Consensus Standards, Consensus Standards, Method Uncertainty and Method Uncertainty and

Quality Assurance in Quality Assurance in Analytical Chemistry Analytical Chemistry

CurriculaCurriculaKevin Ashley, Ph.D.Kevin Ashley, Ph.D.

U.S. Department of Health and Human ServicesU.S. Department of Health and Human ServicesCenters for Disease Control and PreventionCenters for Disease Control and Prevention

National Institute for Occupational Safety and HealthNational Institute for Occupational Safety and HealthCincinnati, OhioCincinnati, Ohio

DisclaimerDisclaimer

Mention of company or product Mention of company or product names does not constitute names does not constitute endorsement by the Centers for endorsement by the Centers for Disease Control and Prevention.Disease Control and Prevention. The The findings and conclusions in this findings and conclusions in this presentation are those of the speaker presentation are those of the speaker and do not necessarily represent the and do not necessarily represent the views of the National Institute for views of the National Institute for Occupational Safety and Health.Occupational Safety and Health.

IntroductionIntroduction

Overview of voluntary consensus standardsOverview of voluntary consensus standards What are they & why are they important?What are they & why are they important?

QA/QC in analytical laboratoryQA/QC in analytical laboratory Need defensible results; verify with QA/QC dataNeed defensible results; verify with QA/QC data

Treatment of analytical method uncertaintyTreatment of analytical method uncertainty Vitally important in commercial / gov’t labVitally important in commercial / gov’t lab

Include ideas in analytical curriculaInclude ideas in analytical curricula Give students background in aspects that they’ll Give students background in aspects that they’ll

have to address in ‘real world’ lab.have to address in ‘real world’ lab.

What are voluntary What are voluntary consensus standards?consensus standards?

Technical documents developed by consensus:Technical documents developed by consensus:

■ ■ Test methods, specifications, guides, practices, Test methods, specifications, guides, practices, terminologyterminology

■ ■ Based on best science for tech transfer:Based on best science for tech transfer: “ “Research to Practice”Research to Practice”

→ → Establish standard of care in field (i.e., analysis)Establish standard of care in field (i.e., analysis)

Voluntary consensus standards Voluntary consensus standards in analytical chemistryin analytical chemistry

Non-profit, non-governmental organizationsNon-profit, non-governmental organizations

Global forums for developing voluntary Global forums for developing voluntary consensus standards (e.g., ASTM consensus standards (e.g., ASTM International, ISO)International, ISO)

Methods/procedures for sampling, sample Methods/procedures for sampling, sample preparation, analysis, estimation of preparation, analysis, estimation of uncertainty, QA/QC requirementsuncertainty, QA/QC requirements

Importance of Importance of StandardizationStandardization

Conformity in methods for sampling and Conformity in methods for sampling and analysisanalysis

Normalized protocols describing Normalized protocols describing laboratory quality assurance / quality laboratory quality assurance / quality controlcontrol

Aim to ensure / maximize data quality, Aim to ensure / maximize data quality, minimize overall analytical uncertaintyminimize overall analytical uncertainty

Provide evidence of acceptable Provide evidence of acceptable performanceperformance

Analytical Method Evaluation Analytical Method Evaluation & Validation& Validation

Literature Search Analysis Development Sampler Development Method EvaluationMethod Evaluation

Analyte RecoveryAnalyte Recovery Sample StabilitySample Stability Limit of DetectionLimit of Detection

Sample GenerationSample Generation Sampler CapacitySampler Capacity Dynamic rangeDynamic range Precision & Bias / Precision & Bias /

UncertaintyUncertainty InterferencesInterferences Reference materialsReference materials

Field TestingField Testing

■ Interlaboratory testing ► Standardization

Sampling & Sample Sampling & Sample Preparation (workplace air)Preparation (workplace air)

Analytical Method Analytical Method DevelopmentDevelopment

ChromatographyChromatography SpectroscopySpectroscopy ElectrochemistryElectrochemistry Bioanalytical chemistryBioanalytical chemistry Other exs.Other exs.

Relative Contributions to Relative Contributions to Analytical ErrorAnalytical Error

Sampling Sample preparation Analysis

Significant error Intermediate errorSmall error

Overall (expanded) error = ∑ (individual errors)

Quality Assurance/ Quality Quality Assurance/ Quality ControlControl

Quality of Sampling and Analytical Method ResultsQuality of Sampling and Analytical Method Results Quality of results limited by method, analytical laboratory Quality of results limited by method, analytical laboratory

and analystand analyst Method evaluation helps define method performanceMethod evaluation helps define method performance Analyst and analytical laboratory contribution to quality Analyst and analytical laboratory contribution to quality

measured by:measured by: Quality Control ProgramQuality Control Program→ → Internal - spiked samples, field blanks, control chartsInternal - spiked samples, field blanks, control charts→ → External - participation in proficiency test programsExternal - participation in proficiency test programs

► Consensus standards describe these elements, processes & procedures

Ex. of an ASTM International standard

D7035: Standard Test Method for Determination of Metals and Metalloids in Airborne Particulate Matter by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)

ScopeReferenced StandardsTerminologySummary of MethodSignificance & UseApparatus & MaterialsSamplingSample Preparation

