EMX User`s Manual
Contents
1. Levels gt Density Plot Layout gt v Stacked Plot DataType gt Scan Plot Settings 3437 5 3450 0 3462 5 3475 0 3487 5 3500 0 3512 5 3525 0 F2 G Figure 5 26 Setting the display mode to Stack Plot 5 28 Helpful Hints 6 Hints for Finding EPR Signals Cryostats shifts the res onant freguency of the cavity and hence the freguency of the spec trometer to a lower value The field for res onance of your EPR sig nals will therefore be lower than you would expect for a cavity with out a cryostat This chapter contains useful and helpful hints to get the most out of your EMX spectrometer and its hardware The first half of this chapter covers advice on what to do if you do not observe an EPR signal from your sample The second half of the chapter concerns itself with optimizing the performance of the EPR spectrometer for your particular sample and operating condi tions It is assumed that you are familiar with the material pre sented in Chapter 2 and Chapter 3 6 1 Make sure that the spectrometer is functioning properly If you followed the directions of Chapter 3 this should not be a problem There are many common mistakes Is the modulation cable connected properly to the cavity and console Is the waveguide gasket installed properly Is everything turned on Advice on troubleshooting is pre sented in the next chapter Scan over the correct magnetic field range If you do2. 8 A MICROWAVE A DE zZ N reeessow E 8 gt A H Figure 6 4 Location of the AFC MOD LEVEL potentiometer Cryostats can protect your cavity from con tamination due to sam ple tube breakage Insert a cryostat in the cavity Ouartz has a dielectric constant of 3 8 but a low dielectric loss Inserting high purity guartz sleeves such as the variable temperature dewar actu ally concentrates the microwave magnetic field intensity at the sample The increased field intensity produces an EPR signal that has a larger signal to noise ratio than is achieved in the absence of the dewar insert If your experiments approach the sensitivity limit and your samples are nonlossy you may benefit from the use of the variable temperature quartz insert dewar even if the experiment is run at room temperature EMX User s Manual 6 7 Optimizing Sensitivity Parameter Selection 6 2 2 e Optimize the receiver gain You need to have sufficient receiver gain in order to see all the details in your spectrum Figure 6 5 shows the results of insufficient as well as exces sive receiver gain If the receiver gain is too low you will see the effect of digitization in the spectrum spectrum b whereas at high gain the signals will clip due to an overload in the signal channel spectrum c A good way to automati cally optimize your receiver g
3. Additional Technigues 5 Manually Tuning a Microwave Bridge MVA A klystron bridge reguires approximately three minutes to warm up after the console is turned on When the Stand By indicator is green the software allows you to switch to Tune mode This chapter provides instructions for procedures that are routine for some users but may be infreguently encountered by others Specifically the chapter will describe manually tuning the EMX spectrometer changing cavities fine tuning the AFC and per forming automated 2D experiments 5 1 The Auto Tune routine of the EMX software is effective at tun ing the cavity and bridge under most circumstances However there are some circumstances where automatic tuning may have difficulties Lossy samples such as water can be problematic particularly when you work at high microwave power levels Following these instructions will help you to tune the spectrom eter under these adverse conditions 1 Open the Microwave Bridge Control dialog box If this window is not already open click its button the button labeled MW in the tool bar The button toggles the dialog box open and closed The microwave bridge con trol dialog box will then appear See Figure 5 1 2 Switch the microwave bridge to Tune mode The bridge status indicator shows the three states or modes for the microwave bridge Stand By Tune and Operate See Figure 5 1 In Stand By the power to the micro
4. not sweep over the correct magnetic field range you will miss your signals This mistake occurs guite often when using a cryostat in the EPR cavity Consult literature refer ences to determine approximate g values for the species in your sample You can then choose the appropriate magnetic field for your sample Most organic radicals will have a g value of approximately 2 This corresponds to a field for resonance of approximately 3480 Gauss at a microwave fre guency of 9 8 GHz Metal ions can have large departures from g 2 as well as large zero field splittings making it dif ficult to guess where the resonance might occur Performing a wide scan in your initial experiment will maximizes your probability of finding the EPR signal EMX User s Manual Hints for Finding EPR Signals Finding an EPR signal Sometimes you may have diffi culty finding the EPR signal from an unknown sample or a sample you are not familiar with Here we provide two exam ples of parameter sets that are useful for finding EPR signals from unknown samples that you suspect will consist of either an organic radical See Figure 6 1 or a transition metal ion See Figure 6 2 respectively These parameters are by no means optimized but they will serve to help you find the sig nal After you find the EPR signal you need to reset the field center and scan range See Section 4 3 You also need to optimize your EPR signal using the method described later in t
5. the dialog box on and off The Interactive Spectrometer Control dialog box will then disappear See Figure 5 5 and Figure 5 6 Open the Microwave Bridge Control dialog box If this window is not already open click its button See Figure 5 5 in the tool bar The button toggles the dialog box open and closed The Microwave Bridge Control dialog box will then appear See Figure 5 7 Attenuation W Microwave Bridge Control Mil Display Frequency 76 6 fF SSS GSO a O UNI Bias 43 0 Iris Oper ae ee oe Stand By nal Phase 46 8 th Button 2 3 AutoTune JT Tris o Fine Tune Buttons Stop Tuning TF Dual Trace Figure 5 7 The Microwave Bridge Control dialog box Cos 2 A A m a K Changing EPR Cavities 6 Switch the microwave bridge to Stand By mode Click the Stand By button in the dialog box to change to the Stand By mode See Figure 5 7 Waveguide kis Screws Motor BA paai AN No Waveguide Gasket Nitrogen Purge Iris Port Motor L Shaft n KN Z Modulation Cable Figure 5 8 Connections on the ER 4102ST cavity EMX User s Manual 5 13 Changing EPR Cavities Store the lock nut in a place where it will not be lost 10 Disconnect accessories If a variable temperature dewar assembly is installed disconnec
6. 5 21 Fine AFC Tuning for Gunn Diode Bridges 5 Adjust the FINE AFC Potentiometer Turn the knob until the AFC needle is once again centered in the AFC meter Figure 5 20 AFC EME Adjust the lt 4 needle to the AFC center Luni m Figure 5 20 Centering the AFC meter Verify that the AFC needle remains centered Vary the microwave attenuation between 0 and 60 dB Note that there may be a drift at 0 dB caused by sample heating if you have a lossy sample in the cavity Also the needle may rush off to the left or right at low powers because the AFC loses lock In most cases the AFC will lock again at higher microwave power levels If not switching between Operate and Tune modes and back again at 30 dB atten uation will lock the AFC once more Then increase the attenuation more slowly than the previous time Repeat Step 2 through Step 6 until the needle remains centered Record the microwave frequency and FINE AFC potentiometer setting The setting is microwave fre quency dependent and reproducible If you record the set ting at that microwave frequency you need not perform this whole procedure every time you use low microwave power levels Because only the insertion of a cryostat sub stantially shifts the microwave frequency you will typi cally only need a setting for a cavity with and without a cryostat 5 22 2 A A m a i Performing 2D Experiments Performing 2D Exp
