Another possibility is apparently European television - there have been a
number of recent reports of very loud amplitude modulated signals from
these television broadcasters on the east coast. An increase in the
severity of interference due to these signals would be consistent with our
current position in the sunspot cycle- we are currently approaching a
maximum, making long distance communication at 50 MHz much more likely
than it is during a sunspot minimum. However, these signals have been
reported at a frequency of approximately 48.5 MHz, which may be too low to
account for these spurious signals. In any case, the sunspot issue would
be relevant in this frequency range to account for an increasing problem
over the past several years.
I hope this helps.
Mark
On Tue, 30 Nov 1999, Mo Latif wrote:
> Dear Lawrence Byrnes,
>
> I operate an AC-400 system and have also noticed a spurious difficult to
> phase peak/multiplet in the 13C (1H decoupled) spectra of weak samples where
> long acquisition times are necessary. I actually noticed this soon after I
> had the spectrometer delivered from new (about 8.5 years ago) and sent them
> the sample and spectra, after complaining and they ran the sample on theie
> demo AMX400 and found other spurious peaks! These looked worse than on my
> system so I did not take the matter further and thought myself lucky to have
> such a small problem. Since then I have found that other users of AC
> spectrometers have found similar spurious signals. My spectrometer has not
> become worse over the years. The signal usually appears around 160ppm and
> most of the time is out of phase and quite weak. I believe it is as you
> guessed a receiver gain (board) problem, as it only occurs on weak samples
> where you need to turn the wick up. The only other theory I have is that we
> have a radio station in London called 'KISS 100 FM' which transmits at
> 100.13Mhz which is almost exactly my 13C frequency perhaps I'm picking this
> up weakly. Have you had a new radio station open up near you operating at
> about 50Mhz (this sounds a bit low, maybe its a police or cab transmitter?).
> I'm sorry I can't help but as a fellow suffer I sympathies and if you do
> find a solution please let me have the answer I'm even willing to pay!
>
> All the best,
>
> Mo
>
> -----Original Message-----
> From: Lawrence Byrnes [mailto:numare@eclipse.net]
> Sent: 28 November 1999 20:05
> To: ammrl@wwitch.unl.edu
> Subject: Spurious peak in 13C data
>
>
> 28 November 1999, Sunday, 1305 EST
>
> Hello Spinlanders,
>
> Our Bruker AC-200 system broadband channel has a problem. There is a SW
> independent peak that appears around 167-168 ppm in 13C data. The problem
> was first observed nearly one year ago. Initially the problem was a minor
> irritation since the S/N for the peak was very low. This is no longer the
> case.
>
> Currently, when running proton decoupled 13C experiments, the signal starts
> out as negatively phased singlet near 168 ppm. During long term
> acquisitions, this singlet becomes a multiplet of positively and negatively
> phased peaks between 168 and 167 ppm.
>
> When the problem was first noticed I was able to eliminate it by reducing
> the
> receiver gain to a value that was less then that determined by the automatic
>
> receiver gain control. This no longer works.
>
> During the last few months I have tried to determine the origin of the
> spurious signal as follows:
>
> Run experiment with air compressor off.
> " " " fluorescent lights orr.
> " " " sample spinner off.
> " " " VT unit off.
>
> None of the above offered any insight into the origin of the problem.
>
> Last Friday I was running a proton decoupled 13C experiment on a sample that
>
> had been prepared outside of our laboratory. The sample contained 4.2 mg of
>
> a C22H36O2 compound in 1.0 ml CDCl3. The straight chain molecule has two
> acetylinic groups and a terminal carboxylic acid. Long term acquistion is
> required. The spurious multiplet has better S/N than that of the COOH.
>
> Yesterday I decided to clean the probe to see if contamination by vapors
> from
> the Mobil Rarus 427 air compressor oil was the source of the problem. The
> results of pre and post cleaning 13C acquisition are the same.
>
> Yesterday I also installed a Balston inline filter in the air line that
> supplies the probe. The filter has not reduced the problem.
>
> I would surely appreciate any ideas that you folks might offer. This group
> has been very generous with their insights and experiences in the past. I
> that you all.
>
> Kind regards,
>
> Lawrence
> --------------------------------------------------------
> 11/28/99 13:05:22
> Lawrence Byrnes,
> NMR Laboratory Manager,
> Numare Spectralab Incorporated
> Research Director,
> The Schoenenberg Institute of Molecular Spectroscopy
> 10 Summit Avenue, Building #6, P.O. Box 96,
> Berkeley Heights, NJ, 07922-0096,
> telephone 908-665-0066
> facsimile 908-464-4192
> email numare@eclipse.net
> www http://www.eclipse.net/~numare/
> --------------------------------------------------------
>
>
>
>
____________________________________________________________________________
Mark Filipkowski
Assistant Professor
Department of Physics
PHYS 226
University of Arkansas
Fayetteville, AR 72701
filipkow@comp.uark.edu
Amateur radio callsign: AC5RR
(501)575-7593 (Office)
(501)575-2506 (Physics Office)
(501)575-4219 (Lab)
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