Handling controls were prepared by dosing ultrapure water (60 ml, n = 4) and kept at room temperature for 48 h. This was done to assess potential losses unrelated to the reagent, temperature or interaction with biological tissue. An additional water blank was run after filtering the samples to assess if any particles from previous samples could have contaminated subsequent ones. Samples were vacuum filtered (25-μm filters) and rinsed with 20 ml of ultrapure water. This amount was chosen because initial trials with tissue samples indicated that larger amounts of water were not filterable. During such trials, particles were observed stuck to the sample containers and filtration funnels. To enumerate potential losses, after sample filtration a new filter was placed in the unit and the sampling jars and funnel flushed with approximately 400 ml of ultrapure water.