analyses low concentration (ppm range) methanol in water

[starfry]

hi there,
I need to analyize low concentration (ppm range) methanol in water.
I use:
Injection volume: 1µl
gas flow: 25 ml/min N2
Column: Porapack Q
Oven: 120°C
TCD Detector 120°C

The peak of water appeared at about 2.1 min was realy huge.
Although there was a small peak appeared (about 3 min) after the peak of water, it seemed no relation with methanol. I tried 100 ppm MeOH in water and 4 ppm MeOH in
water, the peak area were almost the
same. The peak area of 4 ppm MeOH
was even larger than 100 ppm..

Any suggestions are appreciated.

[chromatographer1]

Your present procedure produces a large volume of water to saturate the pores of the Porapak Q. This will broaden and diffuse the later eluting small methanol peak. You are probably not even seeing the methanol peak but the backflash of the water in the injector.

First you should be using an FID.

Second if NA then lower the injector temp to 120°C and your initial temperature oven to 60°C and ramp to a reasonable temperature if you even hope to see your methanol peak.

Inject slowly, 1µL per 3 sec. Increase your flow rate to 40cc/min or more if you have a FID.

I hope you are using glass column.

Good luck

[dcobice]

Hi, well if you have a FID detector in the Lab perhaps you can perform these kind of analysis using Head Space, I the past I used that tecnique and run well.I put the water into a head sapace vial ( with salting out ( using any amount of Na2SO4)) an then evacuated in oven at 80 degres for 30 minutes, then inyect 1 ml of the gass phase using a gass sringe.
Perhaps i hope these help.But using a TCD is too difficult because of the amount of water and the stationary phase of the column colapse.If I m remenber i used a DB-624 or DB-inowax, DB-wax cappilary column.
The cromatography conditions are 1ml/min ( nitrogen as a carrier gas) then 35 to 100 at 5 degres per minute.120 degres in the inyector, and the FID is at 250 degres.
These is my litle experiencie about.

regards

[vballchemist]

EPA Method 8015 Direct Aqueous injection by FID includes Methanol.
Detection limits are between 1-10ppm. Run times with a DB-624 Column are typically <5 minutes.

See the following site for the 8015 method:

http://www.epa.gov/epaoswer/hazwaste/test/main.htm

See the following site for an example chromatogram and application:

http://www.chem.agilent.com/scripts/chromatograms.asp

[Ron]

The TCD is not a very sensitive detector. I doubt if you will ever see low PPM levels of methanol on a TCD. At that level an FID is really your only option.

[oscarBAL]

Hi if you dont have a FID well you could to use SPME injection technic, that is very secitive, and unnexpensive, you could find more information in Supelco, I am sure that is good way to do it.