In some instances, collection of samples poses a significant challenge. In general, the best collection strategy is to submit the entire item. This can be easily accomplished for items such as a small rock, clothing, or a knife, but is more difficult in the case of larger, fixed objects. In cases where the samples can be collected using a slightly moistened swab, such as from a window, the collection process is fairly straightforward. Some problems can arise when a hard porous surface, such as pavement or concrete, is encountered. In these cases, it can be difficult to collect small bloodstains or other biological fluids by swabbing. There are occasions where scraping the substrate containing the biological sample is a viable option. However, care must be taken. For example, if the crime scene is outdoors, wind could cause loss of sample or contamination of another item.

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