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).
| SI Base Units |
• Length = metre
• Mass = kilogram
• Time = second
• Thermodynamic temperature = kelvin
• Electric current =
ampere
• Amount of substance = mole
• Luminous intensity =
candela |
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.
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Silver:
So many useful
reactions |
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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.
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PrimAg®
A
very practical mole! |
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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!
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PrimAg®
primary
reference silver |
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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.
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