Highlights:
Preparation and certification of a new CRM for seven trace elements (Al, Ca, Fe, K, Mg, Na, and Ti) in high purity quartz
Developed by the National Centre for Compositional Characterisation of Materials (NCCCM) of Bhabha Atomic Research Centre (BARC) and CSIR-National Physical Laboratory (NPL)
Validation of an ICP-AES method for accurate determination of trace elements in quartz
Thorough homogeneity and stability assessment of the CRM material
Certification of values through an Inter-Laboratory Comparison Exercise (ILCE) following ISO guidelines
Introduction
High purity quartz is an essential material with diverse applications ranging from optical waveguides and photovoltaic devices to frequency control applications. The presence of trace impurities, even at minuscule levels, can significantly impact the quality and performance of the final products. Therefore, the accurate determination of trace element concentrations in high purity quartz is crucial for quality control and characterization.
To ensure the reliability and comparability of analytical results, certified reference materials (CRMs) are indispensable. However, the lack of matrix-matched CRMs for trace elements in quartz has been a long-standing challenge. To address this issue, the National Centre for Compositional Characterisation of Materials (NCCCM) of Bhabha Atomic Research Centre (BARC) and CSIR-National Physical Laboratory (NPL) collaborated to develop and produce a new CRM for the certification of critical trace elements in high purity quartz.
Analytical Method Validation
The foundation of this CRM development was the validation of an ICP-AES method for the determination of Al, Ca, Fe, K, Mg, Na, and Ti in quartz samples. The method involves the dissolution of the silica matrix using hydrofluoric acid and sulfuric acid, followed by the quantification of the trace elements using ICP-AES.
The method was thoroughly validated in terms of selectivity, linearity, sensitivity, limits of detection (LOD) and quantification (LOQ), accuracy, and precision. The selectivity assessment ensured that there were no significant matrix interferences at the selected wavelengths for each analyte. The linearity was established over the desired concentration ranges, and the sensitivity, expressed as the slope of the calibration plots, was determined for each element.
The LOD and LOQ values were calculated based on the analysis of procedural blanks, and they were found to be suitable for the determination of the target analytes at the desired levels. The accuracy of the method was evaluated using spike-and-recovery experiments, which demonstrated recoveries between 96-102% for all the elements. The precision, expressed as the relative standard deviation (RSD) of replicate analyses, was within 2% for most of the analytes.
To further validate the reliability of the method, the authors analyzed a high purity silica CRM (BCS 313/2) and found excellent agreement between the obtained values and the certified values, with no statistically significant differences.
Homogeneity Assessment
Homogeneity is a critical requirement for the production of a CRM, as it ensures the representativeness of the material and the comparability of the analytical results. The authors conducted a thorough homogeneity assessment following the guidelines of ISO Guide 35.
First, the sampling constant (Ks) was determined to establish the minimum sample intake required for reliable analysis. The Ks values ranged from 0.019 g for Mg to 0.081 g for Al, indicating that the material was homogeneous at around 0.1 g of sample.
The within-bottle and between-bottle homogeneity were then evaluated by analyzing 30 replicate samples (10 bottles, 3 replicates per bottle). The analysis of variance (ANOVA) showed no significant differences between the variances within and between bottles for all the analytes, confirming the homogeneity of the prepared material.
Inter-Laboratory Comparison Exercise (ILCE) and Certification To assign the certified values, an ILCE was organized involving six qualified laboratories, including NABL (National Accreditation Board for Testing and Calibration Laboratories) accredited laboratories and the National Metrology Institute (NPL). The participating laboratories used various techniques, such as ICP-AES, FAAS, and TXRF, for the characterization of the property values.
The assigned property values were calculated using a weighted average approach, where more weight was given to the results with higher associated uncertainties. This approach was justified by the evaluation of the Zeta (ζ) scores, which indicated that the participating laboratories had reported accurate results with appropriate estimates of uncertainty.
The certified values for the seven trace elements, along with their expanded uncertainties (k=2, 95% confidence level), are presented in Table 1.
Table 1: Certified Values of the Quartz CRM
Element | Element Certified Value ± Expanded Uncertainty (k=2) (mg/kg) |
Al | 1181 ± 96 |
Ca | 58 ± 8 |
Fe | 81 ± 9 |
K | 558 ± 35 |
Mg | 20 ± 3 |
Na | 249 ± 16 |
Ti | 17 ± 3 |
The combined uncertainty of the certified values includes contributions from the between-unit homogeneity (ubb) and the characterization study (uchar) by the ILCE. Other uncertainty components, such as long-term and short-term stability, were found to be negligible.
