External Standard Concentrations

[JenI]

I am hoping that someone can clarify the math behind my external standard. In the past when I have purchased my calibration standards for GC, I have always requested the solvent to be absolute ethanol. The reason I do this is because when I report the sample result, I use a multiplication factor to convert the result into an anhydrous value. For example, if my sample was at 40%v/v I would multiply the compound result (which I am calling "as is") by 2.5 to get the anhydrous result.

In my quest for improvement I ordered an additional standard at a level of 95%ethanol:5%water. This standard would match the ethanol level of a sample that I analyze. However now I am unsure if there is a conversion factor needed on the standard when I calibrate, because it is no longer anhydrous.

I use a 1 point calibration and inject the calibration standard as received ie no dilutions.

So I guess my question is Should the % ethanol(solvent) of my calibration standard have any adjustment factor because I adjust my final result to be reported on an anhydrous basis.

Hopefully I have explained myself well enough. I am just not clear on how to calculate the math. So if someone could explain to me the math behind it, I would really appreciate it. Thanks in advance Jeni

[JGK]

Just to clarify

Are you analyzing the ethanol content of the samples or are you analyzing an unknown in an ethanol water matrix?

[JenI]

Sorry, I am looking to quantify the samples' profile which is in an ethanol matrix. There are ~5-7 congeners that I monitor and the external standard contains all of them. I am not analyzing the ethanol level using GC but I use the ethanol level to determine the multiplication factor so I can convert the "as is" sample to an anhydrous result. Does this help? Thanks Jeni

[JGK]

I would suggest that, If you're STD and samples are in the same EtOH:water (95:5) matrix, why dont you calculate the anhydrous concentrations of the STD components and use these values in the calibration. You will only have to adjust values once this way rather than for each sample injection and your sample concentrations will be expressed as "anhydrous concentrations".