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Spectrometer Calibration

1. 17 8 Set the calibration frequency range A calibration is required for each modulation frequency that you intend to use The SCT H signal channel has a range of 100 kHz to 6 kHz in 0 1 kHz steps The SCT L signal channel covers the frequency range of 6 kHz to 100 Hz in 1 Hz steps Most people will normally run all their spectra using 100 kHz modulation but under some special circumstances other frequencies may be desirable A good approach to take is to calibrate the signal channel every 10 kHz from 100 kHz to 10 kHz A sufficiently large range of frequencies is then covered for most EPR experiments Enter the Mod Frequency Start kHz usually 100 kHz Mod Fre quency End kHz as well as the Mod Frequency Increment kHz values The start value must be greater than the end value See Figure 17 6 Note that you should set the start and end values equal to each other for ER 4122SHQE or ER 4119HS cavities so that only one frequencies at a time is calibrated See the warning in the lefthand margin Set initial values for the tuning caps search The parameters in this section set the starting values for the tuning capacitor search See Figure 17 6 The Initial Modulation Amplitude defines the modula tion amplitude used when optimizing the tuning capacitors The default value 10 is appropriate for most cavities and resonators The default Initial Sweep Width of 20 G works well for a DPPH sample If you know an approximate optima
2. Hall Field 3320 000G Figure 17 21 The Receiver Level display User Service Training Course 17 21 Calibrating the Signal Channel EMX 12 Set the center field To interactively set the center field click the Interactive Change of Center Field Parameter button in the tool bar See Figure 17 22 AE AEE Eae xc a Interactive Change Button Figure 17 22 The Interactive Change of Center Field Parameter button in the Tool Bar Clicking this button creates a marker vertical line in the spectrum window that moves with the cursor Place the cursor where you would like the center field to be and click with the right mouse button See Figure 4 13 of the EMX User s Manual This action replaces the cen ter field value with the magnetic field position of the marker For fur ther details on this operation consult Section 4 3 2 of the EMX User s Manual W WINEPR ACQUISITION Spectr JO x File Parameter Acquisition Processing View Options Window Info IES AEE Alki EAE biic E E OPERATE Frequency 9 77 Ghz Attenuation 25 dB mW Power 0 636 Diode Hall Field Q Value 2200 3406 480G Temperature 300 00K Spectr Intensity 600 400 200 0 200 400 600 3460 3470 3480 3490 3410 3420 3430 3440 3450 3500 G NUM 348321G Figure 17 23 The center field marker Acquire the spectrum once more
3. EPR Spectrometer Calibration 17 For many experiments it 1s vital that your spectrometer is carefully cali brated For example it is essential to know the precise values of the magnetic field modulation amplitude in order to obtain quantitative EPR spectra The calibration procedures in this chapter enable you to measure the experimental conditions produced by the spectrometer with considerable accuracy This chapter is not meant to be a general overview of spectrometer calibration and quantitative EPR Therefore we highly recommend the following refer ences which discuss the topic in much greater detail e Poole C P Electron Spin Resonance a Comprehensive Treatise on Exper imental Techniques First Ed Interscience New York 1967 e Poole C P Electron Spin Resonance a Comprehensive Treatise on Exper imental Techniques Second Ed Wiley New York 1983 e Alger R S Electron Paramagnetic Resonance Interscience New York 1968 User Service Training Course Standard Samples Standard Samples 17 1 Standard samples are useful for system performance tests spectrometer cali bration and quantitative concentration measurements Ideally the standard sample should contain stable long lived paramagnetic species be easily pre pared under consistent and controlled methods and should be fully character ized with respect to all spectroscopic parameters such as relaxation times and hyperfine and fine structure splittings In
4. Channel Options command in the Parameter drop down menu The Signal Channel Options dialog box will appear W Signal Channel Options Tuning Caps p2 c winepr tpu st9515al cal E SCT Selftest fi Resonator 1 z X High Pass Filter Resonator 2 X AFC Trap Filter Modulation Signal Input Lock in l external Acquisition Trigger Internal Internal Internal F Lock In Integrator External External External Resonator Figure 17 19 The Signal Channel Options dialog box The AFC trap filter blocks any frequency signal components at the AFC modulation frequency that may contribute to noise in the EPR signal The high pass filter suppresses low frequency signal compo nents that may also contribute to added noise in the EPR signal These two filters influence the calibration values of the signal channel By default they are both selected Ensure that both options are checked Only under very rare circumstances would you acquire spectra without these filters 17 20 Calibrating the Signal Channel EMX 9 Adjust some parameters After centering the DPPH sample most of the parameters should be fairly close to what is needed for the cali bration routine Check the values in the Standard Acquisition Parameter dialog box and modify them so that they correspond to the You do not need to edit values in Figure 17 20 The Center Field value may be somewhat dif the Static F
