[mir]

I have created really good data of a reaction profile through time, with a little dirty method:
I added my internal standard directly into the reaction mixture, before running the reaction.
The internal standard hopefully didn't do anything with the reaction, and GC/MS is not showing any signs of that either.

What methods are you using to quantify chemical species while making reaction profiles?

[movies]

I've used this method for determining conversion and yield of a reaction by GC as well.  It's a very nice method!  You can do the same thing by NMR, but GC is more accurate.  I always get confused when calculating response factors, even though I've done it many times now.

What did you use as your internal standard?  We usually use a relatively high boiling alkane, like tridecane.

[mir]

I am analyzing phenols and Im using 1,3-dinitro benzene as an internal standard. It has retention time close to the retention-time area my analytes is appearing. I read in many articles that long-chained alkanes are used as Internal Standard.

But now I am trying another method, by adding internal standard to my GC-samples when diluting, before running GC/MS.

[movies]

I would really recommend using a different standard.  Things like nitroaromatics can decompose at high temperature and throw off your results.  Something like di-tert-butylbenzene might be a better choice, but again long alkanes are generally the best.  You don't necessarily need the GC retention times to be similar, as long as both are clean, sharp peaks.

I also favor using the standard in your reaction because then the relative concentrations of your standard and substrate/product will stay the same.  Measuring out the standard every time you take a sample is not only kind of a pain, but also induces another opportunity for measurement error.

[mir]

Things like nitroaromatics can decompose at high temperature and throw off your results. 

I havent experienced any trouble with the dinitro-benzene yet. And other have also used this IS for their work. But I will check into the suggestions of yours 

I also favor using the standard in your reaction because then the relative concentrations of your standard and substrate/product will stay the same.

But how do you know if the IS is not affecting your system? It might catalyze the production of a biproduct. Since you are using this method, do you routinely check this in some way?

[movies]

But how do you know if the IS is not affecting your system? It might catalyze the production of a biproduct. Since you are using this method, do you routinely check this in some way?

Yes, that's always a concern, but if you use something inert like an alkane then you shouldn't have a problem.  Nitroaromatics can react with transition metals via redox chemistry, so that's why I would avoid them.  Even electron rich aromatics can be tricky with transition metals.  The other thing to watch out for is to keep the concentration of your IS low enough that it won't significantly impact the dielectric constant of your reaction mixture.  Of course, it also has to be soluble in your reaction mixture, so if you are using a very polar solvent (e.g., acetonitrile) non-polar standards like tridecane won't do the job because they are insoluble!

[mir]

You got right!

1,3-dinitrobenzene is probably decomposing in the reaction mix.

Dp you know if it will decompose during GC and give wrong data?

[movies]

Heh, don't say I didn't warn you!

Decomp in the GC could also be a problem.  You can test that by taking a sample of pure dinitrobenzene and shooting it onto the GC.  You should only see one peak, but if you see others, you're getting decomp.  Be careful though, it might decomp to something that won't fly on the GC.