Using WRT and control charts to evaluate analytical precision

Transcription

1 ICPF - Working Group on QA/QC in Laboratories Meeting of the Heads of the Laboratories 7-8 September 2017 in Verbania Pallanza Using WRT and control charts to evaluate analytical precision Alessandra Mazzoli, Aldo Marchetto and Gabriele Tartari C.N.R. Institute of Ecosystem Study, Verbania (Italy) a.marchetto@ise.cnr.it

2 Field Laboratory Quality Assurance/Quality Control Sampling, transport and conservation of samples Skilled personnel Validated and written analytical methods Properly constructed, equipped and maintained laboratory facilities Use of high-quality glassware, reagents, de-ionised water and other testing material Internal QC Calibration, adjustment, and maintenance of equipment Use of blanks, DL, QL Use of replicate samples Use of control samples and standard samples, with proper records (control charts) Validation and critique of results Archiving results External QC Interlaboratory exercises Certified reference materials

3 Uncertainty: Introduction Degree of doubt about the correctness of a measurement result Expanded uncertainty (U ): Range, expressed in percentage (k = 2), around the reported result within which the true value lies with high probability (95%) According to: ISO 11352:2012, Estimation of measurement uncertainty based on validation and quality control data European Commission, SANTE/11945/2015 document

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5 Expanded uncertainty: Calculations U = k u Where: U : expanded uncertainty k : expanded coverage factor (k = 2) u : combined uncertainty

6 Combined uncertainty: Calculations u (u RSD wr ) 2 + (u bias ) 2 Where: u RSD wr : uncertainty within the laboratory (control charts) u bias : uncertainty associated with the interlaboratory (WRTs)

7 Uncertainty: Calculations u bias RMS bias 2 + (u C ref ) 2 Where: RMS bias : Root Mean Square of the sum of the squared bias m : number of results to add up

8 Uncertainty: Calculations u bias RMS bias 2 + (u C ref ) 2 Where: u C ref : estimation of average Qn : robust standard deviation No. : number of laboratories m : number of results to add up u C ref

9 2.5 Robust average calculation Total N, sample mg L median iteration

10 2.5 Robust average calculation Total N, sample 4 mg L *Median of the absolute differences from the median median iteration

11 2.5 Robust average calculation Total N, sample 4 mg L *MADM median iteration

12 2.5 Robust average calculation Total N, sample 4 mg L *MADM median iteration

13 2.5 Robust average calculation Total N, sample *MADM mg L median mean iteration

14 2.5 Robust average calculation Total N, sample *MADM 1.5*cSD mg L median mean iteration

15 2.5 Robust average calculation Total N, sample *MADM 1.5*(SD*1.134) mg L median mean iteration

16 2.5 Robust average calculation Total N, sample *MADM 1.5*cSD mg L median mean iteration

17 2.5 Robust average calculation Total N, sample *MADM 1.5*cSD 1.5*cSD mg L median mean mean iteration

18 2.5 Robust average calculation Total N, sample *MADM 1.5*cSD 1.5*cSD mg L median mean mean OK! iteration

19 Interlaboratory dispersion Determination of conductivity in sample 3 (WRT 2013) 60 RSD = 4.8% Conductivity (µs/cm) CORR 25 C

20 Interlaboratory dispersion Determination of sulphate in sample 3 (WRT 2013) RSD = 8.9% Sulphate (mg S/L) IC CS IC CS IC WS ICP OES IC WS ICP OES

21 Interlaboratory dispersion Determination of ammonium in sample 5 (WRT 2013) 2.5 RSD = 13.6% Ammonium (mg N/L) SPEC Phe CF Phe CF GD IC CS IC WS Other SPEC Phe CF Phe CF GD IC CS IC WS Other

22 Interlaboratory dispersion Determination of ammonium in sample 1 (WRT 2017) 1.6 RSD = 17.8% Ammonium (mg N/L) DE03 DE05 DE04 DE01 DC01 DC02 DF99 DE99 DZ99 CF Phe SPEC Phe IC CS IC WS EL Other SPEC Other

23 Uncertainty: interlaboratory and control charts Interlaboratory (uncertainty for method and laboratory bias) Control charts (uncertainty for reproducibility within the laboratory) u bias u RSD wr Combined uncertainty u Expanded uncertainty U mg L Average = mg L -1 RSD % = 2.7% UWL UCL LWL LCL

24 Uncertainty: Conductivity (CNR-ISE) RSD 20 WRT U target Conductivity (µs/cm)

25 Uncertainty: Sulphate (CNR-ISE) RSD 20 WRT U target Sulphate (mg S/L)

26 Uncertainty: Ammonium (CNR-ISE) 2 RSD Ammonium (mg N/L) WRT U target

27 Conclusion Uncertainty: Is a good mean to quantify the doubt related to a final result (expanded uncertainty) Allows a laboratory to evaluate its performance Requires values laboratories must already have (control charts and interlaboratory results) Is a required parameter for obtaining and keeping an accreditation