Stability Assessment
The stability of the certified property values was evaluated annually from 2015 to 2020. The observed values were compared with the certified values using the criteria specified in ISO Guide 35, and the material was found to be stable within the stated uncertainties, as indicated by the low Zeta (ζ) scores (all < 2) presented in Table 2.
Table 2: Stability Assessment of the Quartz CRM over the Years
Year | Al (mg/kg) | Ca (mg/kg) | Fe (mg/kg) | Mg (mg/kg) | K (mg/kg) | Na (mg/kg) | Ti (mg/kg) |
2015 | 1171 ± 29 (-0.2) | 64.3 ± 2.1 (1.5) | 79.3 ± 2.4 (-0.4) | 20.0 ± 1.3 (0.0) | 569 ± 22 (0.5) | 252 ± 20 (0.2) | 16.3 ± 1.4 (-0.4) |
2016 | 1232 ± 32 (1.0) | 63.3 ± 2.3 (1.3) | 83.3 ± 2.8 (0.5) | 19.7 ± 1.6 (-0.2) | 546 ± 27 (-0.5) | 247 ± 16 (-0.2) | 15.4 ± 1.1 (-1.0) |
2017 | 1119 ± 24 (-1.3) | 64.2 ± 2.0 (1.5) | 75.2 ± 2.5 (-1.2) | 21.1 ± 1.2 (0.7) | 587 ± 25 (1.3) | 256 ± 17 (0.6) | 19.3 ± 0.5 (1.5) |
2018 | 1210 ± 30 (0.6) | 65.7 ± 2.4 (1.8) | 78.2 ± 2.7 (-0.6) | 21.6 ± 1.0 (1.0) | 543 ± 20 (-0.7) | 263 ± 15 (1.3) | 17.9 ± 1.1 (0.6) |
2019 | 1273 ± 25 (1.9) | 62.3 ± 1.8 (1.0) | 80.8 ± 1.5 (-0.04) | 21.4 ± 0.8 (0.9) | 563 ± 30 (0.2) | 247 ± 14 (-0.2) | 17.5 ± 1.2 (0.3) |
2020 | 1093 ± 30 (-1.7) | 61 ± 2 (0.7) | 77.1 ± 2.5 (-0.8) | 20.4 ± 1 (0.3) | 561 ± 39 (0.1) | 235 ± 15 (-1.3) | 17.0 ± 1.0 (0.0) |
The values are presented as mean ± expanded uncertainty (k=2); Zeta (ζ) scores are given in parentheses.
Significance and Applications
The development of this new certified reference material for trace elements in high purity quartz is a significant milestone in the field of analytical chemistry. The availability of a well-characterized, matrix-matched CRM will have a profound impact on the following areas:
Validation of Analytical Methods: The CRM can be used to validate the accuracy and performance of various analytical techniques, such as ICP-AES, FAAS, and TXRF, employed for the determination of trace elements in quartz samples.
Calibration and Traceability: The certified values of the CRM, which are traceable to the International System of Units (SI), can be used to calibrate analytical instruments and establish the metrological traceability of laboratory results, ensuring the comparability of measurements across space and time.
Quality Control and Assurance: The CRM can be used as a quality control tool to monitor the performance of analytical laboratories and ensure the reliability of their results, particularly in the context of high-purity quartz production and characterization.
Research and Development: The availability of this CRM will facilitate more accurate and reliable research on the distribution and geochemical significance of trace elements in quartz, which can provide valuable insights into the petrogenetic history and provenance of quartz samples.
Conclusion
The National Centre for Compositional Characterisation of Materials (NCCCM) of Bhabha Atomic Research Centre (BARC) and CSIR-National Physical Laboratory (NPL) have successfully developed and certified a new reference material for seven trace elements (Al, Ca, Fe, K, Mg, Na, and Ti) in high purity quartz. This CRM was produced following rigorous validation of the analytical method, thorough homogeneity and stability assessments, and an inter-laboratory comparison exercise in accordance with ISO guidelines.
The certified values, along with their expanded uncertainties, provide a reliable reference for the analytical community involved in the characterization of high purity quartz. This CRM will play a crucial role in ensuring the accuracy, traceability, and comparability of analytical results, ultimately contributing to the quality control and development of quartz-based materials and products. The successful development of this CRM showcases the collaborative efforts of national laboratories in India to address the lack of matrix-matched reference materials and strengthen the country's analytical capabilities.
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