EMX User`s Manual
Contents
1. esaa CIEE Bel Sane ell Blelel Ee AFC Receiver Level MMOPERATENE Frequency 9 81 T waj a Attenuation 0 dB LEI a a Power MNCALBRATEDN Q value 200 7 mW Diode Hall Field o0 A o Signal Noise Measurement Loix 10 3 10 0 noise trace 10 noise 725 33 Receiver gain 02 10 Microwave power 200 68 mW 3470 3480 signal trace Peak to peak height 34722 00 Receiver gain 2 00 10 5 Microwave power 12 66 mW Conversion Time 163 84 ms 3490 3500 G Conversion Time 163 84 ms _ eee 00 _ 0 0 SSS 10 3 0 5 Acquisition done Weak Pitch factor 1 02 Signal noise ratio Figure 9 9 Noise measurement and the final result 9 12 Signal to Noise Ratio Test 10 Check the S N ratio On the right panel the results of the signal intensity and noise level measurements will automatically appear At the bottom of the panel the auto matically calculated signal to noise ratio will be displayed in the box See Figure 9 9 The signal to noise ratio should be higher than 330 to meet the specifications of the Bruker EPR instrument If the result is lower than this value consult Chapter 6 and Chapter 7 Sometimes a large cavity background signal can significantly decrease the test result Refer to the next section Section 9 2 and run a cavity background signal test to verify this If those hints do not hel2. Collet Nut Fiduciary Mark Irradiation Grid Cover J Pedestal Figure 8 4 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 W J A m a i Calibration of the Signal Channel Aly L S gt a 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 DPPH sample is a small point sample and therefore has a fiduciary mark that indicates the position of the DPPH crystal in the sample tube Center the DPPH sample verti cally 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 dia log box Click the Interactive Spectrometer Control button in the tool bar and the dialog box will appear See Figure 8 5 We can now optimize some of the parame ters and adjustments for the calibration routine Set some parameters Set the Microwave Attenuator to approximately 25 dB The Time Constant needs to be set to a low value less than about 0 16 ms A Modula tion Amplitude of 1 Gauss is usually sufficient Set the Sweep Width to 100 Gauss A Receiver Gain of approxima
3. EPR Spectrometer Calibration 8 For many experiments it is vital that your spectrometer is care fully calibrated 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 spectrome ter calibration and quantitative EPR Therefore we highly rec ommend the following references which discuss the topic in much greater detail e Poole C P Electron Spin Resonance a Comprehensive Trea tise on Experimental Techniques First Ed Interscience New York 1967 e Poole C P Electron Spin Resonance a Comprehensive Trea tise on Experimental Techniques Second Ed Wiley New York 1983 e Alger R S Electron Paramagnetic Resonance Interscience New York 1968 EMX User s Manual Standard Samples Standard Samples 8 1 Standard samples are useful for system performance tests spec trometer calibration and quantitative concentration measure ments Ideally the standard sample should contain stable long lived paramagnetic species be easily prepared under consistent and controlled methods and should be fully characterized with respect to all spectroscopic parameters such as relaxation times and hyperfine and fine structure splittings In
4. 3 Turn on the instrument and tune Turn on the instru ment if it is not on yet Tune the microwave bridge and the cavity It is best to wait several hours because the spec trometer is most sensitive and stable after it has achieved thermal equilibrium 9 14 i Cavity Background Signal Test 4 Create a new spectrum window if needed If there is no empty spectrum window create one by clicking on the Create New Spectrum button in the tool bar Performing the Background Signal Test 9 2 2 1 Open the parameter option dialog box Set the parameters for Weak Pitch Measurement as indicated in Table 9 2 See Figure 9 10 Weak Pitch Background Parameter Measurement Measurement Modulation 8 0G 8 0 G Amplitude Modulation 100 kHz 100 kHz Frequency Receiver Gain adjust adjust Phase 0 0 Time Constant 1310 72 ms 1310 72 ms Conversion Time 163 84 ms 163 84 ms Center Field 3480 G 2600 G Sweep Width 50G 5000 G Resolution of 1024 points 1024 points Field Axis Microwave 3 dB 3 dB Attenuation Table 9 2 Parameters for Background Signal Measurement EMX User s Manual 9 15 Cavity Background Signal Test SS Se Experiment x EEE ncv swo F Hall Signal Channel 1 5 Center Field 3490 63 G Receiver Gain 1 00 10 Sweep Width 50 00 G Modulation Frequency 100 00 kHz Static Field 3459 540 G Modulation Amplitude 8 00 G Microwave Brid
5. addition the reso nance line should be narrow and preferably homogeneous Unfortunately the universal standard sample has not been found Many standards have been suggested and each has its own par ticular merit The standard samples supplied with every Bruker spectrometer are discussed below DPPH a diphenyl 6 picryl hydrazyl 8 1 1 DPPH serves as a reference both in the solid state and in the liq uid state when dissolved in benzene or toluene mineral oil The line width measured from the solid is subject to exchange nar rowing 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 as 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 amplitude of the signal chan nel of an EPR spectrometer DPPH has been studied extensively by e M bius K and R Biehl Multiple Electron Resonance Spec troscopy Plenum Press 1979 e Dalal N S D E Kennedy and C A McDowell J Chem Phys 59 3403 1979 i Standard Samples e Hyde H S R C Sweed Jr and GH Rist J Chem Phys 51 1404 1969 e Dalal N S D E Kennedy and C A McDowell J Chem Phys 61 1989 1974 e Dalal N S D E Kennedy and C
