ROMIL PrimAg^® Reference Materials
traceable to SI Units via ultrapure primary reference silver

Key Applications Overview

At Last - International Traceability to SI Units

A global economy needs to be certain that analytical data, generated in one country by one analyst, will be recognised and universally accepted. There are many formal international mutual recognition agreements between national certification bodies, underpinned by ILAC and EA, that should ensure mutual recognition of certified reference materials (CRMs) used to calibrate and control analytical systems.

But the reality is that, despite many efforts by various national and international authorities to bring order to the confusion and chaos, traceability in analytical chemistry remains fragmented. In many parts of the world national standards predominate. All too often the authority of the various international agreements is overlooked and national standards tend to predominate.

At the level of the working analyst traceability has only recently started to become important, thanks to the steady incorporation of ISO 17025 into laboratory quality systems. In both the USA and others parts of the world there remains a widespread, if naïve, belief that simply asking that a CRM be 'NIST traceable' will somehow ensure all necessary traceability!

With the modern analytical chemistry required to deliver data that is accepted and understood on a global scale it has become imperative that we move from national standards and all accept data that shows clear traceability to SI units. Therefore in analytical chemistry, which deals with amounts of chemical substances reacting together, the SI unit to work with is clearly the mole, one of the seven fundamental units of measure units that are defined by the SI System (SI = Système Internationale).

But what, exactly, is the mole? It is defined as the number of discrete particles (in this case atoms) contained within 12 grams of the pure isotope carbon-12. This is all very well and good, but the choice of carbon does create problems for analytical chemists because carbon is so unreactive. One of its forms, diamond, is the hardest known natural substance.

So until now the only way to produce certified reference materials (CRMs) for trace element analysis has been to look at each element in isolation and thus produce separate elemental CRMs, each with demonstrated traceability. This is an extremely costly and complex process, undertaken by a very few organisations. As a result most trace element analysis is calibrated using secondary reference materials or tertiary working standards. Few of these 'standards' are provided with a certificate that demonstrates clear and unbroken traceability to the mole and increasingly accreditation assessors are recording their dissatisfaction by requiring laboratories to undertake further verification of such reference materials' fitness for purpose.

There is, therefore, a real need for a more practical and affordable scheme of traceability. Such a scheme must be based on a material that has similar stability to carbon, but which has just enough reactivity to enable it to link into a chain of reaction schemes with other chemical substances in order to create an unbroken chain of traceability.

The need for such a 'practical mole' is not new: early in the last century in 1912 the analysts working within Nobel's Explosive Company, one of the founding companies of Imperial Chemical Industries, now ICI plc, put in place a system of reference materials to standardise all the volumetric titrations that formed the core their analytical methods.

Silver: So many useful reactions

The system used a sample of specially pure silver as the 'ultimate reference' and had a hierarchy of pure substances as reference materials, all of which would meet the requirements the modern ISO REMCO definitions. Analysts well know that silver can be obtained in a very pure form. It is sufficiently stable over a very long period of time such that its chemical properties do not change. And importantly, it provides the vital link into chemical reactions through its solubility in acids, which can then be reacted with bases and halides. The details of this groundbreaking scheme are recorded in an address given by the President of the Society of Public Analysts on the occasion of their Annual Dinner in London in 1930.

It was this level of attention to analytical excellence, which helped build the strength of ICI over the next 50 years. The concept was taken up other English companies, including the laboratory chemical firm of Hopkin and Williams who used a similar scheme at their Dagenham its site near London in the 1970s.

The proper implementation of this type of scheme of traceability involves a great deal of work; calibration analyses must be performed to a higher level of precision than that to which the routine analyses are done. Also, the calibrations should be regularly performed, say once a year, to ensure that routine results remain within the calibrated scheme. Even so, the overwhelming advantage to laboratories that worked with the silver scheme was that their results withstood close scrutiny with others working with similar schemes, no matter where they were located.

PrimAg® A very practical mole!

Despite the clear advantages there were many barriers to introducing a silver scheme into many laboratories, not least the availability of resources in terms of time and analytical expertise. As cost saving pressures grew during the late 1980s the system's popularity began to fade and it looked as though another great British development would become part of history. Until now.

The introduction of the ISO 17025 international standard for the competence of testing and calibration laboratories is a move in the right direction for chemical metrology. But the need to use CRMs that are traceable to a SI unit is written into ISO 17025, which is a challenge for most laboratories.

Which brings us to ROMIL PrimAg^®. We wanted to provide a range of metal solution and pure substance CRMs that demonstrated direct traceability to the SI unit, the mole. But we wanted to do so in a way that fitted in with the ROMIL concept of not following established trends and so doing things better, but differently!

PrimAg® primary reference silver

So we adapted the well-established concept of silver traceability to the requirements of modern analytical chemistry. Working quietly on our own we proved to ourselves that this was a concept that had been waiting for its time: the adoption of ISO 17025. More importantly we were able to prove to the rigorous technical assessors from UKAS that our concept and our chemistry were ready for the 21st century. This culminated with our accreditation to ISO 17025 as a chemical calibration laboratory in June 2002. Combined with our quality management system registration to ISO 9001:2000 we are now an unbeatable source for the supply of such CRMs.

Now all analytical laboratories may partake in traceability to the internationally accepted SI units through use of certified reference materials that have, through the use of our primary reference silver and PrimAg^® CRMs, a clear demonstrated traceability to the mole and an unambiguous statement of uncertainty so that the final measurement result may be controlled to fine limits due to the application of high precision classical techniques within the PrimAg^® scheme.

So, with PrimAg^® from ROMIL, the goal of true international traceability with controlled uncertainty (or, comparability) has finally become available to analysts, worldwide.

PrimAg is a registered trademark of ROMIL Ltd.

Key Applications Overview