8.4.4 Alternately, subtract the instrumental signal or externalcalibration-
derived concentration of the unknown
(unspiked) sample from the instrumental signals or
external-calibration-derived concentrations of the standard
additions. Plot or subject to linear regression of the
corrected instrument signals or external-calibrationderived
concentrations as the dependant variable versus
the independent variable. Derive concentrations for
unknowns using the internal calibration curve as if it were
an external calibration curve.