APMP.QM-S3 Cd in Rice Powder

Transcription

1 APMP.QM-S3 Cd in Rice Powder (Coordinated by KRISS, NIM and NMIJ) Final Draft Report Sep. 16, 2011 Yong-Hyeon YIM, Euijin HWANG, Jun WANG, Yanbei ZHU

2 Summary The present study, APMP.QM-S3, was performed to demonstrate and document the capability of interested national metrology institutes in the APMP region to determine the mass fraction of Cd in rice. It was carried out as a regional supplementary comparison related with a previous CCQM key comparison, CCQM- K24, and run in parallel with a pilot study, APMP.QM-P12. The comparison is an activity of the APMP-TCQM and was coordinated by KRISS (Korea Research Institute of Standards and Science, Korea) with NIM (National Institute of Metrology, P. R. China) and NMIJ (National Metrology Institute of Japan) as co-coordination laboratories to provide reference value. Seven national metrology institutes (NMIs) participated in the supplementary comparison. The degrees of equivalence for NMIs employing double ID-ICP/MS techniques are excellent while the results obtained using other techniques may need more improvement. 1

3 1. Introduction Rice is the major source of food for a large proportion of the world s population, especially in Asia. Therefore, many countries set regulations limiting the amount of toxic elements, such as cadmium, in rice to protect public health. With ever increasing international trade of food, traceable measurement of hazardous elements in food supported by NMIs has become one of the essential requirements for food safety. The intention of this study is to establish the comparability in the measurement of cadmium in rice. It is expected that participating laboratories, which can demonstrate their capability to measure cadmium in such a matrix, will be able to apply such expertise to a range of other types of sample. The previous regional APMP comparison on cadmium in rice, APMP.QM-P2, was carried out in parallel with CCQM-K24 key comparison. However, many APMP participants failed to show satisfying comparability with the CCQM results. Subsequent APMP.QM-P5 study (Cd content in oyster tissue) also resulted in limited success with small number of participants. The present supplementary study, APMP.QM-S3 (Cd content in rice powder), has been originally proposed and approved by APMP-TCQM in 2007 as APMP.QM-K24 to provide NMIs in the APMP region a chance to demonstrate the degree of equivalence in their measurement and possibly link it to the CCQM-K24 result. The study was run in parallel with the pilot study, APMP.QM-P12. It has been carried out from July to December 2008 and the results were presented at the APMP meeting in Kuala Lumpur, Malaysia, in December Draft A report for the APMP.QM-K24 was circulated to participants in Feb But it was recognized that the RMO comparison should have been discussed and approved by the Inorganic Analysis Working Group (IAWG) and registered in the Key Comparison Data Base (KCDB) in advance. Therefore, the coordinating laboratories asked a retrospective approval of the study and discussions on the report in the IAWG during the CCQM meeting in Paris in April After submission and discussion of draft A report of APMP.QM-K24 in the CCQM meeting, it was decided to register the study as a supplementary comparison of APMP considering the lack of sound scientific background to link the two studies directly. A new comparison number, APMP.QM-S3 was assigned to the previous APMP.QM- K24 in September The decision was reported and the follow-up actions were discussed further in the CCQM-IAWG meeting at Hindas, Sweden on 29 th September

4 2. List of Participants Participants in APMP.QM-S3 are listed in Table 1. Table 1. APMP.QM-S3 participants. Economy Institute/Organization Contributors China National Institute of Metrology (NIM) Dr. Jun WANG Dr. Chao WEI Dr. Qian WANG Hong Kong Government Laboratory Hong Kong (GLHK) Dr. Yiu-chung YIP India National Physics Laboratory-India (NPLI) Mr. Prabhat K. GUPTA Indonesia Indonesian Institute of Science (LIPI) Dr. SUMARDI Japan National Metrology Institute of Japan (NMIJ) Dr. Yanbei ZHU Korea Thailand Korea Research Institute of Standards & Science (KRISS) National Metrology Institute, Thailand (NMIT) Dr. Yong-Hyeon YIM Dr. Kyung-Seok LEE Mr. Charun YAFA Ms. Pranee PHUKPHATTHANACHAI 3. Samples Each participant was sent more than one bottles of rice powder sample containing approximately 30 g of material in each bottle. The sample originated from rice grown in contaminated area. The material was tested at KRISS for stability and homogeneity and was found to be appropriate for the needs of the comparison. The homogeneity test was carried out using isotope dilution ICP/MS (ID-ICP/MS) after microwave digestion of subsamples. Ten bottles were sampled randomly from 1190 bottles with minimum sampling size of 0.5 g. The relative standard deviation of Cd content 3

