Chromatography Forum

Hi at all
I'm trying to analyze pentachloroethane in water with "press and inject" headspace.

The parameters are
20mL vial with 10mL of sample
incubation: 20min at 80°C with shake
valve temp 120°C
line temp 120°C
GC oven temp 40hold 3 min to 200 at 10°C/min
Split 10:1
Flow 1.2ml

The headspace is like new because has 500 injection, while the gc column has a little bit much work (2 years old i think)
My response is very very low and unreproducible, also if another compounds are ok (for example exachloroethane and another classic VOC).
Some idea?
thanks to all

In the U.S., we measure pentachloroethane frequently using "Purge & Trap", which is for all practical purposes "dynamic" (or exhaustive) headspace except that the gas flow is passed through the sample (most commonly water).

For us, the problem is simply getting it out of the water. It is fairly non-volatile, so I would not be worrying so much about the chromatography end of things, but look more towards tweaking my headspace parameters to "push" the more nonvolatile compounds over.

Greg

Daniele,

You could optimize your HS method by tweaking the vial equilibration time (increase it and see if there's any difference) or adding a salt (like NaCl) to decrease the solubility of pentachloroethane (PCE) in water and "push" it in the headspace (as gpronger suggested).
If you know the Henry's law constant of PCE (partition constant between headspace and water for PCE) you could also optimize the headspace-to-sample volume ratio in order to gain decent sensitivity.

Hope this helps

danicrd,

Are you saying that other compounds you are doing at the same time are doing much better in terms of reproducibility? Also, what detector are you using? Poor partition and only one carbon hydrogen bond do not bode well for an FID.

Best regards.

The detector i'm using is MS (varian saturn2000).
My question come from results of other compounds like exachloroethane, naphtalene for example... that i'm currently analyze from 5 to 500ppb.
Why pentachloroethane not?

About equilibration time...i'm using 20 minutes because with longer time some compounds have lower recovery (1,1,2,2-tetrachlorethane for example, 2,2-dicholopropane) due to vial or septa activities I think

Can be an active sites problem somewhere?

About equilibration time...i'm using 20 minutes because with longer time some compounds have lower recovery (1,1,2,2-tetrachlorethane for example, 2,2-dicholopropane) due to vial or septa activities I think

Can be an active sites problem somewhere?

.

It is possible. What type of septa are you using for your HS vials?

Hi

Some properties found by googling:


Properties
Incompatiblities:
alkalis
metals
sodium-potassium alloy + bromoform
water
Notes: Hydrolysis produces dichloroacetic acid. Reaction with alkalis & metals produces spontaneously explosive chloroacetylenes


Physical data
Appearance: colourless liquid with a camphor-like smell
Melting point: -29 C
Boiling point: 162 C

Vapour pressure: 3.4 mm Hg at 20 C
Density (g cm-3): 1.68

Water solubility: negligible

So, just the fact that the water solubility is very poor may cause a unstable gas/liquid equilibrum but perhaps more intresting is that it seems like this pentachloride can undergo hydrolysis (perhaps facilitated by high temperatures).
Would recommend that you look further into this as water seems like an unsuiteble sample solvent for HS analysis in this case.

thanks
probably the HS isn't right choice but this is only instr available and i nedd to check water samples for contamination....

thanks
probably the HS isn't right choice but this is only instr available and i nedd to check water samples for contamination....

Hi

OK. Perhaps this article can give you a few ideas:

Chemosphere
Volume 47, Issue 9, June 2002, Pages 971-976
"Dechlorination of pentachloroethane by commercial Fe and ferruginous smectite "
http://www.sciencedirect.com/science?_o ... 18fce#toc6

In short I think the authors came to the conclusion that even a small pH change from like 7,3 to 8 could increase the degradation speed of PCA.
So perhaps a small addition of a pH 7,0 buffer to the vials could help?

In any case the bp of the analyte is 162°C so you should consider to increase temperatures in valve and transferline just above this to minimize potential surface interactions.