Quality%20Control%20In%20Measurements

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Quality Control In Measurements

• Tom Colella
• CLAS Goldwater Environmental Lab

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Quality Control In Measurements

• Measurement
• Comparison of unknown to known standard
• Validity depends on quality of standard
• Usually indirect, using calibrated system

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Quality Control In Measurements

• Calibration
• What?
• Response of measurement system related to amount
  of analyte
• Why?
• Eliminate BIAS
• When?
• Depends on nature of measurement system
  frequency of use

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Quality Control In Measurements

• Calibration Standards...
• General Considerations
• Purity
• Stability
• Storage
• Homogeneity
• Representativeness

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Quality Control In Measurements

• Calibration Standards...
• Stock Standard Preparation
• Use calibrated equipment (Class M/A, TC/TD)
• Consider stability vs. dilution error
• Work in appropriate measurement range

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Quality Control In Measurements

• Calibration Standards...
• Matrix Matching
• Prepare standards in same matrix as sample
• Applies to extracts, digests, preservation,
  synthetic (lab) samples
• NOT possible for natural samples

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Quality Control In Measurements

• Calibration Standards...
• Process in sample preparation or use process
  controls

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Quality Control In Measurements

• Calibration Standards...
• Range and distribution
• Select high and low standards
• Bracket samples with standards
• Determine of standards
• Distribute evenly throughout range
• Do not extrapolate

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Quality Control In Measurements

• Calibration Standards...
• External Standards
• Same analyte prepared and measured separately
  from samples
• Used to generate cal curve or response factor

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Quality Control In Measurements

• 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

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Quality Control In Measurements

• 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

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Quality Control In Measurements

• 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
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Quality Control In Measurements

• QC Samples
• Categories of error
• Contamination
• Calibration Drift
• Bias
• Imprecision

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Quality Control In Measurements

• QC Samples...
• Contamination
• Field Blanks
• Trip Blank
• Equipment Blank

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Quality Control In Measurements

• QC Samples...
• Contamination
• Lab Blanks
• Storage Blank
• Instrument Blank
• Method Blank

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Quality Control In Measurements

• QC Samples...
• Calibration Drift
• Initial Calibration Verification (ICV)
• Continuing Calibration Verification (CCV)

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Quality Control In Measurements

• QC Samples...
• Bias
• Spikes
• Lab Fortified Matrix (LFM, Spike)
• Trip Spike

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Quality Control In Measurements

• QC Samples...
• Bias
• External Source QC (QCS, LFB, CRM)
• SPEX, VHG, ERA, Ultra, NIST

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Quality Control In Measurements

• QC Samples
• Bias
• Surrogates

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Quality Control In Measurements

• QC Samples
• Imprecision
• Replicates - Combine with External Source QC (LFM)