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Lecture 7

CHEM 212 Lecture Notes - Lecture 7: Isotope Dilution, Internal Standard, Observational ErrorPremium

3 pages45 viewsFall 2016

Department
Chemistry
Course Code
CHEM 212
Professor
Hamilton M.Coreen
Lecture
7

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Chem 212!
Lecture 7!
Sept. 21/2016!
!
Approaches to Quantification:
External Standard!1.
Internal Standard!2.
Standard Addition!3.
!
External Standard:!
-Calibration standards!
-Calibration curves (plot of signal versus concentration)!
-Use calibration curve to "Read" many unknown samples!
!
-avoid single point calibration for deterring Sa =kna or Sa=k[A]!
-Single point assumes linear relationship over large concentration range => Not good!
"-potential error by assuming calibration curve goes through zero!
"-error in reported concentration is dierence in x-axis=>accuracy problem!
""-can not solve this by replicates=>systematic error !
!
-Multipoint external standard method:!
-requires cal curve!
-hopefully y=mx+b or a non-linear equation!
-use least squares on spreadsheet!
-never assume linearity beyond calibration points:!
-dont assume anything past 1st and last calibration points because you don't know if linearity continues forever!
"-sample signal must be in range if too high then dilute it!
-Calibration standards must "bracket" unknown concentration!
"-if its not then fix it adding more sample or diluting sample!
-May need "Matrix Matched" standards or use "method of standard additions"!
"-matrix may have some kind of interference!
"-instead of making standard in reagent water I would also have some other materials to mimic the blood!
!
-Internal standard does nothing for matrix interference!
-if complex matrix you probably won't want to use external standard curve!
!
Internal Standard Quantification:!
-Add a known amount of internal standard to the unknown sample!
"-a related compound that is not the analyte is added to the sample before you do anything else!
-Signal from analyute is compared to signal from internal standard to determine amount of analyte in sample!
-Isotope dilution is the ideal way but not only way!
"-Measure Calcium with magnesium as IS!
"-Pick something that has same chemical and physical properties for internal standard!
!
!
""""" I.S.!
""""""""Analyte (75%)!
!
!
!
!
!
!
""""" 100ng" " 75ng!
!
-measure by ratio compared to standard!
-if we spill sample the ratio is still relevant so we still know how much we started with!
-CON: hard to find analyte and internal signal do not always give you the same intensity of signals!
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