7. bridge Maintain a controlled environment for the best spectrometer performance Air drafts past the spectrom eter especially the cavity may induce temperature fluctua tions or microphonics from sample vibration Large fluctuations in the ambient temperature may degrade perfor mance by reducing the frequency stability of the cavity Very humid environments may cause water condensation You can reduce condensation inside the cavity by maintaining a con stant purging stream of dry nitrogen gas Note that excessive gas flow rates can generate microphonic noise through sam ple vibration Minimize electrical interference Noise pick up from electromagnetic interference EMI noise may be encoun tered in some environments You may be able to minimize EMI noise by shielding or perhaps by turning the noise source off if generated by equipment near the spectrometer There is often less EMI at night EMX User s Manual 6 5 Optimizing Sensitivity Allow the spectrometer to warm up One hour is usu ally adeguate to achieve a stable operating temperature For maximum stability under extreme operating conditions such as any combination of high microwave power high magnetic field modulation amplitudes and variable temperature work allow the system to equilibrate under the same conditions as the experiment will be performed Carefully follow the procedure for positioning the sample inside the cavity This is particularly important
8. in the center of the magnet air gap by moving the bridge on the table Carefully tighten the stabilizers Be careful not to stress the waveguide when expanding the stabilizers Reconnect the nitrogen purge line and adjust the flow rate for a light flow Installing the waveguide gasket properly 5 16 Changing EPR Cavities 18 Reconnect the iris motor shaft to the iris screw The procedure here is like Step 10 performed in reverse Reposition the iris screw motor Screw the lock nut onto the iris screw Click and hold the up iris button in the Microwave Bridge Control dialog box until the iris screw is approximately half way out 01881913 21515112 Bale IIS elm F ETA Ne Experiment Options Button Figure 5 13 The Experiment Options button 19 Read in the calibration file for the cavity Open the Experiment Options dialog box in order to read in the calibration information If this window is not already open click its button See Figure 5 13 in the tool bar The button toggles the dialog box open and closed The Experiment Options dialog box will then appear Click on the Change File button A new dialog box Open Calibration File will appear Select the appropriate cali bration file for your cavity and click OK This will auto matically load the calibration data you have selected Confirm that the calibration file is the correct one for the cavity The calibration file
9. name usually consists of two or three letters that identify the type of cavity ST for ER 4102ST or TM for ER 4103TM followed by the serial number of the cavity This number is located on either the front or back of the cavity Clicking Cancel returns you to the Experiment Options dialog box EMX User s Manual 5 17 Changing EPR Cavities E Signal Channel Options LJalxl Resonator Tuning Caps fe c winepr tpu st9227 cal I SCT Selftest f E e Resonator T a m asar E IX AFC Trap Filter Modulation Signal Input Lock in F external Acquisition Trigger Internal Internal a Change F Lock n Integrator External External O External File Open Calibration File 21x c winepr tpu File name Folders dm9304 cal dpphtest cal hall0121 cal mvt9406 cal st9227 cal st9515 cal st9515a cal st9515b cal List files of type Drives scr Calibration Files ce x J c micron z Figure 5 14 The Experiment Options and Open Cali bration File dialog boxes Service engineers often save the calibration files in the c acquisit tpu directory during the installation of the spectrometer 5 18 sk ser Fine AFC Tuning for Gunn Diode Bridges Fine AFC Tuning for Gunn Diode Bridges 5 3 The AFC Automatic Freguency Control is the circuitry used to lock the microwave source freguency to the resonant fre quency of the cavity In most cases part
10. sure that you have at least 10 data points within the narrowest line that you are trying to resolve This means that for EPR signals with very narrow lines you will need to increase the number of data points that are collected for a given field sweep How ever if the lines of your EPR signal are sufficiently wide increasing the number of data points will not yield any addi tional information but will only result in longer sweep times With the EMX you can select 512 1024 2048 4096 or 8192 data points Remember you will probably want to increase i Optimizing Sensitivity the time constant by a factor of two as you double the number of data points Figure 6 7 shows the enhancement in resolu tion achieved by increasing the number of data points Figure 6 7 Expanded view of narrow lines in an EPR spectrum using 1024 points a or 8192 points b e Optimize the field modulation amplitude Excessive field modulation broadens the EPR lines and does not con tribute to a more intense signal Figure 6 8 shows the results of excessive field modulation You can see how some of the smaller lines in spectrum a were lost in spectrum b even after increasing the modulation only slightly A good rule of thumb is to use a field modulation that is approximately the width of the narrowest EPR line you are trying to resolve Keep in mind that there is always a compromise that must be made between resolving narrow lines and increasi
11. the AFC offset is close to zero Magnetic field drifts Magnetic field drift may produce an asymmetric or distorted line for samples exhibiting very nar row resonance linewidths This problem may arise for line widths less than 20 mG Use a field frequency lock system to eliminate field drift problems 7 16 i Warning Noises No Signal When Everything Works 7 12 Check cables Make sure that all the cables are connected Check the modulation cable and the preamplifier cable Sample position If you have a small sample make sure that the sample is centered in the cavity Magnetic field values Are you using the correct field val ues to see your EPR signal If you are using a cryostat remember that the microwave freguency drops and hence the field for resonance will also be lower Is the Hall probe posi tioned properly in the magnet Warning Noises 7 13 High pitched noise from the heat exchanger The heat exchanger will emit a high pitched noise when it reguires more distilled and deionized water Funny noises from the iris motor Stop turning the iris motor immediately You may be breaking the iris screw EMX User s Manual
12. the new cav ity See Figure 5 11 Visually position them just above the magnet pole caps e i bw 3 Figure 5 11 Installing the waveguide stabilizers EMX User s Manual 5 15 Changing EPR Cavities Steps 14 and 15 are used to set the limit switches in the iris motor The limit switches prevent you from screwing the iris in too far and thereby breaking the iris screw Make sure you connect the modulation cable to the MOD modulation connector and not the R S Rapid Scan con nector 13 14 15 16 17 Figure 5 12 Attach the appropriate size collet and pedestal on the cavity Screw in the iris Manually turn the iris screw until it is almost all the way in The iris screw will stop rotating It may be a good idea to back the screw out 1 2 turn after it hits the bottom This will further decrease your chances of accidentally breaking the iris screw dur ing the tune procedure Click and hold the down Iris Button Activate this button See Figure 5 1 until the iris motor stops this is the lower limit of the motor With the iris motor in its lower limit reattach the iris motor drive to the iris screw Connect the modulation cable to the cavity Reconnect the waveguide sections and tighten the stabilizers Do not forget to install the waveguide flange gasket between the two flanges make sure it is ori ented correctly See Figure 5 12 Position the cavity
