C/N anlyzer users,
I have written a reply to Andrew below about how to calculate the amount
of O2 needed for combustion, at least with the formulas that we use.
However, this is using a Europa Sci. SL analyzer which has the advantage of
having a variable O2 injection.
We now have a CE instruments analyzer in our department operated by another
group for %C and %N only. While the instrument works very well for %C and
%N it was origainally installed with a 25 mL O2 loop operated at 200 kpa,
or 2 atmospheres of O2!! With typical samples contaning 2 mg of C each,
this excessive O2 quickly consumed a copper column in as little as 110
samples. Considering that using 0.5 mm dia copper wire we get 1,800 to
3,000 samples on a Cu column on our Europa SL, 110 samples seemed a very
short life. Fitting a 10mL loop helped considerably, but there are no
guidelines I know of from CE instruments as to how to calculate the amount
of O2 needed for a sample. It would appear that with a 10 mL loop, even at
0 pressure it would still contain 1 atmosphere of pressre, or 10 mL O2. We
were told in installation not to operate the loop below 90 kpa, or about
0.9 atmospheres, so I assume that now at that pressure we are injecting 1.9
atmospheres, or 19 mL of O2. I assume that we with a larger sample we
could simply increase the pressure of O2 in the loop using a similar
formula to the one described below.
Looking at the pneumatic diagram for the CE instruments analyzer, it is
actually much more complicated than the Europa SL, but less flexable. It
would appear that it would be easy to re-plumb it like a Europa SL, in
which the carrier stream was simply switched from He to O2 for a certain
number of seconds, and then back. Bleeding the system at the begining of
the run would be necessary as with the Europa System, as N2 seeps into the
O2 line when the system is not runing. And an additional He purge would
have to be re-plumbed to purge the sample with He rather than the waste O2.
Still, this would seem to be a much more flexible system and use less O2-
has anyone ever tried it on a Carlo Erba or CE analyzer?
Andrew,
With our two Europa Sci. systems we use the following formula to calculate
the amount of O2 needed:-
1.5*[(0.2mL O2/mg tin)+(2mL O2/mgC)]
Some groups use as high as 2*[(0.2mL O2/mg tin)+(2mL O2/mgC).
Thus for a tin capsule weighing 35mg, and a sample contaning 2 mg carbon in
the tin we use:-
1.5*[(35mg tin*0.2mL O2/mg tin)+(2mg C* 2mL O2/mgC)]
or 1.5*[7.2mL O2)+(4 mL O2)]=17 mL O2.
The advantage of the Europa SL analyzer is that the amount of O2 is simply
set in the software. You should be operating your O2 and He carriers at
the same pressure. Then, measure the flow rate through your instrument,
using just He. Putting O2 into the system will appear as a reduction in
flow rate as the O2 is absorbed in the Cu column. If your flow rate is as
low as 60ml min-1, then you simply have to adjust the O2 vale to be on for
17 seconds for 17 mL of O2. If you are using a higher flow rate, such as
95 mL min-1, then you have to calculate the amount of O2 needed as"-
(60 secs/95mL)*17 mL O2= 11secs.
Thus you should easily be able to change the amount of O2 you need for the
weight and size of tin cup and sample C.
The begining of the O2 pulse should be the same as you are using now, then
set the end time for the begining time plus the amount you calculated
above. So if you begin your O2 pulse at 5 seconds, you will have to end it
at 16 seconds for 11 seconds of O2.
Hope this helps,
Paul.
At 02:17 PM 2/25/99 GMT, you wrote:
>Hi
>
>I am using some solvents to transfer an azo dye into tin cups for
>elemental combustion and then ms analysis. This has proved to be
>tricky as the solvent, acetone, has the effect of creeping up the
>sides and over the top of the cup resulting in loss of sample.
>
>To reduce this I have been using Chromosorb and it works to some
>extent but nothing is ever perfect....
>
>so I thought I would use the smooth wall tin cups with a flat base.
>
>They work great and dont need any chromosorb!!
>
>Now this is a whole bunch more tin being combusted
>
>Does anyone have any advice or tips to improve the combustion of the
>sample. I remember someone telling me that it is best to increase the
>O2 time.
>
>We are using an ANCA SL sampler converter attached to a 20-20 IRMS
>Europa Scientific.
>
>Any advice or tips are most welcome.
>
>Andrew Johnston
>
>
>Dr A.M.Johnston
>CEP
>SCRI
>Dundee
>DD2 5DA
>Scotland
>
>Tel No: 44 1382 562731
>Fax No: 44 1382 562426
>
>
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