THM analysis/HeadSpace GC/ECD

[chemengineer]

Dear Friends,
I want to analysis THMs in water by using Headspace GC/ECD.
Any method?

Thank you.

[chromatographer1]

A problem lies in the water vapor in the headspace sample and your ECD.

If you use a FID you can see less than 50ppb of chloroform if you use 22mL of water in a 25mL vial, heated for 15 min @ 85°C, and using a 1cc sample loop on a backpressured 0.53mm or 0.32 mm capillary (use 1 to 5m of 0.25mm ID restrictor).

I would suggest using a 1301 or a 5% phenyl column.

Other choices would be 1701, or VOCOL column.

I would use a flow rate of 5 to 10cc/min.

Have you tried SPME yet?

best wishes,

Rod

[chemengineer]

Dear Rod,
Thank you for your kind reply.
Actually I have to find two way: 1-Extraction sample preparation 2-HeadSpace sample preparation

Actually, I have an Autosampler "Perkin Elmer,TurboMatrix 4.0" but as my GC is Chrompack, it is difficult to match two instruments, however I am trying to do that, and I became successful to connect it to my GC, but as it may take time to start its run , I wanted to find another way of sample preparation too.
I want to have two way of analysis "With/Without HeadSpace".

And please tell me when I use my HeadSpace , what is the suitable method for setting HS?
(Timming,Temperature ...).I attached you a pic of my Turbomatrix software, you can see wich parameters are needed to set a method for it.

Also about extraction method (for sample preparation and manual injection), would you please explain it more? (I know Penthane is a suitable extraction solvent), but I don't know the exact Penthane extraction procedure . Is it Normal Penthane? Liquid form? Should it be in any special grade?

I would be very thankful if my request meets your favorable consideration.
Hope to hear from you soon.
Yours sincerely,
Semiramis

[camino]

Hello,

I actually work with THM, using GC-ECD.

I'm working with L/L extraction with hexane as extraction solvent (50 mL of water and 10 mL of hexano, and 5 min. magnetic stirring), I have a very good detection limits (about 0.1 µg/L of chloroform), is a quick method, you can process many sample at the same time, and you can work with autosampler.

I also work with SPME, 100 µm PDMS fiber, a solvent-free method, but hardful to automatize. And there is less interferents than the extraction method. The detection limits is about 2 µg/L of chloroform.

This two methos work well to complain with legal limits in supply water.

Regards

[chromatographer1]

It is very difficult to produce reproducible headspace results without an automated headspace analyzer. See the manual for details of setting temperature and flow if you have one.

SPME can be done manually using immersion or headspace for the fiber.

Please call Supelco or email techservice@sial.com for details.

Extraction procedure is dependent upon finding a sample that is clean enough to perform the analysis, which is not always an easy task.

Good luck with your analysis.

Rod

[chemengineer]

Dear Camino,
I found Standard Method 6232B for my THM analysis. It is Liquid extraction with Pentane.
And it says to do internal calibration with Hexane and 1,2-dibromopropane.
May you please tell me about internal standards and why we do it? When (the period of time ) we have to do it?

Thank you.
Semiramis
PS: I haven't found any program for my Autosampler HeadSpace sampling machine attached to my GC, yet

[camino]

hello,

You should add the IS after the extracion proccess, directly to the pentane fase after the extraction.

In the L-L extraction there is a problem to known the final volumen of the organic phase. i.e If you use 100 mL of sample and 10 mL of hexane, after the extraction you don't obtain 10 mL of hexane, you get about 9 mL, due you lose solvent because it dissolve with the water and/or it evaporate; and when you calculate the original concentration in the water, how we can calculate the pre-concentratio factor???
If you don't use IS the recovery will be higher (~ 120 - 140%).

This only can resolve working with IS, or extracting the calibration standards, trating it as samples. But this second option is more time consuming. Normally we work with external calibration (the standard are prepared in the same solvent as extraction, rising at determinated volume, and adding IS).

I add 200 µL of IS (5 mg/L) to the hexane directly, in this way the IS correct the loss of hexane (or pentane) and the recovery take down to normal values (~80-90%). Then I take 1 mL to a chromatographic vial, and inject 1 µL in split mode. I use 35ms capillary column.

You may add the IS before the extraction, directly to the sample (the IS must be in a water miscible solvent i.e methanol), and treat it as surrogate.

I hope you understand all, I'm practising my English actually. Tell my if you need any explanation.

Best regard

Best regard

[chemengineer]

Dear Camino,
Thank you for your help.

Step by step I have to add some new analysis to my daily routine analysis, now after THMs analysis I should analysis BTEX compounds in water samples.
I am searching for a standard method "not purge & trap" but GC/FID.
Do you know any standard method for this test ?

thank you,
Semiramis

[mbulentcakar]

Hi,
I agree with Rod (Chromatographer1) on suggesting SPME, also for BTEX in water by GC FID. It should give 2-10 times better MDLs. It may be even more, you see, sample enrichment can be optimized by selecting appropriate film thicknesses etc.
I suggest you to have a look at supelco site:
http://www.sigmaaldrich.com/Brands/Supe ... ntral.html
I should add that I don't work for Supelco.
Good luck.

[chemengineer]

Dear mbulentcakar,

Thank you for your help.
Ok. So I think as you and ROD suggested, using a SPME method for direct injection is ok.

Just may I know if I am using a method and I just change the sample preparation part to SPME extract and direct injectin , shall I change other parts og method for GC?

Thank you.
PS: If you know any standard method from EPA or some other , please tell me.