5. The DPPH signal should now be nicely centered in the spectrum 17 22 GENS Calibrating the Signal Channel EMX Calibrating the Signal Channel 17 4 2 l Open the Calibrate Signal Channel dialog box Click the Cali brate Signal Channel command in the Acquisition drop down menu A new dialog box will appear c winepr tpu st9515a1 cal Change File Frequency List Calibrate from fi 00 0 kHz to fe kHz Increments fi 0 kHz Mod Amplitude Limit fi 00 0 max 1st Resonator IX 2nd Harmonic 2nd Resonator Figure 17 24 The Calibrate Signal Channel dialog box 2 Enter the filename for the calibration file The calibration file 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 found on the back or front of the cavity Click on the Change File button to open the Open Cal ibration File dialog box Signal channel calibration files are normally stored in the tpu subdirectory along with field controller and other cal ibration files Enter a filename and click OK Open Calibration File 21x File name Folders st9515al cal c winepr tpu st9515al cal st9515b cal st9515d_cal test cal tm4103 cal tm4103a cal List files of type Drives sct Calibration Files c c micron Figure 17 25 The Calibrate Signal Channel dialog box User Ser
6. addition the resonance line should be narrow and preferably homogeneous Unfortunately the universal stan dard sample has not been found Many standards have been suggested and each has its own particular merit The standard samples supplied with every Bruker spectrometer are discussed below DPPH a a diphenyl 8 picryl hydrazyl 17 1 1 A DPPH exhibits g anisotropy i e the g value depends on the orientation of the sam ple with respect to the magnetic field DPPH serves as a reference both in the solid state and in the liquid state when dissolved in benzene or toluene mineral oil The line width measured from the solid is subject to exchange narrowing and thus varies from under 1 gauss to over 4 gauss depending on the solvent that was used for recrystallization It has a g factor of 2 0036 0 0003 When dissolved in solution a quintet with unresolved hyperfine couplings is observed because the spin exchange narrowing is reduced as the sample is diluted A small single crystal of DPPH is an ideal sample for calibrating the phase and the field modulation ampli tude of the signal channel of an EPR spectrometer DPPH has been studied extensively by e M bius K and R Biehl Multiple Electron Resonance Spectroscopy Ple num Press 1979 e Dalal N S D E Kennedy and C A McDowell J Chem Phys 59 3403 1979 e Hyde H S R C Sweed Jr and G H Rist J Chem Phys 51 1404 1969 e Dalal N S D
7. for full gain of the modulation amplifier are recorded and saved with the calibration file Once this data is available the signal channel will then vary the input signal to the modulation amplifier to produce the modulation amplitude that you have selected Those familiar with older EPR spectrometers will remember the tuning boxes for tuning the modulation coils The above mentioned software and hardware makes these tuning boxes unnecessary as well as adding great flexibility in the choice of modulation frequencies Theory of Signal Channel Calibration Phase Calibration 17 2 3 Once the modulation amplitude has been calibrated the reference phase 1s easily calibrated by studying the phase angle dependence of the signal inten sity The intensity of the output signal is proportional to the cosine of the phase difference between the reference signal and the modulated EPR signal See Figure 17 3 It is most convenient to determine where the 90 phase difference occurs because first the absence of a signal cos 90 0 is easy to detect and second the cosine function and hence the intensity changes rapidly with respect to the phase angle at 90 In the calibration routine spec tra are acquired at several different values of the reference phase and the 90 phase difference is extrapolated from the signal intensities The phase angle resulting in maximum signal intensity for that particular frequency is recorded and saved with the cali
8. line width the magnetic field modulation brings the sample into resonance before and after the magnet has reached the field for resonance This results in a broadening and distortion of the EPR signal See Figure 17 1 In the limit of an infinitesimally narrow EPR signal the peak to peak width of the first derivative EPR signal will be approximately equal to the peak to peak modulation amplitude User Service Training Course 17 3 Theory of Signal Channel Calibration Figure 1 1 17 4 1G N j S 0a S Q vim a SS The signal shape of the DPPH EPR signal as a function of the field modulation amplitude The first step of calibrating the modulation amplitude involves choosing the correct tuning capacitors The modulation amplifier needs a bit of help to obtain large modulation amplitudes at modulation frequencies greater than 50 kHz This is a consequence of the decreasing skin depth with increasing fre quency The modulation coils on the cavity are tuned or made resonant by adding a tuning capacitor in series with the modulation coil Tuning a Capacitor Modulation Coil LNY Figure 17 2 The LC resonant circuit for high frequencies The calibration routine switches various tuning capacitors in and out of the circuit until the modulation amplitude is maximized The optimal capacitor for that particular frequency as well as the modulation amplitude
