Tuning and Matching
Contents
1. e g enter stop Enter acqu to switch to the acquisition window if this will be used to monitor the tuning and matching Start the frequency sweep by typing wobb The curve that appears in the acquisition window is for 3C Adjust the tuning and matching following the guidelines given above for H Notice that some probeheads e g broadband probeheads have sliding bars instead of screws one set labeled tuning and another matching These probeheads also have a menu of tuning and matching values for several nuclei Set the tuning and matching sliding bars to the values indicated for C on the menu Adjust the tuning bar until the dip is well matched at some frequency and then walk the dip towards the correct frequency as described above for H Once the 3C circuit is tuned and matched the 1 C wobbling must be stopped and H wobbling begun One straightforward way to do this is as follows Exit the wobble routine by typing stop Enter edsp change to NUCI to 1H and exit by clicking SAVE Then start the H frequency sweep by typing wobb After a few seconds the H curve appears in the acquisition window and the H circuit can be tuned and matched as described above Alternatively if the user already has a data set in which NUC1 1H and NUC2 OFF there is no need to redo edsp for the current data set The user may simply read in the H data set and then type wobb Once the probehead is tuned and matched for C and H exit the wobb2. probe needs to be retuned each time the heteronucleus is changed Matching is the process of adjusting the impedance of the resonant circuit until it corresponds with the impedance of the transmission line connected to it This impedance is 50Q Correct matching minimizes the power that is reflected by the probehead and so is lost or equivalently maximizes the power that is transmitted to the coil and so is available to do NMR A probehead is said to be matched when all of its resonant circuits are matched Again once a probehead has been matched it is not necessary to rematch it after slight adjustments to the carrier frequency On the other hand large adjustments to the carrier frequency necessary when changing nuclei do warrant rematching the probehead Tuning and matching are carried out simultaneously using the XWIN NMR command wobb wobble During wobbling a low power signal is transmitted to the probehead This signal is swept over a frequency range determined by the parameter wbsw the default value is 4 MHz centered on the carrier frequency i e sfol sfo2 etc depending on which nucleus is being tuned matched Within the preamp High Performance Preamplifier Assembly or HPPR the impedance of the probe over this frequency range is compared to the impedance of a 50Q resistor The results are shown both on the LED display of the HPPR and in the acquisition submenu in XWIN NMR Both displays show the reflected power of the prob
3. 70 6 6 0 384 100 3 7 0 306 160 0 3 0 220 250 3 9 0 158 400 7 1 0 111 90 600 9 9 0 083 1000 13 2 0 059 1500 15 2 0 047 2000 16 8 0 041 So for example if auto gain determines a lock gain of 100dB the user should set the loop filter to 160 Hz the loop gain to 0 3 dB and the loop time to 0 220 sec see Chapter 4 Menu Description of the BSMS User s Manual for how to set these parameters from the BSMS keyboard AVANCE User s Guide Bruker Goto Locking and Shimming 17 Preparing for Acquisition 18 Bruker AVANCE User s Guide Goto
4. Chapter Preparing for Acquisition 2 Goto Sample Preparation 2 1 The quality of the sample can have a significant impact on the quality of its NMR spectrum The following is a brief list of suggestions to ensure high sample quality Always use clean and dry sample tubes to avoid contaminating the sample Always use good to high quality sample tubes to avoid unnecessary difficulties in shimming Filter the sample solution Always use the same sample volume or solution height This minimizes the shimming that needs to be done between sample changes Recommended values are for 5mm tubes 0 6ml or 4cm of solution and for 10mm tubes 4 0 ml or 4cm of solution Use the depth gauge to position the sample tube correctly in the spinner This is discussed further in Chapter 5 Sample Positioning of the BSMS User s Manual Check that the sample tube is held tightly in the spinner so that it does not slip during an experiment Wipe the sample tube clean before inserting it into the magnet For experiments using sample spinning be sure the spinner especially the reflectors is clean This is important so that the correct spinning rate can be maintained Tuning and Matching the Probehead 2 2 Once the sample is inserted the probehead should be tuned and matched Notice that correct tuning and matching is especially important for higher frequencies In general the probehead should be tuned and matched each time a new samp
5. een tuning and matching it is generally useful to adjust the T and M screws in an iterative fashion Turn the M screw until the dip is well matched at some frequency the dip extends to the x axis and the number of LED s lit in the vertical HPPR display is minimized Most likely this will not be the desired frequency Adjust the T screw slightly to move the dip toward the center of the window or equivalently to reduce the number of LED s lit in the horizontal HPPR display Rematch the dip by adjusting the M screw Again adjust T to move the dip towards the center of the screen and rematch using M In this manner continue walking the dip towards the correct resonance frequency Note that it is possible to run out of range on the M screw If this happens return M to the middle of its range adjust T to get a well matched dip at some frequency and walk the dip towards the correct frequency as described above As mentioned above ideal tuning and matching is when the dip is centered in the window and extends to y 0 the x axis on the acquisition window or equivalently when the number of LED s lit on the preamp is minimized in both the vertical and horizontal display When the H circuit is tuned and matched exit the wobble routine by typing stop Click on return to exit the acquisition window and return to the main window Tuning and Matching C 2 2 2 Since most 1 C experiments make use of H decoupling when tuning and match
