The evidence stated in this case lists two pairs of jeans, shirt and a handgun found at the crime scene. The integrate of each piece of evidence need to be handle to avoid any cross contamination such as DNA or traces of any other matter that may be used to identify additional information to solve the case.
The example use of traces of pollen is a good example of how basic chemistry could influence contamination to the evidence. If the chemical compound is found on one item but excludes other evidence can both crime scene analyst