Chemistry 155 Introduction to Instrumental Analytical Chemistry

Chem 155 Unit 1 Page 53 of 313In other words, if C IS is held constant in all experiments, then theratio of the analyte to internal standard signals will beindependent of instrument sensitivity.When to use an internal standard?When substantial influence on instrument sensitivity is expected dueto variation in things like:Sample matrixTemperatureDetector sensitivityInjection volumeAmplifier (electronics) drift Flow rateAn internal standard must:a. not interfere with your analyteb. ideally have the same dependence on the chemical matrix,temperature (or other troublesome variable) as the analyte.Consider the ubiquitous ‘salt plate’ IR sampling method. A drop ofanalyte is sandwiched between two salt plates and this is placed inthe IR beam. The path-length is highly variable from experiment toexperiment. The signal A = εbC where ε is characteristic of themolecule, b is pathlength and C is concentration. If you are doing anexperiment to measure the increase in amide formation versus timeby the intensity of the amide bands near 1700 cm -1 . You could takesamples and measure them periodically, but the variability of thepathlength would distort the results.On the other hand, if all the samples were spiked with the sameamount of acetonitrile then the sharp nitrile stretch at 2250 cm -1 couldbe used as an internal standard.Page 53 of 313