Retention Time shifting earlier, larger peak area

[gcuser1]

I am running an agilent 6890 on ramped temp program, under constant pressure. The retention time of my peak of interest shifts earlier and earlier with subsequent injections. After inspection, it was discovered that the wool inside liner had shifted all the way to the top of the liner. Switched to a new liner with the wool positioned to the middle. There still seemed to be a shift however, sporatic and not earlier with each subsequent injection, rather sometimes earlier sometimes later. But still the correlation of larger peak area with earlier elution. Later switched to a completely different liner which had a top and bottom taper surrounding wool to prevent movement. After that, the run was much more consistent, however, still same issue of earlier elution times. The shift is very very slight, however. It is not noticed much in six replicate injections, but will be noticable after 20 injections, and much more noticable from one day to the next. It is assumed that the wool is still moving within its confined space, however, less movement than before because of the restrictions from top/bottom taper. A previous analyst did not encounter this problem with the regular liner with wool and no tapers. The other analyst noted a different instrument in the lab was used. That instrument was a liquid only injector on the inlet. The current instrument is hooked to both a liquid injector as well as a headspace loop on same inlet. Is it possible this type of connection is creating back pressure? I want to try using a liner without glass wool to see if the shifting persists, but my supervisor doesn't seem to think the liner is the issue. What are your thoughts?

[Steve Reimer]

When I have peaks, especially early eluting peaks, shifting later and diminishing in area I start by trying to find the inlet leak. Cycling the oven temperature makes it worse, changing the liner tightens it up.

[Peter Apps]

When you inject volatile solvent into a hot inlet you get a pressure pulse. With the large dead volume of a headspacer's plumbing in your carrier gas supply this might cause enough backflow to shift a glass wool plug (in which case it should also give erratic peak areas, how is your area repeatability ?). Try taking the headspacer out of the flow path by connecting the carrier supply directly to the EFC.

If you need a liner with a evaporation surface that absolutely cannot shift then go for a fritted liner.

There is not much else to add without knowing what the compond of interst is and the analytical conditions.

Peter

[gcuser1]

Inj# - Retention time - Area
1 - 9.563 - 15.2801
2 - 9.533 - 15.4151
3 - 9.504 - 15.4863
4 - 9.473 - 15.5962
5 - 9.449 - 15.6343
6 - 9.423 - 15.7000
11 - 9.303 - 16.0964

The above were results with straight liner with wool in center, no tapers. After run, inspected and found wool moved to top of liner. Conditions: Inlet 250C, FID 300C. Split 25:1. 9.6psi constant pressure, Helium. Phenomenex ZB-1 30mx.32mmx1.0um. 1uL injection. Temp: 150C, ramp 10C/min to 280C, hold 280C for 12min.

Later changed to liner that had baffles just above and below wool to hold in position. Similar results obtained with peak eluting earlier with subsequent injections and with higher area counts, however, the results were much tighter

I do not have the priveleges to remove the headspace. With the above info, do you believe that the liner is the issue, aside from the headspace causing backflow?

[Peter Apps]

"There is not much else to add without knowing what the compound of interest is and the analytical conditions."

To quote one of our established experts: "the more you don't tell us, the more we can't help you".

Peter

[dlbenach]

I'm curious about how the system is plumbed. You mentioned that a headspace (HS) loop is also connected to the inlet. Can you give some more detail. On our systems with HS, the carrier gas goes to the HS first and then back to the GC via the HS transfer line. Is your situation the same except that you also have a syringe tower above the inlet to do direct injection? If this is the case, can you plumb your carrier gas directly to the inlet bypassing the HS? Maybe something with the HS is causing the problem.

[AICMM]

Wool should make very little difference in retention times.

Balance gas can make a significant difference in retention times but not variable. Besides, sounds like (but cannot really tell) you are doing a liquid injection.

Moving forward rules out a leak. Therefore, I would argue either an overloaded column (hard to believe at 25:1 with 1 micron film) or you are reacting with column phase and stripping a bit with every injection. Then sitting allows it to passivate. Hard to tell more without more info.

Best regards,

AICMM