9. the microwave bridge and cavity tuned Remove cryostats from the cavity because it is easier to position the DPPH sample properly in the cavity Except for the FlexLine resonators it is necessary to use the ER 4118CF cryostat when calibrating FlexLine resonators In particu lar the ER 4112HV and ER 4113HV helium cryostats prevent the cor rect positioning of the sample Another advantage is that the resonant Collet Nut Fiduciary Mark Irradiation Grid Cover Pedestal Figure 17 16 Proper positioning of the DPPH sample frequency of the cavity will be approximately 9 8 GHz without the helium cryostat and the field for the DPPH signal will be known approximately 3480 Gauss The DPPH sample is a small point sam ple and therefore has a fiduciary mark that indicates the position of the DPPH crystal in the sample tube Center the DPPH sample vertically in the cavity The center of the black irradiation grid cover corresponds approximately to the vertical center of the cavity Open the Interactive Spectrometer Control dialog box Click the Interactive Spectrometer Control button in the tool bar and the dialog box will appear See Figure 17 17 We can now optimize some of the parameters and adjustments for the calibration routine Set some parameters Set the Microwave Attenuator to approxi mately 25 dB T
10. 17 The Interactive Spectrometer Control dialog box 6 Optimize the DPPH sample position Move the sample tube up and down until the maximum signal intensity is attained See Figure 17 16 Avoid moving the sample from side to side Perhaps the best technique is to loosen the collet nuts for the pedestal and sam ple tube and move the sample too low Then use the pedestal to slowly push the sample up Sometimes the process of moving the sample tube in the cavity can cause the AFC to lose lock Retune the frequency if this happens If the signal is clipped decrease the Receiver Gain When you have centered the DPPH sample secure the sample tube by tightening the collet nuts User Service Training Course 17 19 Calibrating the Signal Channel EMX l Transfer the parameters To set the parameter values to a spec trum click on the Set parameters to spectrum button The cursor will turn into the letter P for Parameter Place the cursor on a spec trum window and click the left mouse button to copy the parameters to that spectrum Click the Interactive Spectrometer Control button in the tool bar to close the dialog box 9 a If there is no spectrum available click on the New Experiment but ton in the tool bar to create a new spectrum See Figure 17 18 REHN g Cih jS EE New Experiment Button Figure 17 18 The New Experiment button 8 Check the AFC Trap and High Pass Filters Click on the Signal
11. 1st Resonator should be selected See Figure 17 7 The ER 4105DR dual cavity has two sets of modulation coils By selecting 1st Resonator or 2nd Resona tor you are selecting the set of modulation coils that are to be cali brated Turn on the AFC Trap and High Pass Filters The AFC trap fil ter blocks any signal components at the AFC modulation frequency that may contribute to noise in the EPR signal The high pass filter sup presses low frequency signal components that also contribute to added noise in the EPR signal These two filters influence the calibration val ues of the signal channel By default they are both selected Ensure that both options are selected See Figure 17 7 Only under very rare cir cumstances would you acquire spectra without these filters Start the Setup Scan When the Setup Scan function is activated the magnetic field is swept rapidly up to 50 Gauss at about a 30 Hz rate to provide a real time display of the EPR spectrum on the screen Click the Setup Scan button in the Results folder See Figure 17 7 Setup Scan Button Setup Scan es Time Constant wa TE Figure 17 8 The Setup Scan window 17 10 A Setup Scan window will appear Click the Activate button in the monitoring panel if it is not already activated Then click the Setup Scan button in the Setup Scan window to activate it See Figure 17 8 Leave this window open so you can adjust the parame ters interactively Cali
12. Create button to close the dialog box 17 6 GETS Calibrating the Signal Channel Elexsys Create an Experiment Select Calibration Figure 17 5 Creating a calibration experiment 3 Open the parameter dialog box Click the Parameter button to open the Calibration Parameters dialog box If the Calibration folder is not in front click its tab to bring it to the front See Figure 17 6 Parameter Button Set Frequency f Range Tuning Caps Search Amplitude Search Phase Search Figure 17 6 The Calibration Parameters dialog box User Service Training Course 17 7 Calibrating the Signal Channel Elexsys A The EN 801 ENDOR resonator should not be used above 25 kHz Consult the Elexsys E 560 documentation for instructions on cali brating this cavity A The ER 4122SHQE and ER 4119HS reso nators should only be calibrated one modula tion frequency at a time At lower modula tion frequencies the modulation amplitude can get sufficiently large to damage the coils The Mod Ampli tude Limit needs to be adjusted separately for each modulation fre quency A It is not necessary to calibrate every 100 Hz from 100 kHz to 10 kHz In particular FlexLine resonators in a cryostat with no gas flow can become suffi ciently warm to dam age the resonator if you calibrate more than 20 frequencies in one experiment
