Surrogates are compounds added to the sample either at the initiation of the sample extraction process (Extraction Surrogate), or at a step immediately prior to loading on the instrument (Instrument Surrogate). Surrogates are selected using the following criteria:
1. The surrogate should be chemically similar to the test code parameters. This is of particular importance with respect to the Extraction Surrogate as its recovery is a key indicator regarding the effectiveness of the extraction process.
2. The surrogates should chromatograph or respond on the test instrument similarly to the test parameters.
3. The surrogate’s response on the instrument should be stable, reproducible and resolve effectively, or has inherent properties, to allow it to be differentiated from the parameters of interest.
There are allowances for the failure of surrogates in specific situations.
If an analysis should have a minimum of four Instrument Surrogates, one is allowed to fail.
Failure of the Extraction Surrogate is only allowed under specific circumstances. An Extraction Surrogate make fail “high” if a high background concentration of native components exists within the test sample that interfere with the surrogate.
An Extraction Surrogate may fail “low” if a test sample or extract requires a large dilution factor to bring the concentration of a test parameter within the instrument’s calibration range. Dilution of the Extraction Surrogate may not be allowed if contractually not permitted.