5 between bottles was less than 0.41%. Sample bottle containing about 30 g of material was sent to each participant presented in the Table 1 with information and instruction documents on July 8, The sample for NIM, China, once returned due to customs clearance problem and finally delivered on October 20, The deadline for reporting of results was Dec. 31, Introductions to Participants The following information was provided to all participants General instructions. Result report form Detailed sample information, guideline for sample preparation and reporting are described in the general instructions. The information provided to participants is given in Appendix A and B. 5. Methods Participant The participants were free to choose one or more suitable methods of Cd measurement. Methods and instruments used are given in Table 2. Table 2. Analytical methods and instrumental techniques used by APMP.QM-S3 participants for cadmium measurements. Subsample mass Number of subsamples Digestion Calibration Instrument NIM 0.5 g 12 HNO 3 + H 2 O 2 MW digestion Double IDMS ICP/MS GLHK 0.5 g 5 HNO 3 +HF MW digestion Double IDMS ICP/MS NPLI 0.5 g 6 HNO 3 + H 2 O 2 + HF MW digestion External calibration GF-AAS LIPI 0.5 g 4 HNO 3 + H 2 O 2 MW digestion Standard addition GF-AAS NMIJ 0.5 g 4 HNO 3 + H 2 O 2 + HF MW digestion Double IDMS ICP/MS KRISS 0.5 g 5 5 ml HNO ml H 2 O 2 MW digestion Double IDMS ICP/MS NMIT 0.5 g 4 5 ml HNO ml H 2 O 2 MW digestion Double IDMS ICP/MS 4

6 For sample digestion, all laboratories used microwave with varying mixture of HNO 3, H 2 O 2 and HF. Two different instrumental techniques were used for the cadmium measurement in samples. Four laboratories used double isotope dilution mass spectrometry (IDMS), while the other two participants used atomic absorption spectroscopy (AAS) with either external calibration or standard addition. 6. Participants Results The participants results are given in Table 3. Participant Table 3. APMP.QM-S3 participants measurement results for cadmium. Reported value (mg kg -1 ) Reported standard uncertainty (mg/kg) Coverage factor k (95% confidence level) Expanded uncertainty (mg kg -1 ) Relative expanded uncertainty (%) NIM GLHK NPLI LIPI NMIJ KRISS NMIT

7 7. Discussion In the original proposal, it was suggested to use the results of three NMIs (NIM, NMIJ, KRISS) that have also taken part in CCQM-K24 to calculate the reference value (RV) and its standard uncertainty (RU). Although the present study has been reclassified as an independent RMO supplementary comparison, it is proposed to use the same strategy for calculation of RV. It should be noted that only those three NMIs, among APMP.QM-S3 participants, had demonstrated their measurement capability of Cd in similar matrix at the time of the present comparison by participating in CCQM key comparisons. As shown in Table 4, the arithmetic mean, the median and the mixture model median of the three results used for RV calculation are listed with associated uncertainties. Standard deviation of the three results was used as associated uncertainties of the mean and the median. Scaled median of absolute deviation, MAD E, was not used as a statistical estimator of the standard uncertainty for the median value due to its negative bias for small number of data set. The difference between mean, median and mixture model median values are negligible considering the associated uncertainties. Hence, the simplest estimator, the mean value and standard uncertainty of results from the three NMIs (which is ± mg/kg), is proposed as the RV and RU [1]. Based on the homogeneity test, uncertainty contribution from sample homogeneity is expected to be less than a quarter of the proposed uncertainty of the RV. Therefore, the uncertainty of the RV is set satisfactorily for evaluation of degree of equivalence without noticeable contribution from sample homogeneity. The reported values and standard uncertainties of the participants are graphically displayed in Figure 1 together with the RV and its uncertainty, RU. The double IDMS results from most of participants show decent agreement with the RV within the stated uncertainties. However, the two results obtained by calibration with external standard or standard addition have relatively larger bias and/or uncertainties than those from double IDMS. Table 4. Reference value calculation for APMP.QM-S3. Reference value,rv (mg/kg) Standard Uncertainty, RU (mg/kg) Expanded uncertainty (mg/kg) Relative expanded uncertainty (%) k (95% CL) Mean Median MM median* * Mixture model median [2]. 6

