Why does a negative peak appear?

What instrument are you using, chromatography? I remember that when testing high-purity argon, the oxygen peak of the argon showed as a negative peak. It’s also possible that it’s Ghost Peak

We also observed a negative peak when producing chlorine; I’m not quite sure either

1. In the Detector, the signal polarity of the data processing system is reversed; the signal connections are inverted. 2. In a TCD, if the thermal conductivity of the sample is higher than that of the carrier gas, select “Negative Peak Processing” in the data processing options. 3. The ECD is contaminated, which may result in negative peaks appearing after positive peaks; clean the ECD or replace it (if necessary). 4. In an FPD detector, if the gas is impure, the sample will dilute the background when sampled, leading to negative peaks. It would be helpful if you could provide detailed information about your analysis and the configuration of your instrument, so that others can assist you in resolving this issue

I encountered that once too; why did replacing the liner of the injection port solve the problem?

What is mentioned on the 5th floor refers to the ghost peaks; when the cushion loses air pressure, the formation of these peaks becomes irregular, and any type of peak can appear. You can give it a try; not every time the cushion loses air does a negative peak occur, and the retention time remains more or less the same, right?

There are roughly several types of negative peaks: the signal is reversed； Different polarities ； Unknown Peak ； Pressure fluctuations ； Instantaneous magnetic field fluctuations ； Component interference, and that’s about it – that’s all I know. Each case needs to be handled on its own

It may be caused by the deposition of poorly volatile components on the pad. Personal opinion.

I’m using a FID detector, and negative peaks usually appear during sampling. Sometimes, because the lab needs the test results quickly, the second sample is injected before the first one has finished processing, which leads to negative peaks. The same procedure didn’t cause negative peaks before; is this related to instability in the chromatograph?

According to your description, manual sampling should be used; due to the relationship between temperature and the amount of sample injected, changes in internal pressure occur. If there are also components that are difficult to vaporize or components that cause interference, then anything can happen. If we simply conclude that it is due to the low temperature of your injector, a slightly large injection volume, a low carrier gas pressure, and the presence of high-molecular-weight components in the sample, it can be improved by adjusting certain parameters. For example, increasing the pressure slightly, reducing the injection volume, raising the injector temperature, and so on – depending on the actual conditions, improvements should be possible. Besides, if it is determined that the first sample is still in the column, another set of samples is injected. If the amount of sample injected is large and the carrier pressure is not high, then air resistance will occur. Have you played with water guns when you were young? The effect is an amplified air resistance. With standardized and consistent procedures in the analysis, it is easier to determine the issues. Otherwise, it’s really hard to say.

It must be interference! How can you move on to the second sample before even finishing the first one? Don’t you all wait for the column temperature to return to normal before injecting samples?

Another point is that if there is a persistent residue of samples on the column, its efficiency will be significantly reduced; in worst cases, the column may become damaged. It is recommended to age the columns first and then make samples for testing. Be sure to wait until one sample is completed before moving on to the next; column residue seems to be a significant issue!

Sure, you can cut off a section at the end of the column inlet, about 10 CM long. This way, there is no need for aging.

1 The signal line connector is connected incorrectly. 2 The sample was injected into a faulty column. 3 In a dual-column system, the injection position is incorrect. 4 The carrier gas system is unbalanced during injection with the six-way valve

It can be said that the sampling procedure wasn’t followed properly; this kind of situation has not occurred before with negative peaks. Additionally, since they work on shift rotations and get up in the morning, the food doesn’t have enough time to stabilize. Could it be that sampling was carried out before the food had stabilized enough? Is that also a factor?