“Make up” gas is a gas flow that is used to sweep components through a detector to minimize band broadening. For FID often N2 is used, which is set at a flow of 10-20mL/min. Depending on the GC-brand and detector design, the use of make-up gas can improve sensitivity: check the manuf. recommendations. If you want to optimize make up gas flow for maximum sensitivity, inject a fixed amount of component, using different make-up gas flows. Measure the signal to noise of a component (do NOT look at the peak response only !).
Make up gas needs to be filtered using charcoal filters , same as the air and H2. Note that the signal of mass flow responding detectors, like FID, will not be affected by make-up gas, while the concentration – responding detectors, like a TCD, the signal is related to the dilution of the gas. If column flow is 2 mL/min, and make up gas is set at 8mL/min, we loose a factor 5 in sensitivity. That’s why with TCD-type detectors we need to minimize make up gas.
Further reading: newest reference to measure flows in GC