Chromatography Forum

hi, all
This is dhruvil.
i want to prepare 3000 ppm solution of Ethano. My sample is freely soluble in DMF.(N,N-Dimethyl fromamide). i want to find out the ethanol content of the sample by head space GC. Please suggest me the way in which i should follow stock solution preparation, standard solution preparation and the sample preparation.

I am going to use DB-324 (30x0.53x3 um) column..is it ok ?

so please suggest me the weight of the ethanol to be taken in stock, the dilution factor for preparing standard from stock....and the sample preparation.

hoping for the quick reply...from all..

cu and take care.
bye[/u]

Do you have a rough idea of the concentration in your sample? I am currently developing a method to look for DMSO and nPrOH in a sample using GC-Headspace and DMF as the solvent.

To start with I used very rough standards as I wasn't sure of the concentrations in the standards. I used 0.1 % v/v nPrOH and 0.2 % v/v DMF by spiking 100 and 200 ul into 100mls DMF.

Below are the equations I used to calculate round % w/w for my sample (and comparison results from other techniques I recieved after the run.)
Note: calculations haven't been checked so I can't promise they are right!

DMSO

[%v/v STD / Area STD] * area sample = % v/v sample
[0.2/14.41]*1.83 = 0.025 % v/v DMSO in sample
in 0.5 mls 0.025 % v/v is [0.5/100] * 0.025 = 1.25 x 10-4 ml DMSO
1.25 x 10-4 ml * 1.101g/ml = 1.37625 x 10-4 g * 1000 = 0.137625mg DMSO in 24.38mg sample
[0.137625 / 24.38] * 100 = 0.564 % w/w DMSO (NMR result is 0.5 % w/w)
nPrOH

[%v/v STD / Area STD] * area sample = % v/v sample

[0.1/102.31]*0.4 = 0.004 % v/v nPrOH

in 0.5mls 0.004 % v/v nPrOH = [0.5/100] * 0.004 = 2 x 10-5 ml nPrOH

2 x 10-5ml * 0.8034g/ml = 1.6068 x -5g * 1000 = 0.016068 mg nPrOH in 24.38mg sample

[0.016068/24.38] * 100 = 0.066 % w/w nPrOH (0.1 % w/w by NMR and 0.07 % w/w by EOVI)

You can re-arrange these equations when you know how much is in your sample to make a standard closer with a % v/v that corresponds more closely to the % w/w in your sample. Alternatively run a range of % v/v standards and detemine linearity and read your samples off from the line if it is linear.

Regards, Pat.

Apologies for missing a question from your first post.

To make your 3000ppm ethanol standard I would inject 300ul EtOH in a 100ml volumetric flask with about 60ml DMF and then make up to the mark with DMF.

[3000ppm = 0.3 % v/v right?]

The system I use injects 1 ml of headspace from a 12ml vial which I add 0.5ml of sample/standard but there's plenty of variations that will work I guess. General sample prep here is 25mg dissolved in 0.5mls (but I might have to go higher because the DMSO response is lousy. EtOH should have a good response and be well resolved from DMF.)