Below follows a summary of responses to my recent querry:
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I suspect that most of your background (31p, 13c, 1h) is from oil mist/
droplets that have been deposited on your inserts, coils, dewars, and
tuning circuits over the years. If you are using regular engine oil in
your compressor (which works ok),as opposed to compressor oil or synthetic
oil, you will see lots more background signals because of the additive
packages that internal combustion engine lubricants require to meet engine
manufacturers specs. Some of these additives are: detergents (phosphates?),
anti foaming agents, anti corrosion agents, viscosity index improvement
agents, acid neutralizers, vanadium and molybdenum compounds for high
pressure lubrication, plus many more. Since these "packages" vary form
brand to brand you could just do a quick NMR of the oil (or any other
substitute lube). Compressor oil is typically a non-detergent oil and would
have less "stuff" in your spectra. You could also use synthetic oil, which
might be even better than the compressor oil.
By now you have oil contamination in your probes (inserts, coils dewars,
tuning circuits, VT heater-dewar), upper stack (spinner turbine, sample
eject lines, probe/shim cooling lines), all the air lines in the instrument
plus valves, ant the air lines between the instrument and the compressor!
If your system uses Tygon/beverage-tubing, I'd just replace it with new
tubing. At about 10 cents/foot, it isn't worth cleaning. You have to be
careful cleaning any valves and fittings if they have Nitrile/Buna-N
o-rings or seals (don't use acetone here). The probes, heaters, and spinner
assembly will need to be disassembled and cleaned (again, be careful with
the solvents) and flushed. The compressor to system air lines should be
cleaned or replaced, as well to prevent repeat contamination.
New industrial grade oil-lubed piston compressors are often rated to put
out less than 3ppm of oil (by weight) in the air. There are many filters
available that trap sub-micron particles and droplets and can be followed
by activated carbon filters to clean up the vapors and micro droplets.
Of course, if your compressor is worn out and pumping lots of oil (if the
air lines are "wet" with oil you need to rebuild of replace your compressor.
If your compressor is in good shape, synthetic oil may also help. Synthetic
oils often have better high temp. performance and may reduce oil vapors.
It won't fix a worn compressor!
If your thoughts are leaning toward an oil-free compressor, I have a few
suggestions. The NOT recomended list starts out with oil-free compressors
from Sears (mfgd. by De Vilbiss). They are NOISY, unreliable, and wear out
very fast (less than 1000 hours operation, probably <500 hours). So unless
you want to be a part-time compressor repairman, I would avoid these ones.
In general, avoid "direct drive" types in favor of geared-down, low rpm types
for longer lifespans. Oil-free scroll type compressors also look good.
Most oil-free rotary screw types aren't made in <10 HP sizes. While most
industrial grade compressors will cost more than the hardware-store
compressors, they have longer lifetimes (and don't forget to think about
where you will get parts and service- not at Home Depot/Sears, etc.)
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Do you see (or smell) any signs of oil inside the probe? Did you run 31P NMR
on the compressor oil to see if there is a 31P signal that corresponds to the
anomaly that you are getting?
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How broad is the P31 artifact signal?
Try cleaning your probe first. I have cleaned mine by soaking in a 50 - 50
solution of deionized water and isopropyl alcohol and then run heated dry
nitrogen from a liquid N2 tank through until dry.
Also I use only Wilmad tubes. I have had problems with some strange broad
artifacts and total shift of lock frequency when using Norrell.
I would also sugest using a liquid N2 for gas source. I just have a standard
gas withdrawl pressure regulator connected to the spinner and VT / body air.
I have a coil of copper refrigerator tubing in a 5 galon heated water bath to
stabilize the temperature.
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>From long ago,,, I seem to remember that the phospho-lipids in
cell walls fall in that range. Have you got anything growing
in there :) ? I suppose that you could try running a sample
of the pump oil.
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Thanks for the useful suggestions and additional information. Will run 31P
on air compressor oil this week. The air compressor and associated air lines
were installed in 1994. The autodrain mositure and oil vapor filtration
system has never been serviced. Have a feeling that this will turn out to be
the culprit.
Kind regards to all,
Lawrence
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06/01/99 14:31:42
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/
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