6. ehead versus the frequency of the signal The user observes either one or both of these displays while tuning and matching the probehead Tuning and Matching H 2 2 1 12 When the NMR experiments to be performed are H homonuclear experiments e g IH 1D spectroscopy COSY NOESY or TOCSY it is only necessary to tune and match the H circuit of the probehead Make sure that the sample is in the magnet and the probehead is connected for standard H acquisition Note that there is no special configuration for tuning and matching Also it is recommended to tune and match without sample spinning Set the parameters In XWIN NMR enter edsp and set the following spectrometer parameters NUCI 1H NUC2 OFF NUC3 OFF This automatically sets sfol to a frequency appropriate for H tuning and matching There is no need to adjust sfol carefully now Exit edsp by clicking SAVE Other wobb parameters are wbsw which determines the wobble sweep width in MHz the default value is 4MHz and wbst which determines the number of wobble steps over the sweep width the default value is 256 Both of these parameters may be found in the eda table No other parameters are required Start wobbling Bruker AVANCE User s Guide Goto Goto Tuning and Matching the Probehead Before starting the wobbling procedure ensure that no acquisition is in progress e g enter stop Enter acqu to switch to the acquisition window of XWIN NMR if it i
7. ing a probehead for 3C it is generally a good idea also to tune and match for H When tuning and matching a probehead with multiple resonant circuits it is best first to tune and match the lowest frequency circuit and then to proceed to higher AVANCE User s Guide Bruker 13 Preparing for Acquisition 14 frequency circuits The larger capacitors and inductors found in lower frequency resonant circuits can be expected to have larger stray capacitance and inductance than the smaller elements in higher frequency circuits Thus one would expect tuning and matching lower frequency circuits to affect the tuning and matching of higher frequency circuits more so than vice versa So when tuning and matching a probehead for both H and 3C it is best to make the 3C adjustments first and the H adjustments last Make sure that the sample is in the magnet and the probehead is connected for the appropriate experiment Also it is recommended to tune and match without sample spinning Set the parameters In XWIN NMR enter edsp and set the following spectrometer parameters NUCI 13C NUC2 OFF NUC3 OFF This automatically sets sfol to a frequency appropriate for 1 C tuning and matching Exit edsp by clicking SAVE Other wobb parameters are wbsw and wbst as mentioned above Both of these parameters may be found in the eda table No other parameters are required Start wobbling tune and match Ensure that no acquisition is in progress
8. le is inserted each time a new probehead is put in the magnet and each time the observe or decouple nucleus is changed when using a broadband probe In a probehead there is a resonant circuit for each observe and decouple nucleus indicated on the probehead label e g one for H and one for 1 C in a dual H PC probehead one for H and one for a wide range of nuclei in a BBO probehead There is also a resonant circuit for the lock nucleus but the standard user will never need to adjust this so we will ignore it for now Each of these circuits has a frequency at which it is most sensitive the resonance frequency Tuning is the process of adjusting this frequency until it coincides with the frequency of the pulses transmitted to the circuit For example the frequency at which the H resonant circuit is most sensitive must be set to the carrier frequency of the H pulses which is sfol if the H circuit is connected to the f1 channel s 02 if it is connected to the f2 channel etc A probehead is said to be tuned when all of its resonant circuits are tuned Once a probehead has been tuned it is not necessary to retune it after slight adjustments to the carrier frequency since the probehead is AVANCE User s Guide Bruker 11 Preparing for Acquisition generally tuned over a range of a couple MHz On the other hand large adjustments to the carrier frequency necessary when changing nuclei do warrant retuning the probehead so a broadband
9. le routine by typing stop Click on return to exit the acquisition window and return to the main window Bruker AVANCE User s Guide Goto Goto Locking and Shimming Locking and Shimming 2 3 Before running an NMR experiment it is also necessary to lock and shim the magnetic field Locking To display the lock signal enter lockdisp This opens a new window in which the lock trace now appears The most convenient way for the standard user to lock is semi automatically using the XWIN NMR command lock To start the lock in procedure enter Lock and select the appropriate solvent from the menu that appears Alternatively enter the solvent name with the lock command e g Lock cdc13 During lock in the lock power field value and frequency shift for the solvent are set according to the values in the 2H Lock table also known as the edlock table These values can be edited with the command edlock Note that the lock power listed in this table is the level used once lock in has been achieved The field shift mode is then selected and autolock is activated Once lock in is achieved the lock gain is set so that the lock signal is visible in the lock window At this point the message lock finished appears in the status line at the bottom of the window The lock in procedure outlined above sets the frequency shift to the exact frequency shift value for the given solvent as listed in the edlock table It als