13. wave source is shut off When you switch to Tune the source turns on and you produce a frequency sweep that allows you to see the dip of your cavity Switching to Operate causes power only at the resonant frequency to be transmitted to the cavity When you turn on your spec trometer it should be in Stand By mode which is indi cated by Stand By appearing in the Microwave Bridge Control menu See Figure 5 1 If you have been acquir EMX User s Manual Manually Tuning a Microwave Bridge JE WINEPR ACQUISITION Microwave Bridge Control Eile View Options Window Info Deis EPdeiic STANDBY Frequency 0 00 Microwave Power AFC amp Diode Meter KI ovaie Bridge Attenuation Control Display Ration Tune Button a av Bias E a Slider a Buttons S eel Figure 5 1 The Microwave Bridge Control dialog box ing spectra already your bridge will probably be in Oper ate mode Click the Tune button in the dialog box to change to the Tune mode 3 Set the microwave attenuator to 25 dB The micro Y A wave attenuation is set by clicking the arrows on either ou may notice that LEVELED and side of the attenuation display See Figure 5 1 The UNCALIBRATED arrows on the left side change the attenuation in 10 dB appear in the bridge sta steps the arrows on the right side change the attenuation tus indicator Do not be in 1 dB steps alarmed
14. B Adjust the Bias slider bar See Figure 5 1 until the Diode meter needle is centered You can locate the Diode meter by referring to Figure 5 1 The center corresponds to 200 microamperes of diode current Sometimes particularly when the cavity has a low Q the AFC meter may rush off either to the right or left and lose lock at 50 dB In most cases the AFC will lock again at higher microwave power levels If not switching between Operate and Tune modes and back again at 30 dB attenuation will lock the AFC once more Match the cavity For maximum sensitivity we need to critically couple or match the cavity to the waveguide Critical coupling results in a maximum power transfer between the waveguide and the cavity It also means that no incident microwaves are reflected back from the cav ity If the cavity and waveguide are truly matched the reflected microwave power seen by the detector should remain constant i e 0 when we vary the attenuation This is the criterion we use for critical coupling You control the coupling or matching of the cavity by adjust ing the iris screw First increase the microwave power by 10 dB i e attenuator setting 40 dB Click the T or J iris buttons for the iris screw motor until the diode current again returns to 200 microamperes i e The needle is EMX User s Manual 5 9 Manually Tuning a Microwave Bridge centered Repeat the procedure 10 dB steps in the atten uator setting an
15. Hall incremental Sweep Signal Channel Field Sweep Center Field 484 54 Starting Sweep Width i0000 Microwave MW Power Sweep iver Gain 2 00 ulation Frequency gt 100 00 S 483116 Power ulation Amplitude Select m Dtp Modulation Phase Microwave Value 97097 GHz Offset Power Sweep N i A 5 o z mW Time Constant 5 Power i H db Conversion Time fea 2 msec Step Sweep Time 5 24 sec emperature unit I i Temperature F Set the Number l Step 1 00 4 of Scans to 5 pa Ee Goniometer gt Number of X Sca gt 1 E Angle FF deg Resolution in Y ME Step fi o00 EH deg Repetitive Mode r Save as Defaut Commen Figure 5 21 Sample parameter settings for acquiring a 2D data set 4 Set the starting microwave power to 0 2 milli watts Change the power in the power setting box to 0 2 milliwatts This will be the power that is used to acquire your first spectra See Figure 5 21 5 Use a step value of 5 dB By using a negative step value the power will increase in units of 5 dB between each scan See Figure 5 21 6 Set the number of spectra to be acquired to 5 In the Resolution in Y box change the setting to 5 This will program the spectrometer to acquire 5 scans See Figure 5 21 Click OK to close the window T Click on Run to acquire your 2D data set This will initiate the first of five scans with the power increa
16. ain is to use the set field center and field range button in the tool bar as described in Section 4 3 1 When you draw a rectangle around the entire spectrum the receiver gain is automatically set such that the newly acguired spectrum will fill the display completely a HAAA Saade paha Uu Figure 6 5 Effect of using gain settings that are either a optimal b too low or c too high on an EPR spectrum Optimizing Sensitivity Optimize the conversion time The conversion time you select will affect the dynamic range of your experiments The conversion time is actually the amount of time the ana log to digital converter spends integrating at one field posi tion before moving to the next field value in the sweep If you need to resolve lines that are very intense as well as lines that are very weak i e carbon 13 satellites within the same spec trum you will need to use a sufficiently long conversion time If the conversion time is too short the smaller signals will be lost in the steps of the digitizer The conversion time you select will also determine the sweep time That is the sweep time will be equal to the conversion time multiplied by the number of data points in the spectrum See selecting the number of data points below Optimize the time constant for the selected conver sion time The time constant filters out noise however if you choose a time constant
17. ants Tune the signal reference phase Gunn Diode Microwave Sources While the dip is in the center of the display adjust the Signal Phase slider bar See Figure 5 1 until the depth of the dip is maximized and the dip looks somewhat symmetric See Figure 5 2 We shall fine tune this phase later but this procedure gets us close to the correct phase Tune the signal reference phase Klystron Microwave Sources While the dip is in the center of the display adjust the Signal Phase slider bar See Figure 5 1 until the shoulders on each side of the dip appear to be approximately the same height and the dip looks somewhat symmetric See Figure 5 3 We shall fine tune this phase later but this procedure gets us close to the correct phase 2 A A m a K Manually Tuning a Microwave Bridge 12 13 14 Fine tune the microwave source freguency Click the Operate button in the dialog box to change to the Operate mode Adjust the Freguency slider bar until the needle of the AFC meter is centered You can locate the AFC meter by referring to Figure 5 1 Sometimes the needle may rush off to the right or left edges of the meter This happens when the AFC Automatic Freguency Con trol is no longer locked If this happens click the Tune button to return to the Tune mode Repeat Step 9 and then try again Adjust the bias level Change the microwave attenua tion to 50 d
18. based on this mechanism The spectral profile may change mark edly if the modulation frequency is varied while applying strong microwave fields See Figure 2 17 Magnetic field gradients These may produce highly asymmetric lineshapes Reposition the cavity within the mag net air gap to check the magnet for homogeneity Check for magnetic objects in or around air gap Magnetic field inho mogeneity could also broaden the response to obscure split tings by overlapping spectral components EMX User s Manual 7 15 Lineshape Distortion Anisotropic g matrix A highly anisotropic g matrix natu rally produces asymmetric lines Background signal A strong background signal from contamination of the EPR cavity or the sample can distort your EPR spectrum High conductivity High conductivity exhibited by sam ples with mobile electrons will result in asymmetric lines known as Dysonian lineshapes This results from a mixing of absorption and dispersion components induced in the sample itself Lossy samples If you put large lossy samples in a cavity you can also obtain Dysonian lineshapes Use progressively smaller capillaries until you obtain a symmetric lineshape Microwave reference phase The dispersion signal from easily saturated samples can be very large compared to the absorption signal To minimize the contribution of the disper sion signal carefully adjust the microwave reference phase In addition make sure that
