GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 1

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1 ACTRIS-GAW: Uncertainty Evaluation in VOC Analysis Draft VOC Measurement Guidelines C. Plass-Duelmer, A. Werner, S. Reimann, S. Sauvage, R. Steinbrecher and the ACTRIS VOC workshop participants comparable concepts for uncertainty estimates in GAW, EMEP, users (modelers) can easily assess the quality of the data guide to the expression of uncertainty in measurement (GUM) contributing uncertainties methods to estimate the uncertainties Data delivery with total uncertainty and reproducibility What are the (a) minimum requirements, or (b) recommended procedure in uncertainty evaluation for GAW-VOC. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/ ) under grant agreement n 654. GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 1

2 Standard uncertainty of analysis u = u + u + a CRM calibration u measurement u a standard uncertainty of analysis u CRM standard uncertainty of certified reference material u calibration uncertainty response, which we get from calibration u measurement uncertainty response, which we get from measurement of sample GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B

3 Standard uncertainty of analysis example u = u + u + a CRM calibration u measurement u CRM certificate of calibration GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 3

4 Standard uncertainty of analysis example u = u + u + a CRM calibration u measurement VOC measurement/ppb standard deviation 1 3 ppb benzene 1,047 1,044 1,053 0,00541 s n = i= 1 ( x x) i ( n 1) GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 4

5 Standard uncertainty of analysis example u = u + u + a CRM calibration u measurement VOC measurement/ppb standard deviation 1 3 ppb benzene 0,356 0,365 0,363 0,00468 s n = i= 1 ( x x) i ( n 1) GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 5

Combining standard uncertainties parallel measurements software analysis u = u + c Experiment c benzene, µg m -3 Mearts 1 Mearst GAW Expert Workshop VOC, C.

6 Combining standard uncertainties parallel measurements software analysis u = u + c Experiment c benzene, µg m -3 Mearts 1 Mearst GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B u sampling 1,07 1,09 0,56 0,56 0,86 0,87 0,36 0,35 0,43 0,41 0,98 0,90 0,14 0,14 0,16 0, 0,4 0,5 0,18 0,17 0,16 0,15 0,63 0,6

7 Uncertainty of VOC(s) VOC(s) calculated value standard uncertainty rel.expanded uncertainty ppb ppb % Ethane,19 0,83 5,08 Ethene 0,0 0,39 19,54 Propane 0,69 0,4 3,13 Propene 0,10 0,17 15, methylpropane 0,15 0,09 4, n-butane 0, 0,1 6,0 Acetylene 0,8 0, 16, Ethane -methylbutane 0,1 0,1 8,74 benzene 0,16 0,19 5, toluene 0,09 0,3 10, ethylbenzene 0,0 0,08 16, o-xylene 0,05 0,08 5,81 p,m-xylene 0,05 0,08 5,81 Ethane N Ethane Air GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 7

8 QA: proof that true ambient air mixing ratios are measured Uncertainty: give estimate of deviation from true value Sampling / Analysis and Detection/ Calibration/ Data evaluation Adsorption/desorption (losses/memory) matrix Leaks (losses/contamination) Chemical changes at surfaces matrix Chemical changes due to reactions (O3 ) matrix Standard drifts / transfer problems Blanks matrix Quantitative trapping - matrix Peak integration - Insufficient peak separation GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 8

9 QA should include (beside others) -Leak tests -Regular blank and standard measurements (close to real air matrix and including entire system), log results -Use of multiple standards and check for consistency, check pressure regulator and transfer line -FID-C-Response to check artificial losses and contamination -linearity check -single compound injections for labile compounds -Regular check of process parameters, visual inspection of chromatograms -Maintenance plan (filters, inlet tubing, adsorbent, ) -Regular standard addition measurements with high concentrated standard GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 9 C. Plass-Dülmer ACTRIS NA4 10/011

10 Error evaluation (1) Systematic errors Calibration Gas uncertainty losses i.e. in water trap (= X+/-Y%) blank values deviation from expected behavior (like deviation from C-Response or standard addition) understood? If not = system interacts with VOC in a way you do not understand put in error labile compounds uncertainty in decomposition/rearrangement and consequence for other compounds (gain) Systematic volume errors as small as possible optimize system thorough characterization (whole system, test gas close to ambient air ) If reproducible bias apply correction, put scatter into error GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 10

