Chromatography Forum

We know sulfur chemiluminescence detector (SCD) is very sensitive to sulfur compounds and it's due to the reaction of those compounds with H2 to produce SO, in turn, SO will react with O3 to yield SO2*, finally, SO2* turns into SO2 and light. The light is detected by a PMT to give signal.
My question is why SCD can still detect SO2 ?

According to my Antek 7090 manual, you have the mechanism wrong. This is always how I've understood it:

1) Burn the effluent from the column to CO2 and SO2.
2) Reduce the SO2 to H2S with the hydrogen coming in from the top of the furnace.
3) The ozone oxidizes the H2S to make an excited SO2* species - which emits light.

I believe that initially, all of the SO2 in your sample gets reduced to H2S before it is oxidized again by the ozone, thus you get a signal.

My understanding was that the sulphur containing species are first oxidised to in effect produce sulphur dioxide which is then reduced by reaction with hydrogen to sulphur monoxide, which then reacts with the ozone.

Could be wrong though

ASTM D5504 lists 2 different proposed mechanisms of the SCD chemical reactions in its appendix.

With either mechanism, SO2 native to the sample is reduced by Hydrogen in the burner and then later in the reaction cell it forms excited state SO2 that yields light.

Thanks all for replying me. So SO2 is first reduced to SO. But then SO2 must be the least sensitive because the reaction efficiency is always less than 100%.
I should try to increase H2 flow to enhance the SO2 sensitivity shouldn't I ?

Are you trying to say that the SO2 that is native to your sample is somehow different from that you are generating in the furnace?

It's been a while since I used my SCD but I don't recall any H2 flow optimization. I do recall that it was critical to get the small ceramic tube down into the furnace as far as possible to maximize the sensitivity (Antek 7090). I presume that this was to maximize the contact time between the hot hydrogen (atoms?) to maximize the amount of SO2 converted to the right species for reaction with the ozone.

Regardless of the detailed mechanism, all of the column effluent is burned to CO2 + SO2 (and probably others). The sulfur in all sulfur-containing molecules is converted to SO2. The SO2 native to the sample is now lumped into that whole mish-mash. That's all you're ever detecting.