Chromatography Forum

While doing GC analysis on Packed column ( Porapack-Q) i am getting very high signal value
What could be the reasons and how to rectify

Packed columns tend to have a higher signal simply because there is more material physically coming off the column during normal operation. It is also possible that you are heating it too high, you have a leak, or maybe gas purity problems.

More information would be helpful. What kind of detector (and for that matter, what kind of GC)? Describe the signal better: what is your baseline signal at ambient temperature? And during your analysis? Is the signal at least stable or is it jumping around? When was the last time your did inlet/detector maintenance? Have you changed anything besides the column recently?

For Packed column ( Porapack-Q) Detector is FID, carier gas is Nitrogen with constant oven temp. at 150°C and with this condition signal value is around 1200 and some times it is around 100 on same column.

Is this a peak area, or the background ?

Peter

Background

I'll go with aldehyde on this one - most likely there is a gas purity problem. You need charcoal scrubbers on all the gasses to the FID, and a charcoal and oxygen scrubber on the carrier gas.

Did the increase in baseline follow any changes in the sytem - new column, new gas cylinder, other maintenance ? Is the high signal also noisy ?, if not you could probably just autozero the detector output as long as you can still get the sensitivity, linearity, dynamic range etc that you ned for you analysis. What GC do you have ? - on some instruments the detector readout is in microvolts, and then a baseline of 1200 is not a problem.

Peter

Yeah if you don't have traps on your gas lines do that first. Once you've got the traps on I would let it sit over night with flow turned up a little higher than normal in order to push any gunk out.

Let us know the GC manufacturer/model and the signal's unit.

For Peter Apps
This increased in Signal value is observed 1) After chnage in gases in between 2) Sometimes after change in column
For aldehyde
We are using Agilent GC 6890 with liquid sampler and FID

If the background changes when you change gas cylinders you certainly have a gas purity problem, and gas scrubbers are a requirement. What is the volume flow rate of carrier gas, and are you reducing the FID makeup gas accordingly ? the total volume of nitrogen going to the FID needs to be around 30 ml/min.

Peter

If you change out tanks without purging your regulator there can be oxygen up in the little gauges that slowly bleeds out and will cause an elevated baseline--though this should decrease over a day or so. Could also have a bit of oxygen sneak in when changing out columns, but in both situations I wouldn't expect the baseline to stay so high for long.

You said that sometimes the baseline is as low as 100, is that the usual baseline? When it is elevated does it stay elevated for the whole time you are using that tank or does it slowly get better over time?

I have serviced a 6890 where the analysis (regulated pharma) required a method so old that the use of a packed column with the operating temperature and flow etc required resulted in a sustained background of ~1000 all the time. Nothing that could be done about it, the columns would just bleed and bleed and had to be replaced pretty often. There was nothing they could do about it since the method could not be changed.

Your problem sounds different though .