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Lecture_overheads_-_Ch05

Lecture_overheads_-_Ch05 - CHEM 321 Quantitative Analysis...

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CHEM 321 – Quantitative Analysis Ch. 5 – Quality Assurance & Calibration Methods 5-1 Basics of Quality Assurance 5-2 Method Validation Accuracy Precision Selectivity/Specificity Linearity Limit of Detection (LOD) Overview of Calibration Methods 5-4 Internal Standards (compensates for drift) 5-3 Standard Addition (compensates for matrix effects)
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5-1 Quality Assurance (QA) BASIC REQUIREMENTS Are your data trustworthy? Good accuracy (implying that results are correct ) Good precision (implying that samples are representative ) No false positives (analyte incorrectly indicated as present ) No false negatives (analyte incorrectly indicated as absent ) ADVANCED REQUIREMENTS Can your analysis stand up to legal scrutiny? Confirmation of the presence of the analyte of interest using generally- accepted scientific standards Determination of the concentration of the analyte using appropriate methods (i.e., cal curve, internal standard, and/or standard additions) Documentation in order (SOPs, QA/QC, control charts, validated method, analysts trained/certified, etc.) OJ Simpson Trial (EDTA in blood sample)
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Why do QA/QC? To Avoid the Worst Case Scenario!
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How accurate are results from “accredited labs”? Certified river water sample containing = 62. 3 ± 1. 3 nM Pb Analyzed by 181 labs and by 9 labs with rigorous QA/QC programs Take home lesson: There is no guarantee that the results (even from accredited labs) are reliable Must periodically assess accuracy with blind check samples
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API 5-2 Method Validation Once the data quality objectives of a particular application are specified , must ensure that the method used is acceptable for the intended purpose (i.e., assess analytical figures of merit) Accuracy Assessed by analyzing SRMs and computing % error Assessed by analyzing spike and computing % recovery Precision Assessed by analyzing replicates and computing %RSD < 1% or better using best techniques (internal standards) often ~20% at ppm levels, ~50% at ppb levels Selectivity Ability of method to discriminate between analyte (Specificity) and other potentially interfering species that might be present in the sample matrix Linearity Linearity of response as function of concentration (aka R 2 ) Linear region of cal curve (aka dynamic range) LOD Above this level, analyte detected Below this level, analyte not detected (ND)
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Selectivity of Analytical Methods Gravimetric determination of Cl - - not a selective/specific method AgNO 3 can precipitate out other halide ions (Br - (aq) + Ag + (aq) → AgBr(s)) Titrimetric determination of Ca - not a specific method EDTA can complex other divalent cations (Mg 2+ (aq) + Y 4- (aq) → MgY 2- (aq)) Glass pH electrode - more specific method Glass designed to respond specifically to H + , but not under all conditions (alkaline error: at low [H + ] and high pH, electrode responds to Na + ) Fluoride via ISE - more specific method
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