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There is a difference between Cr2O3 and CuO, namely their usefulness (or lack thereof) to act as oxygen sink at higher temperatures.

At temperatures >800°C, to serve as oxygen sink CuO is about as useful as a chocolate teapot.  At T >800°C, i.e. at typical EA reactor temperatures CuO decomposes into Cu2O and O2.  Cu2O is quite stable and not nearly as reactive as CuO, which is why the (bottom) part of an EA reactor filled with a band of CuO will turn red over time; the colour of Cu2O.  In other words, even if one would run no samples but keep the EA at operational temperature, a CuO filling would deteriorate over time due to decomposition.  To add insult to injury, the reduction reactor would now merely act as oxygen trap by converting Cu into CuO (black).

In other words, in an EA reactor Cr2O3 serves the same purpose as does NiO in a GC/C-IRMS reactor (operated at temperature >940°C), namely as oxygen sink ("store").

Using Cr2O3 instead of CuO ensures the reduction reactor does what is meant to do rather than being an expensive glorified oxygen trap.

For EA-IRMS analyses of organic carbon one could probably get away with using CuO instead of Cr2O3 because the O2 pulse will provide all the oxygen required for sample combustion into CO2.  Before anybody asks, would I try this? No.  If it ain't broke, don't fix it. ;-)


Best wishes,

Wolfram



From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Deborah Rupprecht
Sent: 05 January 2016 06:49
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Chromium vs. Copper Oxide

Hi,
am wondering then  why was chromium oxide ever used/invented as additional filling when/if there is just little difference. There ought to be an effect /advantage on certain analyses because it is more expensive and harmful than copper oxide... *clueless look back in EA history*

Best wishes,
Deborah




Am 04.01.2016 um 20:26 schrieb Stephen Taylor:
Hi Todd,

In our lab we use both a COSTECH 4010 and a Isotope CUBE for wt% and isotopes of CN.   The manufacturers recommended oxidation catalysts are: chromium oxide and copper oxide in the COSTECH and CUBE respectively.  I run the COSTECH combustion at 1000oC and the CUBE at 950oC.  I see no difference in the quality of the data, and assume there is little difference between the efficacy of these oxidation catalysts ...

FWIW: I have tried measuring 15N of sediment (ground whole rock) containing <0.05 wt%N using a single reactor on the COSTECH 4010 (also chromium oxide + reduced copper + silvered cobaltous/ic oxide as described in the Costech manual), but the 15N data was inconsistent/poor.  My suspicion/assumption(?) is that there isn't enough copper at the correct temp (600 to 650oC) in the single reactor configuration, but it could also have just been the samples (!?!).  FYI: I was only interested in 15N and have no data on the quality of the 13C using a single reactor.

Any particular reason you're attempting CN of plant material on a single reactor?

Cheers,
Steve
On 04/01/2016 9:35 AM, BROWN, TODD M [AG/1005] wrote:
Has anyone compared chromium vs. copper oxide as the oxidation catalyst in a Flash 1200? I used copper oxide without any difference in % and isotope CN results when I was running a 2 tube setup that had the reduced copper in the second tube, however, I have switched to a 1 tube packing scheme so there is not enough room for a lot of any 1 catalyst. The 1 tube packing goes chromium oxide, reduced copper, silvered cobaltous/ic oxide as describe in the Flash manual. Since I accidentally ordered a large amount of copper oxide (I really need to pay better attention sometimes) I was thinking of using it for the chromium in this configuration. My samples are typically 2 mg of dried plant material.

Todd Brown
Monsanto
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Chesterfield MO 63017
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