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 ±2%. Use of a protocol that was prepared
using a GMIS may not guarantee such accuracy.
When selecting any specialty gas mixture, be sure you know what
type of standard the mixture you are purchasing will be analyzed
against. To ensure accuracy, insist on traceability to an SRM,
PRM or NTRM. Also remember that, as in the case of emission
monitoring, traceability and subsequent accuracy may be governed
by law.
You can ensure the integrity of your laboratory calibrations by
obtaining gas mixtures from reliable companies such as Scott
Specialty Gas, which has been the leading supplier of precursor
standards for NIST SRMs for more than 25 years, Praxair, the
second largest supplier of industrial and specialty gases to
chemical companies worldwide, and Airgas, which offers an
extensive line of high purity specialty gases as well as a full
range of specialty gas
equipment specifically designed for use with highest
purity gases and high tolerance gas mixtures. Your dedication to
accurate calibrations will be rewarded many times over in the
precision of your measurements, and thus the quality of your
products, which can only make your customers smile.
For more information about EPA protocol gas accuracy, download a
copy of Accuracy Counts from www.scottgas.com.
This article is provided by Scott Specialty Gases. Scott
Specialty Gases, a leading global manufacturer of specialty
gases located in Plumsteadville, PA. More information on the
company can be found at http://www.scottgas.com.
This article is copyrighted by Scott Gases. It may not be
reproduced in whole or in part and may not be posted on other
websites, without the express written permission of the author
who may be contacted via email at
scottgas@digitalbrandexpressions.com.
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