2. Quality Control In Measurements Tom Colella CLAS Goldwater Environmental Lab

3. Quality Control In Measurements Measurement Comparison of unknown to known standard Validity depends on quality of standard Usually indirect, using calibrated system

4. 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

5. Quality Control In Measurements Calibration Standards... –General Considerations Purity Stability Storage Homogeneity Representativeness

6. 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

7. 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

8. Quality Control In Measurements Calibration Standards... –Process in sample preparation or use process controls

9. 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

10. Quality Control In Measurements Calibration Standards... –External Standards Same analyte prepared and measured separately from samples Used to generate cal curve or response factor

11. 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

12. 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

13. Quality Control In Measurements Calibration Standards.. –Calibration acceptance criteria Response Factors –% CV Calibration Curves –Correlation Coeff. Conc.SignalResponse Factor 10 ppb1025 V 102.5 V/ppb 20 1980 99.0 30 2890 96.3 40 4103 102.6 CV = 3.0%

14. Quality Control In Measurements QC Samples –Categories of error Contamination Calibration Drift Bias Imprecision

15. Quality Control In Measurements QC Samples... –Contamination Field Blanks –Trip Blank –Equipment Blank

16. Quality Control In Measurements QC Samples... –Contamination Lab Blanks –Storage Blank –Instrument Blank –Method Blank

17. Quality Control In Measurements QC Samples... –Calibration Drift Initial Calibration Verification (ICV) Continuing Calibration Verification (CCV)

18. Quality Control In Measurements QC Samples... –Bias Spikes –Lab Fortified Matrix (LFM, “Spike”) –Trip Spike

19. Quality Control In Measurements QC Samples... –Bias External Source QC (QCS, LFB, CRM) –SPEX, VHG, ERA, Ultra, NIST

20. Quality Control In Measurements QC Samples… –Bias Surrogates

21. Quality Control In Measurements QC Samples… –Imprecision Replicates - Combine with External Source QC (LFM)