CCQM-K45: Sn in tomato paste key comparison

Transcription

1 CCQM-K45: Sn in tomato paste key comparison Final report September 2006

2 CCQM-K45: Sn in tomato paste key comparison Final report September 2006 Contact Point: Gill Holcombe Tel: +44 (0) Prepared by: Céline S.J. Wolff Briche, Gill Holcombe, Mike Sargent Approved by: Date: 12 September 2006 LGC Limited 2006

3 Contents 1 Summary 1 2 Introduction 1 3 Rationale of this comparison 1 4 Participation in CCQM-K Instructions to participants 2 6 Methods and instrumentation used 2 7 CCQM-K45 participants results Reported results for Sn Total solids determinations 3 8 Agreement on the key comparison reference value 5 9 Equivalence statements 6 10 Acknowledgements 8 11 References 8 12 Appendix A: Protocol distributed to participants 9 13 Appendix B: Sample preparation and homogeneity Sample preparation Homogeneity of the tin content Homogeneity of the total solids content 13 Page i

4 1 Summary CCQM-K45 key comparison was performed to demonstrate and document the capability of interested National Metrology Institutes in the determination of the mass fraction of Sn in a food matrix. This comparison was an activity of the Inorganic Analysis Working Group of the Comité Consultatif pour la Quantité de Matière and was co-ordinated by LGC (Teddington, UK). The following laboratories participated in this key comparison (alphabetical order). KRISS (Republic of South Korea) LGC (UK) LNE (France) PTB (Germany) UME (Turkey) Good agreement was observed between the reported results of four of the participants. The other participant did not use any HCl during the sample preparation, which can affect the stability of the tin in solution, and its reported value was lower than for the other participants. It was not included in the calculation of the key comparison reference value with the agreement of the NMI concerned. The reference value was calculated as the arithmetic mean of the reported participants results (excluding one), and is equal to mg kg -1, with an expanded uncertainty of 2.4 mg kg -1 (1.1%). The equivalence statements were calculated on this basis. 2 Introduction In October 2004, at the Inorganic Analysis Working Group (IAWG) of the Comité Consultatif de la Quantité de Matière (CCQM) meeting in Mexico, LGC proposed a comparison to analyse three elements in tomato paste. It was agreed to organise two studies in parallel: a key comparison, CCQM-K45, for Sn in tomato paste, and a pilot study, CCQM-P72, for Sn, Pb and Cd in tomato paste. The tomato paste analysed by the CCQM-K45 and CCQM-P72 participants was also used for two studies involving field laboratories. These were organised by LGC as part of the certification as a reference material, and by the Central Science Laboratory (CSL) for a round of the FAPAS PT scheme. 3 Rationale of this comparison This key comparison builds on CCQM-P13 (metals in food digest) pilot study and uses a sample spiked with three toxic metals. The main scope was to underpin Calibration and Measurement Capabilities (CMC) for determination of tin in food, for which there is an EU regulatory limit of 200 mg kg Proficiency testing (FAPAS ) 2,3 has highlighted problems with the measurement of tin in food matrices by field laboratories, leading to inconsistent data between laboratories. In a number of cases, low results were achieved, leading to a low consensus value compared to the National Metrology Institute (NMI) reference value. In addition, the use of multi-element techniques for food applications is increasing rapidly and errors arise because multi-element digestion or extraction procedures are frequently unsuitable for the determination of tin. Key comparison CCQM-K45 (for Sn only) was conducted in parallel with the pilot study CCQM- P72. The pilot study also included lead and cadmium, for which accurate determination at low levels in food is of high importance, and was a test of the capability of NMIs to make accurate measurements at very low concentrations in the presence of interfering species. Key objectives of the study were to establish international comparability for measurement of Sn in food samples and to underpin NMI measurements of toxic metals in a food matrix. Page 1

