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Surrogate vs Internal Standards

What are Reference Standards?

Reference Standards are samples of a known concentration of analyte(s). Standards can be used in the identification and confirmation of analytes. A series of standards at known concentrations can be used to produce a calibration curve which can be used for quantitative analysis. See our technical note on the importance of using analytical standards for more information.

What is an Internal Standard?

An Internal Standard (IS) is a chemical compound which is added at the same concentration to all samples during sample preparation. An internal standard can be added to a sample, extract or a standard solution. By using an internal standard it allows for variations between samples and the instrument which will affect the target analyte(s) to be taken into account and adjust results to account for these errors. This adjustment is done by using a ratio of the peak area of the target analyte to that of the internal standard and so changes to the measurement of signal from the analyte should be proportional to that of the internal standard:

π‘ƒπ‘’π‘Žπ‘˜ π‘Žπ‘Ÿπ‘’π‘Ž π‘Ÿπ‘Žπ‘‘π‘–π‘œ = π‘ƒπ‘’π‘Žπ‘˜ π‘Žπ‘Ÿπ‘’π‘Ž π‘œπ‘“ π‘Žπ‘›π‘Žπ‘™π‘¦π‘‘π‘’ / π‘ƒπ‘’π‘Žπ‘˜ π‘Žπ‘Ÿπ‘’π‘Ž π‘œπ‘“ 𝐼𝑆

Using an internal standard improves the precision of results. The internal standard normalizes the calculated results to minimize bias in the measurements and decreasing the need to repeat measurements. For more information on internal standards and how to choose and use them, see our technical note.

What is a Surrogate Standard?

Surrogate Standards (SS) are commonly used in the analysis of organic compounds. Surrogates are organic compounds which are chemically similar and behave in a similar manner to the target analyte(s). Surrogate standards are commonly added, at a known amount, prior to sample preparation and analysis of a field sample or a QC sample. It is important to choose a surrogate that shouldn’t be in your sample. By using a surrogate standard, it allows the sample matrix effects to be assessed by calculating recovery (%):

π‘…π‘’π‘π‘œπ‘£π‘’π‘Ÿπ‘¦ (%) = (πΆπ‘œπ‘›π‘π‘’π‘›π‘‘π‘Ÿπ‘Žπ‘‘π‘–π‘œπ‘› (π‘œπ‘Ÿ π‘Žπ‘šπ‘œπ‘’π‘›π‘‘)π‘“π‘œπ‘’π‘›π‘‘ / πΆπ‘œπ‘›π‘π‘’π‘›π‘‘π‘Ÿπ‘Žπ‘‘π‘–π‘œπ‘› (π‘œπ‘Ÿ π‘Žπ‘šπ‘œπ‘’π‘›π‘‘)π‘Žπ‘‘π‘‘π‘’π‘‘) Β x 100

The surrogate will mimic target analyte behaviour during the sample preparation and analysis. A low recovery (%) can imply that there is an issue with being able to efficiently extract the target analyte(s) from the sample matrix during sample preparation. The recovery could also indicate an issue with the instrument. Surrogate standards are used to determine if the sample matrix is causing any interference to a sample.

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