The order in which samples are taken depends on the analyte being collected: organic analytes (unfiltered, then filtered) are followed by inorganic analytes (filtered, then unfiltered). Samples are filtered for inorganics, nutrients, pesticides and dissolved organic carbon. Unfiltered samples include volatile organic compounds, stable isotopes,tritium and radon. Quality assurance and quality control also require the periodic collection of replicate samples, including field blanks and spikes.