bath temperature and/or flow rate) to assure that the heater current is in a reasonable range.

 

C.  Cooling

 

1.   The cooling provided to the sample will depend on both the temperature of the bath used to produce cooling and the flow of the cooling gas.

 

2.   The temperature of the bath used to cool the gas must be at least 5 °C below the temperature to be achieved.  For example, if you use room temperature air as the cooling gas, you must set the temperature to be controlled at least 5 degrees above room temperature, i.e. the measured temperature when the variable temperature device is not in use.  At very low temperature the difference between the bath temperature and the temperature to be achieved will be greater than 5°C.

 

3.   Adequate flow of an appropriately cooled gas through the probe is of utmost importance in achieving stable temperature control.  Not only does the cold gas serve the obvious function of providing the cooling needed, but it also provides mass flow needed to move the heat away from the heater and into the area of the thermocouple.  For this reason, it is desirable to use as high a flow of the cooling gas as possible.

 

 

II. Setting Up

 

A.  Spinner

 

1.   For high temperature operation, use a ceramic spinner for temperatures above 80°C.  Plastic spinners will melt!

 

2.   You may want to use the ceramic spinner for low temperature because it is heavier and will allow somewhat greater flow before the onset of spinning problems (discussed below).

 

B.  Thermocouple

 

1.   Make sure the thermocouple to the probe is connected properly.  If it is connected, room temperature should be indicated.

 

a.   On the ARX's, the large digits on the VT unit display are the measured temperature in degrees K.  It will be ~296°K.  If the thermocouple is not connected it will flash OR.

 

b.   On the Avance spectrometers, type edte to activate the temperature display.  If the thermocouple is not connected, the actual temperature reading will contain dashes.

 

C.  Set up the appropriate cooling.

 

1.   High Temperature Operation (~ 5°C above room temperature and higher)

 

a.   The maximum temperature specification for 5 mm probes is 180°C and for 10 mm probes is 150°C.  The ceramic spinner must be used above 80°C.

 

b.   Use air for flow through the probe.  No cooling bath is required.

 

2.   Low Temperature Operation

 

a.   For low temperature operation, you will insert a heat exchange coil between the incoming cooling gas and its connection to the probe.  There is a small coil that will fit in a 4 liter dewar for short term operation or you can use the large heat exchange coil that fits in the 25 liter dewars for long term operation.

 

1.   Make sure the heater is turned off, then detach the air line from the ball joint at the probe.  Disconnect the metal socket with green plastic attached from the tygon tubing.  Insert the green plastic piece (with the metal socket) onto the tygon tubing on the black insulated end of the heat exchange coil.  Put the tygon tubing of the air line onto the uninsulated end of the heat exchange coil.

 

2.   To achieve temperatures from ~ 5°C to ~ 5°C above room temperature, you could use a wet ice bath.  Continue to use air for flow through the probe. Place the heat exchange coil into a tall 4 liter dewar and position the dewar under the magnet such that the metal socket can be reattached to the ball joint on the probe without strain.  Pack wet ice around the heat exchange coil.  A small amount of water should be added to improve the heat exchange.

 

3.   To achieve temperatures from ~ -60°C to ~ 5°C, you could use a dry ice/isopropanol bath.  Continue to use air for flow through the probe.  (The air is dried by a catalytic system intended to produce a dew point of -73°C.)  Insert the short copper heat exchange coil into the air line and position it as described above.  Pack crushed dry ice around the heat exchange coil.  Carefully pour isopropanol into the dewar.  (Please do not use acetone since it dissolves the paint when spilled and is considerably more flammable.)  There is a large plastic beaker available for bringing isopropanol near the magnet.  Do not under any circumstances bring metal scoops or solvent cans into the area of the magnet!  To reach temperatures very much below room temperature, wrap cotton around the ball joint where the air line attaches to the probe.

 

4.   To achieve temperatures of ~ -60°C and below, you must use a liquid nitrogen bath.  You can cover the range from very low temperature to room temperature in one session using a liquid nitrogen bath.  It will be necessary to switch all spectrometer functions that require gas flow (sample spinning, sample eject, and gas flow through the probe) to nitrogen gas rather than air when using a liquid nitrogen bath.  On the wall near the spectrometer, there are valves and a diagram that describes how to switch between air and nitrogen gas.  The source of the nitrogen gas is a manifold of "VGL's" located in the NMR Service Corridor (Room 1425).  NMR lab personnel make an attempt to keep a supply available, but use is sporadic and the liquid nitrogen in the VGL's doesn't last forever when not in use.  It is always a good idea to check on the VGL's in advance of your expected usage.

 

For short term operation, insert the short copper heat exchange coil into the nitrogen line and position it as described above.  Wrap cotton around the ball joint where the cooling line attaches to the probe.  Pour liquid nitrogen into the dewar.  There is a white 4 liter dewar available for transporting liquid nitrogen.  Do not under any circumstances bring a metal dewar into the area of the magnet!  Cover the top of the dewar to help keep the bath cold longer.

