Chromatography Forum

Hi all

I am using PE GC MS & head space for BTEX analysis. My standard contanins 15 components( benzene, ethyl benzene, toluene, o,m,p, Xylene, MTBE, n-pentane, n-hexane, n-octane, n-decane, n-dodecane, 1- methyl napthalene, napthalene, 1,2,3 trimethyl benzene.) My GC oven programing is like this. Initial 40 degree and a hold up time of 7 minutes then ramps at the rate of 10 degree/ minute to 240 degree. The problem is initial i am getting a very long solvent peak(methanol) and some of the early eluting peaks especially MTBE, benzene, hexane are coming along with it. Can i get some suggestions how to over come this problem please?

Thanks
nj

What column are you using, what is your matrix (water, soil ?), how are you adding standard to matrix ?

Peter

chemjeeorganic,

Peter is right, have to know what the column is. Also, flow rates, starting temperatures.... The more info the better.

Best regards.

Thanks.
I want to develope method for the said analysis. My colum is Perkin elmer Elite 5MS . The column length is 28 meters with a ID of 0.25 mm and film thickness 0.5. My column flow rate is -initially 2.8 ml with 0 hold and then to 2-0 ml for 0.7 mints and then to 1.0 ml holding for a period of 20.5 mints.Colum program is same as what i have wrote earlier.

Suggestions will be highly appreciated.
Regards
NJ

However surprising that was to me, my colleague suggested me to try with polar column (HP INNOWax). It actually separated VOCs much better than my HP-5ms (with polydimethylsiloxane columns I was never able to fully separate m- and p-xylene, and o-xylene was always overlapped with styrene)!

Hello Chemjeeorganic,

I would make you hold time of 7 minutes less, about 3 minute, that should be sufficent.
After 3 minutes open the split to vent the injector.
Then heat the oven.

Very complex flow program.
I would use 2 ml/min at the transfer than 1 ml/min through the rest of the prgram. otherwise you can't predict what happens with your compounds and the seperation.

chemjeeorganic,

I would look at a lower flow rate for that column (considering the id) and, if you have one available, I would look at a thicker film. As an alternative, you could also look at the volatiles columns, like a DB-VRX, RTX-VMS or SPB-624.

Best regards.

chemjeeorganic,

Sorry, one more comment. As noted by i.horsting, split vent time will have a role in the peak shape. If you have the sensitivity, you probably would be better off running a slight split rate (2:1 or 4:1 for example) rather than splitless/split.

Best regards.

How much spike are you adding to your matrix ?
We add 10ul of spike to 10ml of water (20ml headspace vial) to limit the amount of methanol we are adding.

Rich

Hi
I analize BTEX by HS-GC. 3 years ago i used a wax column. It separated o, m and p-xylene, but benzene eluted very near the MeOH.
Now i used a DB-624 (30m, 0.32mm, 1.8um). m and p-xylene coelutes but benzene elute at 9 min (with a similar program of temperature) . The MeOH is not a problem (I'm interested in benzene)

Many thanks for all the suggestions. Now another problem has encountered. I am getting split peaks for 1.3,5 trimethyl benzene and for napthalene.

Thanks
jn