13. E Kennedy and C A McDowell J Chem Phys 61 1989 1974 e Dalal N S D E Kennedy and C A McDowell Chem Phys Lett 30 186 1975 Weak and Strong Pitch Samples 17 1 2 17 2 Pitch in KCI has emerged as a standard because of its long lived paramag netic radicals and low dielectric loss Because of the long life of the radicals it is unsurpassed as a test of spectrometer sensitivity The pitch is added to a powder of KCI and the mixture is carefully mechanically mixed to obtain a homogeneous sample After mixing the sample is heated pumped and sealed under vacuum Pitch is generally prepared in two concentrations strong pitch which is 0 11 pitch in KCI and weak pitch which is 0 0003 pitch in KCI To correct for variations in spin concentration each weak pitch sample is compared to a standard and assigned a correction factor The peak to peak RUKER GETS Theory of Signal Channel Calibration line width is typically 1 7 G with a g factor of 2 0028 The size very weak of the signal renders pitch ill suited for modulation amplitude calibration The weak pitch samples from Bruker Biospin have an estimated concentration of 10 spins per centimeter The samples are calibrated and the correction fac tor is printed on the side of the tube This sample is prepared for the purpose of measuring instrument performance owing to its high stability however it is not meant as a quantitative spin counting standard Theo
14. a Bruker standard DPPH sample in it and the microwave bridge and cavity tuned Remove cryostats from the cavity because it is Do not attempt to cali m l biaia canis m idn easier to position the DPPH sample properly in the cavity Except for ER 4112HV or ER the FlexLine resonators it is necessary to use the ER 4118CF cryostat 4113HV helium cryostat when calibrating FlexLine resonators Another advantage is that the installed in the cavity resonant frequency of the cavity will be approximately 9 8 GHz with Toe cryostat sample out the helium cryostat and the field for the DPPH signal will be T 2 a a Gon known approximately 3500 Gauss The DPPH sample is a point sam being centered properly ple so it is important to have it roughly centered in the cavity The sam ple has a mark on it indicating where the DPPH is The distance from the collet to the center of the cavity is approximately 70 mm 70 mm il Figure 17 4 The approximate distance from the collet to the center of the cavity PA Create a calibration experiment Click the New Experiment but ton in the monitoring panel and the dialog box will appear See Figure 17 5 Enter a name for the experiment calibration for exam ple in the Experiment Name column Click the Calib tab Make sure the Standard push button is activated green The R F button is for ENDOR calibration Click the
15. able 17 1 stop the calibration routine by clicking the Run Abort but ton in the Acquisition Control buttons Values that are too high can allow excessive current to flow through the modulation coils of the cavity at the maximum modulation amplitude resulting in damaged modulation coils Calibrating the Signal Channel Elexsys Cavity ER 4102ST ER 4105DR ER 41040R ER 4116DM ER 4103TM ER 4108TMH ER 4106ZRC ER 4106ZRAC ER 4107WZC ER 4107WZAC ER 41150DC ER 41150DAC ER 4119HS ER 4122SHQ ER 4122SHQE ER 4114HT ER 4117MX or D MVT ER 4117D R ER 4109EF Maximum Mod Gauss at 100 kHz 32 32 a2 10 16 32 10 10 10 10 10 10 25 15 25 7 10 By 10 Table 17 1 Maximum modulation amplitude for EPR cavities User Service Training Course 17 15 Calibrating the Signal Channel Elexsys 17 16 22 2 24 25 Reset the Mod Amplitude Limit The Mod Amplitude Limit parameter which was set in Step 6 limits the size of the signal sent to the modulation amplifier to prevent any danger of burning out the modulation coils To calculate a safer value for Mod Amplitude Limit use the following formula Mod Amplitude Limit Mod Amp max 2 Mod Mod Amp G where Mod Amp max and Mod Amp G are the values listed in the calibration table and Max Mod is the maximum modulation amplitude listed for your cavity in Table 17 1 Enter this new value for Mod Amplitude Limit in the Calibra
16. brating the Signal Channel Elexsys 12 Set the Microwave Attenuator to approximately 25 dB Click the Microwave tab Set the microwave power to 25 dB See Figure 17 9 Figure 17 9 The Microwave folder of the Calibration Parameter dialog box S o we You should be able to see the DPPH signal by now If you do not see the signal change the modulation phase by 90 degree If you still can not find the signal refer to Chapter 8 of the Elexsys E 500 User s Manual Basic Opera tions A The EN 801 ENDOR resonator should not be used above 25 kHz Consult the Elexsys E 560 documentation for instructions on cali brating this cavity User Service Training Course 13 Figure 17 10 The Absc 1 Field folder of the Calibration Parame 14 Center your DPPH spectrum in the display Click the Absc 1 Field tab See Figure 17 10 Arrange the Calibration Parameters window and the Setup Scan dialog box so that you can see both of them at the same time Make sure that the Center box is selected green Adjust the Center Field value by entering a field value or clicking on the up and down arrows until the signal appears centered in the Setup Scan window For a microwave frequency of about 9 8 GHz DPPH resonates at 3500 Gauss Reducing the Sweep Width in the Setup Scan window can help you to center the signal 3480 000 3480 00 E Wait LED off 00o ters dialog box Adjust other parameters Adjust
