Depending on the type of evidence that you are collecting, it will have to be collected and preserved in a certain way to make sure that contamination does not take place.
Biological evidence - is usually collected using swabs when the evidence is in a liquid form such as blood or semen, however it is highly likely that the liquid biological evidence is on an item such as clothing, in this case the whole item of clothing could be collected and preserved using a sealed bag so make sure that the evidence does not fall out when in transit to the lab. If the biological evidence came in the form of a fingerprint then a technique such as bichromatic powder would need to be used to collect the evidence, the copy of the fingerprint would then be placed onto a slide and preserved in a plastic tube where it can not move around to keep the copy of the fingerprint intact. In terms of collecting hair samples tweezers would be used to pick up the hairs and they would then be placed into a plastic tube.
Physical evidence - is usually a lot easier to collect than biological and chemical evidence as most physical evidence is solid which means it can be easily stored. Weapons such as knives and tools are stores in plastic tubes whereas items of clothing are stored in sealed plastic bags.
Chemical Evidence - is most of the time a lot harder to collect than physical and biological evidence as a piece of evidence like gunshot powder can in most cases not be collected, a spot test has to be done at the scene to prove that it is indeed gunshot power. However, chemical evidence such as drugs can still be collected, this is usually done using a spatula and a plastic tube.
OJ Simpson
In the OJ Simpson case where he allegedly killed Nicole Simpson and Ronald Goldman the scenes of crime officers played a large part in how the investigation played out and a lot of blame was put on them for OJ Simpson being acquitted of the murders. The scenes of crime officers had one role in the OJ Simpson case which was to go to the scene where the crime took place so that they could collect any evidence their that was relevant to the case, once they collected the evidence they had to correctly label and tag each piece of evidence, then the evidence could be sent for further analysis. In reality, what really happened was a disaster, the SOCO team did not wear the correct PPE nor did they collect the evidence in the correct way to stop contamination, this resulted in a lot of the evidence that was found being contaminated which resulted in that evidence being useless. They where also very lazy when labeling the evidence, some pieces of evidence where missing labels and the once which had them were not done correctly which resulted in a lot of confusion when the forensic analysts tried to analyse the evidence. Cross contamination was also a very big problem that the forensic scientists in the lab had to face as a lot of the pieces of evidence were doubled up in single bags which resulted in the pieces of evidence contaminating each other, this meant that it could not be used as evidence in court.
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