GC 5890 FID/Purge and Trap Baseline Drift Problem

[rb6banjo]

We have a 3100 in our lab and the transfer line from the concentrator is spliced into the carrier gas line that feeds a split/splitless inlet. In our system, the standard inlet septum is what makes the pressure seal for the inlet. So, if the septum bleeds badly, it will show up as "ghost peaks" in our chromatography. Is that how your system is plumbed as well?

Was the last trace you posted run after it sat over night with the oven at near ambient temperature (40 °C or so) or did you elevate the oven temperature to "bake out" the system, then cool it before running what you posted?

Only the very old Tekmar systems had a "direct connect" to the GC column. It was a manual system (one sample at a time) and it had a liquid nitrogen trap at the base of the apparatus that sat above the injector. The column was fed all the way up through the GC injector and then through the liquid N2 trap where it was connected to the transfer line via a compression-fitting union. Before desorbing the trap, you hit the liquid-N2 trap with coolant and then inject. Your volatiles were frozen in the trap and at the prescribed time, a ballistic heater was applied to the liquid-N2 trap to liberate the trapped analytes from the column. It worked pretty well. However, the new ones are nicer.

[rb6banjo]

Double post. See the next one below.

[rb6banjo]

We have a 3100 in our lab and the transfer line from the concentrator is spliced into the carrier gas line that feeds a split/splitless inlet. In our system, the standard inlet septum is what makes the pressure seal for the inlet. So, if the septum bleeds badly, it will show up as "ghost peaks" in our chromatography. Is that how your system is plumbed as well?

Was the last trace you posted run after it sat over night with the oven at near ambient temperature (40 °C or so) or did you elevate the oven temperature to "bake out" the system, then cool it before running what you posted?

Only the very old Tekmar systems had a "direct connect" to the GC column. It was a manual system (one sample at a time) and it had a liquid nitrogen trap at the base of the apparatus that sat above the injector. The column was fed all the way up through the GC injector and then through the liquid N2 trap where it was connected to the transfer line via a compression-fitting union. Before desorbing the trap, you hit the liquid-N2 trap with coolant and then inject. Your volatiles were frozen in the trap and at the prescribed time, a ballistic heater was applied to the liquid-N2 trap to liberate the trapped analytes from the column. It worked pretty well. However, the new ones are nicer.

[FIDOttawa]

Presumably the connection form P&T to column is via the GC inlet, because in your first post you wrote; "I have tried changing my liner, septum, .....". Most (all?) capillary GC inlets have a septum purge - check your operators manual for how to set it and where to measure the flow. If the septum purge is off or set too low you will see the symptoms that your system suffers from. NB that the septum purge and the split purge are two separate flows.

How did you check for leaks ?

Peter

Hi Peter,

This is what my manual states "Septum purge and split flow rates for capillary inlet
systems are measured at vents located on the front of the flow panel" I do not have this vent on my front panel. My samples go directly onto my column although I do have a septum and liner on my inlet. Also I checked for leaks with a Restek Leak Detector.

[FIDOttawa]

We have a 3100 in our lab and the transfer line from the concentrator is spliced into the carrier gas line that feeds a split/splitless inlet. In our system, the standard inlet septum is what makes the pressure seal for the inlet. So, if the septum bleeds badly, it will show up as "ghost peaks" in our chromatography. Is that how your system is plumbed as well?

Was the last trace you posted run after it sat over night with the oven at near ambient temperature (40 °C or so) or did you elevate the oven temperature to "bake out" the system, then cool it before running what you posted?

Only the very old Tekmar systems had a "direct connect" to the GC column. It was a manual system (one sample at a time) and it had a liquid nitrogen trap at the base of the apparatus that sat above the injector. The column was fed all the way up through the GC injector and then through the liquid N2 trap where it was connected to the transfer line via a compression-fitting union. Before desorbing the trap, you hit the liquid-N2 trap with coolant and then inject. Your volatiles were frozen in the trap and at the prescribed time, a ballistic heater was applied to the liquid-N2 trap to liberate the trapped analytes from the column. It worked pretty well. However, the new ones are nicer.

My transfer line is not spliced. My transfer line goes directly into my inlet and I have a seperate line for my carrier gas to the GC. You are correct on my septum being used as a pressure seal for my inlet. I will check and change it one more time to make sure that is not what's causing my trouble.

The last trace was run this morning after the GC sat at ambient (45 °C) over night. It was baked out yesterday before I left for the night.

