Chromatography Forum

Hi everyone

I'm doing some reactions using benzothiophene (sulfur compound) dissolved in dodecane. My analytical system includes FID, FPD for sulfur detection, and MS detector.

After experiments I can see some new sulfur compounds products in MS spectra but NOT in FPD spectra. I do not know why? In FPD I can see only one peak for benzothiophene.

My thinking is that the amount of these produced sulfur compounds is under detection limit of FPD as their peaks in MS are very small.

Also, I can see some some tiny peaks for them in FID spectra as they have carbon. Can I use their FID peaks areas to determine weight fraction of each compound?

Thanks for any help!

Unless the new compounds do not have any sulphur in them then an FPD that is running properly will surely detect them.

Remember that the FPD responds to the square of the amount of sulphur so peaks get disproportionately smaller as quantity decreases.

Details of you set up would be very useful.

Peter

Thanks Peter for your reply!

The new compounds surely have sulphur in them and they are products of my reaction as expected. Is there anything to do with splitless. I'm working with split ratio 25. I think I can not use splitless mode because my solvent (dodecane) and benzothiophene may saturate the detectors as they come out of the reaction in high concentration.

As I mentioned earlier I can see some peaks in FID too, in addition to MSD. I'm wondering if I can use them to quantify my products.

I have in my lab Agilent 6890N GC and 5973N MSD. The GC has two detectors FID for hydrocarbons and FPD for sulfur. The two columns are: Agilent 19091Z-205 350C max HP-1 capillary (50m*200micro*0.5 micro).

How is your GC set up - are your running both detectors off one column, two separate injectors with two columns running in parallel to two detectors, on injector and one column to one detector at a time or what ?. If I am going to help I need details, not a quick summary.

Peter

Thanks once again Peter!

As mentioned before we have two columns. Both columns are connected to the back inlet of the GC. While one of the columns is linked to MSD, the other one is splited and coupled to both FPD and FID. In this way, each single injection produces three signals, one per detector.

All samples are coming out of the reactor through a transfer line all the way to GC. I'm not injecting directly to the GC.

I hope this clarify the doubt. If not please let me know.

h

That is quite a complex set up. Because the outlet of the column to the MS is in vacuum you already have a strong bias in flow to the MS rather than the other two detectors, which will give very different gas flow rates in the two columns and thus very different retention times - the peaks that you see on the MS probably do not correspond to the peaks from the FID (and the lack of peaks on the FPD). You then split to the FID and FPD - how have you set that ratio ?, transfer line length and diameter ?, or do you have a fancy splitter with make up gas etc ?

How are you setting your column flows - the software in the GC cannot know that you have two columns in parallel unless you have done some very sophisticated lying about column dimensions. It would be very easy to run too low a total flow to the two columns, split most of it to the MS, and have such slow flows through the FID/FPD column that the target peaks are not eluting.

Keep in mind that the FPD is selective for sulphur - this does not make it more sensitive than an FID, it just reduces interference from non-sulphur compounds.

Peter