Chromatography Forum

Hi,
I have noticed some changes during last analyses. I want to quantify BTEX, but with benzene there is coelution of something that (I think) is present in n-hexane (before and also after extraction - used to microextraction of BTEX).

I use GC-MS 6890, HP-5MS column (30m, 250 um, 0,25 um); flow during analysis 0,7 ml/min (He), splitless mode, front inlet - initial temperature 250°C, 1 rate 6 min 80, run time 17,50 min.

Calibration curves of xylenes and styren are ok, toluene is worse, benzene is horrible (unable to quantify with or without extraction process there are some peaks with large area), there is such low sensitivity and I think that peak in RT of benzene is something coelutioning from n-hexane.

Please, do you have any suggestion what to try to separate impurities and benzene - what conditions do I should try with HP-5MS column? With described method (below) sometimes it was possible to determinate benzene and also toluen without any problems.

Thank a lot.

Run the n-hexane by itself and see if you have co-elutions with the benzene. If you have access to a more polar column, the aromatics will shift then to a later retention.

How much are you injecting? If you dilute the benzene or if you shoot a small amount, how does the benzene peak look? You can 'needle flash' it (fill syringe, then empty it completely, then just insert it into the inlet and press START).

You can slow the chromatography down. Start at 50 instead of 80 and use a oven ramp.

Have you looked at reference methods (EPA, ASTM etc) for BTEX? This is a very common analysis.

If looking for benzene I would definitely start at a lower oven temperature. 50C is the highest starting point, 35-40c would be even better to separate the lighter hydrocarbons from benzene. Using a 624 phase column with thicker film can help with separation of the lighter analytes also.

You'll have an easier time eluting benzene using a db-624 column. I ran a EPA based method using a DB-624, 30m x 0.32mm x 1.6 (ish)um column. Benzene only then coelutes with deuterated benzene. We didn't worry about that as we used an MSD. Just use another internal and you're good to go.

Yes, I have coelution, it is high tailing peak. I have automatic injection 10 ul. If I shoot a small amount, there is only little (almost any) difference between that peak area and peak in chromatogram when hexane run by itself.

I apologize, I hadnt described my temeperature program right. I use initial 35 hold 10 min and then 6°C/min to 80°C (Run time 17,5min).

Run the n-hexane by itself and see if you have co-elutions with the benzene. If you have access to a more polar column, the aromatics will shift then to a later retention.

How much are you injecting? If you dilute the benzene or if you shoot a small amount, how does the benzene peak look? You can 'needle flash' it (fill syringe, then empty it completely, then just insert it into the inlet and press START).

You can slow the chromatography down. Start at 50 instead of 80 and use a oven ramp.

Have you looked at reference methods (EPA, ASTM etc) for BTEX? This is a very common analysis.

thx, yop, I use 35°C hold 10 min, then 6°C/min to 80°C.

If looking for benzene I would definitely start at a lower oven temperature. 50C is the highest starting point, 35-40c would be even better to separate the lighter hydrocarbons from benzene. Using a 624 phase column with thicker film can help with separation of the lighter analytes also.

yes, I try to find out whats wrong. What to do with instruments I have.
Yes, internal standard could be a solution, but it is strange that with so fascinating technique like GC-MS is, suddenly, it doesnt work (in this case) like it used to and I dont know to solve it. But I believe that there is a solution!

You'll have an easier time eluting benzene using a db-624 column. I ran a EPA based method using a DB-624, 30m x 0.32mm x 1.6 (ish)um column. Benzene only then coelutes with deuterated benzene. We didn't worry about that as we used an MSD. Just use another internal and you're good to go.

[quote="Laura._"]Yes, I have coelution, it is high tailing peak. I have automatic injection 10 ul. If I shoot a small amount, there is only little (almost any) difference between that peak area and peak in chromatogram when hexane run by itself.

I apologize, I hadnt described my temeperature program right. I use initial 35 hold 10 min and then 6°C/min to 80°C (Run time 17,5min).


Injecting 10ul of hexane on such a small column could be causing the problem with benzene and toluene because the large amount of hexane could be spreading those analytes out on the head of the column. To make a large injection work you would need a few meters of guard column before the analytical column, with a nonpolar deactivation layer to help focus such a large injection volume.

The items linked in this blog are great to explain what happens when you inject a large injection volume, usually more than about 3ul.

https://blog.restek.com/large-volume-sp ... out-a-ptv/

sorry, 0 was mistake... only 1 ul is injected so problem shloud not be in injection volume

Yes, I have coelution, it is high tailing peak. I have automatic injection 10 ul. If I shoot a small amount, there is only little (almost any) difference between that peak area and peak in chromatogram when hexane run by itself.

I apologize, I hadnt described my temeperature program right. I use initial 35 hold 10 min and then 6°C/min to 80°C (Run time 17,5min).


Injecting 10ul of hexane on such a small column could be causing the problem with benzene and toluene because the large amount of hexane could be spreading those analytes out on the head of the column. To make a large injection work you would need a few meters of guard column before the analytical column, with a nonpolar deactivation layer to help focus such a large injection volume.

The items linked in this blog are great to explain what happens when you inject a large injection volume, usually more than about 3ul.

https://blog.restek.com/large-volume-sp ... out-a-ptv/

Carbowax or TCEP?

Best regards,

AICMM