Chromatography challenge: Does a GC-capillary column produce different retention times when installed in the opposite direction? Share your opinion!

A little challenge for our regular blog-visitors..

Capillary chromatography has become quite an adult science, but we still discover new and better ways to do better chromatography and generate more accurate data.  Besides that GC columns get very inert, have low bleed and are usable up to high temperatures, also the design of columns has changed.

With “design” I mean how columns are physically made. By using “segment” coating we can deposit stationary phase in different positions of the capillary. Instead of having a long, uniform coated capillary, we can make now an Integrated Guards or Integrated Retention-gaps, all in one piece of capillary.  So,  new generation columns often have a “front” and a “back” side..  Now an interesting discussion starts:  “what can I expect to happen if I install my column in the “wrong” way?”..

The challenge is here:

We have a 40m x 0.25mm Rxi-5Sil MS capillary, where we have an integrated guard column of 10 meter. The coated analytical column is effectively 30 meter long. The 10m guard section is only “deactivated” and the retention will be minimal.

The same column, used in 2 different directions: A: with guard section in FRONT; B: with Guard section at the BACK

We position the column in our GC first in position A, see figure. Here the guard is direct connected at the inlet. The column is heated to test temperature and we check the linear gas velocity by injecting methane; Then we run a test mixture and we measure the retention times for all components.

Now we change column position into position B. The Guard section is now directly connected to the detector. We use exactly the same column and do exactly the same exercise as above.

The Question is: What will happen with the retention times for all components?

Note:

• We use the same column, so the length, ID and amount of stationary phase is exactly the same;
• We test at the same temperature and use the same inlet pressure;

Our Options:

 

1:         The components in situation “B” elute at exactly the same retention times a situation “A”;

2:         The components in situation ”B” elute FASTER, so it seems the column has less retention;

3:         The components in situation “B” elute LATER. So it seems the column has more retention;

Please sent your answers.  Just state which option you choose.

We list the top 25 correct answers when we get back to this discussion in about 4 weeks;