Chromatography Forum

Hello,

Could anyone recommend a book or article on the basis behind internal standards, how to select an internal standard, how to make calculations with internal standards?

We are running an analysis on 8 VOC of different chemical classes (Acetone: 56.15C ; Toluene: 110.65C ; Hexane: 68.75C ;Propanol: 97.15C ;Ethyl Acetate: 77.05C ; 1-Heptene: 93.85C; Propionaldehyde: 48.85C ;Pentafluoroethane (R-125):-46.15C) injected into the GC/MS in the vapor phase and making calculations using an external standard. Well, we will at some point. At the moment, I calculate only one compoun d at a time to kind of see where we stand in terms of error.

Someone suggested we use an internal standard so that as the decrease in analyte response isn't a problem overtime. Would we have like a standard for each different species in the gas standard mix? That's what I'm not sure about?

If we have a standard in a gas cylinder, would we need to reorder a cylinder with our standard gases and have the internal standard added to the cylinder? Or do you buy a completely different cylinder with the internal standards?

Hi Juli,

You can find a good description for the internal standard method, and the use of internal standards in most of the analytical books. You asked how to select an internal standard. Well, the major difficulty in applying the internal standard method is that of finding an appropriate substance to serve as the internal standard and of introducing that substance into both samples and standards in a reproducible way. The internal standards should give signal that is similar to the analyte(s) signal(s),
but also sufficiently different to be distinguished by the instrument.
So the internal standard for your analysis should be some pure standard substance that has similar physical and chemical characteristics with your analytes. It is not excluded, an internal standard to be one of the major components in your samples or standards.

One internal standard can be applied for determination of one or more than one component in your samples. If you're separating a mixture of similar compounds, your method should provide reasonable resolution
between your chosen internal standard and the constituents of your sample.

Said with few words, as you said, internal standard helps you to correct the instrument signals when fluctiation of the detector's response occurs.

the relationship between concentration (for example) and signal may be expressed as follows:

(Concentration ratio between analyte and int.std in sample)/(Concentration ratio between analyte and int.std in standard)=
= (signal ratio between analyte and int.std in sample)/(signal ratio between analyte and int.std in standard)

Best regards

YIKES!

Let me explain our plan for our project. To test what is being removed by our filter, we will have what I am calling a gas standard of known concentration of course. We have a tunnel we will release it into and then see what concentration of each analyte in the standard makes it past the filter.

So according to the calculation above, I would run a sample of the standard gas then run a sample of what we recovered on the "other side" of the filter.

I do have one question though that I need to find out. I don't know if the tunnel can have a vacuum pulled on it. I think that would be a good thing to know..lol if not I'm not sure how this would be plausible. Am I thinking right? If we just release some gas into the air then the VOC concentrations are now diluted. I've been hanging around the GC too long today..hehe..there is an oven on one side of me...and hte vacuum pump blowing out it's heat. My face is warm warm warm. It really makes me want to go to sleep. Sorry..I got sidetracked.

Oh. I have my old analytical book from college. I honestly don't remember ever seeing anything like this in there. I must look again. I'm curious now.

Thanks