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FONTI DI ERRORE NEL CAMPIONAMENTO E NEL TRATTAMENTO DEL CAMPIONE

FONTI DI ERRORE NEL CAMPIONAMENTO E NEL TRATTAMENTO DEL CAMPIONE. Roberto Morabito ENEA – UTS PROT morabito@casaccia.enea.it. SCUOLA NAZIONALE DI CHIMICA ANALITICA PER DOTTORANDI Gargnano, 23-27 settembre 2002. ANALYTICAL METHODS FOR ENVIRONMENTAL ANALYSIS. *.

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FONTI DI ERRORE NEL CAMPIONAMENTO E NEL TRATTAMENTO DEL CAMPIONE

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  1. FONTI DI ERRORE NEL CAMPIONAMENTO E NEL TRATTAMENTO DEL CAMPIONE Roberto Morabito ENEA – UTS PROT morabito@casaccia.enea.it SCUOLA NAZIONALE DI CHIMICA ANALITICA PER DOTTORANDI Gargnano, 23-27 settembre 2002

  2. ANALYTICAL METHODS FOR ENVIRONMENTAL ANALYSIS * Sensitive enough to determine contaminants down to environmental concentrations - from ppt to ppb levels for water samples - from ppb to ppm levels for sediment and biological samples * Able to discriminate among different chemical forms in case of speciation analysis * Characterized by good precision and accuracy

  3. Incertezza(VIM 3.9, UNI ENV 13005, B.2.18) Parametro, associato al risultato di una misurazione, che caratterizza la dispersione dei valori ragionevolmente attribuibili al misurando. Incertezza tipo(UNI ENV 13005, 2.3.1) Incertezza del risultato di una misurazione espressa come scarto tipo. Incertezza tipo composta(UNI ENV 13005, 2.3.4) Incertezza tipo del risultato di una misurazione allorquando il risultato è ottenuto mediante i valori di un certo numero di altre grandezze; essa è uguale alla radice quadrata positiva di una somma di termini, che sono le varianze o le covarianze di quelle grandezze, pesate secondo la variazione del risultato della misurazione al variare di esse.

  4. Incertezza estesa(UNI ENV 13005, 2.3.5) Grandezza che definisce, intorno al risultato di una misurazione, un intervallo che ci si aspetta comprendere una frazione rilevante della distribuzione dei valori ragionevolmente attribuibili al misurando. Nota: La frazione può essere interpretata come probabilità di copertura o livello di fiducia dell’intervallo. Fattore di copertura(UNI ENV 13005, 2.3.6) Fattore numerico utilizzato come moltiplicatore dell’incertezza tipo composta, per ottenere un’incertezza estesa. Nota: Il fattore di copertura k è tipicamente nell’intervallo da 2 a 3.

  5. STIMA DELL’INCERTEZZA DEI RISULTATI ANALITICI • UNI ENV 13005 (2000) – (GUM) • APPROCCIO MISTO GUM/CONVALIDA DEL METODO • CONVALIDA DEL METODO • UTILIZZO DI MRC • UTILIZZO DI CRITERI GUIDA

  6. UNI ENV 13005 (2000) – (GUM) La valutazione dell’incertezza di un risultato può essere intrapresa solo dopo aver eliminato tutti gli effetti sistematici conosciuti Valutazione di categoria A: Analisi statistica Valutazione di categoria B: Analisi non statistica

  7. APPROCCIO MISTO GUM/CONVALIDA DEL METODO EURACHEM – Quantifying uncertainity in analytical measurements – 2nd edition, 2000 IUPAC – Technical Report from Symposium on Harmonisation of Quality Assurance Systems for Analytical Laboratories, Budapest November 1999 Valutazione di categoria A: Scarto tipo di ripetibilità

  8. CONVALIDA DEL METODO M. Thompson, Analyst, 125, 2020-2025 (2000) The “ladder of errors” Result = true value + method bias + laboratory bias + run bias + repeatibility error Risultato = valore effettivo  scostamento del metodo  scostamento del laboratorio  ripetibilità intermedia  ripetibilità stretta.

