C_30J_Topic_2_Contamination_and_interferences_part_1

C_30J_Topic_2_Contamination_and_interferences_part_1 -...

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Topic 2 – Contamination and Interferences A contaminant : something that is inadvertently added to or taken from the sample during the sampling and/or analytical process: – may or may not affect the analytical result. In the field Sample collection (containers, equipment, cross contamination, … Sample handling (filtration, sieving, Preservation (acids, poisons, … Sample storage and transportation (adsorption/desorption, evaporation, leakage, precipitation, reactions?? … In the Laboratory a) Sample pretreatment: filtration, sieving, grinding, digestion (reagents / volatilization / …) transferals, drying, dust, …possible from each operation. b) Analysis: equipment, reagents and solvents, ambient conditions, sample carry-over/cross-contamination, Control contamination by good field and laboratory practices and monitor by: Field: blanks (matrix matched reference materials, rinsings, containers, ), in-field spiking, co-located samples, control site samples, … Lab: RMs, blanks, spiking, duplicates, cleanliness,
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Interferences Interferant: A component of a sample that prevents direct measurement of the analyte concentration. Additive enhance (±) the analyte “signal” by a fixed amount A contaminant, or sample component other than the analyte, that the “detector” responds to. Multiplicative – a sample component(s) that interacts with the analyte to increase or decrease the analyte “signal” by a factor but does not generate a “signal” of its (their) own. y = m x + c Must be removed or quantified Can be either removed or can be corrected for by standard additions techniques (matrix correction standard additions) Standard methods generally list known interferences, part of the validation process; so generally the possibility of their presence can be predicted.
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Three ways to use Standard Additions: a) Sampling and analytical method quality control - (recovery calculations: many possible points for the addition) (amount analyte in spiked sample) – (amount analyte in sample) = (amount analyte added). b) Allow for matrix effects – for multiplicative interferences. c) Quantification of analyte concentrations *** a) and c) only meaningful if b) does not exist. *** a) Standard additions in the absence of interferences. Add a known amount of analyte to a sample and determine [analyte] in the sample and in the spiked sample. If the analysis has gone successfully then the difference between the two concentrations should be due to the amount of analyte added. S
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This note was uploaded on 02/23/2011 for the course CHEMISTRY C30J taught by Professor Vaugnrattray during the Spring '11 term at University of the West Indies at Mona.

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C_30J_Topic_2_Contamination_and_interferences_part_1 -...

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