SOME TECHNICAL PROBLEMS (AS HOMOGENEITY AND STABILITY) IN PROFICIENCY TESTING SCHEMES

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1 The First International Proficiency Testing Conference Sinaia, România 11 th 13 th October, 2007 SOME TECHNICAL PROBLEMS (AS HOMOGENEITY AND STABILITY) IN PROFICIENCY TESTING SCHEMES Paula Vaquero, Ramón Sanz, Francisco García and Isabel de Blas Applus + Labaqua, c/dracma, 16-18, Pol. Ind. Las Atalayas, Alicante (Spain) paula.vaquero@labaqua.com Abstract Proficiency testing schemes are a good instrument to evaluate the quality of the laboratories and to demonstrate the technical competence to the accreditation bodies. These proficiency testing schemes must be also recognised by the accreditation bodies to assure the confidence of these results. The requirements of the accreditation bodies for being an accreditated proficiency testing scheme supplier are so many, standing out the homogeneity and stability of the samples and the low detection limits in the used matrix analysis to assure a target value. These requirements can be carried out through working methods that can give us good detection limits in a multielemental analysis. To solve the first problem, the instrument must be calibrated simultaneously for all the elements that are going to be in the sample, obtaining good repetibilities and reproducibilities. On the other hand, the required methodology to assure a target value through a matrix without any trace of the metal element means a new analysis method for the ICP-MS with low detection limits. Key ords Homogeneity, stability, detection limit, optimization, accreditation 1 INTRODUCTION Nowadays, all laboratories must have validated and studied their own methodologies because of the strictest quality rules and the new demanding regulations. According to that, their results would be reliable and they would must have technical confidence and credibility as much as for all the customer as for the accreditation bodies.the proficiency testing schemes, along with the methodology and instrumental validations, are a tool that assure the quality of the results that the laboratory gives to their customers. In order to be recognised with a reliable level of confidence between 113

2 the different accreditation bodies, the proficiency testing schemes must be totally accredited, from the initial design of the circuit to the final emission report. 2 TECHNICAL PROBLEMS IN PROFICIENCY TESTING SCHEMES DEVELOPMENT In the national field, the technical accreditation body for proficiency testing schemes supplier programs is ENAC (national accreditation body), who establishes strict requirements in their technical note [1], that are based on the ISO 43 standard [2]. These requirements must be carried out for everyone who wants to be accredited as proficiency testing schemes supplier (PTS). If we develop some different parts of the technical note, there are two points that stand out so much and involve technical difficulties: one of these points would be the verification of the homogeneity meanwhile the samples are being prepared and before send them, as well as the stability of the samples along the proficiency testing scheme is running. In the other hand, if we want to give a spiked value with a 100% confidence level, it is needed to assure that the element does not exist in the used matrix for the sample. In order to carry out all these requirements, we must use high precision instruments that should be perfectly validated. The ICP-MS is a really powerful instrument to analyse metals in these concentrations levels, which allow us to do a multi-elemental analysis in different ranges of concentrations with low detection limits. 2.1 Homogeneity and stability The homogeneity acceptance criteria of the National Accreditation Body for the proficiency testing schemes means an analysis of lots of parameters in a short time and with low repeatability. For that reason, it is needed to get deviations lower than 2% in a minimum number of 20 analyses over 10 samples, proving the in-sample and inter-sample variability [3]. This acceptance criteria is established by our organization in an internal procedure and it have been accepted by ENAC. In order to obtain this type of results, we need to improve the methodology and instrumental optimization of the ICP-MS [4] & [5]. We make this instrumental fitting over the characteristical parameters of the instrument, as well the simple introduction system as the element detection system. Figure 1 ICP-MS tune 114

3 Figure 2 ppb calibration Figure 3 - ppm calibration We make this instrumental fitting over the characteristical parameters of the instrument, as well the simple introduction system as the element detection system. For that reason, it would be adjusted some parameters such as plasma power, torch position, sample introduction and carrier gas flow, as well as the different ion lens voltages and the different detection mode voltages (pulse-analogue mode). Also, it is very important to consider the instrument methodology optimization, that must allow the multi-elemental analysis in a work range from μg/l (low levels) to mg/l in the same analysis. For this reason, some interference equations, which allow the multi-elemental analysis, have been developed and strict quality controls have been stabilized that guaranteed the stability of the instrument along all the analysis process. Interference equations: 75 As= 75 M *77 M *78M 54 Fe= 54 M-0.15*15 M There are also interference equations as Fe - Cr, Ti - Cr, Se - Kr, etc. In the same way, the ENAC acceptance criterion for the stability of the sample in the proficiency testing schemes is really severe, according to the ISO 43 Standard [2]. Likewise, we need to have a chemical knowledge of the used matrix with the correct preservative and/or stabilizants to avoid possible surprising problems [6]. The technical requirements that are needed for this stability analysis will be similar to the homogeneity analysis previous to send the PTS samples. 2.2 Detection limits In the cases we want to assure the concentration value of the analyte that has been spiked to the sample and give the result as a target value with a 100% confidence, it is necessary to assure the no-existence of any trace concentration of the element in the sample. For that reason, our internal procedures establish that the concentration of the element must be lower than 1% of the value given as target result as the matrix acceptance criteria. This criteria has been accepted by ENAC and involves a methodology and instrumental parameter modification in the analysis of the matrix sample, to improve the instrument sensibility as much as possible. 115

4 At the methodological point of view, we will use a method which is based on the analysis with low detection limits (approximately 0.01 µg/l for some analytes). In order to get this method, we must increase the integration time in every element replica and cut down the lineal range to concentrations near to the detection limit. Also, this analysis will be done in a multi-element way. Figure 4 - Multi-elemental analysis In the instrumental optimization part, this detection limits will be carried out changing the nebulization system, changing from the Babington nebulizer to a Miramist nebulizer, that will give a higher nebulization performance; for this matter, a higher sensibility. The voltages of the ion lenses and detection system will be also changed, as well as the torch position. All these changes will allow to get the wanted detection limits. 3 CONCLUSIONS The technical modifications, as well instrumental ones as methodological ones, we described above, let us solve the technical problems of the ISO 43 Standard requirements. In the homogeneity and stability point of view, the optimization done in the instrument let us get results of the samples with homogeneous and stables values according to the internal procedure manual of the organization [7], based in the international rules [8], in a quick and simultaneous analysis, working in micrograms per litre range as well as milligram per litre range in the same analysis. Likewise, the optimization we have done to analyze the existence of the element in low concentrations allows to give a qualitative view of the analyte over the 1% of the target concentration. With all of these things, Calitax-Labaqua AIE has managed the proficiency testing scheme supplier accreditation, according to the ISO 43 Standard, from the initial design of the circuit to the final emission report. 116

5 REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] CGA-ENAC-PPI. General criteria for the accreditation of the proficiency testing schemes suppliers according UNE and ILAC-G13 guide UNE IN (ISO/IEC 43-1). Proficiency testing by interlaboratory comparisons. Part 1: Development and operation of PTS ISO 13528:2005. Statistical methods for use in proficiency testing by interlaboratory comparisons DEM-D-002 Agilent 7500 ICP-MS. ChemStation. Operator s Manual Internal procedure PE-D/0026 Determination of metals by ICP-MS Standard Methods. 20th Edition EI-002 Verification of the homogeneity and stability in proficiency testing schemes (Internal procedure) ILAC-G13:2000. Guidelines for the requirements for the competence of providers of proficiency testing schemes 117