Chromatography Forum

hi
I would like to know if anyone else has experienced this. I currently analyze chlorinated pesticides with GC-ECD. I perform maintenence- column cutting, changing septum, inlet liner and gold seal. I do this about 2 or three times a week on a 5890 and a 6890. Many times, the areas of all peaks and internal standard change-either double or decrease 2 fold. I use internal standard calibration and my reference standard usually passes with the areas changed.
I use a dual column configuration and have just switched from a two hole ferrule to a y splitter with 1 hole ferrule to see if it was the columns going into the inlet that changed. Nope
So I don't know what it could be. Pressure stays the same and my peak shape is good
Any help would be appreciated

Thanks

Is this a permanent drop? one that goes away after time? One that goes away after your next inlet maintenance?

It isn't easy to keep the columns aligned during installation in a 2 hole ferrule to maintain an even split. You said that response increases or decreases by a factor of two. Does one go down while the other goes up? If so, that would indicate that the split is uneven. If both go up and down together that would indicate potential leaks at the ferrule, thay are also a little tricky to get to seal at time.

Jh1: It is not a permanent drop. It may go down one maintenence and then up the next.

Ron: yeah i thought it may be the problem with the balancing of the columns in the two hole ferrule, but i have been trying to use a Y splitter with one column going up but that doesn't seem to be it either. Sometimes both the areas would go up or down and sometimes it is only one column I wonder if it could be the ECD? I also wonder which area would be right? The lower or the higher?

thanks for the response

Now I am wondering if the column cuts are square and there is no excess polyimide exposed. It sounds like either the columns are not sealing correctly or there is absortion of compounds.

Do you use a glass wool in your liner?
If yes, It's probably your problem...the size of glass wool plug and the position can modify the signal.

I have thought it was the inlet where the gold seal is that is not fitting properly. I changed the inlet and am using a y splitter. So far, no change in area. I have two other instruments that do the same thing. Sometimes the pressure will go down. I wonder if you overtighten the nuts, that might cause the problem?

It is hard to believe that a minor cut in the column would cause such a drastic difference. they seem to be cut square.

I do not use glass wool. I tried it before and the chromatogram was very dirty.
I just always wonder if my data is correct. I use internal standard calibration and when the area is halved, my standards still read correct. Is it true that it compensates for the area change? I usually just recalibrate when this happens.

Thanks

If the area of the internal standards and the analytes drops by 50% you are getting the correct answer, but something is still wrong with the system. Have you tried the seals with the Vespel sealing rings that Restek sells? They are much easier to seal properly than the standard Agilent part. It is hard to overtighten the original Agilent seal, as a matter of fact it is much more likely that the seal is undertightened. Never reuse the Agilent parts, they leak if they are reused.