Paul’s excellent questions on “Liner Selection for HS VOCs”

I had a feeling the blog I posted yesterday was sure to prompt some thought-provoking questions, as some of my peers had already been doing so. So, it came as no surprise that Paul posted the following excellent questions (in black) to which I have responded to in blue. Normally, I would just address all this in the comments section, but in my opinion the comments section tends to get lost in the weeds. In addition, the questions and my responses were all so long it justified the following blog, so I will respond to the following questions from Paul below.

Paul says:

First thanks for this blog series. I like fundamentals. I read the linked article by Jason S. Herrington and got a few questions about it.

Thank you for the thoughtful questions.

1. Why is there a difference in peak area between the 0,75 mm and 2 mm liner? The Peak width should be wider for the 2 mm liner but the Peak area should be the same. (Degradation or lost in the split?)

My alternative theory is that the 0.75 mm liner fits the fiber like a glove. So, thermal transfer to the fiber needle, fused silica, and most importantly phase is more efficient. Add in the increased velocities of the 0.75 mm liner and now we have further pushed the partitioning equilibrium in favor of desorption. This all translates into faster and more complete partitioning of VOCs out of the fiber. Honestly, although never explicitly stated, I thought this was all the “logic” behind other vendors pushing the 0.75 mm liners for SPME. But then again, I say “logic” because I certainly do not see the data to support any of the above, so it appears to be based on what I shall call logical theory. Of course, maybe it was all just in my head in the first place. Bottom line, the lack of data surrounding the subject (i.e., SPME liner dimensions) was the impetus for me to collect this data in the first place.

2. With a split of 1:5 the velocity in the inlet is much higher than with splitless injection. I can imagine with the higher velocity in the liner because of the split, the liner diameter is not that important any more.

My one colleague has been hounding me about this very point. Obviously, you are both correct about the split minimizing the significance of liner dimensions. Confession: in my previous blog, I made a serious mistake by not justifying the 5:1 split due to the chromatography looking so horrible without the split. TEASER: I ran the same type of experiments in splitless, and initial review of the data continues to say that liner dimensions do not make a Tinkers Dam for HS-SPME VOCs. Future blog to follow…

3. Why is the % RSD of the 2 mm liner seams to be better than for the 0,75 mm liner?

Ah, you saw this too!? I am not entirely sure this is a consistent trend. Perhaps we will know further in the future with a more substantial data set. With that said, I will pose the following theory to explain this observation: as stated above, the 0.75 mm liner fits around the SPME fiber like a glove. Well this means that the SPME fiber has occupied a lot of the available real estate in the liner. This could translate into one or both of the following phenomenon taking place, which may explain the observation in question:

  1. Flow has been constricted/obstructed in the 0.75 mm fiber and this impacts the split, thereby causing some inconsistencies. For the record, this may be a contributing theory to help explain the discrepancy we addressed in the first question.
  2. The SPME fiber “clogging” up the 0.75 mm liner results in turbulent flow, which also causes some inconsistencies.

Everything here is theoretical and will be hard to isolate. Is it turbulent flow, higher velocities, better thermal transfer, inaccurate splits, residence time, etc… the list goes on ad infinitum. I say it is probably the culmination of all of the above, but doubt we will truly ever know. However, it is certainly fun to hypothesize and debate some of the theories.

4. Which role does the distance of the column to the liner Play in this case? (keyword tapered liner)

I have only been running straight-walled liners and have not evaluated column distance.

I hope the community got some ideas.

I hope so too, as the data surrounding SPME liners appears to be hidden. Thanks again for your questions.

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