What is the effect of the carrier gas pressure in GC FID on detection results such as peak time?
Thread Content
First, let’s talk about the carrier gas used in our GC-FID: high-purity He is employed. There are two injection ports, with port 1 not being used for sample injection; analyses are typically carried out by injecting the sample into port 2. However, the two main inlets for the carrier gas He are connected to each other; that is, carrier gas 1 (the carrier gas connected to injection port 1) is discharged directly into the air through the chromatography column, while carrier gas 2 (the carrier gas connected to injection port 2) carries the sample to the detector for analysis. After a certain period of time, we make a switch: the sample with a higher concentration, which is about to reach its peak, is discharged into the air, while carrier gas 1, which contains no sample, is directed to the detector for a blank measurement. Once the peak of this high-concentration sample has passed, we switch back to continuing the analysis of other impurities. I would like to ask whether the pressure of carrier gas 1 and carrier gas 2 has any impact on the experimental results? What I know is that as the pressure of the carrier gas increases, the flow rate also increases; therefore, the peak emergence time must decrease. But what is the effect on the impurity concentration? What does it look like? Also, apart from being switched after a certain period of time, does the carrier gas 1 have any impact on the results? Experts, please give your advice! Thank you
It’s made really complicated. In my opinion, as long as the resolution is sufficient, it has little impact on the concentration
Could the original poster please provide a diagram showing the relationship between the column system diagram and the procedure for loading samples? Your description is indeed a bit unclear.
The poster’s explanation of the chromatography system is quite complicated; I’m too stupid to fully understand it. What follows are just my own thoughts :) 1. Carrier gas 2 → Injection port 2 (sample) → Chromatographic column (the column used for separation, right)? → Detector, carrier gas 1 → injection port 1 → chromatographic column (a column with very little stationary phase, right)? → Empty 2: Without separating the sample, how can one know when the recombined component will appear? Is there no column connected behind the injection port 1? Is it directly vented to the atmosphere, or is detection carried out right away? That’s also an option. 3. What does switching refer to – the detector or the venting system? 4. What does it mean that the two main inlets for the carrier gas He are connected to each other? Can the flow rate ensure that the two are the same, especially when one is used for venting and the other for connecting to the detector? 5. Under certain conditions, the pressure before the column will definitely affect the time of peak emergence. The concentration of impurities and whether the separation in the column is complete are likely related to the degree of combustion in the detector. If the operating conditions (whether human factors or instrumental factors) remain similar, and internal and external standardization is used along with correction using correction factors, the resulting values should be reliable
Reply to 4# dalian*aochao: What LZ means is that carrier gas 1 is used as a blank to test the detector and check whether it is contaminated, while carrier gas 2 is used for normal analysis. Perhaps LZ has very strict requirements regarding impurity levels in the sample quality. In fact, this approach is hardly ever used; at least I have never encountered it before. Actually, it’s sufficient to just not inject any sample or to use some carrier gas to check the baseline level in order to meet the requirements. I’m not sure if my analysis is correct; only LZ can make a judgment on this matter
Reply to 5# zbx: Brother Zhu’s analysis makes sense. It’s fine if the switching is automatic, but if it has to be done manually, the error rate will likely be quite high!
Reply to 3# qugd: The diagram on the fourth floor is correct; it shows that the connections are made in this way. We first determine the peak time of the component with a higher concentration, and then make a switch to empty carrier gas 2 and connect carrier gas 1 to the detector. After the peak time has passed, we switch back to the normal setting.
Reply to 4# dalian*aochao: Your understanding of the diagram is correct; the connection between carrier gas 1 and carrier gas 2 is indeed as shown. The carrier gas 1 is not used as a blank control; GC-FID does not require a blank control, right? It is GC-TCD, with its thermal conductivity detection, that needs a carrier gas as a blank control. The function of our carrier gas 1 is to switch to carrier gas 1 for a specific period of time, in order to place the hydrogen ion flame burner away from contamination caused by excessively high concentrations of substances in the sample using carrier gas 2.
Reply to 5# zbx: Carrier gas 1 is not used as a control; the FID does not require a blank control, whereas the TCD needs a stream of carrier gas as a blank control. Our carrier gas 1 is used to switch at specific times, thereby preventing components with high concentrations from reaching the subsequent detectors
Reply to 8# *lihuagong: Regarding the direction of the gas flow in your topic, there are significant disagreements regarding the flow diagram of your chromatograph. Could you please provide a diagram showing the flow before switching: carrier gas 2 → injection port 2 (sample) → chromatographic column (the column used for separation)? → Detector, carrier gas 1 → injection port 1 → chromatographic column (a column with very little stationary phase, right)? There must be some tubing at least → Empty it (it doesn’t matter which tubing; just empty it. What’s the pressure in front of the column with carrier gas 1?) ) After switching, carrier gas 2 → injection port 2 (sample) → chromatography column (the column used for separation, right)? → Empty carrier gas 1 → Injection port 1 → There must be some tubing involved → Detector (Is there any carrier gas flowing through at this point?) ）
Reply to 9# *lihuagong: I’m not sure how you understand the blank control. The blank control in TCD is due to the structure; as the components of the gas flowing through it vary, the bridge current fluctuates, and a sample peak appears after detection by the detector. FID is a mass-type detector that requires the combustion of components, and differences in mass and quantity result in sample peaks of varying concentrations and amounts.