when to change the capillary GC column?

[wl_yang]

Hi there,

I found some siloxance peaks even for hexane blank during the GC/MS run. I baked the column, changed the liner and septum. The peaks are still there. A professor told me those peaks indicate the column breakdown. I am wondering if it is the time to change the column? What are the criteria when you decide to change the column?

BTW, I still find the responses of my target compounds are reducing (The MS part is fine, I believe, based on the tuning results). Is this related to the brokendown column?

Thanks a lot.

[JI2002]

Not sure what causes the siloxane peaks, but it's not caused by stationary phase breakdown. Elevated baseline is the indication of stationary phase breakdown. Siloxane peaks like D3, D4, D5 are caused by contamination. Make a run without injection. If the peaks are not there, then probably hexane is contaminated. If you still see the peaks, double check injector port, carrier gas lines.

Reduction of target compound response can be caused by many factors. First thing I would try is to switch to 2nd filament and see if it makes a difference.

[Peter Apps]

Unless the siloxane peaks elute at the same time as your analytes they are not a problem in themselves. They can come from a variety of sources besides the column; deactivated glass wool in the inlet, septum bleed, use of silicone rubber tube in sample preparation to name just a few.

Column breakdown nearly always causes deterioration of peak shapes; they will become broader and will have tails and their areas might also be smaller (these symptoms can also be caused by contaminants in the inlet).

The time to replace a column is when you can no longer get the analytical performance you need, and when you have eliminated the inlet as the possible cuase of the problem.

Baking a deteriorated column hardly ever (in fact never in my experience) does any good at all. As long as the stationary phase is bonded it is better to rinse it with solvent. The column manufacturers have instructions for how to do this, and also good tips on troublshooting on the websites or in their catalogues.

Good luck Peter

[Earl]

Siloxane peaks can be a sign of column breakdown, but elevated baseline at the end of the chromatogram is more common than actual peaks. If the ions are 73 and 207 then the siloxane is your column breaking down. If the peaks are large or you need to have a peak free blank shot for your analysis, try slicing a meter or two off the front of the column and shoot the blank again. If the peaks do not interfere with analysis and separations of analytes are still satisfactory you can get a little more life time from your column. But it may be close to time to order a new one.

[james little]

We keep an eye on the column with a grob mix containing some methyl esters of fatty acid, a biscyclohexyal amine, an acid, a phenol, some hydrocarbons, an alcohol, a primary aromatic amine, an aliphatic carboxylic acid, etc.

e.g. see

http://www.quadrexcorp.com/new/grobtest.htm

https://www.macherey-nagel.com/web/MN-W ... ent&Click=

I found these and others by doing google search for grob test mixture for GC.

Even when the column fails for one class of compound, we often still use the column if not interested in that class of compound. We use on GC-MS so gives us an idea of how our GC and our MS are working..