6. for testing the performance of your Bruker EPR spectrometer The first test measures the spec trometer s sensitivity The procedure is especially designed to test as many of the components of the spectrometer as possible with one simple test It therefore gives you a good indication of the overall health of your spectrometer It is also an excellent cri terion for comparing the sensitivity of different spectrometers The second test measures the background signal of the cavity Should your spectrometer or cavity not meet specifications first consult Chapter 7 on troubleshooting If none of the hints solve the problem contact your local Bruker EPR service representa tive EMX User s Manual Signal to Noise Ratio Test Signal to Noise Ratio Test 9 1 The signal to noise ratio test is an important part in maintaining your spectrometer It is also helpful in diagnosing possible prob lems you may encounter especially when you deal with very weak signals or quantify your EPR signals A standard signal to noise ratio test uses the ER 4102ST standard cavity and the weak pitch sample that was shipped with your spectrometer The test measures the EPR signal intensity peak to peak height of the weak pitch sample at low microwave power 12 db and then measures the noise level under the same conditions except higher microwave power 0 db and higher receiver gain to char acterize the noise better The formula for calculation of signal to n
7. of the Signal Channel Calibrating the Signal Channel 8 2 3 1 Open the Calibrate Signal Channel dialog box Click the Calibrate Signal Channel command in the Acquisition drop down menu A new dialog box will appear Calibrate Signal Channel xi c winepr tpu st9515al cal Change File Frequency List Calibrate from 100 0 kHz to 6 kHz Increments 10 kHz Mod Amplitude Limit 100 0 max 1st Resonator X 2nd Harmonic 2nd Resonator enhanced gt Start Cancel Figure 8 12 The Calibrate Signal Channel dialog box Enter the filename for the calibration file The cali bration 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 Calibration File dialog box Signal channel cali bration files are normally stored in the tpu subdirectory along with field controller and other calibration files Enter a filename and click OK EMX User s Manual Calibration of the Signal Channel Better safe than sorry It is a good idea to cali brate the phase of the second harmonic when running the calibration routine Open Calibration File a AE File name Folders c winepr tpu st9515al cal st9515b cal st9515c cal Help st9515d cal sto9
8. your spectrometer s signal chan nel reference phase and modulation amplitude in order to obtain maximum sensitivity minimum distortion and quantitatively reproducible measurements The EMX027 in conjunction with the WIN EPR Acquisition software 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 reso nator has its own individual calibration file therefore this pro cedure must be followed for each cavity 8 2 1 Calibration of the signal channel involves two separate yet inter dependent 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 mod ulation amplitude The second procedure is to calibrate the phase difference between the reference signal and the modulated EPR signal Because the calibration and adjustment of the modulation amplitude can affect the phase difference the first procedure is performed first You calibrate the modulation amplitude by overmodulating a narrow EPR signal A crystal of DPPH with a line width of approximately 1 G is a very good sample to use When the mod ulation amplitude is large compared to the line width the mag netic field modulation brings the sample into resonance before and after the magnet has reached the field fo
9. 122 cal Network test cal tm4103 cal tm4103a cal List files of type Drives Jscr Calibration Files ce J c micron fd Figure 8 13 The Calibrate Signal Channel dialog box Set the frequency limits A calibration is required for each modulation 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 frequen cies 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 Choose the harmonics The signal channel can pro duce 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 the option to calibrate the phase for the second harmonic by clicking the 2nd Harmonic box A cross in the box indi cates that the option is selected However the time sav ings 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 8 16 Calibration of the Signal Channel The ER 4105DR dual cavity is different from the ER 4116DM dual mode
10. 4 Avoid moving the sam ple 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 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 8 10 W J A m a Calibration of the Signal Channel If there is no spectrum available click on the New Experiment but ton in the tool bar to create a new spectrum See Figure 8 6 Transfer the parameters To set the parameter values to a spectrum click on the Set parameters to spectrum button The cursor will turn into the letter P for Parame ter Place the cursor on a spectrum 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 ae e S GEIS EnA CS oJ New Experiment Button Figure 8 6 The New Experiment button Check the AFC Trap and High Pass Filters Click on the Signal Channel Options command in the Parameter drop down menu The Signal Channel Options dialog box will appear Signal Channel Options Mil Ei Resonator Tuning Caps 32 c winepr
