AquaSnap Free measures non-microbial ATP (aka dead cells) in a solution. Use AquaSnap Free in combination with AquaSnap Total to identify the microbial ATP in a water sample. First, test a sample of water using AquaSnap Total and record number (Total load). Then, use AquaSnap Free on the same sample and record this number (Free ATP). The difference between Total and Free provides an indication of the microbial contamination in the sample.