The purpose of this work is to contribute for the metrological traceability statement of chemical measurements. Traceability statement for total chromium mass fraction in serpentine soils was developed. Accordingly, the quantity values requiring a high degree of control are calibration and quality control standard solutions, matrix certified reference material and the calibration curve model fit

Alves, Sheila

Santos, M. M. Correia dos

Trancoso, Maria Ascensão

Repositório do LNEG

XIX IMEKO World Congress Fundamental and Applied Metrology September 6−11, 2009, Lisbon, Portugal  TRACEABILITY STATEMENT FOR THE DETERMINATION OF TOTAL CHROMIUM MASS FRACTION IN SERPENTINE SOILS BY ATOMIC ABSORPTION SPECTROMETRY  Maria Ascensão Trancoso1, Sheila Alves2, Margarida Maria Correia dos Santos2  1LNEG – Laboratório Nacional de Energia e Geologia IP, Lisboa, Portugal, maria.trancoso@ineti.pt  2Instituto Superior Técnico, Lisboa, Portugal, mcsantos@ist.utl.pt   Abstract − The purpose of this work is to contribute for the metrological traceability statement of chemical measurements. Traceability statement for total chromium mass fraction in serpentine soils was developed. Accordingly, the quantity values requiring a high degree of control are calibration and quality control standard solutions, matrix certified reference material and the calibration curve model fit. Keywords: Traceability; uncertainty; soils; chromium  1. INTRODUCTION Measurement uncertainty and metrological traceability are properties of the measurement results closely linked. Measurement uncertainty characterizes the dispersion of the quantity values being attributed to a measurand and metrological traceability allows relating the result to a reference through a documented unbroken chain of calibrations [1]. Each link of the chain contributes to the measurement uncertainty showing the relationship between both concepts [2]. In this work the metrological traceability statement for total chromium determination in serpentine soils was developed, following the interrelated steps [3, 4]: i) Selection of a suitably validated method and identification of the acceptable uncertainty for the analytical result; ii) Writing and understanding the measurement function; iii) Identification of any reagents or equipment with specified values and fixed experimental conditions used in the procedure; iv)  Establishment of appropriated stated references for the influence quantities, identified according to the degree of control needed; v) Measurement uncertainty evaluation.  2. MEASURAND AND ACCEPTABLE UNCERTAINTY Our measurand of interest is the mass fraction of total chromium in soils, (mg kgw -1), which was determined by flame atomic absorption spectrometry after soil digestion with HF, HNO3 and HClO4 acid mixture [5], following a quality assurance programme [5, 6]. Before analysis the soil samples were air dried, sieved through a 2-mm aperture sieve and crushed with an agate mortar and pestle (particle size < 150 μm). The analytical procedure is summarised in Fig 1.  Standard solutionscalibration FAAS determinationRecovery, RmSample dilution factor, FdilSample weighing, mSoil digestion solution, VExtractionCalibration functionDry matterwDMCalculation of the result, wSoil digestion concentration, γ Fig. 1 - Analytical Procedure. Target relative combined standard uncertainty, ( )wu relc , close to 0,1 was considered, based on in house criteria, since no standard measurement uncertainty was required by legislation [5]. All the results were expressed on an oven-dry mass basis at 105 ºC. Since a non-dry sample was used for the chemical analysis the chromium mass fraction was corrected by the degree of moisture, at the end.  3. MEASUREMENT  FUNCTION    Chromium mass fraction,  (mg kgw -1) was calculated and reported on a dry matter basis using equation 1:           DMmdil w1R1FmVw ××××= γ        (1)  where γ is the concentration of Cr in the soil digestion solution estimated from the calibration curve (mg L-1), V the volume of the soil digested solution (mL), m the mass of sample used for analysis (g), Fdil the dilution factor, Rm the mean recovery of the Certified Reference Material (CRM) and (g gDMw-1) the dry matter mass fraction, determined by drying an independent aliquot of the sample at (105 ± 5) ºC.  4. STATED  REFERENCES  The traceability of all influence quantity values reported in equation (1), with adequate uncertainty, should be known, since all of them require a high degree of control. The metrological traceability of γ values are related to the uncertainty of the calibration standards and the uncertainty associated to the regression line. The calibration standards were obtained by diluting a (1000±5) mg L-1 Cr standard solution (Merck, Darmstadt, Germany). The calibration function was metrological controlled as previously described [5, 6]. Since the dilution steps influence the uncertainty of the measurement results all volumetric glassware used was of class A, with H conformity. The measurement uncertainty of the volumetric flasks was estimated from their volume tolerance assuming a triangular distribution [7]. In the case of automatic pipettes, they were calibrated by accredited calibration laboratory equipments which results are traceable to national standards. For example in case of Eppendorf 100 μl micropipette, the calibration error was 0,22 μl (SOQUILAB certificate nº 06.10.11A7). The m values were determined using a Mettler AT 200 analytical balance (Mettler Toledo, Switzerland) with 200 mg capacity and 0,0001 g readability (resolution), calibrated by an accredited calibration laboratory using E2 class weights traceable to national standards.  