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TraceCERT® Inorganic Standards for AAS, ICP and IC

By: Matthias Nold, AnalytiX Volume 9 Article 5

Matthias Nold, Product Manager Analytical Standards matthias.nold@sial.com

 

 

With our TraceCERT products, we offer a product line of certifi ed reference materials that meet the highest metrological standards.

TraceCERT standards are produced in a double-accredited laboratory, fulfi lling ISO/IEC 17025 and ISO Guide 34. This is the highest achievable quality standard for producers of certifi ed reference materials.

TraceCERT products are characterised by:

  • Unique (metrological) level of accuracy and lot-specific values and low uncertainties
  • Traceability to at least two independent references (i.e. NIST, BAM or SI unit kg)
  • Produced and certified according to ISO/IEC 17025 and ISO Guide 34
  • Highest-purity starting materials available
  • Comprehensive documentation according to ISO Guide 31
  • Certificates of ICP standards list up to 70 trace impurities
  • ICP standards packaged in light- and gas-tight aluminium foil bags
  • Competitive price

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CRM production at Sigma-Aldrich

In response to the increased importance placed on certifi ed reference materials (CRM) in recent years, Sigma-Aldrich has established partnerships with several metrological institutes that serve as certifying bodies. For certifi ed elemental and anion calibration solutions, Sigma-Aldrich has maintained a long-standing partnership with Swiss Federal Institute for Materials Science and Technology (EMPA). When EMPA terminated their activities in chemical metrology in 2004, Sigma-Aldrich decided to acquire the metrological competency from EMPA and transfer it to its Buchs site in eastern Switzerland. This transfer included not only unique equipment of the highest metrological level, but also the acquisition of technical capabilities and experience by hiring the veteran leadership of EMPA’s Metrological Reference and Certification Labs. These investments enabled Sigma- Aldrich to build an entirely new product line of high-quality certifi ed reference materials in a very short time.

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Production process of the TraceCERT products

To produce CRMs that meet all the criteria listed above, the entire development process must be highly sophisticated, beginning from the selection of an appropriate starting material to the choice of a purpose-specifi c packaging. Figure 1 shows an overview of the production and certification process we employ.

Figure 1 Schematic representation of the production design and certification of Sigma-Aldrich TraceCERT® products. This approach was established for the preparation of intercomparison samples at the highest metrological stage.


TraceCERT® starting materials (Figure 2) are characterized by two diff erent approaches:

  1. direct measurement of the purity by the most accurate method (e.g. titrimetry)
  2. comparison to an internationally accepted reference material (NIST, BAM, etc); the purity is assigned as 100 % minus impurities.

The combination of these two purity assignments leads to two independent traceability chains, and results in higher reliability of the CRM. The starting material is pretreated (surface etching in case of metals, drying in case of salts) before weighing.

Figure 2 Only well-characterised and high-purity starting materials are used for TraceCERT™ calibration standards (clockwise from upper left: copper, zinc, nickel and gold).


The weighing operation is a key step of production, leading to direct traceability to the SI unit kilogram. Gravimetric preparation using pure materials is a practical and, in most cases, the most accurate calculation of concentration units, through conversion of mass and mole fraction to mass fraction.

The weighted starting material is dissolved in high-purity acid and the stock solution is diluted with high-purity water until the calculated total mass of the final solution is reached. This gravimetric approach allows a highly precise adjustment of the final concentration of the calibration solution. Homogenisation of the solution is achieved by overhead tumbling of the container for several hours. In the fi nal bottling step, contamination is prevented by working under clean room conditions. The bottled solution is then compared to a second, independent reference material.

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Certification

For each TraceCERT product, a detailed and comprehensive Certifi cate is supplied, either delivered with the product (for the ICP standards) or downloadable from the internet using product number and lot number (for the AAS and IC standards). An example of a certificate can be downloaded from the internet for every product (sigma-aldrich.com/tracecert). The certificates are in accordance with ISO Guide 31 and contain lot-specifi c information such as exact content, expiration date, traceability measurements, values of up to 70 impurities (only for the ICP standards) and a proper uncertainty calculation. The uncertainties of the TraceCERT standards are obtained by summing up all the contributing influence parameters (“bottom-up” approach). The influence parameters are shown in Figure 3.

Figure 3 Cause effect diagram of the preparation procedure of TraceCERT® reference materials. Only first and second order influence parameters are shown.

 

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Storage and stability

The ideal container for standard solutions is totally inert, will not absorb analyte, does not leach impurities into the solution, is impermeable towards the solvent and atmosphere and is easy to handle and store.

We found the most suitable material available today was high-density polyethylene (HDPE). Leaching of impurities into the solution and transpiration of the solvent through the container wall have been investigated comprehensively for this material (Figure 4).

Figure 4 Transpiration behaviour of an aqueous calibration solution in a 100 mL HDPE bottle at different temperatures

 

The bottles of the TraceCERT® ICP standards are packed in sealed aluminium bags, preventing further transpiration of the solvent. Owing to this packaging technique, Sigma- Aldrich can guarantee the ambitious specifi cation of 0.2 % uncertainty for the certifi ed value over the entire shelf life of the standard.

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Product portfolio and custom standards

Only a few years after the introduction of the TraceCERT product line, it already comprises more than 120 products. The current portfolio includes an extensive range of singleelement standard solutions for AAS and ICP (1000 mg/L), multi-element standards for ICP and standard solutions for ion chromatography (1000 mg/L).

We are continuously expanding the product range. The most recent product introductions are listed in Table 1. For the complete portfolio and for more information about the products please consult our website at sigma-aldrich. com/tracecert If you don’t fi nd the standard you need in our product list, you can solicit a quotation for a customised standard solution using our new online custom standards platform at sigmaaldrich.hil.ch

Cat. No. Description Pack Size
43678 Lanthanum Standard for AAS 100 mL
42887 Niobium Standard for AAS 100 mL
53378 Cerium Standard for AAS 250 mL
68418 Scandium Standard for AAS 100 mL
75159 Thallium Standard for AAS 100 mL
04689 Titanium Standard for AAS 100 mL
73574 Zirconium Standard for AAS 250 mL
11523 Lanthanum Standard for ICP 100 mL
67913 Niobium Standard for ICP 100 mL
16734 Cerium Standard for ICP 100 mL
92279 Scandium Standard for ICP 100 mL
51873 Thallium Standard for ICP 100 mL
12237 Titanium Standard for ICP 100 mL
51244 Zirconium Standard for ICP 100 mL
 
Table 1 Recently added TraceCERT standard solutions (1000 mg/L)

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Materials

     
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