How do I calculate a split ratio?

There are two points at which a split can be employed in a thermal desorption method:

• During primary (tube) desorption, when it is known as aninlet split.

• During secondary (trap) desorption, when it is known as anoutlet split.

The formulae are:

Inlet split ratio = (Desorb flow + Split flow) : Desorb flow
Outlet split ratio = (Column flow + Split flow) : Column flow
Overall split ratio = Inlet split ratio × Outlet split ratio

For example:

• If the desorb flow is 50 mL/min and the inlet split flow is 100 mL/min, then the inlet split ratio is 150:50 (i.e.3:1) – meaning that 1/3 of the sample will go to the cold trap.

• Similarly, if you have a column flow of 2 mL/min and an outlet split flow of 18 mL/min, then the outlet split ratio is 20:2 (i.e.10:1) – meaning that 1/10 of the sample will go to the column.

• If both inlet and outlet splits were set as above, then the total split would be (3 x 10):1 (i.e.30:1) – meaning that 1/30 of the sample will go to the column.

The thermal desorption control software for Markes’ instruments includes a split ratio calculator to work all of this out for you – but remember that this does not set the flows in your method.

See Application Note 022  for more information on selection of gas flows and split flows.

Terminology:

Desorb flow = Flow through the cold trap during tube desorption

Split flow = Flow to split during tube or trap desorption