Chromatography Forum

I am using an HP 6890, the back inlet is On Column. How should my valve settings be in my method in chemstation? I ask because it looks like I am getting drastically reduced peak areas. The retention times, symmetry, and correlation coefficients for my standards are all good. But the peaks are giving 600 pA where I am used to getting 2 million. All my connections from inlet to detector are good, gas flows are good. The front inlet is split/splitless. It just seems that I am losing some sample somewhere in the inlet. I am using a capillary column, maybe its a make up gas issue/leak in the FID?

Thanks

I seem to remember from some of your earlier postings that you were working with a 5890. Is it the case that you have moved from a 5890 to a 6890?

Gasman

Yes, I used to have 3 5890s, one with EPC, one with no EPC, and one with the regulators on the side. Now I have a 6890 and 7694 HSS. I was wondering if it was a unit issue but I have seen loss of my high boilers in my low level standards, so I am suspecting I am losing sample somewhere or not getting enough on the column, I have looked through the valve list on the unit and we have no valves installed (makes sense for the on column inlet). My baseline is at 20-30 pA...wondering now if it is an FID issue, not enough sample getting to the jet maybe due to makeup gas flow...

Are the readings the value on the GC screen, or are you using a data system?

Gasman

those are the integrated peak areas. went back and found some old chromatograms/data and the old area was 2 mil new is 600. but proportionally they are still the same...

My guess is that with the 5890 you had an A to D converter box between the 5890 and your data system. You now have a 6890 with a lan connection and you connect the 6890 directly to your data system. If I am correct, there is nothing to worry about. At first it is a bit concerting, but you will see no overall difference in your results when you calibrate your system.

Gasman

Also I would advise leak checking with the column installed ( stick a septum on the exit of the column ), especially if you are using a retention gap.

Gasman,

Thanks for the info, we are using a 6890N with lan connection. I am using a retention gap with the full 5m installed. I may cut it down to 1-2m and see what happens. Also, I found that the inlet was not set to "track oven" I am hoping some of the high boilers were getting stuck on the inlet.

Thanks again.