19. by the UNCALIBRATED indi cator this is normal during Tune 5 2 wi Manually Tuning a Microwave Bridge There are two types of microwave sources The letter G in the micro wave bridge designation i e ER 041 XG on the front panel identifies a Gunn source The letter K designates a klystron source Perhaps the sur est method to identify the type of source is by comparing the mode pattern with either Figure 5 2 OT Figure 5 3 Observe the mode pattern on the display moni tor Gunn Diode Microwave Sources This mode pattern is a display of the microwave power reflected from the microwave cavity and the reference arm power as a function of the microwave freguency The mode pat tern should resemble one of the mode tuning patterns in Figure 5 2 If the mode pattern amplitude is too small increase the microwave power in 1 dB steps by decreasing the attenuation If the mode pattern amplitude is too large decrease the microwave power in dB steps by increasing the attenuation Observe the mode pattern on the display moni tor Klystron Microwave Sources This mode pat tern is a display of the microwave power reflected from the microwave cavity and the reference arm power as a function of the microwave frequency The mode pattern should resemble one of the mode tuning patterns in Figure 5 3 If the mode pattern amplitude is too small increase the microwave power in 1 dB steps by decreasing the a
20. ceiver time constant modulation microwave gain for selected power scan time Instrument related factors Optimize AFC Operate the EMX Minimize _ Minimize modulation in a stable microphonics electromagnetic depth environment interference Allow the Minimize Flow nitrogen Shield Isolate spectrometer temperature gas through the and turn to warm up fluctuations in cavity system off the noise the lab source Do not place Avoid air drafts Tighten sample Make sure iris vibrating objects collets and screw is not on bridge waveguide loose stabilizer Figure 6 3 Factors to consider when optimizing your EMX for sensitivity 6 4 Optimizing Sensitivity Optimizing Sensitivity 6 2 Instrumental Factors 6 2 1 y For better spectrometer stability keep the spec trometer away from windows and ventila tion ducts Minimize microphonics Microphonics are unwanted mechanical vibrations in the spectrometer Depending on the nature and frequency of the microphonics these vibrations may generate noise in your EPR spectrum The most com mon microphonic sources include the cavity the sample and the bridge Prevent microphonic noise by securing the waveguide with the waveguide stabilizers Rigidly secure the sample in the cavity by tightening the collets on the cavity sample stack Do not place objects on the microwave bridge that may vibrate or are free to move Avoid placing a fre quency counter with a fan on top of the
21. d adjust the current to 200 microamperes with the iris screw until you have reached an attenuator setting of 10 dB You will notice that as you increase the microwave power the diode current becomes more sensi tive to the position of the iris screw Another thing you may notice is that the AFC meter also changes with the iris screw position Simply adjust the frequency slider bar until the needle is centered again When you have reached 10 dB microwave attenuation adjust the Signal Phase slider bar until you achieve a local maximum in the diode current You should not have to adjust it very much Ver ify that you have achieved critical coupling by changing the microwave attenuation from 10 dB to 50 dB with vir tually no change in the diode current Repeat the matching and bias level adjustment procedures if necessary If you need to operate at power levels greater than 20 mW 10 dB set the attenuator to 0 dB and once again adjust the diode current to 200 microamperes with the iris screw The current can sometimes drift because the high micro wave power starts to heat the sample If this happens wait a minute or two and readjust the coupling 5 10 Changing EPR Cavities Changing EPR Cavities 5 2 1 Open the Interactive Spectrometer Control dia log box If this window is not already open click its but ton See Figure 5 5 in the tool bar The button toggles the dialog box open and closed The Interactive Spec trometer Co
22. dge to make sure the AFC is on If this error occurs during fine tune try auto tune Critically coupling cavity The iris motor has reached both of its limit switches and has been unable to obtain a diode current of 200 microamperes Check if the iris motor is still connected to the screw and that the limit switches have been set properly See Section 5 2 If you are using a flat cell when this happens it is likely that you need to adjust the position of the flat cell It is easier to optimize the cavity dip if you adjust the flat cell while you are looking at the tuning picture If this error occurs during fine tune try auto tune EMX User s Manual 7 3 No Tuning Picture No Tuning Picture 7 4 Tune mode delay period not expired klystron bridge only After you turn on the spectrometer a delay of approximately three minutes is reguired before a klystron will activate as you switch from Stand By to Tune This does not apply to Gunn diode bridges Reference microwave power too low klystron or Gunn diode bridge Carefully adjust the Bias slider bar of the Microwave Bridge Control dialog box until you observe a tuning mode pattern on the display Microwave bridge controller automatically switches from Tune to Stand By klystron or Gunn diode bridge There is insufficient cooling for the microwave source The protection circuitry will shut the microwave source off if the temperature rises too high Make sure that the va
23. emperature unit Sweep Time 335 54 sec Temperature EF EH K Harmonic 0 E Step fico E K Resolution in X 4096 Goniometer Number of X Scans fi E E EI deg cuela Resolution in Y fi H Step fi 000 HH deg ae Repetitive Mode r Figure 6 2 Parameters for finding an EPR signal from a transition metal ion e Make sure your sample is positioned correctly in the cavity Only the central region of the cavity contributes significantly to the EPR signal If you place the sample suffi ciently out of this region you may not detect a signal e Optimize the sensitivity You may have a very weak sig nal in which case you will need to optimize your parameter settings for sensitivity The chart on the following page sum marizes common factors that are important for getting the optimum sensitivity from your EPR measurements The pages that follow the chart provide a more in depth discus sion of these factors EMX User s Manual 6 3 Hints for Finding EPR Signals Sample related factors Adjust sample Optimize Make sure the position to Parameters for cavity is critically maximize Q your sample coupled Optimize Optimize Optimize field Optimize re
24. eriments 5 4 Using the WIN Acquisition software you can perform experi ments in which a second parameter i e in addition to the mag netic field can be varied For example you can perform a set of experiments in which the power is increased incrementally over several successive field scans Alternatively you might perform several consecutive experiments in which the temperature is ramped either up or down between each field scan You can then display the 2D dataset using WIN EPR This section will describe how to utilize the Acquisition software to create a 2D data set and how to display it in WIN EPR The procedure is more easily described by performing an example experiment that investigates the response of the strong pitch spectrum to microwave power 1 Insert the strong pitch sample Place the strong pitch sample into the cavity and tune the spectrometer as described in either Section 3 4 or Section 5 1 2 Open the Experimental parameter dialog box If this window is not already open click its button See Figure 5 21 in the tool bar The experimental parameter dialog box will then appear 3 Change the Y experiment setting The Y Experi ment setting will probably be set to No Y Experiment Change this by selecting MW Power Sweep See Figure 5 21 EMX User s Manual 5 23 Performing 2D Experiments Standard Parameter Spectr1 Lx Experiment X Field Sweep gt Y MW Power Sweep gt no Y Sweep
