Chromatography Forum

I am new to gas chromatography. I'm having trouble holding a calibration and keeping endrin breakdown below my requirement. Some feedback on ideal parameters and gc setup would be greatly appreciated. I am running a 6890 GC in splitess mode injecting 1 microliter. Ports are set at 220 C. Gold seals and septas have been replaced. Column is frequently clipped and liners changed almost after every run with liners that have been cleaned and deactivated in house. The ports are also cooled and cleaned with solvent during routine maintenance. What am I missing?

I'm not sure about endrin, but I used to monitor DDT breakdown for 8270. I found that even most store-bought GC liners (with wool) would not perform well enough to pass all the time. I needed the wool for transfer and sensitivity of high BP compounds like PAHs etc., and I found that Restek's Siltek coated SVOC liners worked really really well. They now have liners called SkyBlue. They market that they are even more inert than the Siltek ones as I think the wool is deactivated with the liner in one process. If you are using wool I would try some of these liners. They are pricey but it may make the difference. If you are not using wool in the liner you may still want to try these liners w/o the wool as their deactivation process is very repeatable (I can't say the same about when I do it in the lab as I'm no good at it).

Ty

I doubt that solvent cleanning alone will reduce residue in the port. Agilent suggests "gun brushing" ( check their website for instructions). I have had good results from it. My boss however is not a fan so has discouraged the practise. I now have a few ports on hand and change when I used to gun brush. While the port is out I clean them with the alumina powder ( like the source in MS) . I have had good results with this.
I also use a Restek liner ( drilled uniliner w/hole@ top part # 21055). I inject 1ul ethyl acetate pulsed splitless @ 50 psi.
If you have been using the system for a while don't forget the split vent trap and copper pipe ( change).

OH Yeah! I forgot about the split vent line! Bigbear is right. I have seen awful things in the split vent line and trap. For me the traps always seemed clean but the copper tube connecting to the injection port would eventually build up with what looked like tar. Once properly cleaned or replaced my peak responses would come back and degradation would be reduced. This is especially crucial for Splitless injections.

Ty

I am puzzled by how crud on the metal parts of the liner (which is removed by gun brushing) or in the split line can reduce peak area or spoil peak shape - surely the whole idea of a glass-lined inlet is that the analytes go onto the column without ever having to touch metal, and certainly not the inlet side walls or the transfer line. I can see that a blocked split vent line might increase peak area and cause tailing by stopping the split from opening, but the Endrin and DDT breakdown tests look for reduced peak areas.

Peter

It would seem to be that way, especially in my case using a direct connect liner, but with the split vent solenod closed and flow into the injector there must be some recirculation of analyites in the port. I've seen it myself, change liner to reduce breakdown ( which has very little effect) then clean the port and get almost now breakdown.

I've seen it in the past where a inexperienced organic chem grad student backflashed the injection port and irreversibly contaminated the injection port. Long story short we ended up replacing the whole port with a Restek Siltek coated injection port. We finally regained sensitivity that was lost. The condition of the metal in the port does indeed make a difference (for some analytes).

I've seen it in the past where a inexperienced organic chem grad student backflashed the injection port and irreversibly contaminated the injection port. Long story short we ended up replacing the whole port with a Restek Siltek coated injection port. We finally regained sensitivity that was lost. The condition of the metal in the port does indeed make a difference (for some analytes).

Hi Ty

I am not disputing that it makes a difference - I am just puzzled that it can make a difference. It implies that enough of the sample to significantly affect peak area and shape is escaping from the liner, diffusing into contat with the metal, and then getting back into the sample stream to put a tail onto the peak. This is surely not the picture that I have in my head of how the inlets are supposed to be operating. Similarly, in theory, Silteking the inlet body cannot make a difference (but it does) because the sample is never supposed to touch the metal.

Peter

Hi Peter,

Sorry for any confusion. It wasn't my intention to dispute anything you had said earlier. I actually agree with you 100%. I was just simply sharing an experience I have encountered in the past.

In my case injecting 1 ul ethyl acetete ( not the cleanest of solvents but mandated for 525) its the solvent crud buildup that causes my endrin breakdown. My peak shapes are fine, just in the case of endrin not big enough!

Hi Peter,

Sorry for any confusion. It wasn't my intention to dispute anything you had said earlier. I actually agree with you 100%. I was just simply sharing an experience I have encountered in the past.

I was hoping that an inlet expert would pop up and explain it to us. What I do note is that Agilent is the only maker that specifically recommends (condones ??) gun brushing. Bruker inherited a specific hot bakeout protocol for the 1079 inlet (and others ?), what do Shimadzu, PE, Gerstel and Thermo recommend ? Agilent also has an unuusally short insertion distance for the column- leaving the column tip close to the metal at the bottom of the inlet.

I begin to think that we need a Uniliner with a very narrow hole at the top - the column is sealed to the liner at the bottom, so no metal contact there, and a narrow hole at the top will give fast linear gas flows that should restrict back diffusion of sample (I am assuming that there are no pressure surges, which might be the underlying cause of the problem I suppose - so PTV should also help).

Peter
Peter

The 6890s onwards have terrible issue with analytes flashing back into the upstream plumbing. Our crew ended up with a 50 psi pulse onto a megabore column - this basically loses a lot of resolution but minimizes residence in that horribly active inlet. An inlet temp of 190 to 200 can help. Still a mess really - have never been happy with that solution.

I think the drilled uniliners Bear mentions are an excellent solution for the other side of the inlet - they are essentially a return to the old packed column-megabore conversion. They seem to have come down in price a bit as well. My understanding is they are somewhat expensive to use, since they only seal to the column once and cannot be reused - is that your experience, Big Bear?

My understanding is they are somewhat expensive to use, since they only seal to the column once and cannot be reused - is that your experience, Big Bear?

The taper at the bottom of a Uniliner is steeper than in a Presstight connector that is designed to make a permanent seal (although I have reused Siltek Presstights multiple times), and so the liner-column seal can be redone several times. As with the Presstights, the tip of the column has to be sqarely and cleanly cut.

Peter

You are correct Yama001, I can't re use my liners. They sometimes come out with a little bit of column attached. A couple of Agilent engineers suggested that I don't seal the column to the liner ( push the column up till feeling resistance then pulling it back down a smidge). This did not work well for me so I direct connect per Restek's instructions.

I am as confused as Peter, but all I can add is what I have seen in the past. Even though I use a direct liner, 50 psi pulse and 1 ul volume, "crud" still plates to the metal of the injector. The only way to rid it is to abrasively clean.

Excessive Endrin breakdown can be difficult to fix but I can make a couple of suggestions that might make the difference. First, you should be using a siltek coated liner. I've also found that I get a lower endrin breakdown when I use a double gooseneck splitless liner. Specifically, I have found that the Restek liner Cat #20784 with a siltek coating works the best. Their new Sky blue liners were a big dissapointment to me with regards to Endrin and DDT breakdown.

Also, I have found that a Restek Siltek treated inlet seal will make a significant difference in breakdown when compared to a gold seal.

hope this helps