Why use make up gas in GC?

“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

7 Responses to “Why use make up gas in GC?”

  1. Thank u. this is very important in attempting my assignment

  2. Hitesh Patel says:

    Why makeup gas used in G.C.?

  3. hel2n says:

    I just want to ask about the Vent of the FID detector. Why is it that the Vent of the FID detector is not the same. Based on my experience, We have ABB 5000 GC using FID detector. The Vent line is not hot. I installed flexible plastic tube of the FID vent line & I see condensate or dew in the vent line of FID detector.

    In our CEMS analyzer, We are using also FID detector for the hydrocarbon component. The FID temperature reach around 270’C. The vent line is very hot.

    My question is, why is it that the Vent of our ABB GC is colder compared to our CEMS analyzer. Their combustion air & fuel gas pressure is almost the same

    Thanks,

  4. Hi, this post is about using make up gas. this is adding some extra gas to optimize the detector response and control peak width. If you talk about “vent of FID” its something totally different. The total flow out of the FID are the burning products (CO2 + H2O), Nitrogen and the carrier gas + make up flow. You will see dew as the water formed will condense. As long as the exhaust does not impact the response or give corrosion, you can use any temperature AFTER the FID detector. In lab applications, the detector gases are released in free in the open atmosphere.
    hope this helps, jaap

  5. to maximize detector signal and eliminate peak broadening in detector.

  6. hel2n says:

    Thank you for your explanation boss Jaap. We have 1 ABB GC using also FID detector that doesn’t have makeup gas. We used only H2 & instrument air & our carrier is N2 but we measure only one component on that GC. In our complex GC that having multiple components, we used N2 as makeup gas.

    Is it really advisable to use makeup gas or dilute gas?

  7. ABB GC are typically process GC, which typically run with packed columns. Packed columns do have flows between 10-50 mL/min, meaning that you do not need additional make-up gas.Your compoenents will be swept to the detector fast and sensitivity will be OK.

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