FID and THMs analayzing

[chemengineer]

Dear friends,
Please let me know if I could analayse THMs (Three-Halo-Methane) with FID detector or not? Could the 4 peacks be seen?

Thank you.
Semiramis

[camino]

Hello,

Actually I'm analysing THM but with ECD detector, in theory you can "see" them with a FID detector, but I never tried it.

I can say that I use two colums, and non-polar DB-5, and a medium-polar DB-35. With the DB-35 I can resolve better from other compounds also present in supply water, i.e. trichloroetheno and tetrachloroethene.

With my GC/MS the THMs runs good, the sensitivity are good too. I think that the FID will work well, but you'll have many interferents with the FID.

[camino]

Hello,

Actually I'm analysing THM but with ECD detector, in theory you can "see" them with a FID detector, but I never tried it.

I can say that I use two colums, and non-polar DB-5, and a medium-polar DB-35. With the DB-35 I can resolve better from other compounds also present in supply water, i.e. trichloroetheno and tetrachloroethene.

With my GC/MS the THMs runs good, the sensitivity are good too. I think that the FID will work well, but you'll have many interferents with the FID.

Regards,

Javier Camino

(sorry for my english, but in this moment I'm learnning its)

[Peter Apps]

The signal that you get from an FID is roughly dependent on how many C-H and C-C bonds you have in your analyte molecules. Trihalomethanes have only one and so the response will be very weak - thousands of times weaker than the signal from an ECD for the same compound.

Whether you could detect THMs in a sample with FID depends of course on what their concentration is. I doubt that you would detect THMs at the ppb and lower levels that are usually needed for water analysis for example, but you would be able to do a composition analysis on a solvent blend with THMs at the % level.

Peter

[chemengineer]

Dear friends,
Thank you for your kind replys.
But what concentration of THMs could be good to be detect with FID?
And can I inject just THMs to the column, or THMs in a solvent (Like methanol)?

Thank you.
Semiramis

[Peter Apps]

Hi Semiramis

Why don't you post what it is that you need to do - in other words the problem that you need to solve, the samples that you need to analyse, or the analytical data that you need to generate, and we'll tell you if its possible ?

Peter

[chemengineer]

Dear Peter,
With many thanks to your kind cares, I want to analyze THMs in water samples and I have a GC/FID/ECD, my GC is Chrompack CP9001.I dont have Headspace injection nor Purge & Trap system. So please guide me to how do these analysis.
Thanks a lot.
Sincerely yours,
SEMIRAMIS

[Peter Apps]

Hi Semiramis

You need to use your ECD detector, not the FID. Only the ECD is sensitive enough for THMs in water.

Camino posted some advice an columns.

In the absence of headspace or purge and trap you will have to try to extract the THMs into a solvent that the ECD does not see e.g. pentane, and do liquid injections. The main problem will be to separate the THMs from the solvent because the ECD has some response even to hydrocarbons.

Can you try Solid Phase Microextraction (SPME) ?

Peter

[chromatographer1]

Please remember that water and an ECD do not mix well.

Call Supelco technical service for a suitable fiber for a manual SPME.

You might be pleasantly surprised at how inexpensively and how well the analysis can be performed.

(Yes I work for Supelco)

best wishes,

Rod

[skunked_once]

I have done this analysis in the past and used 2,2,4-trimethylpentane (TMP) to extract the THMs from water. One of the problems I remember was finding TMP that was free from THMs and I had to buy a special type of TMP selected for THM analysis from one of the solvent vendors. It was important to keep the TMP in a separate area from sample prep in order to prevent contamination.

[CE Instruments]

From memory the traditional extraction method is just 9ml water, 1 ml pentane in a vial. Shake and leave for a few minutes , (say five) then inject from the Pentane layer. Biggest problems are Pentane related due to evaporation and absorbing compounds , especially THMs. Thermo's traditional solutions is direct aqueus injection (DAI) 1ul through a cold on-column (Grob). Works fine but you need to dedicate the column to DAI and replace the guard column regularly as hardness salts will build up in the pre column. Still available as a free download on THermo.com
http://www.thermo.com/eThermo/CMA/PDFs/ ... _25679.pdf
Regards Richard

[Peter Apps]

Hi Semiramis

I came across this link while I was looking for something else !

http://civil.colorado.edu/web/grad/envi ... m-cdwo.doc

Peter

[bluejay]

The EPA method 551.1 is used for THM extraction from water. It uses either MTBE or pentane as an extraction solvent, and ECD for detection. You can find the method on www.nemi.gov

Hope this helps.

Bluejay