>A few months ago we replaced our AM300 console with a new DPX console. For
>the
>samples we pass on this instrument (polymers : PP, PE, ...), we are mainly
>interested in the high-field part (60-0 ppm). So taking advantage of the
>digitla filter, we limit our Sweep to this range, dropping out the aromatic
>solvent signals.
>
>For some time everything was allright. Then appeared artifacts : out of phase
>peaks at regular intervals (~800 Hz) to both sides of the main signals. Using
>an extended Waltz, the artifacts seemed (?) to have disappeared for sometime,
>but now they're back "splendid as ever" :-( ...
>
>Did anybody of you encounter similar problems ? Help would be greatly
>appreciated,
A serious problem we encountered with our DPX400 soon after we got it in
1994, was the inability of UXNMR (and presumably still XWINNMR) to detect data
overflow. Thus, in a polymer sample, where you are often trying to see single
carbon signals, you may encounter the dynamic range limit, represented by 24 or
32 bits, whichever is used in the acquisition. The program doesn't know about
this and continues accumulating. The result is usually garbage, but I recollect
that distortions such as you described did occur.
When I complained to Bruker about it they could provide no useful workaround,
and as far as I know the problem is unsolved.
Alternatively, you may be seeing preamplifier saturation, since the 'rga'
command looks at the FID _after_ the digital filter. If the signals from the
section of the spectrum outside the digital filter cut-off are large the gain
may get turned up so high that they saturate the preamps and distort the FID
before it ever gets digitised. Extraneous spikes would then appear at all sorts
of frequencies and pass right through the digital filter.
If you get a fix to this, I'd be interested to hear about it. Fortunately, we
don't get too many polymer samples any more...
Alan
P.S. My old Varian XL100 had two ways to solve these problems back in 1972!
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Dr A S F Boyd Chemistry Department
NMR Spectroscopist Heriot-Watt University
phone: +44-131-451 3214 Riccarton
fax: +44-131-451 3180 Edinburgh
a.s.f.boyd@hw.ac.uk EH14 4AS
http://dava.che.hw.ac.uk/ Scotland
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