11. A McDowell Chem Phys Lett 30 186 1975 Weak and Strong Pitch Samples 8 1 2 Pitch in KCI has emerged as a standard because of its long lived paramagnetic radicals and low dielectric loss Because of the long life of the radicals it is unsurpassed as a test of spectrome ter sensitivity The pitch is added to a powder of KCI and the mixture is carefully mechanically mixed to obtain a homoge neous sample After mixing the sample is heated pumped and sealed under vacuum Pitch is generally prepared in two concen trations strong pitch which is 0 11 pitch in KCl and weak pitch which is 0 0003 pitch in KCl 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 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 Instruments have a nominal concen tration of 10 spins per centimeter The samples are calibrated and the correction factor is printed on the side of the tube This sample is prepared for the purpose of measuring instrument per formance owing to its high stability however it is not meant as a quantitative spin counting standard EMX User s Manual 8 3 Calibration of the Signal Channel Calibration of the Signal Channel 8 2 Basic Theory You need to carefully calibrate
12. ER 4117D MVT 10 ER 4117D R 10 ER 4109EF 10 Table 8 1 Maximum modulation amplitude for EPR cavities EMX User s Manual 8 19 Calibration of the Signal Channel Mod Amplitude Limit Mod Amp max Set the Mod Amplitude Limit The Mod Amplitude Limit parameter 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 Actual Mod 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 amplitude listed for your cavity in Table 8 1 Return to Step 1 e g start the calibration routine again and enter this new value for Mod Amplitude Limit Continue from Step 2 through Step 7 as before Finish the Calibration When the routine is finished the message Acquisition Done will appear in the info line Double click the control menu box in the upper left hand corner of the window to close the window The sig nal channel is now calibrated for your cavity and the data saved in the calibration file The next time that you start the WIN EPR Acquisition software this calibration file will be the default calibration file 8 20 i System Performance Tests 9 This chapter describes procedures
13. Resonator 2 IX AFC Trap Filter lodulation Signal Input Lock in T external Acquisition Trigger Internal Internal Internal F Lock tn Integrator External C External External Figure 9 2 Signal Channel Options EMX User s Manual 9 5 Signal to Noise Ratio Test Measuring the Signal to Noise Ratio 9 1 2 1 Open the Signal Noise Ratio Test window Open the Signal Noise Ratio Test window under the Acquisi tion drop down menu See Figure 9 3 The window has two empty spectra and each one contains a set of default parameters for signal or noise measurement Click either one of the windows with the left mouse button to activate that window The parameters shown on the right will be assigned to that measurement E WINEPR ACQU Eile Parameter AAEE LLL Calibrate Signal Channel OPERATE Interactive Receiver Level LEVELLED Quadrature Detection Spectr1 10 3 2 1 3460 3480 3500 Calculate the Signal Noise ratio of two spectra Figure 9 3 Open Signal Noise Measurement window 2 Input the calibration factor for the weak pitch sample Enter the calibration factor you copied in 9 6 Signal to Noise Ratio Test Step 2 of the previous section into the Weak Pitch fac tor box See Figure 9 4 2 WINEPR ACQUISITION Signal Noise Measurement File Parameter Acquisition Processing View Options Window Info Dose aS IAI Io Bele ELIS le
14. ain 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 Once the modulation amplitude has been calibrated the refer ence phase is easily calibrated by studying the phase angle dependence of the signal intensity 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 8 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 spectra are acquired at several different values of the reference phase and the 90 phase difference is extrapolated from the signal intensi ties The phase angle resulting in maximum signal intensity for that particular frequency is recorded and saved with the calibra tion 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 chann
15. auss Experiment X Field Sweep z Y no Y Sweep M Hall Signal Channel Center Field 3484 54 Ee Receiver Gain Ejoo 10 gt E Sweep Width 100 00 kK G Modulation Frequency 100 00 4 kHz Static Field 3483 116 He Modulation Amplitude 1 00 cm Ke Microwave Bridge Modulation Phase 0 00 deg Frequency 9 766000 GHz Offset 0 00 Power 0 64 mW Time Constant 0 64 Fe msec Step 1 db Conversion Time 5 12 H msec Temperature unit paecniime 524 SEL Temperature A K Harmonic 1 Ste 1 00 K a css E Resolution in X 1024 onome ar Number of X Scans 1 anole eee Resolution in Y E Step hiks deg Repetitive Mode fi Figure 8 8 Parameters for a signal channel calibration 10 Acquire a spectrum Click the RUN button in the tool bar 8 12 Calibration of the Signal Channel 11 Adjust the Receiver Gain Monitor the Receiver Level while the scan is running See Figure 8 9 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 M2 WINEPR ACQUISITION ST_PITCH PAR File Parameter Acquisition Processing View Options Window Info OSs cei Bele Ammara AAEH Joi OPERATE Frequency 978 GHz Receiver Level LEVELLED Attenuation 20 dB ci I A o 2 012 mW Diode Hall Field CALIBRATED Qva
