Naming of Standards in Analytical Chemistry

May 26th, 2020 in Learning

I hope this table helps you understand how standards are named within analytical methods. Standards are generally named according to their functions in an analytical method. 

Basic rule: A "standard" is added to a sample in order to estimate the effect of any sample matrix and handling on the levels of analytes in a sample. 

An important note: In the table below, there is "before sample processing" and "after all sample processing". This is a limitation since sample processing is made of many steps (extraction, cleanup, concentration) and each step could have its own standards and should if a step is known to have matrix-dependent effects on analyte levels.

Method type

Standard added before sample processing

Standard added after all sample processing

Quantification Example Method
Isotopic Dilution Method "Internal Standard" (IS) "Recovery Standard" (RS) IS quantifies native; RS quantifies IS.

EPA 1613B (PCDD/F)
Internal Standard Method "Surrogate Standard" (SS) "Internal Standard" (IS) IS quantifies native and SS; SS indicates efficiency of handling and matrix effects

EPA 537.1 (PFAS)
Isotopic dilution + internal standard method "Internal Standard" (IS) "Labelled internal injection standard"

IS quantifies native; LIIS quantifies IS

EPA 1614A (PBDE)
Isotopic dilution + internal standard method

"Internal Standard" (IS) "Labelled internal injection standard"
IS quantifies native; LIIS quantifies IS

EPA 1668C (PCB)
External Standard Method "Surrogate Standard" (SS) NA: No internal standard added Native is quantified directly off a calibration curve (not corrected with internal standard); SS indicates extraction efficiencies of natives EPA 8327 (PFAS)

Essentially, a standard is used to determine the effect of matrix, handling, processing or degradation of analyte(s). You add a known amount of the standard to a sample before processing it. The result is that any loss/enhancement of that standard mirrors the loss/enhancement of a native analyte in the sample, assuming the standard and the native analyte behave identically throughout the handling or processing. The icing on the cake is that if your standard is an isotopically-labelled version of the analyte of interest, then you can guarantee that the loss/enhancement of the standard is identical to the loss/enhancement of the native (with a small number of exceptions). Therefore, any step throughout the sample handling process could have its own standard and should if the loss/enhancement of an analyte in the processing step is thought to be highly covariant with the handling precision (imprecise handling, large error margins, equipment error). Any error becomes extremely important when analyzing trace levels of analytes.

In summary, the standard added to samples prior to any handling is known as "internal standard", "surrogate standard", and sometimes "extraction standard". Some methods (e.g. EPA 1613B) use a "cleanup standard" to monitor the performance of cleanup after extraction. The standards added just prior to chromatography have been named "recovery standard", "internal standard" and "labelled injection internal standard" or simply "injection standard".