headspace loss of analites

[danicrd]

Dear all
have you some experience about loss of 1,1,2,2-tetrachloroetane and 2,2-dichloropropane in headspace analysis?
I'm using gerstel headspace with 2.5mL syringe, holding sample at 90°C for 45 minutes. Volume injected is 250uL (I don't need very high sensibility) and column is 20m*0.18 RTX-VMS column.
Thank you

[danicrd]

no one??

[Peter Apps]

Why do you think that you are losing analytes ? How do you know that they are there in the first place ?

Why are you sing such a narrow bore column ? What is the rest of the set up - GC inlet etc ?

What is the sample, water ?

If you put some butane (lighter gas) or a small amount of pentane or hexane into a vial and inject from that do you see a peak ?

Peter

[chromatographer1]

Perhaps your syringe is leaking.

Have you trouble-shooted your system for a generic loss of analytes?

Have you clogged your column?

There are many issues with vial septa and chlorinated analytes which you should be aware.

Unfortunately, your question cannot really be answered with the limited amount of information you have supplied.

I hope you discover your problem and are able to correct it.

best wishes,

Rod

[danicrd]

Currently I'm injecting Epa 8260 cal mix standard, within 60 components at 500ppb: 20 mL vial with 10 of headspace, sample water.
The average recovery is good..but for 2 analytes in first post the recovery is erratic. Sometimes appear..sometimes not.
The GC is Varian 3800 whit S/SL, hold at 180 °C.
Thank you.

[chromatographer1]

what kind of septa are you using? No info was given.

How long do you heat the sample and at what temperature? No info was given.

Sample is tap water? waste water? plant process water? sewage?

Most likely situation is septa is absorbing these chlorinated analytes or there is a chemical reaction involving chlorinated species.

best wishes,

Rod

[danicrd]

Thank you Rod
in first topic there were some conditions..like 45 minutes of incubation at 90 degrees...
Anyway: the problem appear also in CV, with deionized water. I hope I haven't a matrix effect
Septa are silicone/ptfe.

[chromatographer1]

Sorry Danicrd I missed-forgot-overlooked that detail you gave. Oops !

The teflon coated seals you are using are generally the best for your analysis. BUT heating that long and at that temperature can cause the loss you are experiencing. Aromatics and esters can also suffer losses. Alcohols generally do not. Teflon is permeable and chlorinated and aromatic pass most easily.

If you can modify the conditions you use I would not use that much sample, that much heat, or that much time.

At levels of 5ng you can have wonderful recoveries and sensitivity using 15 minutes @ 80°C. That is 5 ppb of a 1 mL sample if you split the sample 100:1 that means 500ppb of a 1mL sample in peak response.

So if you use 2mL instead of 10mL and heat it for a shorter time you should be able to avoid the loss of these two analytes.

Back in 1997 I saw 0.5ng of analytes in 25 MICROLITERS of water very reproducibly in 3.35mL vials and saw very low limits of benzene from water (less than 200 ppt) from 10mL samples of water. But I did not split my sample but directly injected 1mL of headspace sample using special techniques to refocus the most volatile analytes onto my 0.53mm ID column. I separated 18 solvents in 6 minutes using my methodology. This was published in Analytical Chemistry. But that might not work for you.

Anyway, your vial septum is absorbing your chlorinated analytes. You have to reduce time and possibly temperature to get a reproducible recovery of these.

Good luck and best wishes,

Rod

[danicrd]

thank you very much for answer.
I will try new settings...
bye