Chromatography Forum

Hi All

I'm trying to use FID to quantify sulfur compound (benzothiophene) using C12 (dodecane) as internal standard. I did direct injection to my GC with 0.2 micro liter volume and different benzothiophene concentration (0.5, 2, 4, 6 wt%) dissolved in C12. I'm getting a different RSF for each concentration. Is this ok? I have an old thesis for one of my group mates and he did the same but using thiophene in C8 and he reported the same RSF for all thiophene concentrations!

In the future, I'm running reactions for C12-benzothiophene mixture in a system where pressure is maintained in the main reactor and the sampling vessel using Argon gas. Can I still apply the RSF from the direct injection to the GC. In fact, I did with empty reactor (no reaction so, in=out) and when calculating RSF, it was different (higher) from the ones from direct injection. Just wondering if the main cause is Argon gas in my sample from reactor to GC.

I appreciate any help!

Hi,
Your response factors must be similar, independent of concentration. If you find different response factors with concentration, there is adsorption happening. This can be in liner, column or detection port liner.
Make sure you use inert liner, with deactivated glass-wool. For Sulfur compounds Restek supplies Siltek treated parts which are inherently inert for sulfur. If split injection, use a precision-type. this should give <1% variation on injection repeatability

Column should not be the biggest issue, although peakshape may tell you something.. I hope the eluting peak is symmetrical

last part is detector: look at this blog and you see how important column position in FID is.
http://blog.restek.com/?p=2254


good luck!
jaap de zeeuw, Restek corporation

Using your solvent as the internal standard is very likely to be causing the problem. In the first place your internal standard has a concentration of up to 200 times your analyte when they should have similalr concentrations, secondly 0.2 ul of dodecane injected on column is overloading the detector.

What happens if you dissolve both dodecane and benzothiophene at similar concentrations in octane or hexane ?

Peter