SCD question...

[SkylineGTR112]

I was wondering if anyone knows if running a sulfur sample with hydrocarbons might cause an inhibited signal versus an inert gas like Nitrogen. I would think it would since an SCD relies on Oxygen to create ozone in order to get a signal. Banging my head against a wall trying to figure this out. Wondering if my reasoning is sound or if I'm losing my mind.

[Peter Apps]

This is a classic case of "don't think, measure !". Run a given quantity of sulphur compound with and without a hydrocarbon interference, and compare the signals. At a guess I would say that unsaturated hydrocarbons might be more of a problem than saturated ones.

Please post the results.

Peter

[Ron]

There is little if any effect at low concentrations of hydrocarbons, but at high concentrations there may be a decrease in signal level. As Peter said, the best way to find out if there is an effect on your samples is to do an experiment.

[SkylineGTR112]

I've been running mostly sulfur in inerts for the longest time, and when I've run samples in 15-20% hydrocarbons, the signal is greatly decreased against the inerts. I was thinking that maybe with an abundance of ozone, that the hydrocarbons were bonding with the ozone to give an alkoxy compound. I'm a numbers guy so mechanisms aren't my strong suit. We've recently had to do a rebuild on our instrument that just so happened to coincide with getting there types of samples coming in. So I was unsure if it was the machine or the samplesw causing the problems.

[Peter Apps]

So you have already done the experiment ! 15-20% is a lot of hydrocarbon - are you doing any separation, GC for example before it goes into the detector ?, or are you running the mixture straight into the detector and relying on the SCD's slectivity to pulll the sulphur signal out of the background ?

Peter

[AICMM]

SkylineGTR112,

Not personal experience but related to me. You will see quenching when you run high hydrocarbon loads on SCD. Issue was raised in a discussion about measuring sulfur compounds in ethylene and propylene. You might consider a heartcut configuration to move the sulfurs and less hydrocarbon to a second column and reduce the detector loading. I assume you are running a split injection?

Best regards.

[Ron]

The SCD is a flameless detector, so it is difficult to completely decompose high concentrations of hydrocarbons. There are two ways to handle this, put less sample into the detector, which can actually improve the sensitivity if there is quenching, or for the Sievers SCD the SCD detector can be mounted in series with some FIDs. The FID effluent goes into the SCD, and the hydrocarbons are combusted in the FID flame reducing the quenching.

[Bricevan]

SCDs are supposed to be immune from hydrocarbon quenching effects.

However, If you're overloading the detector with sample then it may not be getting burned completely which may be reducing the signal. As mentioned, an FID in series will help but your detection limit will suffer greatly; expect a factor of 10 increase.

The better way to go is by increasing the split ratio. Another reason to ensure that you're introducing as little sample as possible, is that non-sulfur containing olefins and aromatics can produce a response on the SCD leading to a false positive identification.