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The column flow should be measured at the detector end using procedures recommended for the GC in
question. Once adjusted and measured as required, all the flows can be entered into the Confirm/Enter
Flows dialog box on the controlling method, see Section 3.6.2.
2.4.5 To exit the Set Gas Flows Function
After the measurement of gas flows is complete the user must press the Stop sequence icon to exit
the Set Gas Flows function and return UNITY 2 to Standby status.
2.4.6 Gas flow constraints - minimum & maximum settings
The system must be set up with at least 10 ml/min flow through the tube - and at least 2 ml/min desorb
flow through the cold trap during tube desorption to provide efficient desorption of the tube and efficient
transfer of analytes to the cold trap.
Use much faster flows through the hot tube (>50 ml/min) and at least 10 ml/min through the cold trap
when analysing high boilers (>n-C
20
).
At least 2 ml/min must be used when desorbing the trap. The total flow can be directed to the GC
analytical column or to a combination of column and split vent. The flow through the cold trap should not
normally be allowed to exceed 100 ml/min during either tube desorption or trap heat.
Some variation in flow will be observed through a needle valve if used at less than 2 ml/min.
Flow through tube during tube desorption = desorb flow + split flow (if selected)
Flow through trap during tube desorption = desorb flow
Flow through trap during trap heat = column flow + split flow (if selected)
2.4.7 Sample splitting
UNITY 2 may be set up to offer sample splitting between zero (splitless) and 10,000:1 (double split). The
split that is required for a particular analysis will be dependent on the analyte mass in the sample tube,
the analytical column capacity and the sensitivity of the GC detector selected.
2.4.8 Analytical column capacity
The capacity of an analytical column will depend on its diameter and the thickness of the film of stationary
phase. As a general rule a 0.25 mm I.D. column with a 0.25 µm film has an approximate sample capacity
of 100 ng per component. Wider bore and thicker film columns will have a higher sample capacity - up to
low micrograms in the case of thick film (5 mm), 0.53 mm I.D. columns. Narrower bore and thinner film
columns will have a lower sample capacity - below 10 ng per component in some cases.
2.4.9 GC system detection limits
The specification of GC detectors varies from type to type and manufacturer to manufacturer and it is
advisable to consult the relevant manufacturer regarding detection limits. However, the following general
guidelines may be useful:
Mass Spec., full scan mode: Modern systems should comfortably be able to detect and quantify a single
compound at 10-100 pg.
Mass Spec., single ion mode: Modern systems should comfortably be able to detect and quantify a single
compound at 1 pg.
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