Use of Surrogate Internal Standards

Under the assumption that and l is are both zero, the theoretical expressions for the analytical signals for analyte and SIS in a cleaned-up extract of a spiked sample are  

On dividing these two equations, (y N ) cancels exactly for each individual analytical sample since analyte and SIS are present in the same extract solution  

A value for Q3 from Equation [8.83c] also requires knowledge of, or assumptions regarding the recovery parameters F, FsIs L and Lsu. Some experimental information can be obtained if sufficient sample is available that several aliquots can be analyzed using a range of values of Qsis- since Equation [8.83b] predicts that a plot of R sis vs Qsis provides a value of (Lsis7Fsis ) from the ratio of the intercept to the slope values for Fsis and 

Lgjs separately can be obtained from the same experimental data if independent information on (Ssis//sis) and (y rV ) is available. The most usual procedure, however, is to assume that F = Fj (in most cases Fsis F if the SIS is an isotopolog, reflecting possible occlusion effects) and that the constant loss parameters Lf and Ljij are both zero. (Note that it is possible that nonzero values of Lf and Lju may interact with one another, due to competition for the active sites in the clean-up procedure.) Finally, the best accuracy and precision are achieved when is determined by direct weighing of an SIS of known purity (both chemical and isotopic) but it is common practice to dispense trace level quantities as measured volumes v jg of a solution of concentration Cgig. Then Qgjg = v i. C is, so that resulting errors and uncertainties in are reflected directly in the values deduced for Q.  

In summary, the simple method employing a SIS spiked into the analytical sample without use of a calibration standard, together with the various assumptions and approximations described above, gives a value for Q given by  

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