UTILIZING INTERLABORATORY COLLABORATIVE STUDY TO HARMONIZE PROTOCOL OF ANALYTICAL METHODS

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1 The First International Proficiency Testing Conference Sinaia, România 11 th 13 th October, 2007 UTILIZING INTERLABORATORY COLLABORATIVE STUDY TO HARMONIZE PROTOCOL OF ANALYTICAL METHODS Queenie S. H. Chui 12 ; Celia O. Iamashita 2 ; João Marcos de A. Bispo 2 1 Universidade São Francisco (USF), Programa de Pós-Graduação stricto sensu em Engenharia e Ciências dos Materiais Rua Alexandre Rodrigues Barbosa, 45, Itatiba, SP, CEP , Phone: , Fax: Instituto de Pesquisas Tecnológicas do Estado de São Paulo S.A. (IPT) Agrupamento de Materiais de Referência, Divisão de Química (AMR/DQ) Av. Prof. Almeida Prado, 532, São Paulo, SP, Brasil, CEP queenie.hang@saofrancisco.edu.br Abstract This work aims to describe the experience of using interlaboratory collaborative work to harmonize analytical method procedure in order to reduce measurements uncertainties. The experiments were made using the loss of ignition assay with nine participant laboratories using three protocols: the one adopted by the laboratories in their routine work, the procedure following ISO 6066 and a modified procedure as an alternative indicated by the participant laboratories. The uncertainties of the obtained measurements were calculated and the protocol that resulted in measurements with the lowest uncertainty was indicated as the one that should be adopted by all laboratories. Key words Collaborative trial; interlaboratory uncertainties; loss of ignition 1 INTRODUCTION Bauxite ore is the raw material for aluminum production. To give traceability to the measurements in laboratories of analytical quality control of bauxite, the use of matrix reference materials is necessary. One of the properties of interest for the quality control of the bauxite raw material is the amount of mass loss in the assay of loss of ignition, that is function of the temperature and the warm up time. 209

2 In routine analytical work, the determination of loss of ignition is usually made by calcination of a sample amount in an electric oven at approximately C. The loss of mass determined by difference of weight before and after this operation is given as the result of loss of ignition assay and this is related to the hydrates forms of aluminum present in the bauxite sample. A small lot of bauxite was prepared to be as a candidate of a matrix reference material. After the preparation its homogeneity was tested. Samples of this material were distributed to nine laboratories of research institutes and industrial laboratories. A preliminary collaborative trial was proposed in order to detect the magnitude of the laboratories variability. The laboratories were allowed to choose their own procedure. This comparison revealed a high Interlaboratory variance obtained by different methods. A need to harmonise the methodologies was observed and a suggested protocol established. For the subsequent trials, all participating laboratories were requested to harmonise their procedure to improve data quality. 2 EXPERIMENTAL 2.1 Batch Preparation About 140 kilograms of bauxite raw material were obtained from Alcoa Alumínio S.A. located at Poços de Caldas city in Minas Gerais State, Brazil. Sampling, sample treatment, homogeneity testing and bottling were done at the Inorganic Reference Material Laboratory (LMRI) in the Chemistry Division at the Institute of Technology Research of São Paulo State (IPT) in Brazil. The raw material was ground with tungsten carbide grinders and passed through sieves, resulting in approximately 82 kg with particles <75 μm in size. It was then thoroughly mixed and homogenised on a superficial plastic film. The well-mixed material was poured into bottles containing about 100 g each. 2.2 Collaborative Trials Each laboratory received two bottles. The experiment plan presented three stages: in the first stage, participant laboratories were allowed to use their routine procedure; in the second stage, a chosen procedure was considered to be adopted by all the participant laboratories: the sample after being dried at C was submitted to calcinations at C, C and C; in the third stage, a procedure was established based on instructions of ISO 6606, when specific conditions of time and platforms of temperature are established as C and C. In all stages, the samples were dried at C before submitting them to higher temperatures. Each laboratory was asked to make six replicate samples. In order to give traceability of their measurements, NIST SRM 696 samples or another traceable standard material were analyzed by the participating laboratories to help in avoiding gross errors in the individual laboratories. Mean values, coefficient of variation, standard uncertainties and the confidence interval of the mean of the analytical results were calculated for the three stages of the experiment. 210

3 3 RESULTS AND DISCUSSIONS The results of the first stage when the laboratories used their routine procedures are presented in Table I. Table 1 - Loss of ignition results: first stage of the collaborative trial with laboratories using their routine procedures General mean: 30.29%; standard deviation: 0.84%; relative standard deviation in percentage (CV): 2.80%; standard uncertainty: 0.28%; confidence interval of the mean: 0.65%(n=10; 95% confidence level). It was observed a high dispersion, resulting in 30.29% with uncertainty of ± 2.1% (standard deviation of 0.84%, standard uncertainty of 0.28%). In a technical meeting with the participant laboratories searching for the possible problems that could represent the causes of the observed dispersion, it was pointed out that the time and temperature conditions of heating were the main factors leading to the observed dispersion among laboratories. It was decided that all laboratories should be repeating the assay maintaining the same calcination temperature of C and time of cooling. The following procedure was suggested: 1) To weigh about 2g of sample in a lipless flask with cover and dry at C for two hours. 2) To cool in dissector at ambient temperature. 3) To take off the cover from the flask and to weigh using analytical balance. 4) To transfer the sample to a tarred platinum crucible (m 1 ) and to repeat weighing the empty lipless flask. To calculate the mass of the sample as (m). 5) To calcinate at C for 30 minutes. 6) After that, to increase the temperature to C maintaining the crucible with the sample for 30 minutes and, in sequence, to raise to C and to treat the sample for 1 hour. 7) To remove from the muffle and to cool the crucible with the sample in a disecator at the same conditions used for the empty crucible and 8)After weighing (m 2 ), to calculate the result considering equation (1). The results are described in Table 2. % Loss of ignition = [(m 1 - m 2 )x 100]: m (1) 211

