I just realized that on the formula for very large samples below it should
mL O2 required= 2.5x[(0.2mL O2/mg tin)+(2 mL O2/mgC)]
so 2.5x rather than 25x !
At 10:14 AM 2/2/00 -0800, you wrote:
>David, I assume that you are using a C/N analyzer connected to an IRMS. I
>routinely analyze up to 7 mg C in plant material with good results (a
>precision of 0.05 delta 13C). The trick appears to be using enough oxygen
>to burn the sample well, and using a low enough GC temperature to seperate
>the N2 and CO2. Without enough oxygen for the combustion strange peak
>shapes can result and the chromium oxide in the combustion tube is reduced
>and the combustion tube has to be replaced.
>I would reccomend replacing the combustion tube, and then setting up the
>method with For samples below 4 mg C I use oxygen according to the formula:
>mL O2 required= 2x[(0.2mL O2/mg tin)+(2 mL O2/mgC)] 
>For example, with a 34 mg 9x5 mm tin and 2 mg C:
>O2 needed is = 2x[(0.2 x 35)+(2x2)]= 22 mL O2
>If you are using a Carlo Erba/CE instruments C/N analyzer, then with a 10
>mL injection loop you will already have 10 mL of O2 in the loop at
>atmospheric pressure. Therefore to get 22 mL of O2 set the O2 pressure
>reulator to 1.2 atmospheres, about 120kpa. This will give you 10mL +
>1.2x10 mL = 22mL.
>If you have a Europa Sci. analyzer then measure the carrier low rate, and
>then calculate the number of seconds to switch from the helium stream to
>the O2 stream, for example at 60 mL min-1 your would need to switch from
>Helium to O2 for 22 seconds.
>For large samples up to 7 mg C I find I have to added even more oxygen to
>get complete combustion, using the formula:
>mL O2 required= 25x[(0.2mL O2/mg tin)+(2 mL O2/mgC)] 
>With a Europa Scientific SL analyzer, I can use a column temperature of 58
>C and get good seperation of N and C with a sample contaning 450 ugN and 7
>mg C. A Carlo Erba/CE C/N analyzer uses a different GC column and may not
>be able to seperate this much material.
>Hope this helps,
>At 10:20 AM 2/1/00 -0600, you wrote:
>>Has anyone experienced a problem with introducing too much sample in a
>continuous flow IRMS application? I have found if I run more than 1 mg
>total carbon my delta values become more negative and I develop a sharp
>fore peak on my ion 44 peak. From the amplitude of my ion peaks, 2 to 3
>volts, I am not overwhelming my collectors. Any advice would be appreciated.
>>National Soil Tilth Laboratory
>Paul D. Brooks.
>Dept. ESPM-Ecosystem Sci.,
>151 Hilgard Hall, MC 3110,
>UC Berkeley, Ca. 94720-3110.
>FAX (510)643-5098 (to Att. Paul Brooks)
Paul D. Brooks.
Dept. ESPM-Ecosystem Sci.,
151 Hilgard Hall, MC 3110,
UC Berkeley, Ca. 94720-3110.
FAX (510)643-5098 (to Att. Paul Brooks)