novice question: split versus splitless injection?

[glenchung]

Hello.
I'd like to do a gasoline analysis procedure that I found online. It uses an injection of 1 uL split 1:40.
My instrument is HP 5890 Series II with Agilent ChemStation software. I do not see any dialog box in the software where I can define the 1:40 split. What does the 1:40 split mean for me, in this case? For example, does it mean that I should define my method to be splitless injection with a sample volume of 1/40 uL? That would be too small, I think, right?

Thank you very much for your time.

[LiVD]

Split 1:40 means that only 1 fortieth of your 1 µL gets to the column, while the rest is diverted by a separate gas stream.

Are you sure you have a split/splitless inlet?

1/40 µL is definitely too small to inject reproducibly.

[CE Instruments]

Manual or Electronic pneumatics ?

If manual you have to measure the split flow and set it via a needle valve.

[glenchung]

Thank you. How do I determine whether or not my machine has a split/splitless inlet, and if I see that it does have such an inlet, how do I determine whether the pneumatics are manual or electronic?

[glenchung]

There is a "SPLIT/SPLITLESS INLET VENT" on my machine's console. There is also a pressure gauge under the printed text "SPLIT".

I had assumed the pressure gauge was for the pressure in my column. Is this not right?

Thank you again for your help, LiVD and CE Instruments.

[skunked_once]

I had assumed the pressure gauge was for the pressure in my column. Is this not right?

You are correct. You set the inlet pressure on the pressure gauge to control the column flow. The column flow can be measured at the detector outlet using the adapter plug that came with the instrument. Make sure that the detector gases and makeup flow are turned off.

There is a "SPLIT/SPLITLESS INLET VENT" on my machine's console.

This is where you measure the split flow, using the knob to adjust the flow. If you column flow is 1 mL/min and your split flow is 100 mL/min, the split ratio is 100:1 for example.

[glenchung]

Ah, thank you very much skunked_once, now I see clearly how I can get the 1:40 split that the protocol states.

[rb6banjo]

To accurately determine your 40:1 split ratio you need to know your column dimensions and the head pressure you apply to it. I have a 5890A with manual pressure control. I now use Agilent's flow calculator to do all of these calculations for my GC's with manual pressure control.

http://www.chem.agilent.com/en-US/Techn ... ators.aspx

Plug in your column dimensions and the flow characteristics you want through the column as well as the split ratio you want and you can find your conditions readily. For a 30 m x 0.32 mm x 0.25 µm column, to get 25 cm/s at 40 °C to an FID (atmospheric pressure detector), I need a head pressure of 7 psig. To get a split ratio of 40:1 I set the flow out the split vent at 58 cc/min.

For these settings, the linear velocity through the column is dead on at 25 cm/s (measured by injecting methane) at 40 °C.

[glenchung]

Thank you, I had no idea!