Chromatography Forum

Are there instances where it is better to not use glass wool? Unless it's headspace, I use glass wool all of the time.

Currently trying to complete the volatile organic impurities test on Alcohol (95%), USP. Resolution for Acetaldehyde and Methanol cannot be less than 1.5. Visually the two peaks are separated. However, the calulated result is 1.34 and when I blow up a print out of the 2 peaks and perform a manual measure, I get 1.33.

My problem is the asymmetry of the Methanol peak, it's over 2.0 and sticks out like a sore thumb. My column is the Restek Rxi-624MS-Sil: 30m x 0.32mm 1.8 µm film.

This column has been great and the test chromatogram that is included with the column shows a nice symetrical methanol peak along with everything else that was on the test injection. Everything I run on it seems to have better symmetry than when I used a different 624 column. So where is my problem.

I was able to correct some resolution issues by switching to a liner with a gooseneck at the bottom. However the peak shape of the Methanol didn't change drastically. I've varied my injection speed also with negligable impact. Currently it's 5µL/s. Past settings; 50, 25 10µl/s.

Detector: Flame ionization
Column: 0.32-mm × 30-m fused silica capillary column bonded with a 1.8-µm layer of phase G43
Split ratio: 20:1
Temperature
Detector: 280°C
Injector: 200°C
Column:
Initial Temperature: 40°C hold for 12 min
Temperature Ramp: 10°C/min
Final: 240°C for 10 min
Linear velocity: 35 cm/s
Carrier gas: Helium
Injection size: 1.0 µL

methanol often does not give a symmetrical peak unless a Wax or carbon support column is used.

Of course increasing your column temp might help a bit.

Good luck,

Rod

Hi

In addtion to Rods comments I would like to point you to chapter <621>, here you find that you actually hav e some room to play around with instrument settings.

If memory serves you might even be allowed to drop down in colum size ie a 20m 0,18mmid 1,0µm column.

The chapter allows you to start 10% up or down in start temp if recalling right and so forth, check it out.

Usually I am a bit suspicious about column manufacturing comparisons, but in tight cases like this, source of column might be an issue.

All is well, I increased my split from 20:1 to 40:1 and the Methanol symmetry improved dramatically.

Just re-reading appendix III of good ole Basic Gas Chromatography by McNair and Miller and there was a split ratio reference that lower splits are good for sensitivity, not so good on resolution.

That book should be one of the standards in any Lab.

Thanks guys for the help.

Chuck