My question is not specific to any two combination of pure compound and volatile solvents. I saw some research papers using such combination as an internal standard for GC analysis of bio-oil, e.g. octadecane in acetone. However, when I tried to do it, I realized that it can affect the calibration and quantification. This is because, the volatile solvent will keep evaporating and it will change the concentration of the pure compound in that solvent. So, why do some people use such mixture instead of using just the pure compound?
P.S. In understand that in some cases, GC calibration is carried out as GC-area against concentration (mg/mL). This may require the pure compound to be in a similar magnitude as the compounds of interest that one is going to detect in the unknown samples. However, it still doesn't resolve the question of volatility of the solvents. How to deal with that problem?
You have to keep lot of things in mind while selecting an internal standard. The IS should be of the same class as the unknown compound (the compound to be quantified). It should elute close to your target, before or after, should be pure and should be soluble in the same solvent in which you prepare your sample. Then there should no problem with the quantification.
The use of an internal standard as described by Girijan Menon means that the evaporation of the solvent is absolutely irrelevant. It affects both the standard and the analyte equally within any given sample, so you will still get the ratios required to calculate the amount of analyte. When you do the calculation, the volume of solvent cancels as it is the same for both. The only problem would occur if either the IS or the analyte(s) became so concentrated that they started to precipitate out. However if you choose your solvent and the amounts of sample carefully you should be able to avoid this. It would in any case only be problem if you are using an autosampler with a very large capacity combined with very long run times, or very small samples which require the use of small volume inserts in the vials. In general, running a few samples at once should not cause a significant problem if the vials are properly capped.
Years ago we used very dilute solutions like that (non volatile in a volatile solvent).
We made up the relatively large volume of 'standard' in a vessel and marked the liquid level. (Small volumetric flask is ideal). We withdrew very small amounts of of the reference solution (e.g. microliters) from the flask (50 ml). Whenever the liquid level was low from evaporation, we refilled it to the mark. This way, we could take many microliter samples and not lose appreciable accuracy. For example, 50 samples of 20 microliters each is only 1 ml removed from a 50ml vessel, so the loss of accuracy is only 2%. A purist might even keep track of the number of samples removed and make the minor correction. We didn't bother because the error from that was relatively small compared to other errors in our procedures and measurements.
I also concur with Scot Abbott. We adopted a similar practice for dealing with the storage of IS of a particular concentration. It does need replacing after a while with a fresh solution but on the whole worked well. Keeping the IS solution in the fridge also lessens the evaporation.
IS is added to gain additional confidence in quantification of analyte from the complex matrix. If you are getting good recovery of your analyte of interest from the sample with pure compound, then you don't need to add IS. There may be a change in calibration curve and quantification due to increase in response of analyte after addition of IS.
