The NTN has over 250 sites that focus on wet deposition chemistry by collecting weekly precipitation samples nationwide. The samples are sent to the Central Analytical Laboratory (The Wisconsin State Lab of Hygiene) for analysis and are then used to determine geographic distribution and annual trends. The sample collection and handling methods follow strict clean-handling procedures in order to ensure accurate results. The analytes monitored are: Free acidity (H as pH), conductance, calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), sulfate (SO), nitrate (NO), chloride (Cl), and ammonium (NH). The CAL also measures orthophosphate, but only for quality assurance as an indicator of sample contamination.