25. for better coupling This can occur when working with lossy samples such as aqueous solutions in flat cells or capillaries You need more microwave power If you are using insufficient microwave power it can be difficult to see the cavity dip We recommend setting the microwave attenuator at 25 dB for the best visibility You are not at the correct frequency By putting the sample in you will cause the cavity to resonate at a lower frequency Thus you will usually need to lower the frequency after you have placed the sample in the cavity in order to see the dip i Tuning Error Tuning Error 7 3 Both the auto tune and fine tune procedures of the microwave bridge controller will terminate with an appropriate error mes sage if a particular parameter cannot be set or optimized Here are the possible error messages Tuning Frequency Both the upper and lower limits of the frequency range 1 e 8 9 9 9 GHz have been reached and no defined dip has been detected Check manually if a dip can be found A very slight dip e g very lossy sample may not be detected by the auto tune routine Adjusting Ref Arm Phase The full 360 range of the signal phase has not resulted in an optimal phase setting Adjusting Ref Arm Bias The system is unable to set the diode current to 200 microamperes at 50 dB attenuation Adjusting AFC Lock Offset The system is unable to set the AFC lock offset to zero Check the back of the bri
26. for samples exhibiting a large dielectric loss Improper sam ple positioning can perturb the microwave field mode pat terns in the cavity resulting in less than optimum sensitivity Periodically check the iris coupling screw for tight ness of fit A worn iris screw thread will make the iris sus ceptible to microphonics which can modulate the cavity coupling Critically couple the cavity Best cavity performance is obtained with a critically coupled cavity Maximum transfer of power between the cavity and the waveguide occurs under this condition Optimize the AFC Adjust the AFC modulation depth to minimize the noise level observed in the absorption EPR spectrum at full incident microwave power Adjustments of the AFC MOD LEVEL potentiometer located on the rear of the microwave bridge Figure 6 4 should be made while in the Operate mode with the sample inserted and the spec trometer tuned as described in Section 3 4 You should make this adjustment for all experiments limited by signal to noise considerations The optimum AFC modulation depth is a function of the loaded cavity Q Consequently slight varia tions in the optimum setting may be anticipated If you are using a finger dewar with a boiling refrigerant such as liquid i Optimizing Sensitivity ER 047 PH BROAD BAND PREAMPLIFIER tretfs60ns STABILIZER 50 s OUTPUTS ol E nitrogen you should turn the AFC modulation level to maxi mum
27. his chapter If you still cannot find the signal you may have to adjust parameters such as the microwave power modula tion amplitude scan time etc Standard Parameter Spectr1 Ei Experiment X Field Sweep el Y no Y Sweep el Hall Signal Channel Center Field asooo G Receiver Gain 10 a Sweep Width pooo HH G Modulation Frequency 100 00 J kHz Static Field 3480 0 G Modulation Amplitude 4 00 G H deg Hx Microwave Bridge Modulation Phase Frequency 9 766000 GHz Offset ate Power 10 0 mW Time Constant msec Step 1 y db Conversion Time E msec Temperature unit Sweep Time 335 54 sec Temperature K Harmonic 1 Step 1 00 Ek Resolution in X 1024 Gonlomerer Number of X Scans 1 Ame E gag Resolution in Y 1 1 000 Siep Ei deg Repetitive Mode r Save as Default Comment Figure 6 1 Parameters for finding an EPR signal from an organic radical 6 2 Hints for Finding EPR Signals Experiment X FieldSweep z Ns no Y sweep aj Hall Signal Channel Center Field 2100 0 H G Receiver Gain Efo 10 5 SEER MERE eooo 4 G Modulation Frequency 190 00 E kHz Static Field 3480 0 e Modulation Amplitude 400 He Microwave Bridge Modulation Phase po E deg Frequency fa76so00 GHz Offset po E Power fi 0 0 mW Time Constant 227 58 H msec Step fi E db Conversion Time 12 H msec T
28. icularly if the micro wave attenuation is less than 40 dB the AFC works very well without any need for you to fine tune it If you are performing experiments in which low microwave powers are reguired fol lowing the instructions in this section will ensure that you will obtain optimal AFC performance Please note that this proce dure is not reguired for klystron bridges You can determine the type of bridge you have by looking at the model designation on the front plate of the bridge A model designation containing a G for example ER 041 XG indicates a microwave bridge with a Gunn diode microwave source In contrast a bridge with a model designation with a K such as ER 041 XK has a klystron microwave SOUTCE The Fine tuning Procedure 5 3 1 1 Set the FINE AFC potentiometer to zero The potentiometer for the AFC can appear in two different locations on the bridge depending on when your bridge was manufactured See Figure 5 15 2 Tune the microwave bridge Follow the procedures in Section 3 4 for automatic tuning or Section 5 1 for man ual tuning The freguency bias phase and iris screw should be adjusted so that the needles of the AFC and Diode meters remain centered as you change the micro wave attenuation from 0 to 40 dB See Figure 5 16 and Figure 5 17 Note that there may be a drift at 0 dB EMX User s Manual Fine AFC Tuning for Gunn Diode Bridges caused by sample heating if you have a lossy sample in the cavi
29. increase the coupling range of the cavity This is particularly important when working with flat cells or capillaries Contact your Bruker service represen tative for advice EMX User s Manual 7 5 Magnet Power Supply Shuts Down Magnet Power Supply Shuts Down 7 6 Insufficient cooling capacity Make sure that the heat exchanger is on and that there is sufficient cold water flowing through it Either the Ext or Temp warning LED s on the magnet power supply will light up with this fault Hall probe inserted with the wrong polarity The mag netic field will go to the maximun field Hall probe fallen out of the magnetic air gap If the Hall probe has fallen from the pole piece of the magnet the power supply may g0 to the maximum current value sD BROKER CO Baseline Distortion Baseline Distortion 7 7 Linear baseline drifts The use of very large modulation fields can produce large eddy currents in the cavity side walls These currents can interact with the magnetic field to produce a torque on the cavity and create a resonant fre quency shift A linear field dependent or modulation ampli tude dependent baseline is indicative of such an effect This phenomenon should not be observed if the cavity end plates are properly fitted and torqued Do not attempt to adjust the torque on the plates Contact your local Bruker EPR service representative Slowly and randomly varying baseline The use of high microwave
