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 http://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.
About the Author R.L. Fielding has been a freelance writer for
10 years, offering her expertise and skills to a variety of
major organizations in the education, pharmaceuticals and
healthcare, financial services, and manufacturing industries.
She lives in New Jersey with her dog and two cats and enjoys
rock climbing and ornamental gardening.
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