Tracing the Accuracy of Calibration Gases
Regardless of how you use specialty gas mixtures to calibrate a
laboratory instrument or monitor, the most important thing you
can do to ensure the accuracy of your measurements is to ensure
the accuracy of your gas mixture. Gas mixture accuracy, in
turn, is largely dependent upon its analytical traceability to a
reference material. For analytical traceability to exist, there
must be an unbroken link of comparison between the measured
value of the gas mixture and a reference that is recognized in
the national or international measurement system as being a true
value. Thus, analytical traceability is the link between the
analyzed concentration and the true concentration of a gas
mixture. The National Institute of Standards and Technology
(NIST) provides Standard Reference Materials (SRMs) for this
purpose. Another such metrological organization, the Netherlands
Measurement Institute (NMi), provides Primary Reference
Materials (PRMs).
Analytical traceability is achieved by laboratory analysis of a
mixture using an instrument that has been directly calibrated
using either an SRM or a PRM. A mixture analyzed in this manner
is said to be "directly traceable." SRMs and PRMs are at the top
of the traceability hierarchy and add the least amount of
uncertainty to a mixture analysis because they are individually
analyzed and certified by NIST or NMi laboratories.
Since 1995, NIST and NMi have maintained an equivalence
agreement in those cases where comparative analyses have been
completed by the two labs. Because of this NIST/NMi equivalency,
traceability to a PRM is considered the legal equivalent of an
SRM.
Direct analytical traceability can also be established using
what are known as NIST Traceable Reference Materials (NTRMs).
NTRMs are produced by gas suppliers and then batch analyzed and
audited by NIST against the same internal standards as SRMs.
NTRM values are assigned by NIST and offer nearly the same
accuracy confidence level as SRMs and PRMs in most cases.
The high cost of producing SRMs and PRMs make it economical for
some specialty gas suppliers to blend intermediate standards
called Gas Manufacturer Intermediate Standards (GMISs). Though
they are analyzed against existing SRMs, they are at the bottom
of the traceability hierarchy. A GMIS introduces more
uncertainty because more steps are involved and because NIST is
not involved in either their analysis or certification. The gas
supplier is responsible for assigning the concentration and
accuracy, based solely on their analysis.
Gas mixtures that are produced using GMISs do not provide direct
NIST-traceability, and so may not be suitable for some
applications. For example, the EPA requires that a Continuous
Emission Monitor be calibrated using a protocol that is at least
as accurate to within