 

For long term operation, fill the 25-liter dewar (not all the way to the top since inserting the heat exchange coil will cause it to boil over if it is too full).  There is a low blue plastic cart available for transporting the dewar to fill it.  Use a small wood dowel to measure the liquid nitrogen level in the dewar.  Insert the dowel for about a minute.  Soon after it is removed, frost will form on the portion that was in liquid.  Insert the long copper heat exchange coil into the nitrogen line as described above.  Slowly lower it into the 25 liter dewar of liquid nitrogen. Position the dewar such that the metal socket can be reattached to the ball joint on the probe without strain.  Wrap cotton around the ball joint where the nitrogen line attaches to the probe.  There is a plastic funnel that can be inserted into the top of the heat exchange coil when it is installed in the 25 liter dewar.  At any time, the dowel can be inserted through the funnel to check the liquid nitrogen level and liquid nitrogen can be added to the dewar.  There is a white 4 liter transport dewar for adding additional liquid nitrogen.  Do not under any circumstances bring a metal dewar into the area of the magnet!

 

 

III. Operation

 

Operation is simple in that you a) set the desired temperature, b) establish a reasonable flow of the cooling gas through the probe, c) turn on the probe heater, and d) allow the temperature to equiplibrate.  Below are comments and specific directions for these steps and some additional considerations.

 

A.  Set the desired temperature

 

1.   Type edte to enter the temperature controller menu.  While this menu is displayed, changes will take effect immediately.  The "Sample temp." is the temperature measured at the thermocouple in the probe.  The "Target temp." is the temperature that you want to achieve at the thermocouple.  To change the Target temp. select the button to the right labeled "Change…".  Select "Sample target temp:"  You will have to delete the value in the box before typing in a new value (in deg K).  The change will be sent to the VT device as soon as you type Enter on the keyboard.

 

2.   Unfortunately, setting the temperature through the convenience of the edte menu does not change the parameter TE that is included with your parameters.  You will have to input that manually to have it correct on your plot.

 

B.  Establish a reasonable flow through the probe.

 

1.   On the ARX's, the flow through the probe is adjusted with the valve in the flowmeter that is mounted on the VT unit.  On the Avance spectrometers, the flow is controlled through the edte menu.

 

2.   Once you have set up for low temperature operation, the probe temperature should begin to change soon after a reasonable flow is established.

 

3.   As a general approach, turn up the flow as much as possible without causing spinning problems (see G. below).  On the other hand, for long term operation a lower flow is good in that it will conserve coolant.  Be sure to check that the flow is appropriate (see F. below). 

 

C.  Turn on the probe heater.

 

1.   ARX's:  Press the button labeled "HEATER".

 

2.   Avance500:  In the edte menu select Heater ON.

 

3.   There may be a fair amount of overshoot when changing the temperature a lot.  When initially setting the temperature, be sure to take this into account to avoid heating or cooling your sample too much.

 

D.  Allow the temperature to equilibrate at the desired setting.

 

E.   Check for leaks.

 

1.   Look at the connection of the cooling line or the transfer line to the ball joint of the probe.  If it appears that there is a leak there, turn off the heater in the probe, reduce the gas flow substantially, and disconnect the ball joint.  If a lot of ice has formed, use the hair dryer to melt it.  (See comments below under “Shutting Down” about using the hair dryer in high magnetic field.)

 

2.   Put a small amount of grease on the ball joint and re-attach the cooling line to the probe.  (Do not use stopcock grease!  Use the grease provided in the NMR lab.)  Re-establish the flow and turn the heater back on.

 

F.   Consider whether the flow is appropriate.

 

1.   Turn up the flow slightly.  If this causes the temperature to rise, then the flow rate was not high enough.  It is difficult to achieve a stable temperature with inadequate flow.

 

2.   In the edte menu, note the value of the heater current.  If the current is at the minimum value, more cooling is needed which can be achieved by higher flow or by using a colder bath.  If it is at the maximum value, less cooling is needed which can be achieved by turning down the flow if it is too high or by using a warmer bath.  The maximum heater current can be controlled through the software.  Select the button labeled "Set Max…" to go to the menu that displays and allows changes to the maximum.

 

3.   It is a good idea to record the flow you use since temperature calibration may be somewhat flow dependent.

 

G.  Dealing with spinning prolems.

 

1.   If the sample stops spinning, the spinning air will be turned up automatically to maximum.  This will usually cause the sample to rise to the top of the magnet.  Turn on the eject air then turn down the cooling flow.  This will turn off the spinning air until the sample is re-inserted.  Re-establish the spinning then turn up the flow more slowly.

 

2.   The ceramic spinner or even a second spinner (if your tube is long enough) can be used for additional weight.

 

H.  Wait for the sample temperature to equilibrate.

 

1.   Once the temperature of the probe has equilibrated with the appropriate flow conditions, wait several more minutes to allow the sample temperature to equilibrate.

