by
jh1 » Thu Apr 09, 2009 10:29 pm
From an Agilent document dealing with 5971 baseline issues, but it does cover some issues with source pressure.
http://www.chem.agilent.com/Library/Sup ... A20339.pdf
Excessive Source Pressure
Excessive source pressure error is frequently caused by excessive
pressure, as measured in the source. Also, foreline pressure and vacuum
manifold pressure should be checked to insure they’re within operational
parameters. However, on the 5971A, excessive source pressure can be, and more commonly is caused by a dirty source. This typically can be confirmed by executing a [STANDARD SPECTRUM] autotune. After
obtaining the Autotune print out, evaluate the tune, paying particular
attention to the ENTOFF (entrance lens offset) parameter. Under most
normal operating conditions, assuming a good clean source, good vacuum
and normal background levels, the ENTOFF parameter should be around
4.0 to 4.5. If the value is considerably higher or lower than those value's
(+/- 2 to 3 volts), there's a good possibility that cleaning the source will
correct the problem.
If there is any doubt, clean the source anyway and re-check to determine if
the problem has been corrected.
If the vacuum is holding steady I'd check your flow rate and try to determine if the source is somehow still dirty after your recent cleaning.
Anyhow CI mode on 5975 Agilent MSDs allows for a weaker vacuum and increased source pressure (I'm not sure if it directly monitors this anymore) due to the addition of a carrier gas in the source which requires a much more enclosed source body and increased vacuum pumping. If this is also true with the 5971 then switching to CI mode would loosen the specs on vacuum and source pressure so that an MSD not working within normal spec. might be allowed to tune.