16. cavity The ER 4116DM has only one set of modulation coils Select the resonator In almost all cases the 1st Res onator should be selected The ER 4105DR dual cavity has two sets of modulation coils By selecting 1st Reso nator or 2nd Resonator you are selecting the set of modulation coils that are to be calibrated Start the calibration routine Click the Start button A new dialog box will appear See Figure 8 14 The spectrometer will then automatically calibrate the signal channel at each of the specified modulation frequencies Calibration File c winepr tpu st9515f cal Freq kHz Res TuningC nF Mod Amp max Mod Amp G Phase 1 Phase 2 50 0 1 25 00 ee 10 3 nana naar naan nina ninna nana ninr nann nina Figure 8 14 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 col umn 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 EMX User s Manual 8 17 Calibration of the Signal Channel the measured maximum modulation amplitude when the corresponding Mod Amp max is used Phase 1 and Phase 2 in columns si
17. e 2700 Receiver 3320 000G Level Figure 8 9 The Receiver Level display 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 8 10 Heisa krkem H A ofa 4 Interactive Change Button Figure 8 10 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 This EMX User s Manual 8 13 Calibration of the Signal Channel action replaces the center field value with the magnetic field position of the marker For further details on this operation consult Section 4 3 2 of this manual M2 WINEPR ACQUISITION Spectr1 Eile Parameter Acquisition Processing View Options Window Info SiS chee Ee EEEH e mE ese ae OPERATE AFC Receiver Level Temperature MORERA Frequency 977 Ghiz Eevee tervation 25 a CEJ CI awoook SS POWSl 0 636 mW Diode Hall Field E ovale 2200 m m 3406 4806 Spectr1 Intensity 600 400 200 0 3420 3430 3440 3450 3460 3470 3480 3490 3500 G NUM 348821G Figure 8 11 The center field marker Acquire the spectrum once more The DPPH signal should now be nicely centered in the spectrum Calibration
18. e 9 12 Acquire a weak pitch signal Open Microwave Control dialog box and set to Stand by Remove the weak pitch sample and retune the bridge and cavity 9 18 Cavity Background Signal Test 10 Duplicate the weak pitch spectrum window Click the Duplicate button in the tool bar M2 WINEPR ACQUISITION Spectr2 Eile Parameter Acquisition Processing View Options Window Info Hesa kkek a Ele Ge ea mee ET OPERATE Frequency 9 82 GHz AFC Receiver Level LEVELLED Attenuation 3 dB Lit eMM Powel 100 6 mW Diode Hall Field MAREE ovau fo cit ewe BACK_WKP PAR ol x 10 3 1 10 3 3470 3480 3490 Figure 9 13 Duplicate weak pitch spectrum 11 Change the Center Field and Sweep Width Open the Experiment parameter dialog box Change the Cen ter Field to 2600 G and the Sweep Width to 5000 G EMX User s Manual 9 19 Cavity Background Signal Test Other parameters should be the same as that for the weak pitch measurement See Figure 9 14 and Table 9 2 Standard Parameter BACK_CVT PAR Lx Experiment x Fieid Sweep F Y no Y Sweep Hall Signal Channel Center Field 2600 00 Ec Bea aa oe po 4 Svespiidth 5000 00 G Modulation Frequency 100 00 kHz iatis pisid 380 000 Ea Modulation Amplitude 8 00 Ea Microwave Bridge Modulation Phase H deg Frequency 819000 GHz Offset H Power 100 60 mW Time Constant 1310 72 msec Step 1 db Convers
19. e tune before Temperature Unit Toa po k Delay betore cach sweep M Temperature Flyback Off gt Delay Time 2 00 one Condition Don t wait X Misntex SE Goniometer Sweep aseline Correction Angle Flyback Off x Delay caling Value Save as Default OK Set Experimental Options Acquire a weak pitch spectrum Click the RUN but ton in the tool bar to acquire a weak pitch spectrum See Figure 9 12 Adjust Receiver Gain if needed If the weak pitch signal clipped return back to Step 2 and reacquire the spectrum Adjust the offset if needed If there is a large offset you can open the Interactive Spectrometer Control dia log box to adjust the offset to the proper position where the indicator of the Receiver Level is in the middle Do not forget to click the Set Parameters to the Spectrum button and move the pointer to the signal measurement window and click the left mouse button again Reacquire the spectrum EMX User s Manual 9 17 Cavity Background Signal Test 7 Save the spectrum Save the spectrum on the hard disk for future reference M2 WINEPR ACQUISITION BACK_WKP PAR BEES Eile Parameter Acquisition Processing View Options Window Info DiS ie aS kkhk Bele hee el E eile ia NOPERATEN Frequency 9 82 GHz AFC Receiver Level LEVELLED Attenuation 3 dB Lis mam ee NOE 100 6 mW Diode Hall Field CALIBRATED gvalue fo Lit 34800006 10 3 3480 3500 Figur
