Co-elution of 2 peak DDD and DDT in GCMS

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

7 posts Page 1 of 1
Hi everyone,
I use VF- Xms column (Agilent) for pesticide analysis.(26 pesticide and 6 PCB).
I use temp program: 60oC (2min)--(30oC/min)--120oC--(5oC/min)--300oC(15 min).
But, peak of p,p DDD and o,p DDT co-elution. They have the same m/z 235,237,165.
I hope to receive your help.
Hi Thuy,

One thing you can try is to place an isothermal hold at 45 degrees Celsius lower than the temperature at which the co-elution is observed. The separation will be slower overall, but the two peaks should be prized apart.
I developed a DDT method beginning of this year. Had the same problem with coelution on a 30 m Thermo 5MS colum. I calculated the temperature at the point of coelution and added a 1 min isothermal plateau 20 °C below that point. Achieved baseline separation.
Thank you Matt and ddoerder for answering my question.
I replaced DB 5ms column. Co-elution still happen, but the compounds have different m/z, so i can use.
But I still don't understand. example: if i want separate peaks and the temperature at the point of coelution is 230oC, so how should the temperature program adjust?
For a good estimate we'll need your average linear velocity (cm/s) and column length. The velocity is calculated based on your column dimensions and flow, and you might find this value in your system software or use online calculator.

Example: your average linear velocity is 30 cm/s on a 25m column and you run under constant flow. Your analytes co-elute at 30min at an oven temperature of 230°C. At 30cm/s it takes an unretained compound ~80s (~1.3min) to travel through the 25m column. Note that this is your column dead time, and your analytes are not unretained compounds.

Now you check what is the oven temperature at 30min - 1.3min. That is where those particular analytes are moving down the column, and where it's a good idea to try an isothermal hold since you want to increase their retention.
Thank you very much for your help, Rndirk.^^
I'll do as you instructed and i'll response later.
thuy wrote:
(...) example: if i want separate peaks and the temperature at the point of coelution is 230oC, so how should the temperature program adjust?

Your current temperature program:

thuy wrote:
60oC (2min)--(30oC/min)--120oC--(5oC/min)--300oC(15 min)

Considering the coelution at 230 °C, try adding isothermal hold at ca. 210 °C

60°C (2min)--(30°C/min)--120°C--(5°C/min)--210°C(2 min)--(5°C/min)--300°C(15 min)

edit: spelling
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