We recently overcame a high mass 46 background on a Delta Plus. After moving the GP interface we apparently jostled it enough to loosen the bottom of the open split. After leak checking with Argon, we isolated the leak, applied Polyimide resin. Fixed. Good luck.
 
 andy
 
 
 Andrew Schauer
 Stable Isotope Facility for Environmental Research
 Department of Biology
 University of Utah
 257 S. 1400 E.
 Salt Lake City, UT 84112 USA
 

----- Original Message ----
From: Penny Higgins <[log in to unmask]>
To: [log in to unmask]
Sent: Monday, March 6, 2006 7:57:19 AM
Subject: [ISOGEOCHEM] High Mass 46 background

Hi all,

We've been running carbonates (CO2) on our Delta Plus XL, via the Gasbench 
and PAL, for nearly a year now.  Everything has been running smoothly. 
Alas, in the past few weeks I've noticed that the mass 46 background has 
increased dramatically from 10-20mV to nearly 50mV (sometimes more). Our 
standard deviation on d18O (based on ten peaks) has increased to 0.15 or 
much higher. Carbon is still running very well, and I am certain that water 
in the sample is not getting to the mass spectrometer source. We check 
linearity every day before running analyses, so I know that the instrument 
is performing magnificently and is linear. The mass 46 background on the 
reference peaks is generally much lower than that of the sample peaks, 
though these days still tops 20mV.

Does anyone know where the excess mass 46 is coming from. There appears to 
still be a clear separation between the nitrogen peak and the carbon 
dioxide peak coming out of the GC column. However, I wonder if the GC 
column may yet be bad. We did once draw phosphoric acid into the GasBench 
(a procedural problem we have since fixed). I wonder if this may have 
damaged the GC column?

Any other thoughts?

Thanks,

~Penny

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                       Dr. Pennilyn Higgins
                        Research Associate

                             "SIREAL"
   Stable Isotope Ratios in the Environment Analytical Laboratory

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