During the Corona pandemic, you surely also regularly performed antigen tests for Covid-19. Did you ever take the tests apart?
We were curious and took a closer look at the Corona tests.
What do you think, does the test only contain a paper strip?
The “paper strip” is actually a thin-layer chromatography strip. In the coating of the paper strip, the stationary phase, there is a protein in a small area that only binds to the antigen of the Covid 19 virus (proteins from the virus’s envelope). If Covid 19 antigens are present in the sample, a colored line on the paper strip signals a positive test result. This spot is marked with T = Test.
In the eluent from the tube, into which we put the sample from our nose or throat, another protein is already contained for control, which specifically binds to a protein in the control line. This spot is marked with C = Control. The eluent runs along the test strip due to capillary forces.
This control line serves to check whether the test was performed correctly and functions correctly. As soon as the eluent reaches the control line, it turns red due to the binding to the protein.
Here you can see the process on the test strip: Click on the image to start the video!
The eluent, into which the sample is placed, has two functions:
It serves as an eluting agent, which transports the sample through the entire stationary phase (= the chromatography paper strip) due to capillary forces.
As soon as the sample reaches the specific protein of the strip, the protein binds to any viruses present and the color of the control line appears (i.e., the result is positive). If there is no virus in the sample, there is no reaction and no colored line appears (negative).
The eluent also serves as a buffer, meaning it stabilizes the pH value. This is very important, as both the protein in the chromatography strip and the Covid 19 antigen are sensitive to pH changes.
If the pH value changes, the proteins also change – as a consequence, they do not bind to the viruses (false-negative test result), or the protein of the control line changes color without viruses being present (false-positive test result).
Task:
Do the antigen tests also work with juice, soft drinks, or tap water?
You can try this out in the blog task. Good luck!