11 Error evaluation () (random uncertainty) - Transfer of standard to instrument (pressure regulator, adsorptive loss ) - Blank values (scatter in multiple determinations) - scatter in measured values (detector stability, ) - scatter in sample volume - labile compounds uncertainty in decomposition/rearrangement and consequence for other compounds (gain) - peak area evaluation (baseline, peak-overlap) GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 11

12 Example reproducibility for n-pentane: Multiple measurements of NPL synthetic calibration gas at 4 ppb: 3% (RSD) Peak area integration of this calibration gas (estimated error): % (RSD) Sample volume reproducibility: 1% (RSD) Detection limit for n-pentane (3-sigma): 6 ppt No break-through, interference, adsorption/desorption, blank, peak-overlap in ambient air samples U repr = ( ppt + 3% * mixing ratio) (1-sigma reproducibility) NPL uncertainty (K=): %, 9 measurements: error of the mean = 1% Deviation from common C-response: 1.0% (within expectation) NPL calibration factor uncertainty: u cal = (1% + 1% ) 0.5 =1.4% (RSD) U = * ( ppt + (1.5% +3% )^0.5 * mixing ratio) (expanded uncertainty, k=) GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 1 C. Plass-Dülmer ACTRIS NA4 10/011

13 Example reproducibility for methylvinylketone: Multiple measurements of synthetic calibration gas at 100 ppb: 6% (RSD) Peak area integration of this calibration gas (estimated error): 4% (RSD) Sample volume reproducibility: 1% (RSD), no break-through O3-Interference (standard addition, 50+/-30 ppb O3): loss by 0+/-10% (correct MR by 0%) adsorption/desorption: after standard, air sample enhanced by 100+/-50 ppt, Blank: 30+/-10 ppt in He, 50+/-30 ppt in catalytically cleaned air Detection limit for MVK (3-sigma): 90 ppt (from air blank) peak-overlap with X, variable, error peak integration: 30+/-10 % (not corrected) U repr = (30 ppt + (10% +10% ) 0.5 * mixing ratio) (1-sigma reproducibility) Calgas uncertainty (K=): 4%, 9 measurements: error of the mean = % Deviation from ECN-response: 15.0% (unexplained add to error) Calibration factor uncertainty: u cal = (15% + % ) 0.5 =15% (RSD) U = * (30 ppt + (15% +10% +30% )^0.5 * mixing ratio) (exp. uncertainty, k=) GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 13

14 Error evaluation (3) - Compare Estimates with Measured Values (1) Compare calculated random uncertainty with repeatability and reproducibility check ok? if not, check calculated uncertainties () Compare deviation in comparisons with combined uncertainties of the compared values in range? if not, check uncertainties abs. standard dev. versus mixing ratio Ref-04 1ppt+1% SUPELCO 74-NMHC 3ppt+3% GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 14

15 Draft recommendation for MG - Reproducibility: Calculated by air samples (in stable meteorological conditions), or the target gas (whole air)/ or other working standard measurements (1SD) series of 5 Covers: random error of peak integration, volume determination and blank variation (sampling: includes reproducibility of sampling system) Draft for Systematic errors: Includes Uncertainty of Standard gas (NPL gives k=, use standard uncertainty), possible non-linearity of detector (MS), uncertainty of working standard (NPL transfer, dilution process), Drifts of Standards, systematic integration error (overlays, not separated peaks), other instrumental problems, estimate uncertainty due to observed but not yet understood problems Off line sampling: error of flow rate of sampling system sample volume Uncertainty: reproducibility + systematic errors (coverage factor k=) Flags: temporal problems in performance, questionable data? Detection limit for substances baseline integration for typical peakwidths GAW Expert Workshop VOC, C. Plass-Dülmer et al., FEHp-B 15

GAW-ACTRIS: OVOC Systems

GAW-ACTRIS: OVOC Systems Jennifer Englert, Werner, Plass-Dülmer, Reimann, Hill, Hopkins, Roukos, and ACTRIS community REFERENCES Apel, E.C. et al. (2008): Intercomparison of oxygenated volatile organic