Jason,

We had a similar problem, but we did not have a good flash all the time. We found out that valve #5 in the EA was bad and was bleeding O2 into the helium stream.  This meant that the loop would fill up for the first sample, but that subsequent samples did not get a full shot of O2.  We found the problem by putting He into the O2 side and leak checking with a hand held He leak detector.

Good luck, it took us several months to figure this one out.

Bill

At 04:37 PM 10/16/00 -0800, you wrote:

>I've been having a few problems with our EA for the last few weeks. Here

>are the details:

>It's a Carlo Erba NC 2500 connected up to a Micromass Optima. The Optima is

>working fine for dual inlet work, but the EA has not been used for about 18

>months.

>Linearity tests for N2 and CO2 reference gases give a range of 0.015-0.020

>permil over the full range of refgas sizes, i.e. there is nothing wrong

>with the mass spec. A run of fifteen solid organic standards of widely

>differing sizes (equivalent to 0.05-0.75mg N) gives a scatter of d15N

>values with a standard deviation of 0.14 permil - in other words nitrogen

>works OK.

>However, the situation for carbon is problematic. A similar run of

>standards, (attempted organic standards as well as graphite) gives a range

>of d13C values, with the largest samples being 1.8 permil heavier than the

>smallest samples.

>I assumed that the problem was probably related to poor combustion - there

>is a good flash combustion - sample drop time is 21 seconds, not dissimilar

>>from the recommended values. He pressure/flow is 150 kPa,  88 ml/min. O2

>pressure/flow is 60 kPa / 20 ml/min - I have had the O2 up to 150 kPa / 55

>ml/min with no effect on the d13C/sample size variation. I've also had the

>GC oven up to 110°C for four hours to rid the column of any crud. The water

>trap and furnace tubes have new chemicals and there are no major leaks.

>Also there is a good (R2 = 0.999) linear relationship between the

>integrated area of mass 44 and the mass of the sample.

>I've also fiddled with the way the peaks are integrated, with no success.

>Any ideas as to what could cause the d13C/samplesize variation, and what I

>can do about it? As I say, it is evidently an EA problem, not the mass spec.

>

>cheers,

>

>Jason

>

>

>*********************************

>Dr. Jason Newton

>University of California Santa Cruz

>Department of Earth Sciences

>Santa Cruz

>CA 95064

>USA

>

>email: [log in to unmask]

>Tel: +1  (831) 459 5751

>Fax: +1  (831) 459 3074