BTEX ANALYSIS

[chemjeeorganic]

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

[Peter Apps]

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

Peter

[AICMM]

chemjeeorganic,

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

Best regards.

[chemjeeorganic]

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

[orcicdejan]

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)!

[i.horsting]

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.

[AICMM]

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.

[AICMM]

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.

[Spuzzin]

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

[fsistere]

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)

[chemjeeorganic]

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