Help: GC-uECD analysis OCPs/PCBs (EPA method 608)

[kjlim]

Hello fellow chromatographers,
I have been struggling with a linearity/calibration issue for almost 6 months while analysing organochlorine pesticides (OCP) via EPA method 608 and I seek your advice and assistance. I will try to start from the beginning with as much detail as possible.

I am using a 7-point calibration:
0.009, 0.010, 0.020, 0.040, 0.100, 0.200, and 0.400ug/ml of OCP.
Linearity is assumed by calculating percent relative standard deviation which should be under 10% for this method. This criteria was never difficult to achieve until about six months ago when suddenly the system became non-linear and I started to see this phenomenon:

The response factors steadily Decrease as the concentrations Increase. In the past, the response factors were almost identical over every calibration level.

I am wondering if anybody has ever seen this phenomenon before. All of the tech support people I talk to have never experienced this kind of trend with an ECD.

I have eliminated the following as probable causes of non-linearity:
-The calibrators
-The supply gas purity

I've also tried running a calibration without the guard column, using a different injection port, and using a different detector. So far, none of the troubleshooting has resulted in a calibration as good (linear) as before the I noticed a problem.

Sometimes I suspect that I may have done something to damage the detector during an ECD wipe test. I had performed an ECD wipe test right around the time the non-linearity problems started occuring. But I've looked inside the detector for debris and found none. I also checked the make-up gas fittings for correct distance and column distance into the detector. Also, The baseline readings are low with no negative or ghost peaks.

I have been able to improve the linearity to an acceptable level by increasing the detector temperature and make up gas flow rate. But this feels like a band-aid solution because I don't know why the system changed so drastically to begin with.

Any help or insight would be greatly appreciated. I have an overwhelming amount of detail to provide, but I did not want to overwhelm the main issue for the initial post.

Thank you!

[jwhite80]

2 things with this:

1. In my lab we would only run as high as 0.2ug/ml and sometimes had to drop that high point because of linearity issues.

2. you only need 3 calibration points for 608.

We hardly ever had samples with actual pesticide hits in them so dropping our high point down to 0.1 or 0.05ug/ml was not a problem. In my experience 608 was difficult with our older instruments and no maintenance ever helped. Maybe just make sure you're using brand new standards.