Chemistry 155 Introduction to Instrumental Analytical Chemistry

Chem 155 Unit 1 Page 52 of 3131.15 Internal StandardsInternal standards can correct for sampling, injection, optical pathlength and other instrument sensitivity variations.An internal standard is a substance added (or simply present) inconstant concentration in all samples, and standards.When something unexpected decreases the sensitivity of theinstrument (m) so the signal drops (S = mC + S B ) – it can beimpossible to distinguish this from a change in analyte concentrationwithout an internal standard.Consider the ratio of the blank corrected analyte ( S'S'ANmANCANCinternal standard (IS) signals (S): = = kS'm C CISISISANANIS= SAN− sB) to theAssumes for the moment that k is a constant, i.e. invariant to factorsaffecting overall instrumental sensitivity. As an example, let’sconsider k to be a correction for injection volume in achromatographic system. It is perfectly reasonable to assume that anaccidentally low or high injection volume would affect the internalstandard and analyte signals identically – e.g. if a given injection were6% high, then both analyte and internal standard peaks would be 6%larger than expected.If k is invariant to instrument fluctuations, then the true analyteconcentration can always be derived from the ratio of the correctedsignals so long as the internal standard concentration remainsconstant.Anlalyte conc. in a sample.S CC = 'AN ISANS'ISkwhere C ISis easily derived from a previously measuredkcalibration standard for which the analyte signal ( S'A − STD) andconcentration ( CA − STD) of the analyte and internal standardS'A−STDCIS−STDmAN( S'IS− STD, CIS−STD) are known: k ==S'C mIS−STDA−STDISPage 52 of 313From a calibration standard.