5 4 Participation in CCQM-K45 The institutes / organisations which registered for CCQM-K45 (as entered in the Key Comparison Database) are listed in Table 1. Table 1. CCQM-K45 participants Institute / Organisation Country Contact KRISS Korea Research Institute of Standards and Science LGC Republic of South Korea United Kingdom E. Hwang R. Hearn LNE Laboratoire National d Essais NMi-VSL NMi Van Swinden Laboratorium B.V PTB Physikalisch-Technische Bundesanstalt UME National Metrology Institute France The Netherlands Germany Turkey G. Labarraque M. van Son D. Schiel I. Akdağ D. Karakas NMi-VSL (The Netherlands) registered to participate in this key comparison but, due to the relocation of its facilities, it was unable to report its results within the deadline for this study. 5 Instructions to participants A protocol (Appendix A) was sent to all participants prior to sample distribution and provided information concerning storage and analysis of the sample. Participants were free to use a method of their choice for tin. They were asked to report results on the sample as received, as this is the usual practice for field laboratories. However, to ensure no changes had occurred in the sample between bottling and analysis, or to explain any differences in tin results, participants were asked to determine the total solids content using a common procedure (see Appendix A). 6 Methods and instrumentation used Participants were free to use a method of their choice for Sn, however, it was recommended that HCl should be used during the digestion, as it stabilises the tin in solution (Appendix A). All participants used microwave digestion, using various reagents, including HNO 3, HCl, and H 2 O 2. However, UME did not use HCl, and LGC did not include H 2 O 2. All participants carried out their measurements by ICP-MS, and four of the five participants applied isotope dilution. One participant (PTB) used a magnetic sector ICP-MS, UME did not detail which instrument was used, and the three other participants used quadrupole ICP-MS. Table 2 gives an overview of the methods applied and the instrumentation used by each CCQM-K45 participant. For the total solids determination, a specific protocol was recommended in order to be able to compare the results directly. This is necessary as excess drying may cause degradation of the sample. Page 2

6 Table 2. Analytical methods and instrumental techniques used by CCQM-K45 participants Participant Digestion Method Instrumentation KRISS LGC LNE PTB UME Microwave digestion, IDMS Quadrupole ICP-MS with HNO 3 /HCl/H 2 O 2 Dynamic Reaction Cell Microwave digestion, HNO 3 /HCl IDMS Quadrupole ICP-MS with collision cell Microwave digestion, IDMS Quadrupole ICP-MS HNO 3 /HCl/H 2 O 2 Microwave digestion, IDMS Magnetic sector field ICP- HNO 3 /HCl/H 2 O 2 MS Microwave digestion, HNO 3 /H 2 O 2 3 point external calibration ICP-MS 7 CCQM-K45 participants results 7.1 Reported results for Sn The CCQM-K45 participants results, as reported to the co-ordinating institute (LGC), are given in Table 3. All uncertainties are expanded uncertainties. The coverage factor used was k = 2, except for KRISS, which used k = All data are reported on the sample as received. Results are represented pictorially in Figure 2. Table 3. CCQM-K45 participants measurement results for Sn Participant Reported result mg kg -1 Expanded uncertainty mg kg -1 Relative uncertainty KRISS % LGC % LNE % PTB % UME % 7.2 Total solids determinations All participants applied the protocol described in Appendix A, subject to the variations noted below. The average and standard deviation calculated from the replicate results reported by the participants are given in Table 4 and Figure 1. Page 3

7 Table 4. CCQM-K45 participants total solids results Participant Average total solids g kg -1 Standard deviation g kg -1 Pre-drying stage KRISS Vacuum oven at 70 C for 1 hour, 300 mbar Final drying stage Vacuum oven at 70 C for 1 hour, 250 mbar (6 times) LGC Vacuum oven Vacuum oven at 70 C LNE Conventional oven at 70 C Conventional oven at 70 C PTB Conventional oven at 70 C for 12 hours UME Conventional oven with air flow Vacuum oven at 70 C Two periods of 3 hours, Conventional oven at 70 C for 120 hour Conventional oven at 70 C LNE applied the protocol supplied for 2 determinations, and a classical protocol for 2 other determinations (using the same protocol as in Appendix A, but without using sand or a pre-drying stage). Using their in-house classical protocol, the resulting total solids mass fraction was equal to 278 g kg -1. (1.2 % lower than with the prescribed protocol). The points in Figure 1 represent the average of the total solids determinations, with the bars corresponding to the standard deviation of those results KRISS Mass fraction total solids (g kg -1 ) UME LGC 280 LNE PTB 275 Figure 1: CCQM-K45 participants total solids results PTB mentioned that in spite of the long drying time, the sample did not reach a stable mass within the uncertainty limits of weighing. LNE also found that after 1 week, the loss of mass continues (few milligrams). As the tomato paste was dried longer than recommended, it is possible that not only the water was removed but other components as well. The extended drying time may explain the lower total solids content compared with results reported by the other participants. Page 4

