Chromatography Forum

Hi all,
I am confused on the concentration of eo in headspace. Require to have a calibration of 0.4,0.8,1.2,1.6,2mg/l eo in ethanol. Then prepare 0.5mg/l PO as internal std. If take 2.5ml volume of 0.4mg/l eo plus 2.5ml volume of 0.5mg/l PO. hence is it my calibration become half =0.2mg/l eo if base absolute weight. But curious part is I still able get back recovery eventhough I assume the concentration maintain as 0.4,0.8,1.2,1.6 &2mg/l.

Thanks
Seet

Hi Seet


"I am confused on the concentration of eo in headspace "

This will be different to the concentration in the liquid and will follow a different concentration/calibration curve in the headspace

Although it doesn't actually matter, I presume by "eo" and "PO" that you mean ethylene oxide and propylene oxide?

"If take 2.5ml volume of 0.4mg/l eo plus 2.5ml volume of 0.5mg/l PO. hence is it my calibration become half =0.2mg/l eo if base absolute weight. "

Yes, in the liquid.

" But curious part is I still able get back recovery eventhough I assume the concentration maintain as 0.4,0.8,1.2,1.6 &2mg/l. "

Now you also have me confused on this description in your query

Would you like to explain more clearly

Hi Ralph,

Yes eo and po is ethylene oxide and propylene oxide. Mean I prepare a calibration on 0.4,0.8,1.2,1.6 &2mg/l of eo in 5ml ethanol. Then I withdraw 2.5ml from the 5ml into headspace with another 2.5ml of 1mg/l of po ( hence the concentration of po will be 0.5mg/l, but I assume the concentration eo still be the same as 0.4,0.8,1.2,1.6&2mg/l that time before realise). Hence I further do a spike recovery of 1.2mg/l eo in 5ml with top up with 0.5mg/l po. But I still can get back the recovery of 1.2 mg/l eo. If let said theory part, it should b half d concentration of calibration, but how would it be still can get back the spike recovery?clear?
Thanks
Seet

You are talking a lot about how you prepare the standards but with what calibration data are evaluating the recovery? When you mix 2.5 mL of an EO standard with 2.5 mL of the PO standard, the concentration must be cut in half.

2.5 * 0.4/(2.5 + 2.5) = 0.2 ppm EO
2.5 * 1.0/(2.5 + 2.5) = 0.5 ppm PO

When you do your analysis, you must have some type of calibration data outside of this to determine the recovery. If you are trying to use these standards to evaluate themselves, you will likely get 100% recovery because it's a self-contained system. You need a referee outside of this game.

If you really want to determine the recovery, make your standards as you have described. Then, perform the method of standard addition (using some other standard to perform the additions) and see if you hit the target that way.

What are your samples in, ethanol? You talked about your standards but what we need to know is how those relate to your samples. If your samples are in ethanol then just calculate your calibration curve in terms of mg/L EO and make sure you standardize on a 2.5 mL sample size (the internal standard needs to stay constant, so use 2.5 mL of that as well). You're trying to make this too hard, unless your samples and your standards are different matrices.

Do you correct your calibration values based on the internal standard concentration in the final mixture, or do you keep them relative to the starting concentrations? Speed Stars