8 Figure 1. Participants results for APMP.QM-S3 displayed with error bars representing reported standard uncertainties. The solid horizontal line in red color is the proposed RV of mg/kg; the dashed blue lines show the standard uncertainty of the reference value. The degree of equivalence between a NMI result and the RV is calculated, as given in Table 5, according to the BIPM guidelines. The degree of equivalence, D i, between the NMI result, x i, and the RV, x R, is calculated according to the following equations: D i = x x, 2 ( 2 2 U = u + u ) i R i i R where U i is the expanded uncertainty (k = 2) of the D i calculated by combining the uncertainty of the NMI result, x i, and the standard uncertainty of the RV, x R. Equivalence statements for APMP.QM-S3 results are displayed in Figure 2. The degree of equivalence for NMIs employing double IDMS techniques was excellent, while the results from other techniques need more improvement. 7

9 Participant Table 5. Equivalence statements for APMP.QM-S3. Reported value (mg/kg) Standard uncertainty (mg/kg) D i (mg/kg) U i (mg/kg) KRISS NIM GLHK NMIJ NMIT LIPI NPLI Figure 2. Graph of equivalence statements for APMP.QM-S3. Error bars represent the expanded uncertainties interval calculated with a coverage factor k=2. 8

10 The participants for the supplementary comparison are asked to complete core capability forms to document core capabilities being demonstrated in the comparison. Summarized tables of the demonstrated core capabilities for APMP.QM-S3 are also shown in the Appendix C. 8. Conclusion The present APMP supplementary comparison, APMP.QM-S3, has been carried out in parallel with APMP.QM-P12 to demonstrate and document the capability of NMIs in the APMP region for determination of Cd content in rice and related food matrix materials. The degree of equivalence for results obtained with double ID- ICP/MS was satisfactory with less than 5% deviation from the RV, while those from other methods may require more improvements including rigorous control of measurement method and method validation. The participated laboratories directly demonstrated their capability to measure cadmium in rice and will be able to apply such expertise to a range of samples and analytes with similar matrices. 9. Acknowledgement The work described here contains the contributions of many scientists as detailed in Table 1. The coordinating laboratory gratefully acknowledges Luis Mussio from BIPM, Linsey Mackay from NMIA, Mike Sargent and all members of the IAWG for their comments and advices for RV calculation. Special thanks are due to Kenji Kato and Della Sin for their assistance as chairpersons of the APMP-TCQM. 10. References 1. CCQM Guidance note: Estimation of a consensus KCRV and associated Degrees of Equivalence, 1 March David L. Duewer, A robust approach for the determination of CCQM key comparison reference values and uncertainties, Working document CCQM, 04-15, BIPM,

11 Appendix A. Introduction for APMP.QM-K24 and P12 comparison (Cd content in rice powder) 1. Sample information The rice powder was prepared from rice grown in a contaminated area. Rice grains were rinsed four times to remove rice bran and other impurities, and freeze-dried. The freeze-dried material was broken down into smaller pieces, mixed, and sieved to obtain particles with diameter between 50 to 250 μm. It was further homogenized with V-blender and aliquoted into amber bottles of 60 ml in volume. The amber bottles containing rice powder were sterilized by gamma ray irradiation. Each bottle contains about 30 g of material. Cd content in rice powder: mg/kg Homogeneity: ID-ICP/MS analysis using 10 samples (about 0.5 g each) taken from 10 bottles were carried out. Relative standard deviation was 0.41 %, which is satisfactory for the present pilot study. Each participant will receive one bottle containing about 30 g of sample. The participants will be informed of the date of dispatching of samples. Participants are required to confirm the receipt of the samples (including the serial number of the sample, receipt data, any damages found in the sample package) by (yhyim@kriss.re.kr or ehwang@kriss.re.kr) or FAX( ). If any damage is observed, please contact us immediately to get additional bottle of sample. 2. Sample storage The sample should be kept sealed in its original bottle and stored at room temperature. The opening of the sample bottle should be carefully planned to avoid contamination and decomposition of the sample. 3. Guidelines for sample preparation and dry-mass correction Sample handling in a laboratory with well-controlled humidity and temperature is recommended. Any kind of contamination should be avoided. Sample preparation for analysis: Sampling for the analysis should be carried out at the same time with sampling for the dry-mass correction. Digest the rice powder material completely, if the method used requires digestion. Dry-mass correction: Sampling for the digestion should be carried out at the same time with sampling for the dry-mass correction. It is recommended to take aliquots of sample before and after the sampling for the digestion. Take at least 3 separate aliquots of sample with sample size of g for drymass correction. The material should be dried in a desiccator for 7 days (168 hours) using P 2 O 5 as desiccant. Calculate the moisture content from the mass change observed, and use it for dry-mass correction. 4. Measurement method Each laboratory has to choose the method of measurement for itself and more than one method can be used simultaneously. It is recommended to use appropriately validated methods. 5. Time schedule Dead line for registration: K24 (May 15, 2008), P12 (July 30, 2008) Sample distribution: K24 (July 9, 2008), P12 (before Aug., 2008) 10

