calibration of gas is easy .How to calibrate GC for Standard gas + Benzene toluene and xylene based on response factor. is response factor same when
How to calibrate GC for Standard gas + Benzene toluene and xylene based on response factor. Is response factor same when the sample is injected as a liquid or as a gas ?
In my experience...
(1) In liquid sample, FID ratio of response factor is the same as that in the gas sample. For example, in the liquid sample if the ratio of response factor benzene/toluene = 0.88, the same ratio should be obtained in the gas sample. However, the criterion is that the samples have to be dilute (e.g. < 1%) due to thermal expansion. If the total concentration of impurity (all compounds except for carrier gas) is low, you should be safe to use methane to quantify all hydrocarbon compounds if you have ratio of response factor of those compounds to methane. In general, the more ideal of your gas mixture, the more accurate RF method.
(2) Injection of liquid and gas can have very different result due to thermal expansion. For example, if you inject 1 micro-Liter liquid sample, with 1000 ppm mole benzene, and 100 micro-Liter gas sample with 1000 ppm mole benzene, you will have way much larger benzene peak area in the liquid sample than that in gas sample. 1 micro-Liter liquid sample can become 500 micro-Liter or more gas volume in the injection port once it vaporizes. Another example is that if you inject 3 liquid samples that have 1000 ppm mole benzene in methanol, ethanol, and isopropanol, respectively, you will get different benzene peak areas which is due to different thermal expansion between the three solvents.
(3) If you're interested in making BT (benzene and toluene) gas STD (standard) by yourself, here is a good method: prepare a gas bag with 500 mL carrier gas (e.g. Helium) using a mass flow controller, inject 1 micro-Liter pure benzene into the gas bag and allow it to vaporize naturally (just wait for seconds), you can now calculate concentration of benzene in the gas bag (ideal gas law is good enough). You can inject multiple components (e.g. B + T). You have to use gas-tight syringes (have PTFE caps at the end of plungers). Those syringes for liquid leak terribly. This method is accurate and simple for making gas samples for very volatile compounds (e.g. furan, acetone, hexane, B, T). However, for heavier compounds (e.g. Xylene and even styrene), you will need to use a reactor system, which is another story, if you're interested in.
Hopefully this helps. If not, never mind.
Yu-Ting Cheng Thank you so much Sir for the explanation ! the information you provided is excellent!
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