8 There does not seem to be a link between results reported for Sn and the total solids of the sample material. The low mass fraction of Sn reported by UME cannot be explained by a low level of total solids. 8 Calculation of the key comparison reference value There is no CCQM rule for the calculation of the key comparison reference value (KCRV). This is left to the discretion of the IAWG of the CCQM. In the past, the mean * has often been used to calculate the KCRV, but other methods can also be used. The use of the median as KCRV is recommended if there are results reported with significantly lower or higher mass fraction compared to the other participants results. The weighted mean is applicable for independent normally distributed results of the same population. Unless the number of degrees of freedom associated with the data is very large (> 30), the weighted mean s uncertainty is usually an underestimate. When the mean is used, the standard deviation of the mean is used to estimate the standard uncertainty of the reference value. The KCRV for CCQM-K45 was discussed and agreed upon during the CCQM-IAWG meeting in October 2005, in Berlin. UME reported using a microwave digestion with only HNO 3 and H 2 O 2, but no HCl (Table 2). The protocol (Appendix A) recommended that the use of hydrochloric acid will help to maintain the stability of tin in solution. As this participant used no HCl, it is possible that some Sn was lost during the procedure, explaining the lower reported value. The pilot study CCQM-P72, which was conducted in parallel with CCQM-K45, included 6 more participants for the determination of tin in the tomato paste. 4 In this study, most participants reported values for tin which are higher than the one reported by UME. UME was contacted and agreed that as they did not use HCl, their value could be omitted from the calculation of the KCRV. As a result, only 4 reported values are available to calculate the KCRV. The different approaches considered for the calculation of the KCRV are given in Table 5. Not all participants gave details of their uncertainty calculations. KRISS used a coverage factor equal to 2.45, which indicates only 6 degrees of freedom. As a result, the weighted mean is not considered the best estimate of the KCRV, as all results do not have sufficient degrees of freedom. As only 4 values can be used to calculate the KCRV, the median is also not considered the best estimate of the KCRV. Finally, the arithmetic mean as the average of all the reported results was agreed to serve as the KCRV for the CCQM-K45. Its associated expanded uncertainty was calculated as the standard deviation of the mean, multiplied by a coverage factor equal to 2. The arithmetic mean is equal to mg kg -1, the standard deviation of the mean is used as an estimate of its associated uncertainty, leading to an expanded uncertainty equal to 2.4 mg kg -1 (k=2). * The terms "mean" or "average" indicate the arithmetic mean of observations unless otherwise stated. Page 5

9 Table 5: Comparison of the different approaches to calculate the KCRV for CCQM-K45, using only 4 values KCRV mg kg -1 KCRV standard uncertainty (u) mg kg -1 Method of calculation of u(kcrv) Mean Standard deviation of the mean Median MADe Weighted mean Participants uncertainties used as weights CCQM-K45 KCRV: ± 2.4 mg kg -1 (U = k u, k=2) The results of the participants are displayed graphically in Figure 2. The diamonds represent the results reported by each participant, with the bars corresponding to their reported expanded uncertainties. The KCRV is represented by the red line together with its expanded uncertainty represented by dotted lines % Mass fraction of Sn (mg kg -1 ) UME LGC KRISS PTB LNE 2% 0% -2% -4% -6% -8% -10% Relative deviation from reference value (%) % Figure 2: CCQM-K45 participants measurement results for tin, with the KCRV (without UME result) 9 Equivalence statements The equivalence statements have been calculated according to the BIPM guidelines. The degree of equivalence (and its uncertainty) between a NMI result and the KCRV is calculated according to the following equations: 2 2 D = x x U = ( u + u ) i i R i 2 i R where D i is the degree of equivalence between the NMI result x i and the KCRV x R, and U i is the expanded uncertainty (k = 2) of the D i calculated by combining the uncertainty (k = 1) of the NMI result u i and the uncertainty (k = 1) of the KCRV u R. The equivalence statements for CCQM-K45 are given in Table 6, and displayed in Figure 3. Page 6

10 Table 6: Equivalence statements D i (mg kg -1 ) U i (mg kg -1 ) KRISS LGC LNE PTB UME The diamonds displayed in Figure 3 represent the degree of equivalence between each participant and the KCRV. The error bars represent the expanded uncertainty associated with the degree of equivalence LNE Di (mg kg -1 ) KRISS LGC PTB UME -25 Figure 3 Graphical display of equivalence statements. The degree of equivalence (and its uncertainty) between two NMI results is calculated according to the following equations: 2 2 D = x x U = ( u + u ) ij i j ij 2 i j Where D ij is the degree of equivalence between the two NMI results x i and x j, and U ij is the expanded uncertainty (k = 2) of the D ij calculated by combining the uncertainties (k = 1) of the two NMI results u i and u j. The equivalence statement between the CCQM-K45 participants is given in Table 7. Page 7