 

I.    Shimming:

 

1.   The sample will have to be re-shimmed at the new temperature.  Furthermore, the shimming will change with time as the metal holding the probe and shim coils changes temperature gradually.  For long term data acquisition, autoshimming (at least for the Z gradient) is recommended.

 

a.   The choice of shims to be adjusted with autoshimming is set in the "MENU" of the shim control unit.  Simultaneously press the keys labeled "2nd" (the orange key in the lower right) and the key directly above it.  Turn the endless knob until "5. Shim-Ampl." is displayed.  Press "2nd".  Turn the endless knob until "5. Ampl. Z" is displayed.  Press "2nd" to view the current value of the step size to be used.  This should be a small number, ~1 or 2.  If you wish to change it, rotate the endless knob until the desired value is displayed.  Press "2nd".  Exit from setting Z by pressing "STD BY".  Rotate the endless knob to display "5. Ampl. Z2", etc.  Only those shims that have non-zero values in this Shim-Ampl. menu will be adjusted when autoshimming.  Press "STD BY" as many times as needed to return to the "Standby" display.  To activate the autoshimming, press "AUTOSHIM".

 

J.   Tuning

 

1.   For careful work, the tuning of the probe should be adjusted at the new temperature.  (Come to the “Hardware Set-up” Checkout to learn to tune.  Please ask for specific instructions if you plan to tune the ARX400 QNP probe.)

 

 

J.   Avoiding Disaster

 

1.   Room temperature air is supposed to be left flowing around the shims to keep the magnet dewar from getting too hot or cold.  Make sure that you can hear it.  For long term operation at extreme temperatures, turn up the flow.

 

 

 

IV. Calibration

 

A.  The temperature indicated is that of the thermocouple mounted near the sample.  The sample temperature is always slightly different.  How different it is depends on how far from room temperature you are working and the flow conditions that you are using.  Take note of the flow conditions you are using when you do the calibration.

 

B.   For careful work that requires accurate temperature measurement, you should calibrate the temperature difference between the thermocouple and the sample by using one of the temperature calibration samples located in the standard sample rack.  Use neat methanol for low temperature and neat ethylene glycol for high temperature.  These samples do not contain deuterated solvents and must be run unlocked, i.e. make sure that the field sweep is off!

 

C.  Charts of the chemical shift differences vs. temperature are posted on the side of the VT cabinet.

 

 

V. Shutting Down

 

A.  Do not put a different sample into the probe while the probe is still too hot or cold for the solvent of the new sample.

 

B.   Bring the probe back to room temperature.

 

1.   After high temperature operation:

 

a.   Simply turn off the probe heater.  At the ARX’s, press the “HEATER” button.  At the Avance spectrometers, turn the Heater OFF in the edte menu.

 

b.   Leave the flow through the probe fairly high until the probe has cooled down.

 

2.   After low temperature operation:

 

a.   Set the probe temperature to 295°K.

 

b.   Once the probe is around room temperature, turn off the heater in the probe.  Wait a short period of time until the probe temperature just begins to drop.  !!GENTLY!! remove the connection at the ball joint of the probe dewar.  If the connection is very frosty and does not come off readily with a gentle tug, heat up this connection with a hair dryer from the NMR lab.  (Don't ever use "heat guns".)  The hair dryers are mostly (but not entirely) plastic.  The hair dryer will begin to be pulled out of your hands at about the same time that the magnetic field begins to stop the motor from turning.  Carefully bring it close enough to heat the joint without stopping the motor.  Use the "high" setting on the hair dryer if there is a choice.  When the frost is mostly gone, try again gently to remove the connection.

 

c.   Remove the heat exchange coil from the cooling line.  Re-connect the cooling line to the ball joint on the probe. Leave the flow through the probe fairly high until the probe has warmed up sufficiently.

 

d.   If you were using gaseous nitrogen, switch the spectrometer from nitrogen gas back to air.  (See diagrams on the walls.)

 

e.   Turn the heater in the probe back on by pressing “HEATER” on the ARX systems or turning Heater ON in the edte menu of the Avance spectrometers.

 

f.    Leave the variable temperature unit operating with the temperature set to 295°K until essentially 0 heater current is required to maintain that temperature.  Then turn off the probe heater.  At the ARX’s, press the “HEATER” button.  At the Avance spectrometers, turn the Heater OFF in the edte menu.  Don’t leave the lab until the probe heater can be left off.

 

g.   If you used the 25 liter dewar, remove the heat exchange coil and put the lid on.  If the dewar is closed up it will keep the liquid nitrogen that is left for quite a while.

 

C. Turn the flow back to a reasonable value.

 

1.   On the ARX's, turn the valve on the flowmeter mounted in the VT unit to leave the ball at ~5.

 

2.   On the Avance spectrometers, set the flow through the edte menu to 270 l/hr.

 

D.  Return the variable temperature items that you used to the VT cabinet.  Especially, do not leave the ceramic spinners sitting out.  They are somewhat brittle and very expensive.  The large heat exchange coils may be left at the spectrometers since they are too large for the cabinet.