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Queen's University
ENCH 213
Diane Beauchemin

Externalcalibration:series ofstandard solutions, and samples → not good whenhave complexmatrices Method of standard addition and basics of quality assurance Detection limit • detection limit = minimum detectable concentration under ideal conditions ≡ 3s/m (sensitivity) ◦ where: s = standard deviation for n≥ 7 replicate measurements of a sample whose concentration is 1-5 times the detection limit • lower quantitation limit (aka the determination limit) ss= smallest concentration that can be measured with reasonable accuracy ≡ 10s/m • method detection limit > detection limit ◦ s = standard deviation for n≥ 7 individual samples, where each one is analyzed once Selectivity (or specificity) • ability of an analytical method to distinguish the analyte from everything else in the sample • avoids interference • robustness: ability of an analytical method to be unaffected by small, deliberate changes in operating parameters Ways to demonstrate accuracy • analyze a certified reference material with a similar matrix to your sample: the concentration found should agree with the certified value within the method’s precision • compare results from two or more independent analytical methods: they should agree within their precision. • analyze 3 replicates matrix blank spiked with a known amount of analyte at 3 levels spanning the expected concentration range. • use the method of standard additions. Precision • instrument precision = when the same quantity of sample is introduced ≥10 times • intra-assay precision = when one person analyzes aliquots of a homogeneous sample several times on one day with the same equipment • intermediate precision = when different persons analyze the same sample on different days with different equipment in a lab. • interlaboratory precision = when different persons analyze the same sample in different labs. Control chart Quality assurance with control charts • Aprocess is shut down if: ◦ 1 observation outside the action lines ◦ 2 out of 3 consecutive measurements between the warning and action lines ◦ 7 consecutive measurements all above or all below the center line ◦ 6 consecutive measurements all increasing or all decreasing ◦ 14 consecutive points alternating up and down ◦ an obvious nonrandom pattern Use of spike recovery to assess matrix effect • matrix = everything else in the sample other than the analyte • spike the sample with known amount of analyte • check its recovery: ◦ %recovery = (C spike sampleunspiked sampadded ◦ can have signal supression or enhancement • %recovery ≠ 100%: ◦ matrix effect i.e., matrix interferes with analysis Matrix effect • the ground water matrix suppresses the sensitivity for the analyte Standard addition when the analysis consumes solution • an increasing known amount of analyte is added to constant aliquots of the sample. • the total volume of all the solutions is kept constant to keep the matrix constant. Standard addition when the analysis consumes solution • the final concentration is found by extrapolation • when y=0=mx+b then x=-b/m Standard addition equa
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