AnalysisExpression of ResultsMethod PerformanceRecordsReportingAppendicesAnnexesReferences

Analytical Method Analytical Method UncertaintyUncertainty

Precision & BiasPrecision & BiasASTM International:ASTM International: ASTM E691 – ASTM E691 – Standard Practice Standard Practice for Conducting an Interlaboratory Study to for Conducting an Interlaboratory Study to Determine the Precision of a Test MethodDetermine the Precision of a Test Method

UncertaintyUncertaintyInternational Organization for Standardization: Guide to the Expression of Uncertainty in Measurement (ISO/GUM)

Analytical PrecisionAnalytical Precision

For n analyses, determine μ, s & RSD (CV)Propagation of random errors:Propagation of random errors:

sstottot = [ = [∑ ssii22

]]½½ = [ = [ss22(weighing) + ss22(pipetting) + ss22(dilutions) + ss22(analysis) + ssi’i’

22]]½½

Then estimate (95%) CL’sPerform outlier tests (etc.)

→ → For estimation of overall method precision, also include For estimation of overall method precision, also include estimate of contribution to error from estimate of contribution to error from samplingsampling

Limits of Detection and Limits of Detection and QuantitationQuantitation

LOD LOQAnalyte conc.

95% Confidence Limits

Calibration Line

Response

Analytical BiasAnalytical Bias BBii = ( = (μμii – – RRii) ) RRii

-1-1 Estimate by comparison of measured results Estimate by comparison of measured results

against reference analytical methodagainst reference analytical method Estimate by analysis of reference materialsEstimate by analysis of reference materials

Is bias negligible?; ideally <<10% (+ or –)Is bias negligible?; ideally <<10% (+ or –) Is bias correctable or not?Is bias correctable or not?

Is cause of systematic error identifiable and, if so, Is cause of systematic error identifiable and, if so, does corrective action ameliorate the problemdoes corrective action ameliorate the problem

Consider contributions to bias from steps Consider contributions to bias from steps preceding analysis (e.g., sampling errors, stability preceding analysis (e.g., sampling errors, stability & transport)& transport)

UncertaintyUncertainty

“Type A” evaluation: u = s/√nApplicable when n repeat measurements are possible.

► Exs: Compute std. dev. from repeated absorbance or temperature measurements

“Type B” evaluation: u = s/√3Applicable when upper & lower limits are known (or estimated) but repeat measurements cannot be made. ► Exs: Volumetric glassware tolerances; analytical balance

+/– specs

Uncertainty Budget & Uncertainty Budget & Expanded UncertaintyExpanded Uncertainty

Include contributions of all aspects Include contributions of all aspects uu11, , uu22, , uu33, etc., etc.

to measurement uncertaintyto measurement uncertainty

Compute Compute combined uncertaintycombined uncertainty::

Uc = [u12 + u2

2 + u32 +... + (etc.)]½

Then obtain Then obtain expanded uncertainty expanded uncertainty with coverage with coverage factor factor kk::

U = kUc

Comparison of Uncertainty Comparison of Uncertainty Estimate to Acceptance Estimate to Acceptance

CriteriaCriteria

5

15

25

35

A B C

AcceptedRejected

Inconclusive

95% Confidence Limit

Uncertainty (%)

Method

5% Confidence Limit

AcceptanceCriterion →

Example: Method for Example: Method for Organonitrogen Pesticides in Organonitrogen Pesticides in

AirAir

0 10 20 30 40 50

Carbendazim

Thiobencarb

Propoxur

Diuron

Propham

Methomyl

Oxamyl

Formetanate

Chlorpropham

Captan

Methiocarb

Aldicarb

Carbaryl

Carbofuran

Methomyl

Carbendazim

5% Lower Confidence Limit

95% Upper Confidence Limit

Uncertainty (Accuracy)

← 25% acceptance criterion

SummarySummary Overview of use of analytical uncertainty, QA/QC & Overview of use of analytical uncertainty, QA/QC &

consensus standardsconsensus standards

Criteria for acceptable analytical performanceCriteria for acceptable analytical performance

Myriad applications & requirements in commercial & Myriad applications & requirements in commercial & government laboratoriesgovernment laboratories

Coverage of statistics in analytical chemistry curricula Coverage of statistics in analytical chemistry curricula could be augmented by discussion of these areascould be augmented by discussion of these areas

Questions / Comments?Questions / Comments?

Contact InfoContact Info::

Dr. Kevin AshleyDr. Kevin Ashley

U.S. Department of Health and Human ServicesU.S. Department of Health and Human Services

Centers for Disease Control and PreventionCenters for Disease Control and Prevention

National Institute for Occupational Safety and Health National Institute for Occupational Safety and Health

4676 Columbia Parkway, Mail Stop R-74676 Columbia Parkway, Mail Stop R-7

Cincinnati, Ohio 45226-1998 (USA)Cincinnati, Ohio 45226-1998 (USA)

Tel. +1(513)841-4402; e-mail: Tel. +1(513)841-4402; e-mail: KAshley@cdc.govKAshley@cdc.gov

www.cdc.gov/nioshwww.cdc.gov/niosh