11 Table 7: Matrix of equivalence between the CCQM-K45 participants Lab j KRISS LGC LNE PTB UME Lab i D ij U ij D ij U ij D ij U ij D ij U ij D ij U ij KRISS LGC LNE PTB UME Acknowledgements The work described here contains the contributions of many scientists: Euijin Hwang from KRISS, Ruth Hearn from LGC, Guillaume Labarraque from LNE, Detlef Schiel and Reinhard Jaehrling from PTB, Ibrahim Akdağ and Duran Karakas from UME. Sample preparation was organised by Joanne Croucher (CSL, York), and the homogeneity measurements were performed by John Lewis (CSL, York). Linda Evans of LGC assisted with testing of the protocol for total solids determination. Claire Carter at LGC assisted with sample packaging and distribution. This work was supported under contract with the United Kingdom s Department of Trade and Industry as part of the National Measurement System Valid Analytical Measurement Programme. 11 References Commission Regulation (EC) No 242/2004. FAPAS report 0738, Central Science Laboratory, York, UK (September 2002) FAPAS report 0745, Central Science Laboratory, York, UK (January 2004) C.S.J. Wolff Briche, G. Holcombe, M. Sargent; CCQM-P72: Sn, Pb and Cd in tomato paste pilot study, LGC/2005/025. Page 8

12 12 Appendix A: Protocol distributed to participants Key Comparison CCQM-K45 Determination of Tin in Tomato Paste Protocol Introduction This key comparison builds on CCQM-P13 (metals in food digest) and uses a sample spiked with tin, lead and cadmium. The main scope is to underpin CMCs for determination of tin in food, for which there is an EU regulatory limit of 200 mg kg -1. Proficiency testing (FAPAS) has highlighted problems with the measurement of tin in food matrices by field laboratories, leading to inconsistent data between laboratories. In a number of cases low results are achieved, leading to a low consensus value compared to the NMI reference value. In addition to this, the use of multi-element techniques for food applications is increasing rapidly and errors arise because the multi-element digestion or extraction procedures in wide use are frequently unsuitable for the determination of tin. A key objective of this comparison is to establish equivalence between reference values provided by NMIs for CRMs or PT schemes and hence to aid international traceability for measurement of tin in food samples. Sample Description The sample consists of a glass bottle containing approximately 50 g of frozen moist tomato paste spiked with tin in the mass fraction range of mg kg -1. The sample should be stored in a freezer at 20 C and thawed when required for use. If more sample is needed contact Gill Holcombe at LGC. Analysis At least three replicate analyses should be carried out. Participants are free to use any suitable method but please include a full description of your method of analysis when reporting the results. A full uncertainty budget should also be included with your results, as indicated below. Results should be reported on a moist weight basis, that is, on the thawed sample as received. Normally field laboratories carry out this measurement on the moist sample for regulatory purposes and it is also quite difficult to dry. Participants are, however, requested to determine the total solids content on a portion of the sample as this will assist in discussing the results in the event of any unexpected variability between laboratories. The recommended protocol for total solids determination is given below and for this part of the study participants are requested to adhere to the protocol to ensure consistent data between laboratories. The sample should be thawed to room temperature prior to analysis. The minimum recommended sample size for each determination is 2 g, which is in accordance with the homogeneity testing of the material. Since the sample contains moisture great care must be taken to avoid moisture loss prior to and during weighing. It should never be left uncovered and weighing out of the sample should be carried out quickly to avoid loss of moisture. It should be noted that the use of hydrochloric acid will help to maintain the stability of tin in solution. Page 9