20. el used can all change the phase difference However the reference 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 beginning of this chapter are very good references for the details on the theory of phase sensitive detection and the calibra i Calibration of the Signal Channel tion of signal channels We encourage you to explore this topic further to learn more about calibration Figure 8 3 Signal intensity as a function of the reference phase angle EMX User s Manual 8 7 Calibration of the Signal Channel Preparing for Signal Channel Calibration 8 2 2 A Do not attempt to cali brate a cavity with an ER 4112HV or ER 4113HV helium cryostat installed in the cavity 1 Follow the instructions of Sections 3 2 through 3 5 of this manual You should have the spectrometer turned on the cavity properly installed with a Bruker stan dard DPPH sample in it and the microwave bridge and cavity tuned Remove cryostats from the cavity because it is easier to position the DPPH sample properly in the cav ity Except for the FlexLine resonators it is necessary to use the ER 4118CF cryostat when calibrating FlexLine resonators In particular the ER 4112HV and ER 4113HV helium cryostats prevent the correct position ing of the sample Another advantage is that the resonant
21. ersion Time 163 84 ms noise trace 10 noise Receiver gain 5 02 10 Microwave power 200 68 mW 3470 3480 3490 3500 G Conversion Time 163 84 ms 10 3 linear Baseline Correction 50 Weak Pitch factor 1 02 0 RE Signal noise ratio 50 0 25 50 75 100 125 sec SPECTR EIEIES Figure 9 7 Signal measurement 6 Activate the noise measurement Click the lower window to activate the noise measurement window T Check the parameters Open the parameter dialog box Make sure the X axis is set to Time Scan the power is 200 mW gain is 5 x 10 and the field center at 3300 G The other parameters should be similar to that in signal measurement See Figure 9 8 9 10 Signal to Noise Ratio Test Standard Parameter Signal Noise Measurement Lx Experiment x Time Scan F Y no Y Sweep Hall Center Field Sweep Width Static Field po 3300 000 G Modulation Amplitude 8 00 Signal Channel SE Receiver Gain fo 10 P E sja Modulation Frequency 100 00 kHz Modulation Phase Microwave Bridge Frequency Power 9 795000 GHz Offset 200 68 mW Time Constant 1310 72 msec esi Step 1 db Conversion Time 163 84 msec Temperature unit Sweep Time 167 77 sec Temperature K Harmonic 1 Step 1 00 adk Resolution in X 1024 ea Number of X Scans 1 Anale goo Resolu
22. ge Modulation Phase 0 00 deg Frequency 9 814000 GHz Offset 2 00 Power 100 80 mW Time Constant 1310 72 21 msec Step 1 H db Conversion Time 163 84 a msec Temperature unit Sweep Time 167 77 sec Temperature 300 00 a K Harmonic 1 H Step 1 00 Hk Resolution in X 1024 al SEITE Number of X Scans 1 Angle deg Resolution in Y 1 St 1 000 stg Ha ao Repetitive Mode r Figure 9 10 Set the parameters for weak pitch sample Set receiver gain properly Since the microwave power 3 db is higher than in the signal noise ratio test you may need to adjust the receiver gain accordingly The suggested receiver gain is 1 x 10 Set a time delay Since a very long time constant is used set a delay time of 2 5 seconds to avoid overshoots or undershoots in the first few data points when you acquire the spectrum Open the Experimental Options dialog box found in the Parameter drop down menu and set the Delay before each sweep option and a delay of two to five seconds See Figure 9 11 9 16 ip Cavity Background Signal Test See Section 4 5 for help with interactive spectrometer control Figure 9 11 4 Experiment Options Signal Noise Measurement Field Controller Microwave Settings Sweep Direction Up zi Power Se Sweep Flyback On Zi Power Flyback Off Del ay Settling Condition Wait LED off hd A z Option Field Offset Correction fo co G IX MW Fin
23. ing Condition wait LED off Option Field Offset Correction fo co G IX MW Fine tune before Temperature Unit ae ro Ek Delay before cacheweep Temperature Flyback for zi Delay Time 2 00 sec Condition Don t wait d Misni nA Goniometer Sweep aseline Correction Angle Flyback Off x Delay caling Value Save as Default OK Figure 9 6 Set Experimental Options 5 Acquire a signal spectrum Click the RUN button in the tool bar to acquire a weak pitch spectrum See Figure 9 7 If the spectrum is off center you can use the center field tool to set the correct field center If there is a large offset you can open the Interactive Spectrometer Control dialog box to adjust the offset to the proper posi tion where the indicator of the Receiver Level is in the middle Do not forget to click the Set Parameters to the Spectrum button and move the pointer to the signal mea surement window and click the left mouse button again EMX User s Manual 9 9 Signal to Noise Ratio Test WE WINEPR ACQUISITION Signal Noise Measurement Jorx Eile Parameter Acquisition Processing View Options Window Info heia 2 EIT Bele EL Ie elm 2 ole Ele AFC Receiver Level E eoe fom oe EEE Attenuation 12 dB LEI P 12 66 mW Diode Hall Field EE ovau 2500 LEI 3465 440G Signal Noise Measurement Mi E 10 3 signal trace Peak to peak height 34722 00 10 Receiver gain 20010 Microwave power 12 66 mW 0 Conv
