Analysis 4,4'-DDT with GC-ECD

[brightsun]

Dear bros,

I'm running calibaration curve of 4,4'-DDT.

I bought solid 4,4'-DDT with purity of 99.7% from Sigma-Aldrich ( 4,4'-DDT PESTANAL)

And I use the GC-ECD to make calibration curve with 3 points 3,6,9 ppm in acetone.

The below image is my result :



The first peak : solvent peak
The second peak : doesn't change peak area when running 3,6,9 ppm
The third, fourth peak : area change and both create the linear calibration curve.

My question is which one (3rd or 4th peak) is 4,4'-DDT ?

I appreciate with your reply.

Thanks in advance !

[Don_Hilton]

Suggestion: Watch the inlet conditions! It is very easy to convert DDT to DDE in a GC inlet. Be sure you have a clean inlet liner and inlet seal. And be sure your inlet temperature is appropriate.

Assuming that we are looking at converstion of DDT to DDE, before stating which peak is which - there is one other bit of information that would help: What is the stationary phase in in the column? On a 5% phenyl methyl silicone column, the DDE elutes first - I've seen it too many times...

[brightsun]

Dear Don_Hilton,

I'm using Elite-5 column. You mean there is a conversion from DDT---> DDE in injector ?

I will try changing the temperature of injector until there is one peak only

Thank you !

[Don_Hilton]

While temperature is important, lack of chemical activity is the BIG factor.

Some of the folks who do pesticides on a regular basis probably have some more specific advice and warnings...

[AICMM]

brightsun,

Try a much bigger dilution. ECD's do ppb (ng/mL) analysis (for this compound) such that ppm may simply be saturating your detector.
Unless you are doing a split injection with a decent split, which you don't say.

Best regards,

AICMM

[Steve Reimer]

From method 8081 in EPA SW-846:

9.3.3 DDT and endrin are easily degraded in the injection port. Breakdown
occurs when the injection port liner is contaminated with high boiling residue from sample
injection or when the injector contains metal fittings. Check for degradation problems by
injecting a standard containing only 4,4'-DDT and endrin. Presence of 4,4'-DDE, 4,4'-
DDD, endrin ketone or endrin aldehyde indicates breakdown. If degradation of either DDT or endrin
exceeds 15%, take corrective action before proceeding with calibration.

[tlahren]

As most people here are saying it is most likely breakdown in the liner. Most people use deactivated liners for DDT and Endrin analysis. I used to buy Siltek coated liners from Restek. Now they offer SkyBlue which is supposed to be better. If you use the wrong liner you can fail the method requirements for breakdown to DDE with a brand-new non- or poorly-deactivated liner. If you do not need to use glass wool then I would not use it as it will increase the surface area for reactions to take place in the liner. Any steps you can take to logically reduce the "activity" of the liner will help. Temperature, deactivation and surface area are your biggest control variables to play with.

[Steve Reimer]

The other variable to consider is how long the compounds stay in the liner. Increasing flow through the inlet by using split injection or a larger column diameter or a Y connector and two columns (different phases and detectors) will reduce residence time and opportunity for DDT degradation.
If you are using 3 to 9 ppm standards try a split injection.

[brightsun]

Dear AICMM, Steve Reimer, tlahren

Thanks for your useful suggestions !

I will try using it.

Regards,

[tlahren]

Thanks Steve,

How could I forget about Time?

[Alexandre]

You have to run System Suitability test before analysis of DDT and alike. It is a must.
The test is mentioned by Steve, without PASS the OCs results can not be accepted for regulatory/compliance/legal purposes.