  9. UTILIZZO DI CRITERI GUIDA W. Horvitz, Anal. Chem., 69, 789-790 (1997) SR = 0.02  c 0.8495 Valore della ripetibilità stretta accettabile se compreso tra 1/2 SR e 2/3 SR

  10. UNCERTAINITY OF ENVIRONMENTAL MEASUREMENTS UT = USampling + UStorage + UTreatment + UAnalysis + ……

  11. PRIMARY SAMPLING Location : outside the laboratory Responsability : often undefined Qualification : often questionable SECONDARY SAMPLING Location : inside the laboratory Responsability : analyst Qualification : often questionable ANALYSIS Location : inside the laboratory Responsability : analyst Qualification : usually excellent

  12. SAMPLING • CROSS-CONTAMINATION • LOSSES OF ANALYTES • CHANGES IN SPECIATION • REPRESENTATIVENESS

  13. SAMPLING • NON PROBABILISTICIf some element has probability 0 to be sampled • PROBABILISTICIf all the elements have probability higher than 0 to besampled • CorrectIf all the elements have the same probability to be sampled • UncorrectIf the elements have different probability to be sampled

  14. CHOICE OF SAMPLES Filtered water - Particulate matter Undisturbed surface sediments Biota COLLECTION OF SAMPLES Sampling devices Sampling vessels NUMBER OF SAMPLES Representativeness of sampling point Representativeness of sampling area SPATIA L VARIABILITY Microlayer Interface water-sediment Vertical stratification Enclosed and flushed areas Dockyard activities TEMPORAL VARIABILITY Tides - Seasons SAMPLING STRATEGIES

  15. CHOICE OF SAMPLES Filtered water - Particulate matter Undisturbed surface sediments Biota COLLECTION OF SAMPLES Sampling devices Sampling vessels NUMBER OF SAMPLES Representativeness of sampling point Representativeness of sampling area SPATIA L VARIABILITY Microlayer Interface water-sediment Vertical stratification Enclosed and flushed areas Dockyard activities TEMPORAL VARIABILITY Tides - Seasons SAMPLING STRATEGIES

  16. CHOICE OF SAMPLES Filtered water - Particulate matter Undisturbed surface sediments Biota COLLECTION OF SAMPLES Sampling devices Sampling vessels NUMBER OF SAMPLES Representativeness of sampling point Representativeness of sampling area SPATIA L VARIABILITY Microlayer Interface water-sediment Vertical stratification Enclosed and flushed areas Dockyard activities TEMPORAL VARIABILITY Tides - Seasons SAMPLING STRATEGIES

  17. CHOICE OF SAMPLES Filtered water - Particulate matter Undisturbed surface sediments Biota COLLECTION OF SAMPLES Sampling devices Sampling vessels NUMBER OF SAMPLES Representativeness of sampling point Representativeness of sampling area SPATIA L VARIABILITY Microlayer Interface water-sediment Vertical stratification Enclosed and flushed areas Dockyard activities TEMPORAL VARIABILITY Tides - Seasons SAMPLING STRATEGIES

  18. Reduced direct inputs Resuspension Degassing Typical Tributyltin concentration profile in the sediments Depth

  19. CHOICE OF SAMPLES Filtered water - Particulate matter Undisturbed surface sediments Biota COLLECTION OF SAMPLES Sampling devices Sampling vessels NUMBER OF SAMPLES Representativeness of sampling point Representativeness of sampling area SPATIA L VARIABILITY Microlayer Interface water-sediment Vertical stratification Enclosed and flushed areas Dockyard activities TEMPORAL VARIABILITY Tides - Seasons SAMPLING STRATEGIES

  20. CRITICAL STEPS IN ENVIRONMENTAL ANALYSIS • SAMPLING • STORAGE • TREATMENT Extraction Derivatization Clean up • ANALYSIS

  21. Adsorption on the container walls Contamination from the container walls Stability SAMPLE STORAGE

  22. WATER SAMPLES Temperature Type of containers pH BIOLOGICAL AND SEDIMENT SAMPLES Temperature Type of containers Humidity PARAMETERS CONTROLLING THE STABILITY OF ANALYTES DURING STORAGE

  23. SELECTION OF METHODSFOR SPECIATION ANALYSIS Stabilisation / Storage (examples) «  Preserver the original speciation » STABILISATION - Acidification (waters) - - Lyophilisation (biota, sediments, etc.) - - Oven-drying (sediments) - - Pasteurisation (sediments) - - Gamma-irradiation (biota, sediments,waters) - STORAGE - Freezing recommended for butyl- and phenyl-tins (biota, sediments) - - Storage at ambient temperature (n the dark) for MeHg - - Storage at 4°C in the dark for trimethyllead -

  24. STABILITY STUDIES STABILITY STUDIES STABILITY STUDIES STABILITY STUDIES WATER SAMPLES e.g. Se VI, Se IV Sb V, Sb III As V, As III, Organoarsenic compounds Organolead compounds Organotin compounds SEDIMENT SAMPLES e.g. Methylmercury Organotin compounds BIOLOGICAL SAMPLES e.g. Cr VI, Cr III Methylmercury Organotin compounds Arsenobetaine, DMA

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