24. ion Time 163 84 msec Temperature unit Sweep Time 16777 sec Temperature 300 00 K Harmonic Step 1 00 K Goniometer aie Resolution in X 1024 Number of X Scans SN a gog Resolution in Y Step 1 000 deg mal im Mh Repetitive Mode Figure 9 14 Parameters for cavity background signal measurement 12 Set a time delay Set a 2 5 seconds time delay in the Experiment Options box as in Section 9 2 2 Step 3 13 Acquire a cavity background spectrum Click the RUN button in the tool bar to acquire the cavity back ground signal See Figure 9 15 9 20 Cavity Background Signal Test 14 Save the spectrum Save the spectrum on the hard disk for future reference MB WINEPR ACQUISITION BACK_CVT PAR ol Eile Parameter Acquisition Processing View Options Window Info AFC Receiver Level E eor e o a E LEVELLED Attenuation 3 dB Let bested Power 100 8 mW Diode Hall Field EE ovale 2200 Lo ms 100 000 G L BACK WKPPAR in BACK_CVTPAR SE 10 3 10 3 30 25 20 15 10 3470 3480 3490 3500 1000 2000 3000 4000 Figure 9 15 Acquire the cavity background signal 15 Transfer the spectra to WinEPR Transfer the two spectra you acquired to WinEPR You can either open the WinEPR program and then load the data files you just saved or you can click the Transfer to WinEPR button in the tool bar which
25. l 2 isle Elz Frequency 9 78 GHz Attenuation 30 dB Power 0 200 mW Q Yalue 2700 103 Signal Measurement Window 3460 3470 Indicator of the Active Window 3480 3490 G 10 3 5 Noise Measurement Window Figure 9 4 0 50 100 sec AFC Receiver Level Si nal Chis CI 8 Diode Hall Field Measurement Lis 3480000 Parameters signal trace Peak t k height x eak to peak heig Receiver gain 2 00 10 2 Noise Microwave power 12 00 mW Measurement Conversion Time 163 84 ms Parameters noise trace noise J Receiver gain 5 02 10 5 p Microwave power 200 00 mW Weak Pitch Conversion Time 163 84 ms Calibration linear Baseline Correction Factor Weak Pitch factor 1 02 J4 1 453e 006 Signal noise ratio Signal Noise Measurement Window Activate the signal measurement Click the signal window the upper one A blue bar will appear on the right upper corner Check the parameter settings by open ing the Standard Parameter dialog box The parameters should look like those in Figure 9 5 EMX User s Manual 9 7 Signal to Noise Ratio Test Figure 9 5 Experiment x Standard Parameter Signal Noise Measurement Lx Fieid Sweep F Y no Y Sweep Hall Center Field 3480 00 Sweep Width 50 00 G Static Field Signal Channel 5 3480 00 G Receiver Gain H 200 10 Mod
26. oise ratio is As Gy P S a fil BENN 9 1 N Ay Gs APs JTXC where A and Ay are the peak to peak height of the weak pitch and amplitude of the noise respectively Gs and Gy are the receiver gains used in signal and noise measurements respec tively We use their ratio to correct for the gain difference Ps and Py are the powers used in two measurements and we use the square root of the ratio of powers to correct for the power differ ence The factor of 2 5 translates the peak to peak noise level to a RMS Root Mean Square noise level T is the time constant in seconds and we use the square root of the time constant to normalize the S N to a one second time constant C is the weak pitch correction factor that is printed on the label of the weak pitch sample The standard instrument settings for signal and noise measurements are listed in Table 9 1 There is a built in subroutine to measure the signal to noise ratio which has the default values of standard settings If you want to measure the amplitudes of the signal and noise on a print out by hand make sure that you use the same scale for both signal and noise spec i Signal to Noise Ratio Test tra Otherwise you need to multiply the result by the ratio of the scales Parameter Modulation Amplitude Modulation Frequency Receiver Gain Phase Time Constant Conversion Time Center Field X Axis Setting Sweep Width Resolution of X Axis Microwave A
27. p contact your local Bruker service repre sentatives EMX User s Manual 9 13 Cavity Background Signal Test Cavity Background Signal Test 9 2 Cavity background signals can sometimes be disturbing particu larly when they overlap with your EPR signals or with the area you need to integrate They can distort the EPR signals of your sample and make quantification difficult The best way to avoid these problems is to keep your cavity clean Here we provide a standard procedure to test your cavity background signal The standard cavity background signal test compares the weak pitch signal with the spectrum acquired with an empty cavity over a wide scan range The parameter setting for a standard test is shown in Table 9 2 The ratio of the cavity background signal over the peak to peak height of the weak pitch signal should be less than 1 4 to meet the specifications Preparing for the Background Signal Test 9 2 1 1 Install an ER 4102ST standard cavity See Section 5 2 for instructions The specification for the background signal is based on an ER 4102ST standard cavity and using the weak pitch sample We strongly sug gest that you keep a record and verify the specification periodically 2 Insert the weak pitch sample The weak pitch sample should be inserted in the cavity until the bottom of the label and tape on the sample tube is flush with the collet You also should use the pedestal to hold the weak pitch rigidly
28. r resonance This results in a broadening and distortion of the EPR signal See Figure 8 1 In the limit of an infinitesimally narrow EPR sig nal the peak to peak width of the first derivative EPR signal will be approximately equal to the peak to peak modulation ampli tude i Calibration of the Signal Channel 32G aes 16G Ds 8G R yp PS 4G ae ial ga Sy 2S en ee s amp v 1G e Figure 8 1 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 ampli fier needs a bit of help to obtain large modulation amplitudes at modulation frequencies greater than 50 kHz This is a conse quence of the decreasing skin depth with increasing frequency The modulation coils on the cavity are tuned or made resonant by adding a tuning capacitor in series with the modulation coil Tuning es Capacitor m Modulation Coil AS ae Figure 8 2 The LC resonant circuit for high frequencies EMX User s Manual 8 5 Calibration of the Signal Channel 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 for full g
