head space btex

[bublle]

Questions:
-is there a certain ratio of water:hs volume (for example 75 %) that colud be taken as an optimum ratio?(I use 50 mL vials)
-does acidification have impact on detectibility?
-can external calibration be avoided by using one peak external calibration(in which concentration?)
-(how to count concentration of compound x, just by comparing concentration of external standard-one peak area to area of compound x founded)
-which internal standards are preferable?Probably it has to be deluted at the first place and than applied to your hs sample

Thank you.
Bye

[chromatographer1]

Bublle

When performing headspace analysis the key is remembering that you must have the same matrix in order to get a linear partition according to the changes in concentration of the analyte in solution.

Is your sample matrix going to be something other than pure water?

Is there going to be other volatiles which can vary in concentration?

If the answer to both questions is NO, then assume the partition of benzene and the other aromatics will be very close to 100% partitioned into the gas above the liquid, the headspace. Thus you will be able to measure without an internal standard or any salts to improve partition and your analysis should be very linear and very sensitive.

I measured 50 to 5000 ppt of benzene in water with a FID and a 20ml sample in a 22 mL vial in 1997 with a Tekmar 7000 and a Varian 3500 GC.

But if your water is dirty and variable in content then these factors can affect the partition and an internal standard may be necessary.

If you are measuring hundreds of ppm or above then a smaller amount of water (down to less than 0.5mL) is recommended.

You may email me to discuss if you wish.

Rodney George

[AICMM]

Bublle,

To add just a bit to Rod's comments. BTEX will want to partition to headspace so the sample volumes used will depend largely on the detection limits you wish to obtain. I would definately add internal standard (some options; dichlorobenzene, trimethylbenzene or TCMX) since you will probably have some matrix issues and salting or acidifying is probably not necessary.

I have an application note on my web site and I have a white paper with more information on BTEX by headspace. If you send me your e-mail I would be happy to forward it to you.

Best regards.

[bublle]

I would like to detect very low btex concentrations , above 2 ppm up to concentration of 100 ppm (drinking water and waste water).
Just expect btex from compunnds that will vary in concentration.
How to establish apropriate concentration of int.standard which I would like to add?(If I add just small amount of int.stn.(pure)I get enormous, unmeasurable peaks which I can't use for quantification and I have a big problem.
To dilute it and to add in concentration close to concentration expected?
Is there a catch in establishing optimal volume when having smaller concentrations(smaller concentration---more water,less gas,or???)

I would like to thank you both on your help and am for sending personal mail but have problems in sending it by using your profile details---it seems unpossible!Catch too?

Thank you,
Bublle

[chromatographer1]

Rodney George
Senior Research and Development Scientist
Supelco
rodney.george@sial.com

[AICMM]

detectorguru@detectorsbyaic.com

Best regards.