Peaks delayed significantly and huge solvent peak

[susiedai]

In my recent runs I have seen huge solvent peak( hexane) around 8-14 minutes and my limonene peak changed from 12 minute to 23 minute.
I am using Agilent 7890 GC and 5975C MS, with a HP-5MS column. The inlet was set for 220 degree with splitless mode. The purge flow is 50 mL/min and the carrier gas helium is at 1 ml/min.

The method runs very well in the past until recently I had two manual injections since I had very limited sample volume. All of a sudden, this huge solvent peak appears and masks everything.And even I can see my analyte at a much alter time, the sensitivity was reduced a lot.

I thought it might be the inlet problem and I have opened the insulation cap, cleaned the gold seal, replace the septum, the liner, the o ring, cut my column twice,each time around 15 cm, re-installed the column many times.................The pressure at the inlet reads OK as normal at 7 psi at that flow rate at 220 degree. How can I check anything else?

The peak is still there, even if I inject nothing, the residue from the wash station of hexane produces a big peak at 8 minute.I think this should never happen.

Should I replace my column?

[dim56625]

Hi there,
if you inject air and you get a peak, you have a contamination problem or a carry over. Try to run an analysis without injecting anything (to make sure that your syringe is not the problem). If you get the same peak at the same time, then the problem is with your column (since you have already changed a liner, septa...). Try to bake out the column keeping the oven temp a bit lower than the maximum temp that the column can stand. If this does not help then you have to consider to change your column.

Good luck
Dimitris

[Peter Apps]

Increased retention plus reduced peak aea is most likely due to a serious leak at the inlet, or just before it. Have you run an air and water check on the MS ? Use a leak seeker to check the inlet and all its associated plumbing.

Peter

[gcguy]

Does the pressure spike when the split valve opens and closes? You might have a blocked split line. This can be checked simply by setting a split flow and measuring the flow at the split outlet.

GCguy

[larkl]

I agree with the suggestions re- checking flows. I'd check the column flow, split vent flow.

[Peter Apps]

I agree with the suggestions re- checking flows. I'd check the column flow, split vent flow.

But remember that the flow through the split vent continues even when the inlet is in splitless mode.

Peter

[susiedai]

Thank you all for all your inputs. I did run a air and water check together with several auto tunes. The peaks and peak ratio of water,nitrogen to 69 are all in the low ends below 1%. I also checked the flow rate during the run, it was stable in the first three minutes at 7 psi (oven temperature at 40 degree and flow rate of helium is 1 mL/min in splitless mode). The purge gas was 50 mL/min starting at 0.75 min, and the gas saver is on after two minutes for a flow rate at 20 mL/min. Then the carrier gas flow increases after the oven temperature gradient starts to run after three minutes hold time.

When everything is connected, I cut the column for the third time with a longer length(about 40 cm) then put this short end of column into hexane to see the air bubble, it was perfectly fine, with the pressure reduced to around 2 psi when it is injecting steady bubble stream into the solvent.

I then reconnected the longer column to the GC inlet and injected my standard sample again. This time I got hexane peak ends around 6.4 minutes and my limonene peak showed up around 14.4 minutes. Previously it was at 12.3 minutes. So right now I can probably live with it if I set solvent delay to 7 minutes. But this is pretty late for such a solvent, right?

I also tried a air peak test. For this HP-5MS column, air is supposed to be an non-retained peak. With the right flow, it should be eluting from 1.8 min to 2.5 min. But I did get a wide and ugly hump for the air and it ends around 4 minutes.

My peaks should not elute later but elute earlier, as I have cut my column for so many times, about 80-90 cm now.

Taken those together, What does this mean? Do all these suggest I still have a leak right at where the GC column is connected to the insulation cap?

[edmgc7788]

i think you can try to trim the column on the detector side. I had this situation before, but not much serious, after i trimed the column on the detector side, everything was back to normal.
good luck

[Peter Apps]

If there is a leak at the inlet it could be at any of the connections, the septum, or even a crack where the pipes are welded to the inlet body. You must check all of these with a leak seeker.

It is possible that the EPC has a fault - do you have any way of measuring the inlet pressure without using the onboard electronics of the GC ? - a simle dial gauge will be a start.

Peter

[susiedai]

All your inputs are greatly appreciated. I will try.

Peter, you mentioned a leak seeker. What model do you recommend? If you can give me a link for the product I can then order one.

[Peter Apps]

All the GC and consumables companies do leak seekers, and they are all similar. I have one from Varian (now Bruker) and I know that the Restek works well.

Peter

[JChen]

How was the problem solved? I am really interested in knowing, as I am having the same situation.

[Don_Hilton]

One other thing to check. The column configuration stored in the GC. You show the correct flow rate - but that assumes the GC knows the correct column dimensions so the flow rate can be calculated from the head pressure and exit pressure. If column length, diameter, or detector type have been inadvertently changed, the gas pressure applied to the column may be to low to drive the sample through at the expected rate.