12 Deadline for reporting: December 31, 2008 Draft A report: March 31, Reporting The report should be sent to the coordinating laboratory by before Dec. 31, For rigorous evaluation of the inter-laboratory comparison, detailed information should be included in the report (final report form will be distributed with the sample) Final result with relevant uncertainty should be reported as mass faction [mg/kg] based on dry mass. If the final result has been calculated from more than one method, the individual results from the contributing methods must also be reported. One final result should be reported in the key comparison. However, more than two results can be reported for the pilot study, when different methods are used. Detailed description of analytical methods used. If more than one method is used, each method should be described in detail. Information about sample digestion and preparation. Information about the reference material used for calibration or other materials used in the analytical procedure. Details of the uncertainty evaluation: uncertainty should be evaluated according to the ISO Guide to the Expression of Uncertainty in Measurement, 1993, which includes - Complete specification of the measurement equation including corrections. - Description of all uncertainty sources and their typical values 7. Contact Person. Drs. Yong-Hyeon Yim & Euijin Hwang Principal Research Scientists Health Metrology Group, KRISS 1 Doryong-dong, Yuseong-gu, Daejeon , Republic of Korea Tel: , 5361 Fax: yhyim@kriss.re.kr, ehwang@kriss.re.kr 11

13 Lab. Identification Name: Institute: Address: Country: Tel. number: Fax. Number: Date: Appendix B. Result report form APMP.QM-K24 Cd Content in Rice Powder Result Report Please report all your results and uncertainties in the unit mg/kg. Cd content (mg/kg) Combined standard uncertainty (mg/kg) Coverage factor k (in 95% level of confidence) Expanded uncertainty (mg/kg) Result from individual aliquout (dry-mass corrected) Aliquot number Cd content (mg/kg) Average Standard deviation Standard deviation of the mean 12

14 Questionnaire 1. Analytical method used: 2. Description of the measurement procedure including sample preparation: 3. Correction for dry mass (% of weighted): 4. Amount & number of sample aliquots taken for dry mass correction: 5. Uncertainty for dry mass correction: 6. Amount & number of sample aliquots taken for Cd analysis: 7. (Isotopic) Reference materials used for calibration purposes: 8. For IDMS, indicate reference & spiked isotopes used: 9. Details of the uncertainty evaluation - Complete specification of the measurement equation including corrections - Description of all uncertainty sources and their typical values 13

15 Appendix C. Tables of demonstrated core-capabilities Method Summary of APMP.QM-S3 (Cd in rice powder) NIM GLHK NPLI LIPI NMIJ KRISS NMIT ID-ICP/MS Cd Cd Cd Cd Cd GF-AAS Cd Cd 14

16 CCQM Study: APMP.QM-S3 Inorganic Core Capabilities Summary Table Institute(s): GLHK, KRISS, NIM, NMIJ, NIMT Method: ID-ICP-MS Analyte(s): Cd Capabilities/Challenges Degree of Difficulty Explanation Low Moderate High Contamination control and correction techniques and procedures employed to reduce potential contamination of samples as well as blank correction procedures. The level of difficulty is greatest for analytes that are environmentally ubiquitous and also present at very low concentrations in the sample. Digestion/dissolution of organic matrices techniques and procedures used to bring a sample that is primarily organic in nature into solution suitable for liquid sample introduction to the ICP. Digestion/dissolution of inorganic matrices techniques and procedures used to bring a sample that is primarily inorganic in nature into solution suitable for liquid sample introduction to the ICP. Volatile element containment techniques and procedures used to prevent the loss of potentially volatile analyte elements during sample treatment and storage. Pre-concentration Techniques and procedures used to increase the concentration of the analyte introduced to the ICP. Includes evaporation, ion-exchange, extraction, precipitation procedures, but not vapor generation procedures. Vapor generation Techniques such as hydride generation and cold vapor generation used to remove the analyte from the sample as a gas for introduction into the ICP. Matrix separation Techniques and procedures used to isolate the analyte(s) from the sample matrix to avoid or reduce interferences caused by the matrix. Includes ion-exchange, extraction, precipitation procedures, but not vapor generation procedures. Techniques and procedures used to isolate the analyte(s) from the sample matrix to avoid or reduce interferences caused by the matrix. Includes ionexchange, extraction, precipitation procedures, but not vapor generation procedures. Spike equilibration with sample The mixing and equilibration of the enriched isotopic spike with the sample. Signal detection The detection and recording of the analyte isotope signals. The degree of difficulty increases for analytes present at low concentrations, of low isotopic abundance, or that are poorly ionized. Memory effect Any techniques used to avoid, remove or reduce the carry-over of analyte between consecutively measured standards and/or samples. 15

