3.1. Definition of Surrogate

The surrogate is a compound that likely behaves like the analyte of interest. The analyte of interest, A, is what we are interested in finding the concentration of.

A surrogate is a chemical substance that mimics the behavior of a target analyte during an analytical process. For example, when analyzing a water sample for a certain pollutant (analyte A), a surrogate can be added to the sample to help determine how much of the analyte is present based on the surrogate's recovery rate. It is expected that the surrogate will behave in a similar manner to the analyte throughout the analysis.

Think of a surrogate like a stand-in actor in a movie. Just as the stand-in performs the same actions as the primary actor during filming, the surrogate mimics the analyte's behavior. If the primary actor is unwell and cannot perform their scene, the stand-in will help ensure the scene can still be captured accurately.

We calculate the recovery of A in the process of analysis, so the surrogate is expected to behave like the main compound, and we calculate the efficiency of recovery of A by using the efficiency of recovery of the surrogate.

Using a surrogate allows analysts to estimate the recovery efficiency of the target analyte. The recovery is determined by comparing how much of the surrogate was detected after the analysis process. If the surrogate has a known recovery rate, it can help infer the recovery of the analyte, thereby providing a clearer picture of the analyte's concentration. This is essential for ensuring the accuracy of analytical results.

Consider baking a cake and using a test cake as a surrogate to check whether your recipe works. If the test cake rises perfectly, you can be confident that the actual cake will also turn out well. Similarly, the surrogate indicates the success of the extraction and analysis method for the analyte of interest.

For instance, we add 1 ml of a 100 milligram per liter solution of a surrogate, which results in 0.1 milligram added to the sample. We find the recovery by determining how much of this is recovered at the end by the instrument.

To calculate the efficiency of the surrogate and thereby estimate the effluent concentration of the analyte, we begin by adding a known quantity of the surrogate to the sample. After processing the sample through various analytical steps, we measure how much of the surrogate remains. The difference between the initial and final amounts gives us the recovery rate. This recovery rate is then used to infer the concentration of the analyte in the original sample.

Imagine filling a container with 10 balls, then removing 2 to see how many are left. You started with a known quantity, so if you calculated that 8 balls remained, you could determine that 2 were lost during the process. In scientific terms, this helps us understand how much analyte was also lost during testing. The difference in numbers gives us essential information for our analysis.

This process is critical because sometimes we may not completely separate the extracted surrogate from the sample. Keeping track of the amounts, such as removing some hexane and measuring exactly what is left, is vital for accurate results.

Accurate measurement throughout the analysis process ensures that we can trust the final results regarding the concentration of the analyte. If some components are lost during extraction or analysis, it impacts the recovery calculation. Therefore, careful tracking of how much surrogate is added, what is retained after extraction, and what ends up in the instrument aids in achieving reliable data.

Think of this process like measuring ingredients for a recipe. If you added 2 cups of sugar but accidentally spilled a bit, the final cake won't turn out as sweet as you intended. Similarly, accurate tracking of chemical amounts during analysis ensures that you understand exactly what has been extracted and what results can truly be expected from the analyte.

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