17. bration file The phase difference between the modulated EPR signal and the reference signal depends on several experimental conditions The length of the cable leading to the modulation coils the inductance of the coils in the particular cavity the gain setting of the modulation amplifier the tuning capacitors and the signal channel used can all change the phase difference However the ref erence phase calibration is performed automatically during the routine described in this section The two editions of the book by C P Poole that are mentioned at the begin ning of this chapter are very good references for the details on the theory of phase sensitive detection and the calibration of signal channels We encour age you to explore this topic further to learn more about calibration af 150 SN Pa yk 120 lt a 105 N A 90 a ee 7 l o A Vv V i l a o MERTA aO 45 7 Ye 30 TAREN is A eR U Figure 17 3 The signal intensity as a function of the reference phase angle User Service Training Course 17 5 Calibrating the Signal Channel Elexsys Calibrating the Signal Channel Elexsys 17 3 l Get ready for the calibration routine Follow the instructions of Section 3 1 and 3 2 of the Elexsys E 500 User s Manual Basic Opera tions You should have the spectrometer turned on the cavity properly installed with
18. est modulation frequency and continuing for each selected modulation frequency As each parameter is determined it is displayed in the table See Figure 17 26 Wait until the 100 kHz calibration is completed and note the value in the fifth column of the table Mod Amp G This value can allow excessive current to flow through the modulation coils of the cavity at the maximum modulation amplitude resulting in dam aged modulation coils Compare the Mod Amp G at 100 kHz with the values listed for your cavity in Table 17 2 If the value obtained by the calibration routine exceeds the values listed in Table 17 2 first record the values for Mod Amp G and Mod Amp max because you will need them for the next step Then stop the calibration routine by clicking the STOP button in the Tool Bar twice If the value is less than or equal to the value listed in Table 17 2 allow the calibration routine to continue its task and proceed to Step 11 User Service Training Course 17 25 Calibrating the Signal Channel EMX 17 26 Maximum Mod Gauss at cavity 100 kHz ER 4102ST 32 ER 4105DR 32 ER 41040R 32 ER 4116DM 10 ER 4103TM 16 ER 4108TMH 32 ER 4106ZRC 10 ER 4106ZRAC 10 ER 4107WZC 10 ER 4107WZAC 10 ER 41150DC 10 ER 41150DAC 10 ER 4119HS 25 ER 4122SHQ 15 ER 4122SHQE 25 ER 4114HT 7 ER 4117MX 10 or D MVT ER 4117D R 32 ER 4109EF 10 Table 17 2 Maximum modulation amplitude for EPR cavities 9 Set the Mod Amplitude Lim
19. he Time Constant needs to be set to a low value less than about 0 16 ms A Modulation Amplitude of 1 Gauss is usually sufficient Set the Sweep Width to 100 Gauss A Receiver Gain of approximately 1 x 10 works well Calibrating the Signal Channel EMX 4 Click the Enable button for the Set Up Scan When this option is enabled the magnetic field is swept rapidly up to 50 Gauss to pro vide a real time display of the EPR spectrum on the screen 5 Center your DPPH spectrum in the display Adjust the Field slider bar until the signal appears centered in the Setup Scan window For a microwave frequency of about 9 78 GHz DPPH resonates at 3480 Gauss Adjust the Receiver Gain so that the signal fills approxi mately half of the vertical display range Make sure that the signal channel is set to 100 kHz modulation and first harmonic detection W interactive Spectrometer Control JO x Hall Jnal Channel Center Field 3474 00 Magnetic Calibrated Sweep Width o 00 Field aceiver Gain L 2 52 10 B EY Sweep Address fi 525 7 Modulation Freq fi 00 00 krz a gt Modulation Amplitude fi 00 o Field 3474 000 G Modulation Phase 24 0 00 deg 3 0 00 Setup Scan Enable H Poi Button tant 0 64 H msec u Sweep Width fi 00 00 Time 5 12 msec Level lt c__ 4 Harmonic fi J MW Attenuator fe p H dB Temperature 300 00 fejk SET Options Figure 17
20. he warning in the lefthand margin Choose the harmonics The signal channel can produce either a first harmonic first derivative or a second harmonic second deriva tive spectrum If you have no need for second harmonic spectra and wish to save a bit of time in the calibration routine you may deselect the option to calibrate the phase for the second harmonic by clicking the 2nd Harmonic box A cross in the box indicates that the option is selected However the time savings are minimal and you never know when you may need a second harmonic display it is probably best to always calibrate the second harmonic phase Select the resonator In almost all cases the 1st Resonator should be selected The ER 4105DR dual cavity has two sets of modulation coils By selecting 1st Resonator or 2nd Resonator you are select ing the set of modulation coils that are to be calibrated Adjust the Mod Amplitude Limit The Mod Amplitude Limit parameter limits the size of the signal sent to the modulation amplifier This prevents damage to the coils owing to excessive current Enter a value of 75 for an ER 4122 SHQ cavity For Flex line resonators a value of 10 is more appropriate A value of 30 works well for ER 4122SHQE and ER 4119HS cavities 40 is appropriate for most of the other cavities It is always safe to set this value a bit low as you will adjust this value later A 100 G Sweep Width is sufficient for most resonators Calibrating the Signa