4 Table 2 - Results of the 2 nd stage of the experiment - calcination at C grand mean: 29,97%; standard deviation: 0,33%; relative standard deviation in percentage (CV): 1,10%; standard uncertainty: 0,10%; confidence interval of the mean: ± 0,24% (n=10; 95% confidence level) Table 2 shows that the grand mean was 29.97% with standard deviation of 0.33%, coefficient of variation in percentage (%CV) of 1.10% and standard uncertainty of 0.10%, indicating a lower dispersion when compared to the values obtained in the 1 st stage. The confidence interval, for the 1 st trial, was (30.29 ± 0.65) % (Table I) being an uncertainty of ± 2,1% (= 0.65x100/30.29). In the 2 nd trial, the confidence interval was (29.97 ± 0.23) %, representing an uncertainty of ± 0.80% (= 0.23 x 100/ 29.97) that is about 3 times lower. In the 3 rd stage of the experiment the laboratories followed the procedure based on ISO Considering the various different temperatures that could promote the aluminum and iron hydroxides or oxihydrates separations and several other species that are possible present in minor proportions, it was chosen the following conditions: with approximately 1g of a dried sample at (105 ± 2) 0 C for one hour in an calibrated oven the calcination should start at (375 ± 25) 0 C for 10 ±1 min; after that, the sample was cooled in disecator (with activated alumina or magnesium perchlorate or phosphorous pentoxide as the dryer). After reaching constant weight, with the temperature of the oven at (1075 ± 25) 0 C the sample was maintained at this temperature for (60 ± 2) minutes, after that it was cooled in a disecator for an hour and finally having the mass determined. If the difference of mass within two successive weighing was higher than 0,0005g the sample should return to the oven and again treated at (1075 ± 25) 0 C, until obtaining the above-mentioned requirement. The results of this stage can be seen in Table 3. The 3 rd trial resulted in grand mean of 30.01% with uncertainty of ±0.15% (standard uncertainty of 0.18% and confidence interval with inferior and superior limits of 29.56% and 30.46%). The mean value is almost the same as the obtained in the previous stages. The uncertainty (±1.5%) is higher than the one obtained in the 2 nd stage of the experiment (± 0.80%), and lower than the observed response in the 1 st the stage (±2.1%). Not all the participant laboratories could follow the procedure based on ISO 6606, because their oven did not present the technical conditions to reach the temperature of C. Only six laboratories could attend this condition. 212

5 4 CONCLUSIONS Table 3 - Results of loss of ignition at C (with the procedure based on norm ISO 6606) Grand mean: 30.01%; standard deviation: 0.43%; relative standard deviation in percentage (CV): 1.45%; standard uncertainty: 0.18%; confidence interval of the mean: ± 0,45% (n=6; 95% confidence level) The procedure usually adopted by the routine work in quality control laboratories is simple and easy to apply. When the bauxite samples are treated directly at C, sample losses due to sputtering of material and/or factors of random effect can happen. The introduction of small particles in the interior of the crucible that contains the sample of the analyzed material also must be considered. The alternative procedure (2 nd stage) considers platforms of temperature, until reaching C, resulting in lower dispersion of results and consequently lower uncertainty (±0.80%). It is observed that the values of the grand means, for the 3 stages (30.29%, 29.97% and 30.01%) are comparable in despite of the uncertainties, that were higher for the results of the 1 st trial followed by the 3 rd one. The procedure based on ISO 6606 requires oven with higher temperature capacity that the oven-muffles usually found in the quality control laboratories and this type of oven was not available in all the participant laboratories. It is also important to point out the necessity to maintain temperatures calibration conditions of the oven. In this study, the comparisons in collaborative trials revealed useful and contributed to improve the quality of the routine quality control laboratories. It was possible to indicate the main variabilities of the methods, in order to help analyst to eliminate them. With this study, one can emphasize the importance of using interlaboratory comparison not only to evaluate laboratories performance as well as to harmonize methodologies and when aiming to revision of norms. Acknowledgements To the laboratories of the companies: Alcan, Alcoa, Alcoa Kwinana, Alumar, Alunorte, CBA, L.A. Teixeira, MRN, CVRD and Lakefield for the participation in the studies of interlaboratory comparisons. REFERENCES [1] Pereira, Nedir Dorta S.; Moro, Sylvia Lourdes; Tsai Soi Mui; Perda ao Fogo (Loss of Ignition) em Métodos de Análise Química adotados para Certificação de Amostra Padrão de Calcário. Publicação IPT 1187, séries Técnicas e Métodos n 0 12, São Paulo, SP, 1981, reimpressão 1985, p.37 [2] International Organization for Standardization - ISO 6606: Aluminum ores - Determination of loss of mass at C - Gravimetric method, 1st Edition, Switzerland, 1986, 3p 213