quality control in measurements tom colella clas goldwater environmental lab

Quality Control In Measurements

Tom Colella

CLAS Goldwater Environmental Lab


• Measurement

• Comparison of unknown to known standard

• Validity depends on quality of standard

• Usually indirect, using calibrated system


• Calibration– What?• Response of measurement system related

to amount of analyte

– Why?• Eliminate BIAS

– When?• Depends on nature of measurement system

& frequency of use


• Calibration Standards...

– General Considerations• Purity• Stability• Storage• Homogeneity• Representativeness



– Stock Standard Preparation

• Use calibrated equipment (Class M/A, TC/TD)

• Consider stability vs. dilution error

• Work in appropriate measurement range



– Matrix Matching• Prepare standards in same matrix as sample

• Applies to extracts, digests, preservation, synthetic (lab) samples

• NOT possible for natural samples



– Process in sample preparation or use process controls



– Range and distribution

• Select high and low standards• Bracket samples with standards• Determine # of standards• Distribute evenly throughout range• Do not extrapolate



– External Standards• Same analyte prepared and measured

separately from samples • Used to generate cal curve or response

factor


• Calibration Standards...– Internal Standards• Related analyte added to each

calibrant, sample, QC; measured simultaneously– Corrects errors due to variation

in measurement technique– Must behave similar to analyte– Use with instruments that

measure multiple analytes simultaneously


• Calibration Standards…– Method of Standard

Additions• Form of internal calibration• Corrects matrix interference

errors due to mismatch• Analyte of interest is added to

samples at different levels• Concentration = negative x

intercept


• Calibration Standards..

– Calibration acceptance criteria

• Response Factors– % CV

• Calibration Curves– Correlation Coeff.

Conc. Signal Response Factor

10 ppb 1025 V 102.5 V/ppb

20 1980 99.0

30 2890 96.3

40 4103 102.6

CV = 3.0%


• QC Samples– Categories of error

• Contamination

• Calibration Drift

• Bias

• Imprecision


• QC Samples...– Contamination• Field Blanks

– Trip Blank

– Equipment Blank


• QC Samples...– Contamination• Lab Blanks

– Storage Blank

– Instrument Blank

– Method Blank


• QC Samples...– Calibration Drift• Initial Calibration Verification (ICV)

• Continuing Calibration Verification (CCV)


• QC Samples...– Bias• Spikes

– Lab Fortified Matrix (LFM, “Spike”)

– Trip Spike


• QC Samples...– Bias• External Source QC (QCS, LFB, CRM)

– SPEX, VHG, ERA, Ultra, NIST


• QC Samples…– Bias• Surrogates


• QC Samples…– Imprecision• Replicates - Combine with External Source

QC (LFM)

quality control in measurements tom colella clas goldwater environmental lab

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