Theory of Sampling. Guide to Quality Sample Processing. Jo Marie Cook NACRW 2015

Transcription

1 Theory of Sampling Guide to Quality Sample Processing Jo Marie Cook NACRW 2015

2 2011 Food Safety Modernization Act Integrated Food Safety System Section 202(a)(6): Standards for Sampling and Testing Partnership for Food Protection Uniformity in sample collection and laboratory preparation is essential

3 Harmonized policies and procedures for sample collection, shipment, analysis, storage and retention of food and feed materials Achieve Data Equivalency Estimate Overall Error

4 Goal of Sampling Make a decision about a unit of material based on analytical data derived from a small mass extracted from the DECISION UNIT Lab Sampling : Size Reduction to obtain test sample Question: What is the smallest test sample size?

5 Theory Pierre Gy Compositional Heterogeneity Leads to Fundamental Error Distributional Heterogeneity Leads to Grouping and Segregation Error Sample Correctness and Tools

6 Compositional Heterogeneity Fundamental Error Collect sufficient mass to represent all the particles of varying composition d = Diameter of largest particle m = Mass of sample collected C = constant unique to material and analyte s 2 (FE) = Cd 3 / m Collect the appropriate mass or reduce the particle size!

7 Fundamental Error Constant C = cbfg c = constitution factor: concentration of analyte in critical particles and density of critical particles B = liberation factor: extent critical particles are liberated f = shape factor: cube = 1, sphere = 0.524, wafer = 0.1 g = size range: uniform = 1, large distribution = 0.25

8 Distributional Heterogeneity Grouping and Segregation Error Assure equal likelihood for all increments of the lot to be selected Collect a sufficient number of random increments Mix and/or collect many increments!

9 Sample Correctness Does your sample contain all of the particles of interest in your decision unit? Eliminate bias generating errors

10 Theory of Sampling and Measurement Uncertainty TOS focuses on the Sampling Process TOS minimizes all sampling contributions to MU If the sampling error effects not reduced the MU estimates are compromised Esbensen and Wagner, Analytical Scientist, 2014 Eurachem/CITAC Guide: Measurement Uncertainty Arising from Sampling, 2007 DS-3077, Representative Sampling Horizontal Standard, 2013

11 Survey of Current Lab Practices Mix, chop, blend all subs to make a composite Vertical cutter/mixer >> Cryoprep? Not homogenous!

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13 Repeat analyses from processed samples Method Analytical Sample (g) % Absolute Difference +/- std. dev. Number FL Method 50 g 18% +/- 15% 149 QuEChERS 15g 18% +/- 16% 234 QuEChERS 26 Commodities 42 Pesticides

14 % Diff F & V - QuEChERS, 15g % Absolute Difference vs Concentration Mean Concentration (ppm)

15 % Difference by Pesticide Analyte % n Acephate 19 6 Captan 25 7 Carbendazim Chlorothalonil Cypermethrin Dinotefuran 20 9 Endosulfan Sulfate Iprodione 27 6 Methamidophos 11 6 Prochloraz 16 6 Thophanate Me 13 12

16 % Difference by Commodity Commodity % n Green beans Cabbage Cilantro Cucumbers Culantro Ginger Malanga Mint Papaya Snap Peas Peppers, hot Summer Squash Strawberry Thyme

17 Processing Repeatability PDP Peaches 9 samples of the same commodity with similar detections Peaches frequently always have detections Peaches can be problematic during the homogenization process and is a commodity that is not peeled Original batch extracted- 2/11/15 Re-extracted - 3/12/15

18 Analyst Temperature of Peaches after John Doe 16.6 Jane Doe 15.9 Bob Doe 17.2 Homogenization ⁰C after homogenization Note: Peaches are stored at ~ 4 ⁰C prior to processing

19 Peaches Repeatability = 7% +/- 6% % Difference Methoxyfenozide (0.017 Original & re-extraction) Mean Concentration (ppm) % diff

20 PDP Peaches Repeatability Analyte % diff n ACETAMIPRID 10 6 BUPROFEZIN 0 2 CHLORANTRANILIPROLE 23 2 CYHALOTHRIN TOTAL 13 2 CYPRODINIL 0 2 FLUDIOXONIL 7 6 IMAZALIL 2 2 IPRODIONE 4 10 METHOXYFENOZIDE 10 4 PROPICONAZOLE 0 3 PYRIMETHANIL 8 6 SPIRODICLOFEN 8 6 TEBUCONAZOLE 5 8

21 TOS Sample Processing Quality Criteria Definition: Size reduction to obtain test sample Goal: Represent the Decision Unit Small fundamental, grouping & segregation errors: Appropriate mass Appropriate # of Increments Sample Correctness: Every particle has the same probability of selection Appropriate tools, procedures Unbiased QC to Assess Representativeness = replicates

22 Minimum Analytical Sample Size? Particle size Size Distribution Contaminant Level Contaminant Distribution Analyte interaction with the matrix Cross Contamination Competing Contaminants

23 Minimum Analytical Sample Size? M = 10 * λ*d 3 E 2 M = Mass of sample in grams λ = Density in g/cm 3 d 3 = diameter of largest particles in cm E = Tolerable error (0.15 = 15%) Caution! This assumes a constant of 10. Your constant may be very different. Guidelines for Preparing Laboratory Samples, AAFCO.org

24 Cryogenic Blending Minimum Analytical Sample Size? M = 10 * λ*d 3 E 2 M = g (Only an Estimate!) λ = 0.8 d 3 = (250 µm) E = (0.15 = 15%)

25 Room Temperature vs Freeze & Blend Do we need Cryogenic Milling for Oranges? Smaller particle size, mm -> um (Advantage) Samples are kept Frozen (Advantage for heat sensitive compounds) Very time consuming (Disadvantage) Is it fit for purpose?

26 Room Temperature Blending Oranges 2.46 x 2.06 mm Microscopy: E.R. Noche

27 Blending of -80 C Oranges

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29 Still a few large chunks

30 Thawing -80 C Blended Oranges 0 sec 10 sec 30 sec 240 sec

31 0 sec 2.22 mm 235 sec

32 Thawing -80 C Blended Grapefruit 0 sec 230 sec 527 sec

33 0.94 x 1.51 mm -80 C Blended Valencia 1.36 mm 85 sec mm

34 Repeat Analyses (ppb) - Orange Date 1/29 4/24 4/24 4/24 Subs S1 S3 S3 S3 Frozen cup 1 Frozen cup 3 Frozen cup 3 Frozen cup 3 Penillic Acid (ppb) Penilloic Acid (ppb) *S2 was used for qualitative confirmation of original result

35 Repeat Analyses (ppb) - Grapefruit Date 3/17 4/24 4/24 4/24 Subs S1 S2 S3 S4 original Frozen cup 2 Frozen cup 3 Resample/blend Penillic Acid (ppb) Penilloic Acid (ppb)

36 March/April 2015, Open Access Special Issue 13 Papers on Theory of Sampling TOS Forum

37 Theory of Sampling Guide to Quality Sample Processing Questions?