21. ield param ferent from what is displayed in Figure 17 20 but the Sweep Width eter for a signal channel mist be 100 Gauss calibration Experiment x Field Sweep x WE no Y Sweep x Hal Signal Channel 3 Center Field 340454 HHG Receiver Gain fioo 10 Po Sweep Width fi 00 00 G Modulation Frequency fi 00 00 Es kHz Static Field 3483 1 16 H G Modulation Amplitude fi 00 G Frequency 9 766000 GHz Offset 0 00 Power 0 64 mW Time Constant 0 64 lt msec Step fi db Conversion Time 5 1 2 a msec T z Sweep Time 5 24 sec rl emperature unit Temperature K Harmonic fi Step fi 00 E K a Resolution in X fi 024 rGoniometer Number of X Scans fi Angle deg oe Lad Resolution in Y fi St fi 000 d i Repetitive Mode E Figure 17 20 Parameters for a signal channel calibration 10 Acquire a spectrum Click the RUN button in the tool bar 11 Adjust the Receiver Gain Monitor the Receiver Level while the scan is running See Figure 17 21 If the needle deflects more than 1 4 of the display lower the Receiver Gain Reacquire the spectrum and lower the Receiver Gain until the needle does not deflect more than 1 4 of the display You may have to repeat this last step a few times W WINEPR ACQUISITION ST_PITCH PAR File Parameter Acquisition Processing View Options Window Info LEVELLED Attenuation 20 B Power 2 012 mW Diode REE vce 2700 Receiver Level
22. it The Mod Amplitude Limit param eter in the Calibrate Signal Channel dialog box allows you to limit the size of the signal sent to the modulation amplifier to prevent any danger of burning out the coils To calculate a safer value for Mod Amplitude Limit use the following formula Max Mod Mod Amplitude Limit Mod Amp max aa where Mod Amp max is the value determined by the calibration routine Actual Mod is the value for Mod Amp G determined by the calibration routine and Max Mod is the maximum modulation ampli tude listed for your cavity in Table 17 2 Return to Step 1 e g start the calibration routine again and enter this new value for Mod Ampli tude Limit Continue from Step 2 through Step 8 as before Calibrating the Signal Channel EMX 10 Special Instructions for ER 4119HS cavities It was mentioned on page 24 that these cavities should be calibrated one frequency at a time An additional requirement is that the Mod Amplitude Limit be set properly at each modulation frequency Follow Step 8 and Step 9 for each of the individual modulation frequencies after you have started the calibration routine for that frequency 11 Finish the Calibration When the routine is finished the message Acquisition Done will appear in the info line Double click the con trol menu box in the upper left hand corner of the window to close the window The signal channel is now calibrated for your cavity and the data saved in the ca
23. l Channel EMX 7 Start the calibration routine Click the Start button A new dialog box will appear See Figure 17 26 The spectrometer will then auto 7y matically calibrate the signal channel at each of the specified modula EO tion frequencies i The ER 4105DR dual cis Signal Channel BEF ity is di Calibration File c winepr tpu st9515f cal cavity is different from Aeau aT y ca the ER 4 l l 6DM dual Freq kHz Res TuningC nF ModAmp max Mod Amp G Phase 1 Phase 2 50 0 1 25 00 mode cavity The ER 4116DM has only one set of modulation coils 3474 3476 3478 3480 3482 3484 3486 3488 3490 G Figure 17 26 The Calibration routine The calibration file consists of a table of parameter values and settings for each modulation frequency The first parameter is the modulation frequency The second column indicates the resonator that was selected in Step 5 For modulation frequencies greater than or equal to 50 kHz the optimal tuning capacitor value is listed in the third column The fourth column contains the value of Mod Amp max Mod Amp G in the fifth column is the measured maximum modulation ampli tude when the corresponding Mod Amp max is used Phase 1 and Phase 2 in columns six and seven respectively are the phases at which the first and second harmonic signals are nulled 8 Check Mod Amp G at 100 kHz The calibration routine performs its task sequentially starting with the high
24. l value for the tuning capacitors at 100 kHz you can enter that value in Tuning C Start Value This will save you time because otherwise the searching routine will start from 0 nF Mod Frequency Limit is always set at 50 kHz because skin depth poses less of a problem at lower frequencies Adjust the Mod Amplitude Limit The Mod Amplitude Limit parameter limits the size of the signal sent to the modulation amplifier This prevents damage to the coils owing to excessive current Enter a value of 75 for an ER 4122 SHQ cavity For Flex line resonators a value of 10 is more appropriate A value of 30 works well for ER 4122SHQE and ER 4119HS cavities 40 is appropriate for most of the other cavities It is always safe to set this value a bit low as you will adjust this value later A 100 G Sweep Width is sufficient for most resonators See Figure 17 6 Calibrating the Signal Channel Elexsys 7 Enter values for the modulation phase search The parameters in this step depend on your calibration sample The default values of 1 G Sample Line Width and 20 G Sweep Width work well with the Bruker DPPH standard The signal channel can produce either a first harmonic first derivative or a second harmonic second derivative spectrum If you have no need for second harmonic spectra and wish to save a bit of time in the calibration routine you may deselect this option by clicking the Calibrate 2nd Harmonic button The green color indicates that the
25. libration file The next time that you start the WIN EPR Acquisition software this calibration file will be the default calibration file User Service Training Course 17 27 Notes 17 28