30. lves for the coolant lines leading to the bridge are open See Section 3 2 Make sure that the heat exchanger is on and has sufficient water flow Microwave bridge controller automatically switches from Tune to Stand By klystron bridge only There is protection circuitry which protects the microwave source from voltage spikes To reset the protection circuitry turn the console power off for approximately three seconds and turn it on again The voltages used in the Gunn diode bridge are not sufficiently high to reguire this type of protection circuitry sD BROKER CO Unable to Critically Couple Cavity Unable to Critically Couple Cavity 7 5 Sample position If too much of a lossy sample is in the microwave electric field in the cavity you will not be able to critically couple the cavity Move the sample until the cou pling becomes better The sample position is particularly crit ical for flat cells and capillaries Microwave reference phase If the microwave reference phase is not set properly you will not be able to critically couple the cavity Carefully follow the instructions in Section 3 4 when tuning the spectrometer Iris motor limits improperly set If the iris motor limits were improperly set the iris can not be screwed in suffi ciently Follow the procedure in Section 5 2 to properly adjust the iris motor limits Iris tip size When working with lossy samples it is advis able to use a larger iris tip to
31. nce Verify that laboratory eguipment is not a source of electromagnetic interference EMI If possible turn off all other equipment in the labora tory and observe spectrometer noise output Determine if radio microwave or TV broadcasting stations are operating in proximity to the spectrometer Record the noise level while operating at various times of the day and night EMI related noise will often be reduced at night Power line noise Check the noise content of the AC power lines feeding the spectrometer Line transients or momentary blackouts will drastically degrade the perfor mance of high gain detection systems such as EPR spectrom eters Ground loops Ground loops are very common and often difficult to avoid Disconnect accessory eguipment espe cially if it is plugged into remote AC outlets and observe the noise level Turn off the magnet power supply and observe the noise level If the noise level changes during either of these tests consult your local Bruker EPR service representa tive for alternate installation planning EMX User s Manual 7 9 Excessive Noise Output Microphonic generated noise Secure the waveguide and cavity assembly by using the plastic waveguide stabiliz ers Secure the sample firmly in the collet If you use a cry ostat make sure that the cryostat sits firmly in the cavity Make sure that an excessive nitrogen gas flow rate through the cryostat does not vibrate the sample Worn iri
32. ng your EMX User s Manual 6 11 Optimizing Sensitivity signal to noise ratio If you have a very weak signal you may need to sacrifice resolution i e by using a higher field mod ulation in order to even detect the signal However if you have a high signal to noise ratio you may choose to use a much lower field modulation in order to maximize resolu tion Figure 6 8 Effect of using progressively higher field modulation a d on an EPR spectrum Optimizing Sensitivity e Optimize the microwave power level The intensity of an EPR signal increases with the square root of the micro wave power in the absence of saturation effects When satu ration sets in the signals broaden and become weaker EPR signals with very narrow lines are particularly susceptible to distortion by excessive power Figure 6 9 shows the result of excessive microwave power You should try several micro wave power levels to find the optimal microwave power for your sample A convenient way to find the optimum power is to use the 2D experiment routine described in Section 5 4 tA a Figure 6 9 Effect of using progressively higher power a c on an EPR spectrum EMX User s Manual 6 13 Optimizing Sensitivity Signal averaging With a perfectly stable laboratory envi ronment and spectrometer signal averaging and acguiring a spectrum with a long scan time and a long time constant are eguivalent Unfortunately perfect
33. ntrol dialog box will then appear See Figure 5 6 0135143 SX Bs 20509 994 E 5 EI Microwave Interactive Bridge Spectrometer Control Control Button Button Figure 5 5 The Interactive Spectrometer Control button W Interactive Spectrometer Control lol Hall Signal Channel Center Field 3480 00 H G Calibrated Sweep Width 0 00 H G Receiver Gain H 6 32 10 p Sweep Address 2048 Modulation Freq 100 00 E kuz af gt Modulation Amplitude 0 96 ie Field 3480 000 G Modulation Phase 24 0 00 HH ideg Setup Scan Offset 0 00 Hx T Enable Time Constant 5 12 HH msec Sweep Width 100 00 ai ConversionTime 6 12 HH msec Harmonic 1 MW Attenuator Ege jo dB Temperature Kk Set parameters to spectrum SCT Options Figure 5 6 The Interactive Spectrometer Control dia log box EMX User s Manual 5 11 Changing EPR Cavities AN 3 Setting the magnetic field to the minimum 4 value avoids the risk of i magnetizing your watch when changing cavities Set the modulation amplitude to zero Enter a value of 0 00 in the Modulation Amplitude box Set the magnetic field to the minimal value Enter in a value of 0 00 in the Sweep Width box and a value of 0 00 in the Center Field box Close the Interactive Spectrometer Control dia log box Click the Interactive Spectrometer Control the button labeled I in the tool bar The button toggles
34. power or large modulation fields can heat the cavity and the sample The ensuing thermal drifts in the coupling of the cavity as well as the frequency of the cavity can result in a fluctuating offset in the signal Allow the tuned cavity and sample to come to thermal equilibrium before per forming the final tuning of the cavity Once the cavity is equilibrated and properly tuned under the equilibrated condi tion you can start acquiring a spectrum Avoid air drafts around the cavity as they can randomly change the tempera ture of the cavity and sample and hence the baseline of the spectrum EMX User s Manual 7 7 Baseline Distortion Variable temperature operation Cavity freguency and coupling instability may be induced during variable tempera ture operation especially at very low or very high tempera tures Increase the flow rate of the cavity and waveguide purging gas as the operating temperature departs further from room temperature Wait for the cavity to stabilize at each new operating temperature before recording the spectrum Retune the cavity to compensate for any freguency shift and re establish critical coupling at each temperature Background signal Your cavity cryostat sample tube or sample may be contaminated Call your local Bruker EPR Service representative for advice Never take the cavity apart to clean it 7 8 Excessive Noise Output Excessive Noise Output 7 8 Electromagnetic interfere
35. r Many problems are easily solved by the user The flow diagram on this page will help you diagnose the majority of problems that occur during the tuning phase of operation If you fail to find a solution to your problem after reading this chapter call your local Bruker EPR service representative Unable to couple cavity Sample not aligned Reference phase not set correctly Iris motor limits not set correctly No Tuning No picture dip Power too low Adjust 7 Attenuation as to 25 dB Bridge switches to standby a automatically requency Waveguide Over voltage Insufficient gasket occured cooling misaligned Figure 7 1 Flow Chart for diagnosing problems EMX User s Manual not ready not ready No Cavity Dip 7 1 If a warning dialog box appears when you first start the Acquisition program with a message such as Field Control ler not ready or Signal Channel not ready you have probably forgotten to turn the console power supply on 7 2 Waveguide gasket installed improperly See Figure 5 12 for the proper orientation of the gasket Cavity undercoupled or overcoupled First look at the microwave frequency where you normally expect the cavity to resonate and then adjust the iris screw
