What FID flows should I use?

Question

Is there a table / document somewhere on what Agilent GC FID flows I should be using for a particular column ?

Right now I have a 7890B with a 30m 0.32 column film thickness 0.25. We use it for petroleum / Diesel Range Organic analysis with a ALS injector. I guessed and set the h2 to like 40mL/min and the air to 350mL/ min.

I am not using any make up gas. Should I?

Thanks

krishna · Answer

Most detectors use a makeup gas to increase the flow rate through the detector body. This sweeps peaks out of the detector quickly, avoiding mixing of components and loss of resolution. This is particularly important with capillary columns because the column flow rates are so small. The makeup gas line of your detector parameter list changes depending on your instrument configuration. If you have an inlet with the column not defined, the makeup flow is constant. If you are operating with column defined , you have a choice of two makeup gas modes. Constant makeup This mode provides a constant flow of makeup gas to the detector. Column + makeup = constant This mode provides a variable flow of makeup gas to the detector. As column flow increases or decreases, the makeup flow changes to provide a constant combined flow to the detector. If you choose this option, enter a value under Combined flow . The Combined flow line always displays the same value, while the Makeup line changes as the actual makeup flow changes.

gabcaron · Answer

Just like Krishna said, using makeup gas helps in cases to get peaks out faster. What is usually recommended for the FID is (For packed column FID - if you have a capillary column adapter at your detector inlet / column outlet) Hydrogen 30 mL/min Air 400 mL/min makeup 25 mL/min Otherwise for Capillary FID Hydrogen 30 air 400 Makeup around 10 should be fine (you can play with it if you have issues with the flame) These are the recommended default values. After that, you can do some testing to see if changing those increases your signal and responses. Also as a side note, the detector temperature is normally recommended to be above 300-350 Celcius.

paulsalverda · Answer

The required flame height is directly related to the distance between the tip of the jet and the collector. Before the 5890, the collector height was set by the operator. Starting with the 5890, the collector height is fixed. That means the flame height needs to be fixed. That means that the optimal total flow coming out of the jet is one flow rate and that, while other flow rates will work, they are not optimal. 30 ml/min of carrier + makeup gas, 30 ml/min of hydrogen, and 400 ml/min of air is optimal for the FID. The optimal column flow rate is a bit less exact as there is a bit of a range for each column diameter. Your 0.32 column works best between, oh, say 1.5 ml/min and 3 or so. Remember that increased column flow rate does not push the peaks through all that much faster, that's the oven temperature's job. So if you are running that 0.32 column at 2.5 ml/min, you want your makeup gas at 27.5 ml/min. You will also get slightly better sensitivity running nitrogen makeup than helium due to the physics of the two gases. And ask as many questions as you need! Don't forget that you can talk with experts by calling the Agilent 800#, too.

Lance Nelson · Answer

I agree with Krishna, use makeup gas when using capillary columns. The total column + makeup flow should be approximately the same as the hydrogen flow. The hydrogen flow should be about 1/10 of the air flow. Also, 7890B adds the makeup gas option "Carrier + Fuel = Constant." Use this option if you have H2 carrier and the column flow changes during the run, so the total amount of H2 going to the FID remains constant.

What FID flows should I use?

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