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

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

3. 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

4. Quality Control In Measurements • Calibration Standards... • General Considerations • Purity • Stability • Storage • Homogeneity • Representativeness

5. 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

6. 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

7. Quality Control In Measurements • Calibration Standards... • Process in sample preparation or use process controls

8. 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

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

10. 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

11. 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

12. 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%

13. Quality Control In Measurements • QC Samples • Categories of error • Contamination • Calibration Drift • Bias • Imprecision

14. Quality Control In Measurements • QC Samples... • Contamination • Field Blanks • Trip Blank • Equipment Blank

15. Quality Control In Measurements • QC Samples... • Contamination • Lab Blanks • Storage Blank • Instrument Blank • Method Blank

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

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

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

19. Quality Control In Measurements • QC Samples… • Bias • Surrogates

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