At a crime scene, many pieces of chemical evidence can be collected. These include blood (liquid or dried), hair, fibers, bones, seminal traces, chemicals (including toxins or poisons, narcotics, explosives, etc.), etc. These samples are collected and analyzed in forensic laboratories (or independent laboratories, where specialized labs are not available). Careful collection of samples is just as important as the actual analysis and interpretation of these chemical evidences. For example, the body fluids, including blood, should be carefully sampled in sample vials or cotton and sent for analysis after refrigeration (to prevent degradation of samples). Seminal fluids should be allowed to dry and then carefully sampled in paper bags before transportation. Use tweezers to collect hairs or fibers and seal them in envelopes. For flammable fluids, collect the fluid (if available) in a sample vial; if it has already dried onto some other object, sample the object itself (a piece of wood, a window, etc.).
The types of samples available and collection procedure will vary from crime scene to crime scene.
Hope this helps.