The spiked sample is analyzed and its expected concentration calculated as the sum of the original concentration and the spike concentration, normalized for the constituent volumes. A comparison is made between the actual value and the expected value. These concentrations (original, expected and actual) are recorded in a separate QA/QC data file along with sample number, sample collection date, analysis date and the amount of spike added. In the case of particulate phosphorus, the volume filtered is not used in the calculation to determine percentage recovery.
If a value of >115% or <85% is observed for percentage recovery of the spike, then all of the spikes for that particular analytical run are carefully reviewed. If only one of the spikes is in question, then only that sample is re-analyzed. If all show poor recovery, then instrumentation/reagent problems are suspected and the analytical run is halted until such time that the problem is resolved. This procedure is adhered to for all dissolved analytes and for particulate phosphorus and biogenic silica.