Chromatography Forum

Hi all,
I'm trying to detect aqeous dichloromethane (methylene chloride) in GC-FID (Nucon 5670, India) using a capillary column (Permabond SE54HKW) with the folowing conditions, Inj: 220, Det: 240, oven: 120 degree C. with a split ratio of 1:25.

For a concentration range of 25 to 1000 ppmv gas phase, the above conditions works well. But for the aqeous phase, even for a higher concentration (> 2000 mg/l) , no peaks were observed even after adjusting the sensitivity settings.

Can anyone please suggest me a packed column and required operating conditions to detect dichloromethane in aqeous phase < 2000 mg/l ?

expecting reply with thanks,
Ravi

Hi Ravi,

To be honest, I think your oven temp may be a too high for the analysis of dichloromethane coz I am currently analysing it on an Elite 624 Capillary Column with a starting temp of 38oC. I am using such a low temp because I am only analysing solvents but you may be ok starting at 50oC. Let me know how you get on.

Good Luck.

Salma

Hi Ravi,

Dichloromethan is not easy to dissolve in water. It tends to adsorb to the wall of container or get lossed very quickly through evaporation. It may help to shake the sample, prevent loss, and add some solvent to increase the solubility.

Promochrom
PromoChrom Technologies
www.promochrom.com

Promochrom

The solubility of dichloromethane in water is 13,000 mg/L at 25C. I don't see any solubility problem there.

I agree with promochrom,

I work with waste solvents and when I get a mixture of Dichloromethane and water, they are usually separate layers so though DCM may be soluble in water, it is not totally soluble and therefore you might want to use another solvent to dissolve your DCM in.

Good Luck


Salma

Hi All

I'll go with Salma and Promochem on this one. The concentration of DCM in water saturated with DCM (which is what the solubility figure reflects) is not really the issue when you have to analyse the solution. DCM partitions strongly into the headspace and so aqeuous solutions very quickly lose the DCM when a vial is opened or a stock standard is diluted for instance.

I suspect that there are multiple problems with the analysis. The column temperature is too high, DCM is probably eluting very close to the dead time. An FID is not very sensitive to DCM which has only one C-H bond.
Assuming that Ravi is injecting water the split ratio is probably jumping around due to pressure surges in the inlet, and a plug of water vapour is probably co-eluting with the DCM, which disrupts the FID sensitivity.

Ravi, you analysed gas phase DCM, presumably by injections from a gas-tight syringe. You can adapt this method for the aqueous samples by doing an equilibrium headspace analysis. Seal the water sample into a 10 or 20 ml vial with a septum top, heat it to 60 deg C for 20 min or so, take a sample of the headspace with a heated syringe, and then inject it as if it was a gas phase sample (which it is !!). Working manually will not give good repeatability - for that you need an automated headspace sampler, but it might be good enough to get the job done.

Peter