26. option is selected See Figure 17 6 Better safe than sorry 8 Enter the filename for the calibration file Click the Results tab Se eee See Figure 17 7 The calibration file name usually consists of two to ee four letters that identify the type of cavity ST for ER 4102ST or second harmonic when running the calibration SHQE for ER 4122SHQE followed by the serial number of the cavity routine This number is found on the back or front of the cavity For FlexLine series resonators the serial number is found at the top part of the mod ule block Enter the calibration file name into the Calibration Data Set box If you are re calibrating you can click the file name in the drop down list of the Calibration Data Set The new values will be appended to the calibration file or replace previous values in the file If you want to start fresh click the Delete Data Set button to delete the old calibration data Iy A a Calibration File Name Delete a f PAn AFC Trap Data Set High Pass T Filter Button Filter Figure 17 7 The Results folder of the Calibration Parameters dialog box User Service Training Course 17 9 Calibrating the Signal Channel Elexsys S a a The ER 4105DR dual cavity is different from the ER 4116DM dual mode cavity The ER 4116DM has only one set of modulation coils Activate Button 10 11 Select the resonator In almost all cases the
27. ry of Signal Channel Calibration 17 2 You need to carefully calibrate your spectrometer s signal channel reference phase and modulation amplitude in order to obtain maximum sensitivity min imum distortion and quantitatively reproducible measurements The SCT H and SCT L signal channels in conjunction with the Xepr or WIN ACQ soft ware make this calibration easy to perform The results of the calibration are saved on disk for future use We recommend recalibration at least once a year to ensure quantitative and reproducible results Each cavity or resonator has its own individual calibration file therefore this procedure must be followed for each cavity Introduction 17 2 1 Calibration of the signal channel involves two separate yet interdependent procedures The first procedure is to calibrate the peak to peak modulation amplitude For the sake of brevity modulation amplitude will be used in place of peak to peak modulation amplitude The second procedure is to cali brate the phase difference between the reference signal and the modulated EPR signal Because the calibration and adjustment of the modulation ampli tude can affect the phase difference the first procedure is performed first Amplitude Calibration 17 2 2 You calibrate the modulation amplitude by overmodulating a narrow EPR signal A crystal of DPPH with a line width of approximately G is a very good sample to use When the modulation amplitude is large compared to the
28. table of parameter values and settings for each modulation frequency The first column is the modulation frequency For modulation frequen cies greater than or equal to 50 kHz the optimal tuning capacitor value is listed in the second column The third column contains the value of Mod Amp max Mod Amp G in the fourth column is the mea sured maximum modulation amplitude when the corresponding Mod Amp max is used Phase 1 and Phase 2 in columns five and six respectively are the phases at which the first and second harmonic signals are nulled See Figure 17 14 SHOS7104 y ae Figure 17 14 The calibration results 2i Check the Mod Amp G at 100 kHz The calibration routine per forms its task sequentially starting with the highest modulation fre quency and continuing for each selected modulation frequency As each parameter is determined it is displayed in the table See Figure 17 14 Wait until the 100 kHz calibration is completed Click the Pause button of the Acquisition Control buttons Note the value in the fourth column of the table Mod Amp G Compare the Mod Amp G at 100 kHz with the values listed for your cavity in Table 17 1 If the value obtained by the calibration routine is slightly less than or approximately equal to the value listed in Table 17 1 allow the calibration routine to continue its task by clicking the Pause button again and proceed to Step 23 If the value exceeds the values listed in T
29. the Receiver Gain so that the signal fills approximately half of the vertical display range in the Setup Scan window Make sure that the signal channel is set to 100 kHz modulation or the maximum modulation frequency you entered in the Calibration folder and first harmonic detection A 1 gauss Modulation Amplitude usually is sufficient 17 11 Calibrating the Signal Channel Elexsys 17 12 15 Optimize the DPPH sample position Move the sample tube up and down until the maximum signal intensity is attained See Figure 17 11 Avoid moving the sample from side to side Perhaps the best technique is to loosen the collet nuts for the pedestal and sample tube and move the sample too low Then use the pedestal to slowly push the sample up Sometimes the process of moving the sample tube in the cavity can cause the AFC to lose lock Re tune the frequency if this happens If the signal is clipped decrease the Receiver Gain Rotate the sample tube until the maximum signal intensity is attained When you have maximized the signal intensity secure the sample tube by tightening the collet nuts Check the matching if needed You may need to re center the signal by adjusting the Center Field in the Absc 1 Field tab DPPH Sample Figure 17 11 Fine adjustment of the DPPH sample height in the cavity Calibrating the Signal Channel Elexs
