Surrogate Internal Standard SIS

Surrogate Internal Standard (SIS), often just referred to as Internal Standard (IS) A compound of known chemical purity that is added, in a known amount, to the sample for analysis before any extraction and work-up procedures used to monitor and correct for analyte losses and variations in instrumental sensitivity with mass spec-Iromelric detection isotope-labeled versions of the analyte (isotopologs) are the best (this procedure is referred to as isotope dilution mass spectrometry, IDMS) but isomers or analogs can be used, see Table 2.1. 

The importance of the surrogate internal standard (SIS) has been discussed throughout this book (Sections 2.2.3, 2.6.3 and 8.5.2b) and the type and availability of the SIS wiU have a profound impact on method design and rehabUity. In addition to what has been discussed previously, several practical considerations with respect to the SIS are covered in Section 9.4.5. 

The need for a surrogate internal standard (SIS) and the ways in which it is used in practice were discussed in Sections 2.6.3 and 8.5.2. Questions concerning choice and availability of an SIS were introduced in Section 2.2.3 and Table 2.1, and are discussed further below. All of the concerns about chemical and chiral purity, applicable to the analytical standard (Section 9.4.4), apply also to the SIS however, the absolute chemical purity is not as crucial for the SIS in cases where both calibration and measurement are performed using response ratios of analyte SIS where the absolute quantity of SIS detected does not enter into the final determination (Section 8.5.2b, essentially the signal from the SIS is used as a normalizing factor for the analyte responses). In the few instances where a volumetric internal standard is required (Sections 2.2.4 and 8.5.2a) the only concern is that the VIS is stable and provides no interferences (direct or indirect) that co-elute with the analyte. 

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