What should be done if spikes do not give the correct recovery?
Spikes not giving good recoveries
Verify good recovery can be obtained when spiking blanks. This can be lab water or carrier for off line digestion, distillation or extraction methods.
- If there is not good recovery when spiking blanks, the problem is related to the spiking procedure. This could be from the spike solution concentration, volume delivered, or spiking technique. One can also deliver a spike to a beaker on a balance to check the accuracy of the volume delivered.
- If spikes recovered well for standards or blanks, verify the spike level is adequate. If possible, it is best to deliver a spike level similar to the concentration of the unknown.
- If spike level is low relative to the unspiked result, some of the variability of the actual result may significantly affect the calculated recovery.
- Verify the spiked sample is still within the range of the method. If the spiked result is too high, it may not be an accurate result and could calculate as poor recovery.
- The purpose of the spike is to determine if the sample contains an interference for the test. If blanks and/or standards recover well when spike if may mean the sample interferes with the test. Do not report the result. It may be possible to work past an interference by diluting the sample. If a diluted sample can give good recovery for a spike it would determine that dilution resolves the concern.