30. tion tab Click the Results tab to bring it to the front Click the Run button to restart the calibration rou tine Finish the calibration When the routine is finished the Run button will no longer be highlighted and the calibration data for the lowest modulation frequency is completed e the calibration data in the Results tab are no longer 1 Special Instructions for ER 4122SHQE and ER 4119HS cavi ties It was mentioned on page 8 that these cavities should be cali brated one frequency at a time An additional requirement is that the Mod Amplitude Limit be set properly at each modulation frequency Follow Step 21 and Step 22 for each of the individual modulation frequencies after you have started the calibration routine for that fre quency Adding additional frequencies If you wish to add more frequen cies to the calibration file for example lower frequencies below 10 kHz the remaining instructions guide you through the necessary steps Click the Calibration tab Change the Mod Frequency Incr for example to 2 kHz Change the Mod Frequency Start for example to 5 kHz Change the Mod Frequency End for example to 1 kHz See Figure 17 15 For modulation frequencies less than 10 kHz you need to reset the Mod Amplitude Limit to 10 Calibrating the Signal Channel Elexsys Figure 17 15 Changing the Frequency Range 26 Start the calibration routine Click the Run button to start the cal ibration routine The lo
31. vice Training Course 17 23 Calibrating the Signal Channel EMX The ER 4119HS reso nator should only be calibrated one modula tion frequency ata time At lower modula tion frequencies the modulation amplitude can get sufficiently large to damage the coils The Mod Ampli tude Limit needs to be adjusted separately for each modulation fre quency VL Better safe than sorry It is a good idea to cali brate the phase of the second harmonic when running the calibration routine It is not necessary to calibrate every 100 Hz from 100 kHz to 10 kHz In particular FlexLine resonators in a cryostat with no gas flow can become suffi ciently warm to dam age the resonator if you calibrate more than 20 frequencies in one experiment 17 24 Set the frequency limits A calibration is required for each modula tion frequency that you intend to use The standard signal channel the EMX 027 SCT H has a range of 100 kHz to 6 kHz in 0 1 kHz steps Most people will normally run all their spectra using 100 kHz modula tion but under some special circumstances other frequencies may be desirable A good approach to take is to calibrate the signal channel every 10 kHz from 100 kHz to 10 kHz A sufficiently large range of frequencies is then covered for most EPR experiments Note that you should set the start and end values equal to each other for the ER 4119HS cavity so that only one frequencies at a time is calibrated See t
32. w modulation frequency calibration data will be appended to the table If there is one or more calibration frequencies already existing in the previous calibration file a warning message will appear on the screen asking you if you want to overwrite the existing value Click the Yes or No button depending on your wishes to con tinue the calibration routine Notice that the previous calibration data for other frequencies are still kept in the file 27 Close the Calibration Parameters dialog box Click the Close button in the Calibration Parameters dialog box The signal channel is now calibrated for your cavity and the data saved in the calibration file The next time when you connect the Xepr software to the spec trometer this calibration file will be the default calibration file User Service Training Course 17 17 Calibrating the Signal Channel EMX Calibrating the Signal Channel EMX Preparing for Calibration A Do not attempt to cali brate a cavity with an ER 4112HV or ER 4113HV helium cryostat installed in the cavity S a az It is not possible to change the actual Sweep Width while the Set Up Scan is enabled Change the Sweep Width before the Set Up Scan is enabled 17 18 I 17 4 17 4 1 Follow the instructions of Sections 3 2 through 3 5 of the EMX User s Manual You should have the spectrometer turned on the cavity properly installed with a Bruker standard DPPH sample in it and
33. ys 16 Close the Setup Scan window Click the Close button in the Setup Scan window 17 Reset some parameters After centering the DPPH sample most of the parameters should be fairly close to what 1s needed for the cali bration routine Set the Receiver Gain to approximately 36 dB 18 Start the calibration routine Click the Result tab Move the Cali bration Parameters window to the corner so that you can see the acquisition and the calibration result at the same time Click the Run button in the monitoring panel to start the calibration routine Stop Button Run Pause Button Button Figure 17 12 The Acquisition control buttons 19 Adjust the Receiver Gain Monitor the Receiver Level while the scan is running See Figure 17 13 Receiver Level Meter Figure 17 13 Monitoring the Receiver Gain If the needle deflects more than 1 4 of the display stop the calibration routine by clicking the Run Abort button Lower the Receiver Gain and restart the calibration by clicking the Run button again A warning message will appear asking if you want to overwrite the existing value Click the Yes button to continue Repeat until the needle does not deflect more than 1 4 of the display You may have to repeat this last step a few times User Service Training Course 17 13 Calibrating the Signal Channel Elexsys 17 14 20 Observe the calibration results The Results folder consists of a

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