--- Perhaps there is a problem with the dwell clock generation in the computer. You might try scans with different sweep widths to see if any of them give the correct result. --- I have seen a similar problem on a WM-250 where the sampling rate was controlled by a free-running oscillator of the type used as computer clocks. It was not referenced or phase-locked to the main system clock. You might check your schematics to see if this is the case on your system. If so, it should occur with all nuclei and sweep widths with a constant factor. There is also a divide-by circuit. Check to see if the problem occurs at extreme maximum and minimum sweep widths. ___ do you see 13C satellites for CHCl3? if yes, is 1JCH also scaled by a factor of 8.18/7.27? ___ I once had this happen on an AM-250. Turns out that someone had set the base frequency to some random number. Loading the standard job file did not help. Do a "cf" and check the basic spectometer proton frequency. ___ Are you sure the lock solvent is really CDCl3? CHCl3 chemical shift in acetone-d6 is 8.02 and DMS0-d6 is 8.32 relative to TMS. ____ What I don't understand is how the TMS is still at zero. This sounds like a digitizer problem to me, with the chnages in sampling rate not being correctly interpreted during the FT. Try powering the whole thing down and then starting it again and see what happens, very often these kind of problems (with AC's)just go away after a power cycle! However if it is the digitzer board this could be expensive. There are a lot of junked AC/AM's lying around so I would post an appeal for parts rather than go to Bruker! ____ This may sound off-the-wall, but are you sure there is no DMSO in your sample ? Chloroform in DMSO resonates _at_ 8.2 ppm. Even 1 drop of DMSO in a chloroform solution will make the CHCl3 signal move a long way. ____ Did you checked the parameters file? Maybe it was altered. Based on my instruments (DPX-300 and DRX-500) one of the paramters that affects the chemical shift scale is [sf} which can be found in [edp]. Maybe a user changed the value and save it with other solvent defined. The value of sf depends on the solvent used. ____ A possible solution: are you using the EXACT same sample on the 250 and 300? A common problem encountered with chloroform is that the chemical shift of chloroform is highly dependent on the sample matrix and concentration. Therefore, when different samples are referenced to TMS at 0, you may observe the CHCl3 in the sample to have a shift of anywhere from 7 to about 8.5 ppm, depending on the sample concentration and matrix. Everything is relative. ____ Have you checked SF0 and SY to make sure that they are set to correct frequency? Incorrect settings would lead to the problem that you describe. ____ Have you CAREFULLY checked your "processing" parameters? (Not just the acquisition parameters.) We had experienced something similar and found one of our processing parameters was inadvertently altered -- I don't remember which one now, but I suspect it was the SF for processing.) I suspect this is a software issue, since TMS is unchanged. ____ (A) Software -- I assume that the system software is properly configured, etc. In particular there is usually a way to input the nominal spectrometer frequency as a software variable to calculate correct chemical shift information. If this variable is wrong, then you would see a result as you describe. (B) Hardware -- An NMR spectrometer is designed to maintain a fixed ratio between the various frequency sources. The absolute accuracy of the master frequency source is not necessarily controlled that closely, but the ratios are maintained to extremely high accuracy. The denominator term in these ratios is the nominal lock frequency. Typically, the reference frequency for all synthesizers is a single, high spectral purity, crystal -controlled master oscillator. Its absolute frequency may be accurate from anywhere to 1 part in 10E6 of its nominal value to 1 part in 10E9 if the master oscillator is in a temperature controlled oven. The idea here is to have a master oscillator with high spectral purity. The absolute frequency accuracy is irrelevant within limits. Each frequency synthesizer will have the means to generate offset frequencies. If an offset synthesizer has failed, then unexpected frequencies would be produced. In your case, if the 2-H lock offset synthesizer is not operating correctly, the system may still manage to maintain a stable field-frequency relationship between the 2-H lock transmitter (which is in error) and the 1-H observation transmitter frequencies (which will also be in error). The offset error in the lock frequency (in Hz) will be about 1/6 the error in the 1-H transmitter frequency. In your example this would be about 6 MHz producing a 36 MHz error in the 1-H transmitter frequency. The effect of operating at the resultant higher 1-H frequency would be to produce the results that you describe with respect to chemical shift -- provided that the software variable was set to a nominal 300 MHz. I might add that if the above scenario (B) is correct, then the 2-H frequency was probably in error at the time the system was originally installed. The installer ran the field up to the point that he or she observed a resonance for either 2-H or 1-H in a test sample and never noticed that the spectrometer was in fact operating at a higher field strength. Assuming that there is not an error in a software variable (a) check the 2-H frequency source and the 1-H frequency source with a good digital frequency counter (b) also check the frequency of the NMR master oscillator. In the second case (b) if its temperature controlled oven has failed, it may be in error by several MHz. BTW, you must locate a place in the circuitry where the frequencies are available on a CW basis.Received on Thu Feb 17 2000 - 14:01:43 MST
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