36. s screw Check for a worn iris coupling screw An iris screw that does not fit snugly in the waveguide may gen erate noise by modulating the cavity coupling Replace the worn iris screw with a new one Boiling liquids If you are using a dewar with a boiling refrigerant such as liquid nitrogen you will need to increase the AFC modulation level 7 10 Poor Sensitivity Poor Sensitivity 7 9 Excessive microwave power The microwave power may be settoo high which will cause your sample to satu rate Optimize the power for your sample by recording spec tra at a variety of power levels Wrong cavity type for sample The type of cavity you use for a particular sample can make a large difference in sensitivity Consult the Bruker literature on the full line of EPR cavities to determine which one is best for your sam ples Low cavity Q The cavity Q can be degraded because of improper sample positioning Having your sample positioned in the microwave electric field will reduce the sensitivity by degrading the cavity Q especially for samples with high dielectric loss This can happen if you are using flat cells or capillaries Observe the Q value read out in the microwave bridge dialog box when you are adjusting the sample posi tion Cavity not critically coupled Maximum power is trans ferred between the cavity and waveguide when the cavity properly matches the impedance of the waveguide i e is critically coupled A dras
37. sing in 5 24 sk ger Performing 2D Experiments units of 5 dB between each scan You will notice the scan number updating in the box in the upper right corner of the spectrum window See Figure 5 22 Spectr1 lol 10 3 7 Current Scan Display 3440 3460 3480 3500 3520 GI Figure 5 22 Current scan display 8 Transfer your 2D data set to WIN EPR By clicking the WIN EPR button you will launch the WIN EPR pro gram and automatically load your dataset See Figure 5 23 0251818 2 21 E Ja le ee WIN EPR Button Figure 5 23 Launching the WIN EPR program from Win Acquisition 9 Display your 2D data set Select 2D Processing from the WIN EPR System menu See Figure 5 24 Your data should automatically appear as seen in Figure 5 25 If your data does not appear as in Figure 5 25 make sure the display mode is set to Stack Plot See Figure 5 26 EMX User s Manual 5 25 Performing 2D Experiments Figure 5 24 Opening a 2D dataset 5 26 ehoner Performing 2D Experiments WINEPR System 2D Processing Eile 2D Processing Parameters Display Options Help anes Se ar Eee a Na ao 10 3 3 3437 5 3450 0 3462 5 3475 0 3487 5 3500 0 3512 5 3525 0 F2 G Figure 5 25 Stack plot display of 2D dataset EMX User s Manual 5 27 Performing 2D Experiments WINEPR System 2D Processing Options Help Scaling Ree ad
38. stability is usually impos sible to attain and slow variations can result in considerable baseline drifts A common cause of such variations are room temperature changes or air drafts around the cavity For a slow scan the variations cause broad features to appear in the spectrum as shown in spectrum b of Figure 6 10 You can achieve the same sensitivity without baseline distortion by using the signal averaging routine with a small time constant and shorter scan time For example if you were to signal average the EPR spectrum using a scan time that was signifi cantly shorter than the variation time these baseline features could be averaged out In this case the baseline drift will cause only a DC offset in each of the scanned spectra Spec trum a shows the improvement in baseline stability through the use of short time scans with signal averaging when the laboratory environment is not stable Saas p e Till Figure 6 10 a Signal acguired with short time sweeps and signal averaging b Signal acguired with long time sweep and long time constant 6 14 Troubleshooting 7 This chapter lists some common problems you may encounter with your Bruker EMX EPR spectrometer Major hardware mal functions are not covered We concentrate on problems due to operator errors set up errors or protective circuitry The mate rial presented in Chapters 2 3 and 4 is useful in understanding much of what is discussed in this chapte
39. t the coolant trans fer line and the thermocouple connections from the cavity Disconnect the modulation cable from the cavity This is the twinax cable labeled with a white connector and attached to the front of the cavity See Figure 5 8 Disconnect the nitrogen purge line from the port on the waveguide The port is half way down the waveguide attached to the cavity See Figure 5 8 Disconnect the iris motor shaft from the iris screw First unscrew the lock nut from the iris screw Lift the shaft upwards to disconnect Move the iris motor to the side where it is out of the way See Figure 5 9 Iris Motor Shaft Iris Screw Figure 5 9 Disconnecting the iris motor shaft from the iris screw 5 14 2 A A m a i Changing EPR Cavities 11 Disconnect the cavity See Figure 5 8 While grasp ing the waveguide attached to the cavity with one hand unscrew the four waveguide screws joining the two sec tions of waveguide Loosen the waveguide stabilizers rotating the screws and carefully remove the cavity from the air gap of the magnet See Figure 5 10 Take care not to lose the gasket which was between the two wave guide flanges Seal the cavity with the solid collets and put the cavity in a safe clean place Figure 5 10 Loosening the waveguide stabilizers 12 Install the waveguide stabilizers on
40. that is excessively high relative to your sweep time you may actually filter out your signal You should adjust your time constant to fit the conversion time you have selected These two parameters are actually very related because the conversion time will determine the total sweep time You need to use a time constant that will be suf ficiently long to filter out undesirable noise yet short enough that you do not distort your signal Therefore if you want to use a longer time constant you will need to increase the scan time as well Figure 6 6 shows the effect of progressively increasing the time constant while maintaining the same sweep time All the spectra are at the same scale A safe rule of thumb is to make sure that the time needed to scan through an EPR signal i e one EPR line is ten times greater than the length of the time constant A time constant that is 1 4 that of the conversion time will guarantee that your spectrum is not distorted However for samples limited by a low signal to noise ratio you may want to make the time constant equal to the conversion time or greater EMX User s Manual 6 9 Optimizing Sensitivity se ANS 1 A E ee Figure 6 6 Effect of using a progressively longer time constant a d on an EPR spectrum Selecting the number of data points The number of data points is the other parameter that will determine the appropriate sweep time A general rule is to make
41. tically undercoupled iris will not transmit power to the cavity and so will not excite EPR tran sitions A drastically overcoupled cavity will have a lower Q resulting in lower sensitivity These effects can occur when using lossy samples such as aqueous solutions or conducting samples EMX User s Manual Poor Sensitivity Water condensation During low temperature operation water can condense inside the cavity Water being a high dielectric loss material will absorb the microwave power in the cavity and destroy the cavity Q Avoid condensation by using a purging nitrogen gas flow through the cavity Signal channel not calibrated The modulation ampli tude and phase of the signal channel may not be properly cal ibrated Make sure that you load the proper calibration file into the data system Also make sure that the Calibrated check button in the Interactive Spectrometer Control dia log box is not un checked Receiver gain or modulation not optimized See Section 6 2 2 Sample not positioned properly Center your sample in the cavity i Poor Resolution Poor Resolution 7 10 Microwave power set too high Saturating microwave power levels will broaden your resonance line Verify that the linewidth is independent of the microwave power level by recording the spectrum at various power levels Modulation amplitude set too high Large field modu lation amplitudes will broaden your resonance line partic
