EPR spectrometer & components

Transcription

1 EPR spectrometer & components Water lines µ wave bridge VC41 Gas Flow Controller Temperature Controller Vacuum pump for cryostat on left hand side water chiller HASKRIS EMX EPR Spectrometer EMX Magnet Power Supply Diaphragm pump

2 Getting Started (turning the instrument on) 1. Open the two yellow handled water valves simultaneously. Check for leaks and report if any are found. 2. Turn on the Haskris water chiller. The power may require cycling a few times (Chuck states 6 times in his instructions) if it has been off for a while. Again, check for leaks. 3. Turn on the computer, monitor, and printer if they are not already on. 4. Turn on the bridge power supply with the big red switch on the back of the spectrometer console. The liquid crystal frequency display on the µ wave bridge should display something. 5. Check the water chiller to confirm the orange liquid level indicator is on and the green temperature light is flashing if not, the chiller requires water. 6. Allow ~15 minutes for the temperature to stabilize. 7. Turn on the magnet power supply at the bottom front of the console. Two steady green lights should indicate all is okay. The magnet and bridge require ½ to 1 hour to stabilize, during that time open the WINEPR acquisition software (see step 8), prepare samples, etc. 8. Launch software with Bruker EMX icon located on the computer desktop

3 Preparing to Tune Make sure the bridge is in stand by mode Disconnect the iris motor shaft from the white Teflon iris screw Using the down arrow, run the iris control all the way down until arrow greys, indicating maximum depth is reached. Manually screw the white Teflon iris screw clockwise to the maximum down position, then back it out ½ turn. Re insert the motor shaft, and reposition the motor so that the shaft is vertical. Back the iris out using the mouse and the up arrow 2 3 full turns. This is most easily done with a second person watching. Tuning the Bridge Go to tune mode Set the attenuation to 30 db. Move the frequency slider slowly until a dip is observed on the view screen. This may be hard to find if the bias, signal phase, and iris are far from optimal. Once observed, center the dip with the frequency control. Adjust the signal phase for a symmetrical dip, resulting in a signal like this Set the bias control approximately center scale (50%)

4 Fine Tuning the Bridge Change to operate mode and set the attenuation to 40 db Adjust the frequency to lock the AFC meter to center scale Adjust the Bias level: Set the attenuation to 50 db; if the AFC meter falls out of lock, readjust with the frequency control. Then, adjust the Bias slider until the Diode meter needle is centered. Match the cavity: Set the attenuation to 40 db and re center the diode meter with the iris control (up and down arrows). This may take several turns of the iris and the diode may go down before it comes back up. Keep the frequency centered with the frequency control. Then repeat the procedure at 30 db, 20 db, and 10 db. Adjust the signal phase control in a continuous single direction, until you reach a local maximum in the diode current. Leave the signal phase at this maximum setting and do not adjust it further. Re center the diode with the iris control (up and down arrows). Decrease the attenuation in small steps, keeping the diode meter centered with the iris control. If the AFC drops out of lock increase the attenuation and relock it with the frequency control. Then, increase the attenuation to 50 db in 10 db increments. The meters should remain centered with out adjusting anything.

5 Inserting the sample Make sure the instrument is in stand by mode. Carefully remove the compression fitting at the top of the cavity and locate one with the proper size tube opening (if the cryostat is in, use the appropriate sized top hat). Insert the fitting and lightly tighten. Wipe the sample tube clean and place it in the cavity. Adjust the sample depth by gently pushing up on the pedestal (white adjustment plug, at the bottom of the cavity) or down on the sample as necessary to center the sample in the cavity. (If the cryostat is in place, there is no pedestal / white adjustment plug.) After inserting a sample it may be necessary to retune.

6 Parameter table See subsequent pages for discussion of parameters to optimize for best sensitivity

7 Parameters: Optimization Candidates Conversion Time the amount of time the A to D converter spends integrating one field position before moving to the next value. Affects the dynamic range Time Constant filters out noise Beware, if you choose a time constant that is excessively high, relative to your sweep time, you may filter out your signal Resolution in X (Number of Data Points) Determines the appropriate sweep time; you should have at least 10 points within the narrowest line that you are trying to resolve to properly define it. Number of points available are 512, 1024, 2048, 4096 or 8192 As you double the number of data points, you should also double the time constant Modulation Amplitude Use a field modulation that is approximately the width of the narrowest EPR line you are trying to resolve Excessive modulation broadens the EPR and does not contribute to a more intense signal Shown below is the effect of using progressively higher field modulation on the EPR spectrum

8 Parameters: Optimization Candidates Receiver Gain Effect of using gain settings that are either optimal (a), too low (b), or too high (c) Microwave Bridge Power The intensity of the EPR signal increases with the square root of the power, in the absence of saturation effects Effect of using progressively higher powers on EPR spectrum: Number of X scans or signal averaging Signal averaging can achieve the same effect as a long scan time when in the absence of a stable laboratory environment

9 Instrument Shut Down Go to standby mode in the MW bridge control window of the EPR software Remove sample and cover the top of the cavity Shut off the power to bridge in back and magnet power supply at the bottom front. Tidy up while additional cool water circulates around the bridge control unit. Shut off the Haskris water chiller Take your stuff back to lab, but remember to come back. All the warm water in the chiller needs to flush out. Turn off the water with the two yellow handles

10 Low Temperature Why? EPR frequencies (1 100 GHz) are in the microwave range o Aqueous solutions will warm up in the EPR cavity at RT The energy difference between the 2 energy levels due to the Zeeman splitting is very small, ~ 0.3 cm 1 for X band EPR. o Based on the Boltzmann distribution, n 1 = n 0 exp-(δe/kt), it can be shown that only at low temperatures there will be enough difference in the population of the S = ½ level (n0) and the S = + ½ level (n1) to create a signal. Spin Lattice Relaxation Rate decreases with decreasing temperature o The relaxation rate is anisotropic (i.e. is different for different parts of the spectrum). Due to the uncertainty principle the EPR spectra will broaden beyond detection at higher temperatures. At lower temperatures the spectra will sharpen up. o This sharpening up of the spectrum by cooling the sample is, however, limited by a temperature independent process: inhomogeneous broadening. For low temperature experiments, please see the facility manager about training and usage