Preservation is the last step of sampling process. It should be done as soon as possible. Remember that collected material is nothing else, but your future scientific results. You want it to be as similar  to the natural state as possible. Without any protection and conservation collected samples  can just dry out, or spill. All natural, ongoing processes have to be stopped as well. In this case just add the appropriate preservatives into the bucket, bottle or any other container for the particular sample. Sometimes you will just have to freeze the collected material, or add in some formalin or alcohol. In other cases the whole procedure will be more complicated and will require more than single step. For chemical analyses it is also very important to choose a proper shape, color and material of used containers. It all depends on what and how you want to study. I am not here to tell you about all the procedures, but to point out the reason of this process. The main aim of it is just to protect the thing you want to study.