42. to accessories in the air gap Spectrometer not thermally stabilized Be sure that the spectrometer has been turned on for several hours Verify that the laboratory conditions are within specified limitations i e temperature fluctuations efc 7 14 Lineshape Distortion Lineshape Distortion 7 11 Microwave power too high The effect of saturating microwave fields is to broaden the resonance This is easily apparent for single structureless lines however small split tings may become unresolvable if strongly saturating levels of microwave power is used Lower the microwave power until you obtain a power independent lineshape Modulation amplitude too high Large field modulation will broaden the resonance line Lower the modulation amplitude to a region where the lineshape is independent of the modulation amplitude See Figure 6 8 Time constant too long for sweep time used A safe rule of thumb is that the time required to sweep through an EPR line should be ten times the length of the time constant See Figure 6 6 Modulation frequency too high The modulation fre quency can determine the resolution of the experiment The spectral profile may also change due to the effect of molecu lar dynamics if saturating microwave fields are applied These effects are especially pronounced if the motional fre quency for the spin dynamics is similar to the applied modu lation frequency The technique of saturation transfer is
43. to the left or right of the square steps the parameter value downward or upward faster than when using the arrows Keeping the mouse button pressed repeats the action auto matically The value of the parameter is indicated graphi cally by the position of the square in the slider bar You can also vary the parameter by clicking and dragging the square The dip corresponds to the microwave power absorbed by the cavity and thus is not reflected back to the detector diode By centering the dip on the display monitor the microwave source is set to oscillate at the same frequency as the cavity resonant frequency Clean the sample tube to be inserted into the cavity Wiping the outside of the sample tube with tissue paper is usually adequate It is vital to avoid contaminat ing the microwave cavity as paramagnetic contaminants may result in spurious EPR signals or distorted base lines in your EPR spectra 5 6 Manually Tuning a Microwave Bridge Insert the sample tube carefully into the cavity See Figure 5 4 Make sure you have the appropriate collet size for your sample tube size The tube should be slightly loose before you tighten the collet nut The bot tom of your sample should rest in the indentation on the pedestal This ensures that your sample is centered hori zontally If you have a small sample less than 2 cm in length you should visually judge how far the tube should go into the cavity in order to verticall
44. ttenuation If the mode pattern amplitude is too large decrease the microwave power in dB steps by increasing the attenuation EMX User s Manual 5 3 Manually Tuning a Microwave Bridge Figure 5 2 Mode tuning pat terns for a Gunn diode microwave source a Off resonance b Slightly off reso nance c On resonance phase 180 off d On resonance phase 90 off e On resonance correct phase undercoupled f On resonance correct phase overcoupled On resonance correct phase critically coupled wa 8 5 4 78 Manually Tuning a Microwave Bridge Figure 5 3 Mode tuning pat terns for a klystron microwave source a Off resonance b Slightly off reso nance c On resonance phase 180 off d On resonance phase 90 off e On resonance correct phase undercoupled f On resonance correct phase overcoupled On resonance correct phase critically coupled wa 8 EMX User s Manual 5 5 Manually Tuning a Microwave Bridge The resonant freguency of a Bruker ER 4102ST cavity is usually approximately 9 8 GHz A cryostat will drop the frequency to approxi mately 9 4 GHz Tune the microwave source Adjust the Frequency slider bar to locate and center the mode pattern dip on the display monitor Clicking the left or right arrows will step the parameter value downwards or upwards Clicking
45. ty t a ji Fine Ed Model Pot 10 a Designation A BR KER ER041X6 MICROWAVE BRIDGEX BAND S OX FINE AFC Fine AFC Pot etna A i BROAD BAND 2 5 oan 80000000 8 kt jt x8 3 aatomi E A 3 e burur O a E S7 Va N N a FINE AFC son ampus A on ede ACCESSORY a FREQUENCY i ON R arc g g 2 39 COUNTER amp amp amp B kd RS232 z 5 Figure 5 15 Two possible locations for the fine AFC potentiometer AFC we Figure 5 16 Properly centered AFC meter 5 20 Fine AFC Tuning for Gunn Diode Bridges BB WINEPR ACQUISITION ST PITCH PAR Eile Parameter Acquisition Processing View Options Window Info li E3 0821218 GCI Bele eos inf 31S E OPERATE Frequency 9 76 GHz Receiver Level LEVELLED Attenuation 20 e a a P 202 Diode n Field KE Q Value 270 AFC ee Meter Diode Meter Figure 5 17 Location of the AFC and diode meters 3 Switch the microwave attenuation from 40 dB to 50 dB The AFC meter may drift to the right See Figure 5 18 AFC A H E Figure 5 18 The AFC needle drifting towards the right 4 Increase the microwave attenuation slowly Increase the attenuation in dB increments between 50 and 60 dB until you observe a significant deflection of the needle See Figure 5 19 AFC E MII Figure 5 19 A significant AFC needle deflection EMX User s Manual
46. u larly as the modulation amplitude approaches the linewidth Reduce the modulation amplitude to ensure that the spectrum is independent of the modulation amplitude See Figure 6 8 Modulation freguency set too high The spectral reso lution is limited by the field eguivalence of the modulation freguency used Reduce the modulation freguency to verify that the linewidth is independent of the freguency See Figure 2 17 Time constant too long for sweep time A larger time constant will begin to filter out the high freguency compo nents of your signal Conseguently if the sweep rate is too fast relative to the time constant the spectrum will appear distorted and broadened To avoid this problem make sure that the time reguired to sweep through one of your EPR lines is at least ten times the length of the time constant See Figure 6 6 EMX User s Manual 7 13 Poor Resolution Magnetic field inhomogeneities or gradients Extremely narrow lines less than 20 milliGauss may be lim ited by magnetic field irregularities Vary the position of the cavity in the magnet air gap If the linewidth changes check for magnetic objects in or around the magnet If possible suspend these objects by a string and watch for a deflection in the same field strength as used in the experiment Do not attempt this with the cavity in the magnet The force of a fer romagnetic object being pulled into the magnet air gap can cause serious damage
47. y center the sample in the cavity You can adjust the sample position by loos ening the bottom collet nut and moving the pedestal up and down Make sure that the pedestal is not in the cavity as it can give an EPR signal Tighten the top collet nut to firmly hold the sample tube in place and the bottom collet to firmly hold the pedestal Sample Tube 4 a Collet and Collet Nut 73 w Pedestal Figure 5 4 Cutaway view of a Bruker ER 4102ST cav ity EMX User s Manual 5 7 Manually Tuning a Microwave Bridge A Steps 10 and 11 assume you have bias in the ref erence arm You usu ally do If the Bias slider bar See Figure 5 1 is all the way to the left side move it towards the mid dle to ensure sufficient bias Otherwise leave it be 10 11 Retune the microwave source Repeat the procedure of Step 6 You may notice a shift in the frequency width and depth of the cavity dip when you insert the sample This is an indication that the microwave field patterns in the cavity are perturbed by the sample and tube Lossy and conductive samples will appreciably perturb the field patterns resulting in large shifts in the resonant frequency Highly conductive samples tend to increase the resonant frequency by decreasing the effective cavity volume Lossy samples will decrease the resonant frequency because of their large dielectric const
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