Comments on Off-Duty Columns by John V.Hinshaw, LC-GC Europe, nov.2000

Chromatography Forum: GC Archives: Comments on Off-Duty Columns by John V.Hinshaw, LC-GC Europe, nov.2000
Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Tuesday, December 5, 2000 - 03:36 am:

In the article "Off-Duty Columns" I have a comment.
See p 816 of LC.GC Europe, November 2000 in Storing columns:

"....... First, cool the instrument but leave the carrier-gas flowing. Next, remove the column from the detector and carefully insert the free end into a septum. Then, remove the inlet end of the column and insert it into the same septum. Residual carrier-gas pressure will maintain a positive flow out of the inlet of the column long enough to place it in the septum.
Never store columns with one or both ends open because columns will trap atmospheric contaminants very efficiently, creating a column contamination problem upon subsequent use".

I agree with this.

Mij comment: if carrier-gas is Helium this technique is not usable.
If the He-filled column is flame-sealed at both ends then, within 7 hours, a total vacuum has been created in the column. When sealed with a septum the column will trap atmospheric contaminants and air.

The pores of the fused silica wall are in the order of 2.3 ; Helium has a "volume" of about 1,9 which means that He can penetrate quickly through these pores.
On the otherhand, Hydrogen-gas H2, can also penetrate through the fused silica wall at a higher temperature. (Att room temperatures this will occur very slowly.
At 100C after 24 hours round 60% of the hydrogen penetrates out of the column giving a vacuum and, if the ends are not flame-sealed, atmospheric contaminants will be trapped in the columns.

Thank you,
jan b.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By H W Mueller on Tuesday, December 5, 2000 - 11:49 pm:

It would be nice to get some references to the original literature (its difficult to find or nonexistent) on matters as discussed above, so that one can judge what is correct or what is conjecture. I have always heat-sealed columns for storage,as a precaution, but I am beginning to wonder. For instance, is it assumed or known that a vacuum forms (the polymer cover has no effect? How about aluminum covered columns?)? If there is a vacuum one will fill the column immediatly with air and "dirt", therein, on reopening (or worse: If there is a small fault somewhere). Are there any careful studies that indicate the auto-oxidation rate of different phases, or whether adsobed (absorbed)"dirt" is so tenacious as to bother analyses over a long period?


Top of pagePrevious messageNext messageBottom of pageLink to this message  By John Hinshaw on Friday, December 8, 2000 - 06:15 pm:

Dear Jan,

You are quite correct about He diffusing through fused silica. See J.E. Cahill and D.H. Tracy, "Effects of Permeation of Helium through the Walls of Fused Silica Capillary GC Columns," J. High Resol. Chromatogr. 21 (10), 531-539 (1998). The authors describe observing lower-than-expected average carrier gas velocites for helium at temperatures above 200 C, attributable to diffusion through the column walls. Nitrogen did not exhibit such losses, experimentally. The authors cite a diffusion rate for hydrogen that is about 10x less than helium. However, helium losses are generally less than 15% under conditions usually selected for capillary column GC.
The article titled "Off-Duty Columns" originally appeared in LC/GC North America 18 (2), 134-140 (2000). The diffusion rate for helium at room temperature is about 35x less than at 250 C, according to the data presented in the Cahill article, and it does seem reasonable that most of the helium in a sealed column will diffuse out in less than one day.
However, the cautionary statement I made concerning contamination from the atmosphere is related to leaving one or both column ends open. In this case, atmospheric contaminants will collect continuously at the open end(s) of the column when they diffuse a short distance into the column and become trapped in the stationary phase. This adsorption process produces an effective positive flow of atmosphere into the column that is essentially constant, and is limited by the diffusion rate of the contaminants into the column cross section. I didn't calculate it, but I would guess it's on the order of tenths of a milliliter per minute.
With both column ends sealed, the diffusion of atmospheric contaminants into the column ends will be essentially nil. The volume of air that could be sucked into the column through a less than perfect septum seal or upon opening the column for installation is equal to the column volume; a few milliliters at most. There would seem to be no difference between flame sealing and a good septum seal on this basis.
After helium has stopped diffusing from the column, any additional contamination incursion will be limited to the diffusion of contaminants through the septum material and into the column entrances, which is again essentially zero because the contaminants will be trapped in the silicone septum material. Diffusion through the essentially zero cross-section presented by the septum-sealed column ends to the atmosphere will be negligable.
There is a difference of several orders of magnitude between leaving the ends open and permitting contaminants from many liters of atmosphere accumulated over a column storage time of days or weeks to settle into the stationary phase, versus allowing one column volume's worth of atmosphere into the column upon opening it after sealing with helium inside. Such minor incursions are removed upon first conditioning the column.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By John Hinshaw on Tuesday, December 19, 2000 - 08:39 pm:

Jan,

I'm going to put parts of this exchange in LC/GC Magazine this Spring. I'd like to be able to give you credit for your interesting comment, but cannot do so without your full name, location, and affiliation. Please send an email to me at jhinshaw@chromsource.com with that information and any additional comments you might have.

Thanks,

John Hinshaw


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