At least one Matrix Spike sample analysis shall be performed on each group of samples of a similar matrix type (i.e., aqueous/water, soil/sediment) or for each SDG, whichever is more frequent. The spike is added before the digestion (i.e., prior to the addition of other reagents). The analyte spike shall be added at 1 μg/L for aqueous/water and leachate samples, or at 0.5 mg/kg for soil/sediment samples. Adjustment shall be made to maintain these spiking levels when the weight of sample taken deviates by more than 10% of these values. This is the level of spike present in the final digestate.