Chromatography Forum

Dear Experienced GC Chromatographor,

What is best way to prepare volatile standard solution? Weigh it into volumetric flask and dilute it with solvent to volume or weigh it into a seal bottle and then weigh solvent into bottle? For example, how do you deal with ethanol?
Thanks for sharing your experience.

Dear Jan,
Try to choose a solvent of high BP, and put first a little of solvent into the bottom of your flask, and then tared before put the EtOH, that should increace the vapor pressure while you finish. Iwait taht could be useful for you.

Here's how we deal with ethanol. We use 99.7% ethanol (200 proof) such as 40B as standard (you might be surprised how many wrongly assume HPLC ethanol or HPLC reagent alcohol is 100%, but that's another story). For ethanol, we pipet 99.7% ethanol into a volumetric flask already containing some water, then quickly add more water and stopper; after allowing to cool slightly to room temperature, we then mix and dilute to volume with water (somewhat similar to oscarBAL's esponse). Obviously, the question is that some of a volatile standard will evaporate while weighing: so how to get a constant or real weight? So, instead of trying to weigh 0.2 grams toluene (for example), I'd weigh maybe 5 grams toluene on a top loader, dilute with solvent to volume, then dilute to desired concentration. We've also weighed by difference using a capped disposable syringe into a volumetric flask containing some solvent.

If you call some standard manufactures like Restek or Supelco, they probably can give you some good ideas. This is what I will do if I can't get any information from them:

Add solvent into a volumetric flask, measure the volume of the VOC needed using a syringe, add it into the volumetric flask, add more solvent to bring it to the final volume.

Take ethanol as an example: if you want to make 100 ml 1000 ug/ml ethanol std in water, you need 100 mg of ethanol. Add about 90 ml water into a 100 ml flask, assume the density of ethanol is 0.80 g/ml, then you add 125 ul of 100 % ethanol into the flask, add more water to bring it to 100 ml. Depending on the purity of ethanol, make some adjustment to the exact conc of the ethanol.

Making VOC std by volume should be more accurate than doing it by weight.

And for the record, our volumetric addition of the ethanol makes it easy for us to deliver results in volume/volume %, commonly used for ethanol. And like JI2002 stated, one can readily convert the volume to grams using the density of the ethanol standard.