Chromatography Forum

We recently built a greenhouse gas acquisition system for a field project measuring CH4, CO2, and N2O. The unit was working fine until moved and an improved injection system installed. While working through another issue, I started getting negative peaks resolving on the downslope of the N2O peak. This occurs regardless of using our automated injection or bypassing with manual injection. The trace returns to baseline after 30 seconds. I cannot separate the peaks with temp. The baseline is clean with no injection. We never had negative peaks prior. This started after I switched columns for a period of time from HS-N to HS-Q and back. I baked out the column and polishing filter on this SRI GC (which has been reliable in the field) then the neg peaks started.

Thank you for your advice ahead of time!!

Try baking the ECD. It may have become contaminated during the column switches and transport.

I raised the temp of the ECD to 350 for several hours. The problem persists. Also, I took apart the manual injector (which I borrowed) and it was full of tygon tube or septa residue, could this give negative peaks on the ECD. What could give negative peaks?

Thank you very much for your help.

Kevin

Negative peaks with an ECD usually mean contamination. You should clean the system and then bake all components at their maximum temperature for several hours.

What do you mean by clean the system exactly? I cleaned the polishing filter and maybe connected the column back to the ECD to soon. Possibly why I'm having this issue. Do you know what the max temp is for ECD bakeout?

Thanks again

By cleaning I mean a system "bake out." I do not know your max temperature limit, as many systems are unique. Check your user manual or the manufacturer.

The instrument was functioning fine until an improved injection system was installed. I suspect the new injection system was not clean and contaminated the system. Try removing the new injector, putting the system back to the orginal configuration, then seeing if it is working properly. There is a chance the new injection system is steadily adding more contamination to the system.

Hi
Hidrocarbons (pentane, hexane, isooctane ....) can be detected as negative peaks in ECD

Well, what do suspect would happen if the injection sampling loops were cleaned with an alcohol solution and not throughly baked out?

Also, I reduced the oven temperature to 50 and slowed the flow rate, the N2O peak separates from the negative peak. But the negative peak remains on injection whether automated or manual.

K2

kahmark,

Alcohols might cause your negative peaks but more likely I would guess that the valve actuation upsetting the flow to the detector. That being said, you might reduce the column flow a bit and see if the negative peak separates more from your N2O.

You might also be able to use make-up to reduce the perturbation but it might decrease your sensitivity.

Best regards.

Thank you for your reply. I am using about 25ml/min of makeup flow (N2) and I changed the carrier flow to 19ml/min and turned the oven temp down to 50deg. This did in fact separate the N2O and neg peak. However, it still doesn't explain the negative peak. If this is alcohol, will the peak decrease with time?

Kevin

I decided to install a O2 trap on our N2 carrier and makeup gas line. This completely changed the signal and dropped the baseline approximately 40%. A negative peak still exists on the front end of the N2O measurement so I think I'll install a universal trap. Any comments?

Gas filters for ECD are VERY important so it is not surprising that you get a much better signal with them. I only have experience with Agilent ECDs but depending on how old the detector is and how heavily it has been used you may need to send it in for cleaning (a more thorough cleaning than bake out). It is very easy to permanently damage them and it is not recommended that you clean it yourself. You can tell the "age" of an ECD depending on the level of your baseline and how bad signal:noise is. Check with the manufacturer for information specific to that brand as well as their policy for cleaning the cells. There are 3rd parties that will do this as well.

I had a customer who was getting rhythmic small negative peaks that had gotten so bad that they were interfering with their calibration. Happened on both ECDs at once even though the baseline was much worse on one than the other. We tried many, many things but nothing resolved it until we exchanged the cells, then they worked great.