Bill, on our Europa Sci. C/N analyzers the O2 tank lasts for years, and
thats at 20,000-45,000 samples per year.  On these systems the O2 doesn't
flow unless it is actually passing into the combustion chamber, and can be
adjusted to the sample size so that copper coumns also last a long time.
We generally get 2-3,000 samples per copper tube using 0.5 mm copper wire.

However, when the system is started up it is necessary to vent O2 that was
in the line from the O2 tank as this will have a significantly higher O2
concentration.  At least one user I know has got around this problem by
bleeding the line continuously at about 15 mL a minute.

It sounds as though you could simply re-plumb your new EA so that it has a
continuous O2 purge of 15 mL a minute, and then purge the actual sample
chamber with cheap Helium.  As long as the cycle time for analysis is long
enough for the O2 tube to fill that should not be a problem.  For example I
would suppose that if you are operating your 15 mL O2 loop at 2 atmospheres
on the gauge then there is 30 mL of O2 in it (10 mL at atmopheric pressure,
and 10 mL for each extra atmosphere on the gauge).  If you then purge the
loop for 3 minutes at 15 mL a minute of O2 between injections then this
should 45 mL of O2 put through the loop, well enough to flush it with
essentially a new charge of O2.

Even running 30 mL of O2 per sample (wich I know many Carlo Erba users do)
seems very excessive to me.  On the Europa we usually use 22 mL of O2 for a
8x5 mm tin capsule weighing about 36 mg, and 2 mgC.  If the sample contain
less C we adjust the amount of O2 down accordingly.  Even over runs of 131
samples this still generally returns plus or minus 0.15 delta units on 15N
and plus or minus 0.05 delta units on 13C, for NIST 1547 Peach leaves, so I
am sure the combustion is good.

Hope this helps,  Paul.



At 12:18 PM 4/28/99 -0400, you wrote:
>Isogeochemists,
>
>I have a question about the oxygen purge on our new CE 2500 EA hooked up to
>a F MAT Delta +.  The oxygen tank hooked up to the CE 2500 only lasts about
>6 months.  On our old EA, a CE 1108, the oxygen tank lasts over 1 year.
>The difference between the 1108 and the 2500 is the autosampler on the 1108
>purges with He and on the 2500 the autosampler purges with O2.  What are
>the other folks experience with these new EA's, is the oxygen consumed at
>twice the rate as the old EA's or do we have a problem with our new
>instrument that we need to fix?  The O2 blank size varies somewhat with the
>tank but is generally low, below 0.15 Vs.  We have installed the smaller O2
>loop on both EA's.
>
>Bill Showers
>----------------------------------------------------------
>William J. Showers
>Dept. of Marine, Earth, & Atm Sciences
>North Carolina State University
>
>(919) 515-7143 - voice
>(919) 515-7802 - fax
>[log in to unmask] - email
>
>