29. round signal 18 Measure the weak pitch signal Click the weak pitch spectrum and repeat the same procedure to get the signal height of the weak pitch signal EMX User s Manual 9 23 Cavity Background Signal Test 19 Calculate the result The ratio of the cavity back ground signal over the weak pitch signal is the test result cavity background signal high val low val y 1 weak pitch signal high val low val 4 The ratio must be less than 1 4 to meet the specifications If the ratio is greater than 1 4 contact your local Bruker EPR service representative 9 24 i
30. tely 1 x 10 works well 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 provide a real time display of the EPR spectrum on the screen 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 approximately half of the vertical display range Make sure that the sig nal channel is set to 100 kHz modulation and first har monic detection EMX User s Manual 8 9 Calibration of the Signal Channel Hal R nal Channel Center Field Magnetic Calibrated Sweep Width Field ceiver Gain H 2 52 10 B Sweep Address 1525 Va Modulation Freq 100 00 HHkez Kil tn gt Modulation Amplitude 1 00 Hye Field 3474 000 G Modulation Phase 0 00 HH deg B 0 00 Setup Scan Enable KE Button tant psa HH msec Sweep Width 100 00 Ha Time 5 12 HH msec leve H i 4 Harmonic k MW Attenuator p 4 dB Temperature 300 00 S K Set parameters to spectrum SCT Options Figure 8 5 The Interactive Spectrometer Control dia log box Optimize the DPPH sample position Move the sample tube up and down until the maximum signal inten sity is attained See Figure 8
31. th the collet You also should use the pedestal to hold the weak pitch rigidly Turn on the instrument and tune Turn on the instru ment if it is not on yet Tune the microwave bridge and the cavity It is best to wait several hours because the spec trometer is most sensitive and stable after it has achieved thermal equilibrium Set the AFC depth You can find the AFC depth adjust ment knob on the back of the microwave bridge See Figure 9 1 Full scale is ten Set the AFC depth or amplitude at around 2 Check the signal channel options settings Open the Signal Channel Options dialog box Make sure you have the right calibration file for the standard cavity you are using High Pass Filter and AFC Trap Filter See Figure 9 2 are checked in the default settings In case the default settings have been changed set them back W J A m a i Signal to Noise Ratio Test S MICROWAVE D OUTPUTS o Figure 9 1 Location of the AFC MOD LEVEL potentiometer HB WINEPR ACQUISITION Signal Channel Options View Options Window Info glxlalz AHEL bE Eeo EE OPERATE Frequency GHz AFC Receiver Level LEVELLED Attenuation 30 dB Li Parar mW i Hall Field MNCALIBRATED Q value Signal Channel Options sonator Tuning Caps 0 a cAwinepr tpu st951 Sal cal I scrsettest H e IX High Pass Filter
32. tion in Y 1 Step 1 000 deg va Repetitive Mode E Figure 9 8 Parameters for noise measurement See Section 4 5 for help with interactive spectrometer control Get ready to acquire a noise spectrum Click the RUN button in the tool bar to acquire the noise spectrum Frequently the baseline will drift since 200 mW micro wave power is going to heat up the cavity and the sample Wait a few minutes to achieve thermal equilibrium Check the tuning and coupling of the system Retune the system if necessary You may also have a rather large offset due to the excessive power and high gain Use the interactive box to make the offset adjustment so that the indicator of the receiver level is in the middle Click the left mouse button on Set Parameters to Spectrum move the pointer to the noise measurement window and click again If you experience overshoots or undershoots you can set a 2 5 second delay time in the Experimental Options box as in Step 4 EMX User s Manual 9 11 Signal to Noise Ratio Test WE WINEPR ACQUISITION Signal Noise Measurement Jor Eile Parameter Acquisition Processing View Options Window Info Acquire a noise spectrum Click the RUN button in the tool bar and acquire the noise spectrum again Two horizontal lines will automatically emerge indicating the noise level If the baseline still drifts you can click the lin ear baseline correction button to compensate for linear drifts
33. tpu st9515al_ cal Ena h Fil Resonator 2 Change ite Modulation IX AFC Trap Filter Signal Input Lock in F external Acquisition Trigger F Lock In Integrator Internal Internal Internal External External External Figure 8 7 The Signal Channel Options dialog box The AFC trap filter blocks any frequency signal compo nents at the AFC modulation frequency that may contrib ute to noise in the EPR signal The high pass filter suppresses low frequency signal components that may EMX User s Manual 8 11 Calibration of the Signal Channel You do not need to edit the Static Field param eter for a signal channel calibration 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 circum stances would you acquire spectra without these filters 9 Adjust some parameters After centering the DPPH sample most of the parameters should be fairly close to what is needed for the calibration routine Check the val ues in the Standard Acquisition Parameter dialog box and modify them so that they correspond to the values in Figure 8 8 The Center Field value may be somewhat different from what is displayed in Figure 8 8 but the Sweep Width must be 100 G
