Principles of Quantifying Lipid Species by Mass Spectrometry

2 PRINCIPLES OF QUANTIFYING LIPID SPECIES BY MASS SPECTROMETRY 

Quantification of the concentration of an analyte with MS analysis, in principle, employs a correlation between the concentration and the ion intensity of the analyte, which is linear within a predetermined linear dynamic range  

Therefore, quantification of an analyte by MS analysis usually requires comparisons to either an external standard that is itself in most cases or an internal standard that is an analog to the analyte (e.g., its stable isotopologue). When an external standard is used, a calibration curve of the standard is established at a series of concentrations each of which should be analyzed under identical conditions that will be applied to the MS analysis of the analyte of interest. When an internal standard is used, the standard is added at the earliest step possible during sample preparation and its concentration should be in an appropriate ratio to the analyte (see Section 13.3.2). 

The advantage of using an internal standard is its simplicity and accuracy resulting from its being processed and analyzed simultaneously with the analyte of interest. However, selection of an appropriate internal standard might be difficult because different systems may need different internal standards and specifically synthesized standards may be necessary to avoid any potential overlap with endogenous species in the analyzed system. Moreover, addition of an appropriate amount of internal standard is not straightforward, but needs some expertise and predetermination (see Section 13.3.2). 

When a standard is used for quantification of the concentration of an analyte, many researchers simply use a formula of ratiometric comparison  

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