I’m not sure you understand what I’m saying. Here’s an article on instrument development of a sars-cov-2 biosensor.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7172500/
I don’t believe it’s the same design as the one you used. But the principle is the same. If you read it, the researchers go to great lengths to optimize the device for the required sample matrix, which is human saliva in universal transport medium (salt solution). Samples that are not stored appropriately, or samples which contain unknown compounds, CAN throw off the test. In the case of this biosensor, it is possible that some compounds in ketchup or toothpaste have an affinity for graphene and cause a false positive. This device is also sensitive to pH changes, so anything with a non-neutral pH can yield a false positive or a false negative… (orange juice?)
I realize that some of this info could be used to argue for the possibility of false positives in clinical covid-19 testing. Which is perfectly fine. My point is, clinical testing is performed according to specified SOPs. Your little “experiment” was not. Therefore, your experiment is invalidated and we can draw no conclusions from it, concerning the accuracy of clinical testing. Clinical tests (in theory) will have been rigorously tested before being approved by the FDA. The false positive rate should be known as long as SOPs are followed. (this is assuming that the test is actual approved and not “emergency authorized”) If SOPs are not followed we don’t know the false positive rate and cant trust the test.

Note: I am in no way defending the accuracy of any clinical test. I’m simply explaining why your experiment does not prove anything. It only misleads people into using easily falsifiable arguments. It makes the opposition to covid tyranny look gullible and incompetent.