17 Capabilities/Challenges Degree of Difficulty Explanation Low Moderate High Correction or removal of isobaric/polyatomic interferences Any techniques used to remove, reduce, or mathematically correct for interferences caused by mass overlap of analyte isotopes with isobaric or polyatomic species. Includes collision cell techniques, high resolution mass spectrometry, or chemical separations. The relative concentrations and sensitivities of the analyte isotopes and the interfering species will affect the degree of difficulty. Detector deadtime correction Measurement of, and correction for, ion detector deadtime. Importance increases in situations where high ion count rates are encountered. Mass bias/fractionation control and correction Techniques used to determine, monitor, and correct for mass bias/fractionation. Spike calibration Techniques used to determine the analyte concentration in the enriched isotopic spike solution. Correction/removal of isobaric/polyatomic interferences (e.g., Sn, MoO) or alternative selection of isotopes is required to prevent isobaric interferences. 16

18 CCQM Study: APMP.QM-S3 Institute(s): NPLI, LIPI Method: ETA-AAS (or GF-AAS) Analyte(s): Cd Inorganic Core Capabilities Summary Table Capabilities/Challenges Degree of Difficulty Explanation Low Moderate High Contamination control and correction techniques and procedures employed to reduce potential contamination of samples as well as blank correction procedures. The level of difficulty is greatest for analytes that are environmentally ubiquitous and also present at very low concentrations in the sample. Digestion/dissolution of organic matrices techniques and procedures used to bring a sample that is primarily organic in nature into solution suitable for liquid sample introduction to the ETA-AAS. Digestion/dissolution of inorganic matrices techniques and procedures used to bring a sample that is primarily inorganic in nature into solution suitable for liquid sample introduction to the ETA-AAS. Volatile element containment techniques and procedures used to prevent the loss of potentially volatile analyte elements during sample treatment and storage. Pre-concentration Techniques and procedures used to increase the concentration of the analyte introduced to the ETA-AAS. Includes evaporation, ion-exchange, extraction, precipitation procedures, but not vapor generation procedures. Matrix separation Techniques and procedures used to isolate the analyte(s) from the sample matrix to avoid or reduce interferences caused by the matrix. Includes ion-exchange, extraction, precipitation procedures, but not vapor generation procedures. Hydride preconcentration/matrix separation of volatile species. Coupling of a hydride system to the ETA-AAS and optimization of conditions. Calibration of analyte concentration The preparation of calibration standards and the strategy for instrument calibration. Includes external calibration and standard additions procedures. Also use of matrix-matched standards to minimize effect of interferences. Signal detection The detection and recording of the absorption signals of analytes. The degree of difficulty increases for analytes present at low concentrations, of low atomic absorption coefficient. Requires selection of operating conditions such as light source, absorption line, Zeeman background correction conditions. Includes selection of signal processing conditions (peak area or height). Memory effect Any techniques used to avoid, remove or reduce the carry-over of analyte between consecutively measured standards and/or samples. Optimization of the furnace temperature 17 Careful matrix matching for external calibration, or checking linearity of calibration by standard addition is required to achieve reliable calibration.

19 Capabilities/Challenges Degree of Difficulty Explanation Low Moderate High program Optimization of temperature and duration of steps for sample drying, pyrolysis to remove (residual) organics, and atomization. Furnace temperature program to minimize analyte loss in the drying/pyrolysis steps, while maximizing analyte vaporization in the atomization step. Correction or removal of matrix effects or interferences Chemical or instrumental procedures used to avoid or correct for spectral and non-spectral interferences. Includes effects of differences in viscosity and chemical equilibrium states of analyte between the standard and sample. Selection of matrix modifier to adjust volatility of analyte and/or matrix to eliminate these effects is also included. Addition of reactive gases (eg oxygen) to the carrier gas to improve matrix separation. Also included is Zeeman or other background correction techniques to remove interference due to absorption and scattering from coexisting molecules/atoms in the sample. Correction for additive interferences in standard addition method is difficult. Use of matrix modifier, optimized temperature program for selective volatilization, background correction and removal of matrix interferences (inhibiting formation of free analyte atoms in the atomization step) are required to get right results in external calibration. 18