The TCD detector on our 6890 Gc shows negative peaks for water acetylene , propadiene .

We have a Gc (6890) with TCD . The Gc & the method setup has been around for years . It measures some permanent gases & some Hydrocarbons .


The method starts with negative polarity to detect H2 (He is the carrier ) & then the polarity is switched to detect N2, O2, Ar, CO, CO2 ,H2S, methane , ethylene , acetylene , propane , propylene , butanes , pentanes


Off lately the TCD is giving us issues . It shows negative peaks for acetylene , propadiene, water  sometimes H2S.


These peaks never used to be negative . This is a new TCD ( changed in Oct 2020). We had similar issues with previous TCD So we had to change it .I am not sure why we have started seeing this negative peaks .

We never had this issue before


  • Check the quality of the carrier gas as it can be contaminated at the source or farther down the line.  Perhaps switch the carrier gas tank for another with a different lot number, make sure any gas traps on the lines have been changed and aren't saturated, and double check there aren't any leaks.  All three items can be sources for gas contamination that is causing the peaks to invert.

  • Hello,

    Yes, most likely your carrier gas are contaminated. If this is the case, the TCD will truly change response. If the TCD is exposed for contaminated carrier gases for a longer time, the TCD filament can be heavily oxidized. In the old days, with TCD`s on 5890 GC`s, the TCD filament could be regenerated by heating to 400C over night (Using pure helium carrier gas!)



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