Chromatography Forum

Hi!

I want to find the "volatile organic compounds" with my gcms. My method is sth like that;

oven program: 35°C for 10 minutes
flow: 0.5 mL/min
split: 20/1 split flow: 10mL/min
inlet temperature: 125 °C
inlet: agilent 5190-2195 universal, low pressure drop
column: hp-5ms ultra inert

My solvent and wash solution is methanol.

What i am looking for is; benzen, carbon disulfide, chloroform, diklorometan, ethanol, ethyl acetate, IPA, n-heptane, n-hexane, tetra hydro furan.

But i have a problem. My peaks are nested. How can i get seperate and net peaks?

I would say that you need a different column for your separation. If you can get to subambient oven temperature, you might do a little better. You might also try a thicker film than what you're using right now.

Your analytes are all listed in this one (obtained from http://www.restek.com). Try these conditions:

https://1drv.ms/b/s!AkH-uI0tnY5LdikM7GBX1kX2o9U

At least some of your peaks are broadened by overloading. Increase your split flow to 100 ml/min.

What is the internal diameter of your column ? - your flow rate is unusually slow for the usual column sizes.

Peter

0.5ml/min isn't too low if you are using a 0.18mm ID column, but the slow flow will broaden the peaks even with that column. I normally run my 0.18 and 0.25 at 0.8 and 1.0ml/min respectively.

For those compounds try the Restek Rxi-624SilMS column in the 20mx0.18x1um film, or the Rtx-502.2 40mx0.18mmx1um film. These work great for volatiles analysis and you can run a high split flow also. The 5 phase with thin film just doesn't work well for such light compounds without sub ambient oven temps.

Your analytes are all listed in this one (obtained from http://www.restek.com). Try these conditions:

https://1drv.ms/b/s!AkH-uI0tnY5LdikM7GBX1kX2o9U

I will try this conditions and write here again.

At least some of your peaks are broadened by overloading. Increase your split flow to 100 ml/min. What is the internal diameter of your column ? - your flow rate is unusually slow for the usual column sizes.

I can try increase my split flow to 100 ml/min or try the splitless mod. Which will be more effective?

My column HP-5MS is length 30 m - Diam. 0.250 mm - Film 0.25 µm
Temperature limits from -60 °C to 325 °C (350 °C)

0.5ml/min isn't too low if you are using a 0.18mm ID column, but the slow flow will broaden the peaks even with that column. I normally run my 0.18 and 0.25 at 0.8 and 1.0ml/min respectively.

First i want to try this column for this analysis but i can't do that i will buy a new one that you said.

I can try increase my split flow to 100 ml/min or try the splitless mod. Which will be more effective?

When someone tells you your column is overloaded then stay away from splitless mode.

My column HP-5MS is length 30 m - Diam. 0.250 mm - Film 0.25 µm

Use column with thicker film

You need to do three things:

Increase the flow rate to 2 ml/min (35cm/s linear velocity)

Dilute your samples x 10

Increase your split flow to 200 ml/min with the new column flow.

Peter

DB5 is not a great residual solvents column. Most recommendations are for 624 or specialized columns.

Usually residual solvents are done on a thicker phase column 1um or so.

You are overloading the column so increase the split ratio.

Use a column flow rate in the ideal portion of the van deempter curve (are you using He or H2?).

...
oven program: 35°C for 10 minutes
...
What i am looking for is; benzen, carbon disulfide, chloroform, diklorometan, ethanol, ethyl acetate, IPA, n-heptane, n-hexane, tetra hydro furan.
...

I doubt you will succeed with this isothermal program.

Hi!

I changed my column. My new column is;

DB - VRX
Length - 60m / Diam - 0.250 mm / Film - 1.40 um
Temperature limit
From -10 to 260

My oven programme is:
60C 2 minutes
15C increase to 180C 0 minutes
45C increse to 225C 5 minutes

Flow 1.3

The peaks that my looking for are coming first 4.7 minute.

My solvent is methanol and blank chromatogram something like that:



On the 4th minute coming a strange peak, before the first my peak Etanol.
How can i clean the peak or how can i develop the method?

Hello

I changed my column. My new column is;

DB - VRX
Length - 60m / Diam - 0.250 mm / Film - 1.40 um
Temperature limit
From -10 to 260

My oven programme is:
60C 2 minutes
15C increase to 180C 0 minutes
45C increse to 225C 5 minutes

Start temperature is too high. Try to start with 35/40

Regards

Tomasz Kubowicz

What makes you think that all your target peaks come out before 4.7 minutes ?

Peter

What makes you think that all your target peaks come out before 4.7 minutes ?

Peter

rb6banjo share this method conditions about VOC under this topic. The link is here:
https://onedrive.live.com/?authkey=%21A ... ot&o=OneUp

What makes you think that all your target peaks come out before 4.7 minutes ?

Peter

rb6banjo share this method conditions about VOC under this topic. The link is here:
https://onedrive.live.com/?authkey=%21A ... ot&o=OneUp

It's not your column. But bearing in mind mentioned 4.7 min. what is the retention time of tetrahydrofuran in the same conditions ?

What makes you think that all your target peaks come out before 4.7 minutes ?

Peter

rb6banjo share this method conditions about VOC under this topic. The link is here:
https://onedrive.live.com/?authkey=%21A ... ot&o=OneUp

It's not your column. But bearing in mind mentioned 4.7 min. what is the retention time of tetrahydrofuran in the same conditions ?

On my column and my conditions tetrahydrofuran is coming 7.07 min.