Chromatography Forum

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.
Thanks.

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.

Hi,
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