Voc analysis using hp7694 headspace and hp 6890 gc ms

[Rolando]

Hello. I am new to headspace analysis and in need of some help.
Im using through needle injection, having almost no response.
I think the problem migh be the pressure setup of the auxiliary epc module, inlet pressure or split ratio.
The 7694 headspace has no pressure control.
Im using a hp5 column 25m×0.200mm×0.33um
If someone could guide me, thank you.

[AaronAIT]

What is your:

- column head pressure
- transfer line pressure
- vial pressure
- split ratio
- oven temp program

Also,

- What are your analytes and matrix?

Also - check the top of the transfer line needle after pushing it through the septum to attach to the inlet. Sometimes a bit of septum gets stuck in the very top of the needle.

[Rolando]

What is your:

- column head pressure
- transfer line pressure
- vial pressure
- split ratio
- oven temp program

Also,

- What are your analytes and matrix?

Also - check the top of the transfer line needle after pushing it through the septum to attach to the inlet. Sometimes a bit of septum gets stuck in the very top of the needle.

I have tried different pressure settings, what would be the correct pressures and split ratio.
The analytes are: vinyl chloride, 1,1-diclhoroethylene, chloroform, carbon tetrachloride, 1,2-dichloroethane, trichloroethylene and tetrachloroethylene.

The oven ramp is init.30°C 4.5min, 30°C/min to 135°C 4min and 10°C/min to 220°C.

Inlet temp is 175°C
Column head pressure 20

[AaronAIT]

Ok, a few things to consider. If you are using the setup where your transfer line is connected to your inlet manually, (i.e. NOT directly piped into the inlet) you always want your transfer line pressure higher than your column head pressure so that you actually get analytes on to your column.

Generally if your column head pressure at your start temperature is at, e.g. 20.0psi, try and set your transfer line pressure to something like 20.3psi as a starting point. Do this by adjusting the dial on top of the 7694. If your transfer line pressure is set much higher than your column head pressure, the excess flow will go out the split vent, essentially increasing your split ratio inadvertently.

Set your split ratio to 5 as a starting point, and your transfer line pressure at about 0.2-0.3psi higher than your column head pressure. Also try and inject a more concentrated standard to see if you are getting analytes transferred onto your column at all.

[Rolando]

Ok, a few things to consider. If you are using the setup where your transfer line is connected to your inlet manually, (i.e. NOT directly piped into the inlet) you always want your transfer line pressure higher than your column head pressure so that you actually get analytes on to your column.

Generally if your column head pressure at your start temperature is at, e.g. 20.0psi, try and set your transfer line pressure to something like 20.3psi as a starting point. Do this by adjusting the dial on top of the 7694. If your transfer line pressure is set much higher than your column head pressure, the excess flow will go out the split vent, essentially increasing your split ratio inadvertently.

Set your split ratio to 5 as a starting point, and your transfer line pressure at about 0.2-0.3psi higher than your column head pressure. Also try and inject a more concentrated standard to see if you are getting analytes transferred onto your column at all.

I am trying what you said, my headspace doesn't have a pressure dial, so i'll set the presure by the aux in the gc epc. I knew the transfer line pressure had to be higher that the inlet, but didn't know how much higher.
And also i will try with that split ratio and submit the results.

[AaronAIT]

Good stuff, let me know how you get on.