13 Determination of total solids content This method is derived from AOAC Official Method (Total solids in canned vegetables). For each measurement, dry a flat-bottomed metal dish (containing approximately 15 g sand and a short glass rod), at 100 C for at least 1 hour. At the same time dry a metal lid for the dish. Place the lid on the dish and cool in a desiccator to room temperature and weigh (Weight A). Add approximately 2 g well-mixed tomato paste into the dish, place the lid on the dish and reweigh (Weight B). Weigh quickly to avoid moisture loss during weighing. Mix the tomato paste with the sand using the glass rod, adding a little de-ionised water if necessary to facilitate distribution. Take care not to lose any of the sand during the mixing process. The rod should be left in the dish at all stages of the procedure. Carry out a preliminary drying stage using one of the following methods: (a) Dry the dish in vacuum oven, with the lid to the side of the dish, at 70 C with the release valve left partly open to allow a flow of air through the oven at a pressure > 300 mbar (< 700 mbar vacuum). Examine the dish at 30 min intervals and remove when the sample has reached apparent dryness. (b) As for (a) but use a force-draft oven at 70 C with sufficient flow of outside air to remove moisture rapidly. (c) With the lid removed, dry the dish over a boiling water bath with occasional stirring until the sample reaches apparent dryness. Dry the partially dried sample by placing the dish in vacuum oven, with the lid to the side of the dish, at 70 C, 250 mbar pressure (750 mbar vacuum) for one hour. Place the correct lid on the dish and cool in a desiccator to room temperature and weigh. Repeat drying for one-hour periods until constant weight is achieved (Weight C). Weighing should be done as soon as the dish reaches room temperature, as the sample will rapidly absorb moisture if left standing over most desiccants. Note: some laboratories may not have a vacuum oven. We have also determined the solids content using the procedure described above with a conventional oven at 70 C and ambient pressure. No apparent difference in solids content was seen with this sample. Total Solids (g/100g) Uncertainty Evaluation ( Weight C - Weight A) ( Weight B - Weight A) 100 = Equation 1 Each laboratory should make an assessment of the experimental uncertainty according to ISO principles (Guide to the Expression of Uncertainty in Measurement, ISO, Geneva, 1993, ISBN ). Each variable contributing to the uncertainty of the results should be identified and quantified in order to be included in the combined standard uncertainty of the result. A full uncertainty budget must be included, as part of the results. Contributions to the overall uncertainty will arise from the repeatability of the sample preparation, the repeatability of instrumental determination, determination of masses and volumes, concentration of primary and internal standards, and any other parameter specific to each method of analysis chosen by the participant. Results should be submitted using the results report form provided and sent to Gill Holcombe at LGC, by post, or fax, no later than 31 March Page 10

14 Gill Holcombe Study Co-ordinator LGC Limited Queens Road Teddington Middlesex TW11 0LY United Kingdom Fax: Page 11

15 13 Appendix B: Sample preparation and homogeneity 13.1 Sample preparation Forty-nine cans from the same manufacturing batch of tomato paste, each containing 800 g, were used in the preparation of the test material. The natural levels of lead, cadmium and tin were determined in the paste prior to spiking with the metallic contaminants and were found to be ~ 11µg kg -1 of lead, ~ 21 µg kg -1 of cadmium and ~ 1 mg kg -1 of tin. The paste was spiked with appropriate quantities of lead (12 mg), cadmium (4 mg) and tin (8624 mg) using standard solutions and mixed for 1 hour 30 minutes. Sub-samples (50 g) of the bulked material were weighed into glass bottles. Each bottle was individually numbered and stored at 20 C. The bottles were irradiated at a dose between 14 and 18.3 kgy, and refrozen Homogeneity of the tin content Twenty-two bottles were analysed in duplicate in order to assess the homogeneity of the batch for tin content. Aliquots of 2 g were taken and microwave digested in a mixture of nitric and hydrochloric acid. The measurements were carried out by ICP-MS. The results of these analyses are shown in Figure 4. The points represent the average of the two determinations performed on each bottle, and the bars represent the corresponding standard deviation Mass fraction of Sn (mg kg -1 ) Bottle Figure 4: Results obtained for 22 bottles analysed in duplicate for tin for homogeneity determination The average of all the results is equal to mg kg -1, with a standard deviation equal to 3.5 mg kg -1 (1.6 %). An analysis of variances was performed on these results, and it was concluded that there was no significant differences between the bottles. As a conclusion, this material was fit for the intercomparison. Page 12

16 13.3 Homogeneity of the total solids content The total solids content was also checked, in 10 bottles, in order to check homogeneous distribution of the material. The recommended procedure described in the protocol (Appendix A) was used. The result of these determinations is given in Figure 5. The points represent the average of the two determinations performed on each bottle, and the bars represent the corresponding standard deviation. The red line represents the overall mean. Analysis of these results revealed that there was no significant difference between bottles. The average total solids content, calculated from these 20 data (red line in Figure 5), is equal to g kg -1. An uncertainty contribution was calculated and is equal to 1.8 g kg -1 (0.61 %, with 9 associated degrees of freedom) Total solids (g/kg) Bottles Figure 5: Determination of total solids content in 10 bottles Page 13