What method should be used to collect liquid blood evidence?

The most appropriate method for collecting liquid blood evidence is using a clean white cotton cloth to absorb the blood and then allowing it to air dry. This technique ensures that the sample is collected without contamination and preserves the integrity of the evidence. The use of a cotton cloth helps to absorb the liquid while minimizing the risk of contamination from other materials. After absorption, air drying is important because it prevents the growth of bacteria and the degradation of the sample, which can affect subsequent analysis.

Pouring blood into a container could lead to splattering or contamination, while using a vacuum device typically isn't practical for this type of evidence collection. Similarly, using a needle and syringe is more suited for medical practices rather than crime scene evidence collection, as it can risk degrading the sample and may not be necessary for the purpose of evidence collection. Thus, the method of absorption with a clean cloth followed by air drying is optimal for collecting liquid blood evidence.