Hi Penny
we experienced a similiar increase in mass 46 background when 
analysing N2O in our Precon-delta-XL system a few weeks ago. We 
suspect a contamination on the Poraplot-Q column of the Precon. 
Baking out the column in the GC oven at 200°C did not solve the 
problem.However, after removing the column and baking in another 
oven at 250°C the mass 46 background is fine.
We suspect that contamination of our sample vials with traces of oil 
vapor might have caused the problem. Some of our sample vials are 
pre-evacuated using a vacuum stand and we suspect that the zeolite 
trap of the pump had been  exhausted. The problem occured when  
running large ( 500 mL ) gas samples. We are still not certain if our 
interpretation is correct. Therefore we would be interested to know if oil 
vapor contamination would also be a possible explanation to your 
mass 46 problem. 
Cheers, 
Reinhard

Date sent:      	Mon, 6 Mar 2006 09:57:19 -0500
Send reply to:  	Stable Isotope Geochemistry 
<[log in to unmask]>
From:           	Penny Higgins <[log in to unmask]>
Subject:        	[ISOGEOCHEM] High Mass 46 background
To:             	[log in to unmask]

> 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
> 
> *******************************************************************
>                        Dr. Pennilyn Higgins
>                         Research Associate
> 
>                              "SIREAL"
>    Stable Isotope Ratios in the Environment Analytical Laboratory
> 
>          Department of Earth and Environmental Sciences
>          University of Rochester
>          227 Hutchison Hall
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