I am not using this N2 that you are talking about but I am using a Tekmar that has my transfer line directly connected to my GC. Its a Tekmar 3000 that I have. My volatiles are back flushed onto my column via my carrier gas (He). But other than that may be very similar to this old system as its definately not a new one.

[rb6banjo]

Hummm? It must be that you have a packed-column inlet. This is what a split-splitless inlet on a 5890 should look like:



What brand of septum do you use? I use the Thermogreen LB-2 (Catalog number 20654) from Supelco and don't have too many bleed problems. I usually heat my systems first thing in the morning to get rid of the overnight accumulation of "stuff" but after that, I'm good while I'm running.

If you can, try a different brand of septum and see if the impurities pattern changes.

[FIDOttawa]

Hummm? It must be that you have a packed-column inlet. This is what a split-splitless inlet on a 5890 should look like:



What brand of septum do you use? I use the Thermogreen LB-2 (Catalog number 20654) from Supelco and don't have too many bleed problems. I usually heat my systems first thing in the morning to get rid of the overnight accumulation of "stuff" but after that, I'm good while I'm running.

If you can, try a different brand of septum and see if the impurities pattern changes.

Yes that is the type of inlet I have. We also have a 5890 split/splitless running for PCB's so I was able to look at the difference. I am using the same Septum as you as well. When you heat your system in the morning, what temperature do you set it to and for how long? Might be something I will try if I ever get it running properly again.

[rb6banjo]

So, your P&T transfer line goes into an inlet that is controlled like the one I photographed?

My typical methods have a stopping point of 240 °C. I generally run it up to that before I start for the day.

[FIDOttawa]

My tranfer line goes from my P&T directly into my inlet. I do have the column head pressure gauge and controller knob but not the 2 bottom controllers for total pressure and septum purge.

[rb6banjo]

It's likely a packed-column inlet. That shouldn't have an effect on the impurities.

What is your inlet temperature?

[Peter Apps]

Just to be sure - does your front panel look like the one that r6banjo posted or not ?

Hummm? It must be that you have a packed-column inlet. This is what a split-splitless inlet on a 5890 should look like:



What brand of septum do you use? I use the Thermogreen LB-2 (Catalog number 20654) from Supelco and don't have too many bleed problems. I usually heat my systems first thing in the morning to get rid of the overnight accumulation of "stuff" but after that, I'm good while I'm running.

If you can, try a different brand of septum and see if the impurities pattern changes.

Yes that is the type of inlet I have. We also have a 5890 split/splitless running for PCB's so I was able to look at the difference. I am using the same Septum as you as well. When you heat your system in the morning, what temperature do you set it to and for how long? Might be something I will try if I ever get it running properly again.

Because the picture shows a septum purge outlet, which earlier you said that you do not have ?????

ALL septa bleed, they bleed more when they are first installed, so by continually changing septa you are probably making the problem worse. The hotter you keep the inlet the more the septum will bleed. Having the inlet hot overnight with the column cool will bleed a lot of septum junk onto the top of the column.

Peter

[FIDOttawa]

I do have a packed column inlet. My inlet temperature is set to 240 degrees. My front panel does not look the same as the one r6banjo posted. I do not have a total flow or a septum purge knob on mine.

[Peter Apps]

I do have a packed column inlet. My inlet temperature is set to 240 degrees. My front panel does not look the same as the one r6banjo posted. I do not have a total flow or a septum purge knob on mine.

OK, let's go with that. You have a packed column inlet - are you using a packed column ?

What flow rates and pressures are you using for the carrier gas, and does the GC control the gas flow through the P&T, or does the P&T have its own gas controls ?. If the P&T has gas controls, do these control the carrier gas flow through the column, or is the GC adding gas flow at the inlet.

I ask this because if the P&T is supplying all the gas then the packed column inlet is acting only as a connector from the transfer line to the column - a connector with a large internal volume and a piece of hot rubber at one end. There are better ways of making that connection.

Peter

[rb6banjo]

If the inlet is acting as a large union anyway, and the carrier gas pressure is supplied by the P&T, I'd get a union and connect the transfer line directly to the column. Just heat the inlet enough to make your GC happy and keep from creating a cold spot.

That would get the rubber septum out of your flow path. That's how the old Tekmar systems worked so I know it can be done.

[Peter Apps]

r6banjo, you read my mind !

Peter