Chemistry 155 Introduction to Instrumental Analytical Chemistry

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Chem 155 Unit 1 Page 46 of 3131.13 Reagent Blanks for High Accuracy:A reagent blank is a blank that is made by doingeverything for the sample prep etc. but without thesample:Acids etc usedin sample prep.digestfilterDilute to100 mLvolumeAnalyze 2.3ppm0.23 mgFe found“No Sample” sample.Ultrapurewater blankIn this example the reagent blank is analyzed againstultrapure water. The 0.23 mg Fe found in the reagentblank may be due to Fe impurities in the acids, but italso may be a matrix effect. In either case, itsuggests that we should do what in order to arrive ata more accurate result?Either:a. analyze all standards and samplesagainst reagent blanks orb. subtract the signal from the reagentblank against result from sample resultsmade with pure water blanks!Page 46 of 313
Chem 155 Unit 1 Page 47 of 3131.14 Standard additions fix matrix effects:Use the method of standard additions when matrixeffects degrade the accuracy of the calibration curves.There is one important assumption built into thecalibration curve idea. That assumption is thefollowing:the sensitivity of the instrument to the analyte in the standards isEQUAL TOthe sensitivity of the instrument to the analyte in the sample matrixWhy would the sensitivity be different?1. Many Instruments are sensitive to things like:1. pH 2. ionic strength 3.organiccomponents of the solvent matrix2. Sometimes other chemicals can change thecalibration sensitivity by:chemically binding to or interacting with theanalyte atom/moleculeThese two things are examples of:Matrix EffectsPage 47 of 313
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Chemistry 155 Introduction to Instrumental Analytical Chemistry

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