## ISOGEOCHEM@LIST.UVM.EDU

#### View:

 Message: [ First | Previous | Next | Last ] By Topic: [ First | Previous | Next | Last ] By Author: [ First | Previous | Next | Last ] Font: Proportional Font

Subject:

Re: sample size and C13 results

From:

Date:

Wed, 2 Feb 2000 10:14:05 -0800

Content-Type:

text/plain

Parts/Attachments:

 text/plain (71 lines)
 ```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, Paul. 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. > >Thanks, > >David DenHaan > >National Soil Tilth Laboratory >Ames, Iowa > Paul D. Brooks. Dept. ESPM-Ecosystem Sci., 151 Hilgard Hall, MC 3110, UC Berkeley, Ca. 94720-3110. phone (510)642-3155 FAX (510)643-5098 (to Att. Paul Brooks) ```