The U.S. Geological Survey (USGS) initiated the Interlaboratory Comparison Program in 1982 as part of the external quality assurance activities conducted for the National Atmospheric Deposition Program (NADP) . The objectives of the Interlaboratory Comparison Program are:

1. Verify the quality of chemical analyses of precipitation samples determined by the Central Analytical Laboratory (CAL) and Mercury Analytical Laboratory (HAL)
2. Estimate the analytical precision of participating laboratories
3. Determine if statistically significant differences exist among the analytical results of participating laboratories

Samples from the following sources are used in the Interlaboratory Comparison Program:

1. Ultrapure deionized-water samples prepared by the USGS (DI Samples)
2. Standard reference samples with certified U.S. National Institute of Standards and Technology (NIST) traceable values, prepared by High Purity Standards and diluted and spiked by the USGS (Simulated or SIM Samples)
3. Excess natural wet-deposition samples collected at NADP sites and prepared by the USGS or CAL (Natural or NAT Samples) - Not used for mercury samples