The bias of the method was evaluated as recovery, Rm, obtained using the reference material river clay sediment LGC 6139, with Cr concentration of (126 ± 18) mg kg-1. Dry matter mass fraction, wDM, was determined by gravimetry [8] using a MEMMERT UM 400 oven, thermostatically controlled and calibrated at (105 ± 5) ºC, by an accredited calibration laboratory using Pt 100´s thermometer which results are traceable to national standards. According to the ISQ calibration report nº ETEM 1274/06, the expanded uncertainty for each temperature measurement was ± 0,2 ºC. The measurement uncertainty of wDM, was estimated using the procedure described in [9].   5.  PERFORMANCE CHARACTERISTICS OF THE METHOD In table 1 the performance characteristics of the method used for the determination of total Cr mass fraction in soils are shown.  Table 1 - Performance characteristics of the method used for the determination of total Cr mass fraction in soils. Selectivity / Interference Free from interferences    Linearity, working range Linear range 0,05 – 2,0 mg L-1Limit of quantification 7 mg kg-1Repeatability RSDr  = 0,04 at 3000 mg kg-1Within-lab reproducibility  RSDRw = 0,04 at 3000 mg kg-1Recovery  (tolerance) (1,00± 0,10) using CRM RSDr – Relative standard deviation in repeatability conditions; RSDRw - Relative standard deviation in within laboratory reproducibility conditions; CRM – certified reference material.  6. MEASUREMENT  UNCERTAINTY  The combined standard uncertainty for , w ( )wuc , was estimated using an intralaboratory approach. Each measurement uncertainty associated with individual input parameters (eq. 1) was quantified and combined following the law of propagation of uncertainties for independent input quantities [7]. Fig. 2 shows the cause and effect diagram for , summarizing all the input influence quantities. A branch combining all the individual repeatability contributions (fwprecision), given by the standard deviation of replicate analysis, was added. The requirements for the digestion procedure are included in the recovery branch.     Fig. 2 - Cause and effect diagram.   In table 2 the input quantities values and associated standard uncertainties, ( )ixu , for total chromium mass fraction are shown.  For a soil sample with  = 3,05 g kgw -1 the ( )wuc  was 0,49 g kg-1. The target measurement uncertainty was fulfilled since  = 0,08 which is lower than 0,1.  ( )wu relcTable 2 - Input quantities values, ,  and associated standard uncertainties, , for total chromium mass fraction, , in serpentine soils. iX( )ixu wiX  ix  u(xi) γ   0,419 mg L-1 0,017 mg L-1V  100 mL 0,084 mL Fdil 50 0,068 m  0,7518 g 0,0001 g wDM  0,913 mg kg-1 0,015 mg kg-1Rm 1 0,06 fprecision  1 65 mg kg-1 w  =  3,05 g kg-1( )wuc  = 0,49 g kg-1ix  is the best estimate of the ith input quantity iX The assessment of each input quantity contribution into  value was carried out using the relative variance, as shown in Fig. 3. Precision (f( )wucprecision ), γ  and recovery (Rm) are the contributions that control ( )wuc  values.    Fig. 3 - Relative variances. 7.   CONCLUSIONS Evaluation of traceability requirements for the determination of total chromium mass fraction in soils identified five property values requiring a high significant degree of control, which means that “special” stated references should be selected: - Chromium standard solution - Working calibration standard solutions - Matrix certified reference material for chromium in soils - Calibration curve model fit   Significant degree of control for mass and volume measurements, minerals acids, ultrapure water, temperature of the drying oven and the wavelength of the instrument are also needed. They are guaranteed by the quality assurance system of the laboratory that provides appropriate stated references.   8. REFERENCES  [1] BIPM IEC, IFCC, ILAC, ISO, IUPAC, IUPAP and OIML,International vocabulary of metrology—basic and general concepts and associated terms (VIM), JCGM 200:2008.  [2] W. Bitch, “interdependence between measurement uncertainty and metrological traceability” Accred Qual Assur 2009 , on line DOI 10.1007/s00769-009-0500-4. [3] V. J. Barwick, S. Wood, “Meeting the Traceability Requirements of ISO 17025. An Analyst’ Guide”, LGC Limited, Teddington, 3rd Ed., 2005 [4] EURACHEM/CITAC, “Traceability in Chemical Measurement - A guide to achieving comparable results in chemical measurement”, 2003.  (http://www.eurachem.org/guides/Traceab.htm)  2009-01-09. [5]  M. A. Trancoso, M. M. Correia dos Santos, M. L. S. Gonçalves, “Quality assurance program for the chemical characterization of soils”, Accred Qual Assur, vol. 8, pp. 323-333, 2003. [6]  S. Alves, M. M. Correia dos Santos, M. A. Trancoso, “Evaluation of measurement uncertainties for the determination of total metal content in soils by atomic absorption spectrometry”, Accred Qual Assur, vol 14, pp 87-93, 2009. 0 0,2 0,4 0,6 0,8 1,0F dil wDMV m g Rm fprecisionrelative variance γ [7]      ISO (1995) GUM, Guide to the expression of the uncertainty in measurement, ISO, Geneva. [8] International Standard 11465:1993, “Soil quality - Determination of dry matter and water content on a mass basis - Gravimetric method”, International Organization for Standardization, Geneva, 1990. [9]  F. C. Mouro, A. I. Fernandes, S. C. Calisto, M. A. Trancoso, “Evaluation of measurement uncertainty for the moisture and dry matter mass fractions in industrial residues and sludges. XIX IMEKO World Congress – Fundamental and Applied Metrology, Setember 6-11, Lisbon, 2009.     

Traceability statement for the determination of total chromium fraction in serpentine soils by atomic absorption spectrometry

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Traceability statement for the determination of total chromium fraction in serpentine soils by atomic absorption spectrometry

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