I am analyzing herbicide mixtures by GC using an ECD. My standards range from 0.05ng/ul to 0.75ng/ul. As my standards increase in concentration my responce factors decrease. To meet the method reqiurerments the %RSD must be below 20%. I have always seen this decrease in responce in the past but it is now at a point that I can not meet the 20%. I have backed off on the amount of sample put on column and have checked the standards. If any one has any suggestions please respond.

What type of column (dimensions and phase) are you using? What gases and flow settings do you have?

The linear range of the ECD is low and there are two main things to try if you are convinced this is a detector issue. First if the ECD board has the ability to adjust the signal, you might want to increase the signal to 50 Hz or so. Some new ECD cells are so clean, the background signal can drop the detector out of linear range. By increasing the background signal you may be able to overcome this. Second try switching makeup or aux gas to N2 with total flow thru the cell near 60 ml/min.

I am using two columns, a 0.25u x 30m DB-XLB and a DB-5.625. I have a 0.53u guard column which goes to a Y and is then hooked to my two analytical columns then each go to a separate ECD. My flow is 1.2 and my total flow is 74. I did increase the signal from 13 to 35 which did not seem to help. The same problem is occuring on both systems. I will try increasing the signal further. I'm not totallly sure it is a detector problem. Thanks for your input Steve and if you have any further suggestions please forward them.

I had a similar problem and solved it by reducing the make-up flow to the detector to about 30 ml/min using N2 and increasing the dwell time the syringe needle stays in the injector to 3-5 seconds. I hope this helps.