10. n using the X WIN NMR lock command is the lock power In some instances the power level listed in the edlock table is too high meaning that the lock signal is always saturated Usually in this situation lock in can be achieved but since the signal is saturated it oscillates A quick fix is simply to reduce the lock power by hand once lock in has been achieved A better fix is to change the power level in the edlock table Note that the appropriate lock power level depends on the lock solvent the field value and the probehead Shimming AVANCE User s Guide Bruker 15 Preparing for Acquisition 16 If the probehead has just been changed the first step in shimming the magnetic field is to read in the shim file corresponding to the new probehead Enter rsh and then select the appropriate file from the menu that appears Assuming that the shim file is a good one or that a prior user has shimmed the field for the current probehead the user need only adjust the Z and Z shims and possibly the X and Y Generally the shims are adjusted while viewing the lock signal and the best shim values correspond to the highest lock level height of the lock signal in the window For further discussion of shimming see Chapter 6 Shim Operation of the BSMS User s Manual Optimize lock settings optional Once the magnetic field has been locked and shimmed the user may wish to optimize the lock settings as described below It is strongly rec
11. o sets the field value to the value which is the same for all solvents listed in the edlock table and then adjusts this slightly to achieve lock in As a result the absolute magnetic field is now nearly the same no matter what lock solvent is used This has the advantage that offsets can now be defined in ppm since the absolute frequency corresponding to a given ppm value no longer depends on the lock solvent Another advantage of following this lock in procedure is that it automatically sets the parameter solvent correctly in the eda table This is especially important if you wish to use the automatic calibration command sref as described later see Spectrum Calibration and Optimization on page 25 It is recommended that each time the probehead is changed the user adjust the phase of the lock signal while monitoring the sweep wiggles i e while the field is not locked but is being swept This is necessary if the original lock phase is very far wrong in which case autolock may fail to achieve lock in If the original phase is reasonably close to correct then lock in can be achieved and the phase can be adjusted afterwards using autophase Please note that the lock phase for each probehead is stored in the edlock table To make shure that XWIN NMR selects the edlock table assigned to the current probe enter edhead then click on Define Current and select your current probe The other lock parameter that may possibly be problematic whe
12. ommended to follow this procedure before running any experiment requiring optimal stability e g NOE difference experiments After the field is locked and shimmed start the auto power routine from the BSMS keyboard see Chapter 2 Key Description of the BSMS User s Manual For lock solvents with long T relaxation times e g CDC13 however auto power may take an unacceptably long time and the lock power should be optimized manually Simply increase the lock power level until the signal begins to oscillate i e until saturation and then reduce the power level slightly approximately 3dB For example if the lock signal begins to oscillate at a power of 15dB the optimal magnetic field stability can be expected when a level of approximately 18 dB or even 20dB is used The field stability will be significantly worse if a power level of say 35dB is used instead When the lock power is optimized start the auto phase routine and finally the auto gain routine Take note of the gain value determined by auto gain Using this value select the appropriate values for the loop filter loop gain and loop time as shown below in Table 2 Bruker AVANCE User s Guide Goto Table 2 Lock Parameters BSMS Firmware Version 940614 Lock RX Gain Loop Filter Loop Gain Loop Time after auto gain Hz dB sec dB 120 20 17 9 0 681 30 14 3 0 589 110 50 9 4 0 464
13. s desired to use this to monitor the tuning and matching Notice that being in the acquisition window slows down the wobbling procedure so if the HPPR LED display will be used to monitor tuning and matching it is best to remain in the main XWIN NMR window and not switch to the acquisition window Start the frequency sweep by typing wobb The curve that appears in the acquisition window is the reflected power as a function of frequency Unless the probehead is quite far from the correct tuning and matching there will be a noticeable dip in the curve When the H circuit is properly tuned the dip will be in the center of the window denoted by the vertical marker and when the circuit is properly matched the dip will extend all the way down to the x axis Similar information is conveyed by the LED display on the HPPR The horizontal row of LED s indicates tuning and the vertical row matching When the circuit is properly tuned and matched the number of LED s lit is minimized This usually means that only green LED s not red are lit in both the horizontal and vertical displays Tune and match Adjust the tuning and matching screws labeled T and M at the base of the probehead with the special tool provided Note that the screws are color coded and those for the H circuit are usually yellow Also note that the screws have a limited range and attempting to turn them beyond this range will damage the probehead Since there is interplay betw
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