Chromatography Forum

Hi,

I am trying to improve resolutiuon and separation of Endrin and 4,4-DDD. I am using a method shared to me from another lab, whereby they acheive separation on an older model Agilent 6890 GC dual microECD. We have a 7890 GC dual microECD. The peak containing the two analytes appears as a peak with a total width from 19.16-19.32 minutes and at the top of the peak there is a small valley at 19.26 minutes that is about 1/5th in size of the the height of the entire peak.

We are using the following:
Restek liner #23308.5 skiliner splitless double taper gooseneck 4mmX6.5mmX78.5mm
Guard column: Agilent # 160-2535-10 FS, deactivated 0.530X 10m (cut to 5m)
Connector: Restek # 21388 siltek MTX y-connector
Col 1: Restek #11139 30m 0.32IDd 0.5microm df
Col 2: Restek #11324 30m 0.32IDd 0.25microm df

Oven Program:
50 degree C for 0.5 min
50 degree C/min to 100 degree C for 0 min
10 degree C/min to 300 degree C for 4 min
Run time 25.5 min

inject 1 microL

Split/Splitless inlet mode is in pulsed splitless at 210 degree C
pressure 4.9551 psi
total flow 154 mL/min
Septum Purge flow 3 mL/min
injection pulsed pressure 50 psi until 0.5 min purge flow to split vent 15mL/min at 0.4min

average velocity 19.148 cm/sec
holdtime 2.6112 min

Please advise.....

Thank you!

Some questions:

1. Is this a pesticide degradation method? If so the temperature in your injection port could be causing degradation.

2. If this is indeed a degradation method, are you absolutely sure your DDD is co-eluting with endrin? It may be endrin aldehyde or 2,4'-DDT that DDD is co-eluting with with the method you have listed below. In addition, if you have endosulfans in your standard, both Endo II and Endo sulfate can be quite close to DDD.

If you are sure that 4,4'-DDD and endrin are the co-eluting culprits, I would suggest stretching your method out some to separate them. Also, if this is an older CLP-1 column, and you haven't adjusted for your column maintenance over time, your pesticides are likely to shift on you. Of course if you're clipping the guard column this point is moot.

If you are injecting only DDD and endrin and they are co-eluting, you're going to have to change the method. I have several methods that will separate these analytes if you want to get in touch with me. They range from 6 minutes to 120 minutes.

Good luck otherwise,

Stunt