34. ttenuation Signal Measurement 8 0G 100 kHz 2 0 x 10 0 1310 72 ms 163 84 ms 3480 G Field Sweep 50G 1024 points 12 dB Noise Measurement 8 0G 100 kHz 5 0 x 10 0 1310 72 ms 163 84 ms 3300 G Time Scan 1024 points 0 dB Table 9 1 Parameters for Signal Noise Measurements EMX User s Manual 9 3 Signal to Noise Ratio Test Preparing for the S N Test 9 1 1 F Vly lt The calibration factor is found on the weak pitch sample s label It is listed as C Cgx calibration factor It is usually approxi mately equal to one and corrects for variations in the sample concen tration 1 Install an ER 4102ST standard cavity See Section 5 2 for instructions The specification for the signal to noise ratio is based on an ER 4102ST standard cavity and using the weak pitch sample We strongly sug gest using the standard cavity for this standard test and keep a record and verify the specification periodically If you use other types of cavities to do the signal to noise ratio test the results and the settings will be different due to different Q values and the microwave field distribu tions of the cavities Insert the weak pitch sample Copy down the cali bration factor posted on the label of the weak pitch before you insert it The weak pitch sample should be inserted in the cavity until the bottom of the label and tape on the sample tube is flush wi
35. ulation Frequency 100 00 kHz 3483 116 G Modulation Amplitude 8 00 Microwave Bridge Modulation Phase Frequency 9 766000 GHz Offset Power 12 00 mW Time Constant 1310 72 msec or Step 1 db Conversion Time 163 84 msec Temperature unit Sweep Time 167 77 sec Temperature K Harmonic 1 Step 1 00 ik Resolution in X 1024 maa Number of X Scans 1 Anale deg Resolution in Y 1 Step 1 000 deg or Repetitive Mode 5 Parameters 4 for signal measurement Set a time delay Since a very long time constant is used set a delay time of 2 5 seconds to avoid overshoots or undershoots in the first few data points when you acquire the spectrum Open the Experimental Options dialog box found in the Parameter drop down menu and set the Delay before each sweep option and a delay of two to five seconds See Figure 9 6 We also advise you to select MW Fine Tune before each sweep option to ensure the acquisition is made under proper coupling conditions 9 8 ip Signal to Noise Ratio Test See Section 4 3 2 for help in setting center fields and Section 4 5 for help with interac tive spectrometer con trol Experiment Options Signal Noise Fine Field Controllesp Tune i Microwave Settings Sweep Direction Up Power Se Sweep Option Flyback On Power Flyback Off Del ay Settl
36. will automatically launch the WinEPR program and transfer the spectrum of the active window To transfer the other spectrum you need to activate that spectrum window in Acquisition program by clicking the EMX User s Manual 9 21 Cavity Background Signal Test spectrum window and then click the Transfer to WinEPR button in the tool bar again The WinEPR appli cation will appear See Figure 9 16 Bruker WINEPR System Data Tansfer Options Help Bl te alee SEIS DRM ees Algebra Show Document Figure 9 16 Transfer to the WinEPR for data processing 16 Click 1D processing under WINEPR System Select the cavity background spectrum 17 Measure the cavity background signal Click Expand under Display A box contains Expand Display Values will appear On the right side of the box there are low val and high val of the Y Scale The difference between these two values is the signal height of the cavity background signal See Figure 9 17 9 22 i Cavity Background Signal Test WINEPR System 1D Processing Eile 1D Processing Parameters JB ptions Help Scaling CSS ees Jie Reset z i s Expand Display Values Split Screen Co Scaling Y Scaling i i is Start 100 00 low val 1663 5 Substitute 1 00 00 wen i 5100 00 i fess View End high val Distance DataType gt 1500 4000 Figure 9 17 Measure the signal height of the cavity backg
37. x and seven respectively are the phases at which the first and second harmonic signals are nulled Check Mod Amp G at 100 kHz The calibration routine performs its task sequentially starting with the highest modulation frequency and continuing for each selected modulation frequency As each parameter is determined it is displayed in the table See Figure 8 14 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 modula tion amplitude resulting in damaged modulation coils Compare the Mod Amp G at 100 kHz with the values listed for your cavity in Table 8 1 If the value obtained by the calibration routine exceeds the values listed in Table 8 1 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 8 1 allow the calibration routine to continue its task and proceed to Step 9 i Calibration of the Signal Channel i Maximum Mod Gauss Cavity at 100 kHz ER 4102ST 32 ER 4105DR 32 ER 41040R 32 ER 4116DM 10 ER 4103TM 16 ER 4108TMH 16 ER 4106ZRC 10 ER 4106ZRAC 10 ER 4107WZC 10 ER 4107WZAC 10 ER 41150DC 10 ER 41150DAC 10 ER 4122SHQ 15 ER 4114HT 10
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