Dear Tao,


Your observations do point to a problem with reactor temperature, i.e. possibly a faulty thermocouple giving a higher T reading than the actual temperature (you have checked of course the reactor temperature is set correctly to 1020°C).

Organic compounds with their relatively "low" melting and boiling points combust quite readily even if the reactor temperature is not at 1020°C which is why you get decent d13C values for organics even though the reactor is not up to operational temperature.

The story with carbonates is quite different. For starters, carbonates are not being combusted (= oxidised) to CO2 but thermally converted (= broken down) because carbon in carbonate [CO3]2- is in the highest state of oxidation (+IV) carbon can assume.  Carbonates decompose rather than show a defined boiling point. Since carbonates typically have melting points well above 800°C it is quite important for the reactor temperature to be higher still.

Converting graphite into CO2 also requires high temperatures, typically above 700°C and therefore temperatures well in excess of the boiling point of organics (which kind of level out at about 500°C).

Given your observation one would be tempted to conclude the hot zone of your combustion reactor has a temperature of around 700°C rather than the required operational temperature of 1020°C.

Hope this helps.


Best,

Wolfram


-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Sun, Tao
Sent: 02 February 2018 05:43
To: [log in to unmask]
Subject: [ISOGEOCHEM] Flash HT not hot enough?

Dear all,

  This is about Thermo Flash HT plus system that is connected to Delta V.
We have been running organic matter, some ammonium/nitrate salts for C and N isotopes, respectively. Run has been very good, for both signal intensity and isotope numbers.
  Recently when we tried to do carbonate and graphite for bulk carbon isotope, the signal gets significantly low (~1 magnitude lower than that of organic matter with equivalent quantity of C), and d13C became lighter. It appears to  be incomplete combustion either due to lower temperature, or insufficient O2 supply.
  When I looked through the peephole, I did not observe any sample "flash burn" after being dropped, and the reactor does not look bright hot. Instead, the reactor wall is merely dark red, and the hotzone top is dark.
  It looks like that the furnace is not as hot as the method setup, i.e. 1020C. Anyone have experienced such issues?
Just had a quick run using organic matter, still no flash burn, but the signal intensity and isotope numbers are very good.

Any insights would be greatly appreciated!

Tao

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