Chromatography Forum

I'm hoping someone can explain how to calculate the concentration of etoh in a sample from the following info.

100ul of 75.5% ABV alcohol(everclear) was injected into a 40 ml headspace vial. The vial was heated, and 50ul of headspace was taken using a gas tight syringe and injected into the GC.

How would I determine the ppm of etoh from that injection. Once I know the concentration of etoh in the above sample I plan on running dilutions to set up a curve so that I can determine etoh in other unknown samples.

The concentration is 75.5%.
If you are injecting it into water in the vial you should tell us that and state what the volume is.
As it is you are injecting 75.5uL of ethanol into your vial. Multiply by the density to get the mass and divide by the mass of the water. If you want the concentration in the headspace the calculation is a bit more complicated.

100 uL of 75.5% Everclear is 84 mg (0.84 density). Since your vial volume is 40 mL, you have 75.5% * 84 mg / 40 mL, or 1.59 mg/mL ethanol in the vial if you assume that all of the ethanol volatilized (you have to make that assumption for this to work).

You inject 50 uL, or 0.05 mL, so you injected 1.59 mg/mL * 0.05 mL, or 79 ug of ethanol. You can make your calibration curve in terms of ug injected (this is the best way). Once you calculate how many ug you have in your injection then all you need to know to calculate the concentration in the sample is to know the volume of sample added to the vial and the density of the sample (or, better, do it by weight).

Thanks. My 50 ul injection caused my peak to flatten out, so I tried a second injection using only 10 uL. Using your calculations I came out to 15ppm

100uL of 75.5% everclear =84mg
84mg / 40ml =1.59 mg etoh

inject 10 uL

1.59 mg/mL * 0.01 mL, or 15 ppm of ethanol

Assuming (as we often have to when insufficient information is provided) that the 10 ul is the same stuff as the 50 ul, then the concentration of ethanol in the sample is the same in the 10 ul as in the 50 ul - in other words 75.5% (whether vol/vol, mass/mass or what we do not know). If you want to go from one non-standard unit (%) to another (ppm) then multiply by 10 000.

Peter

We did not use headspace to assay for ethanol levels in hand sanitizers, which were 62 to 75% ethanol. We added internal standard, diluted with water or methanol, used polar capillary and FID.

Peter is (as normal) correct. My formula gives you ug; you mad the leap from mg (mg/mL * mL) to ppm, which is a concentration (mg/L). Can't do that - you have to use the initial weight of the sample to get there.