Recently, we bought a GC valve system using Porapak Q column. We have std gas consist of 10 ppm 1,3-butadiene and 100 ppm methyl chloride. We have 2 peaks detected, where 1st peak is much smaller than 2nd peak. The question is:
Can we assume that the 1st peak is 1,3-butadiene and while the 2nd peak is methyl chloride if Porapak Q pack column is been used?? However, the bp for 1,3-butadiene is -4.4cel while methyl chloride is -99cel. But since methyl chloride is more polar (more interaction with Porapak Q)than 1,3-butadiene, will the methyl chloride elute later than 1,3-butadiene ??We do not have individual gas.Therefore, we would like to seek advise from anyone who have experience with gases stated here. Please assist me. Thank you very much.
By Anonymous on Friday, March 28, 2003 - 06:34 am:
As you have infered, the easiest way and smartest thing (in any situation like this) would be to obtain and inject the two analytes separately.
By Rodney on Friday, March 28, 2003 - 08:31 am:
Yes, inject them both if you have them separately.
The primary separation mechanism of 'Q' is molecular size and shape, not boiling point or polarity. For Example (1,2,3,4):
Diethyl Ether, Pentane, Methylene Chloride, and Acetone:
bp 34.5°C, 36°C, 40°C, and 56.5°C
elution order is the reverse of boiling point:
4, 3, 2, 1: 0.89, 0.98, 1.24, and 1.48 min.
on a Porapak Q 80/100 mesh 1 meter X 2.3mm ID 25cc/min Nitrogen
Note however, that C5 and ether are nearly the same molecular size, the polarity of ether does retard it after pentane.
Senior Research and Development Scientist
Gas Separations Research
595 North Harrison Road
Bellefonte, PA 16823
By jos vloet on Monday, March 31, 2003 - 05:46 am:
I have data of methyl chlorid and propylene and propane.
Methyl chloride elutes first, so you can assume that butadiene elutes later.
By Ralph Calvert on Monday, March 31, 2003 - 08:05 am:
First, the relative peak sizes will depend on the detector e.g. FID or TCD.
Second, the relative retentions on Por Q are as follows:
methyl chloride 3.2
methylene chloride 19
I knew those old tables would come in handy one day!
hope that helps
By Anonymous on Tuesday, April 1, 2003 - 06:27 am:
The last time I measured the specific FID mass response of methyl chloride relative to C4 hydrocarbons it was about 0.28, which does not quite square with you seeing all of both components. If you are using FID and you do mean areas, your 100 ppm methyl chloride peak should still be bigger than the 10 ppm butadiene peak.