| Regardless of how you use specialty gas | | | | Standards (GMISs). Though they are analyzed |
| mixtures to calibrate a laboratoryinstrument | | | | against existing SRMs, they are at the bottom |
| or monitor, the most important thing you can | | | | of the traceability hierarchy. A GMIS |
| do to ensure the accuracy of your | | | | introduces more uncertainty because more |
| measurements is to ensure the accuracy of | | | | steps are involved and because NIST is not |
| your gas mixture. | | | | involved in either their analysis or |
| | | | certification. The gas supplier is |
| Gas mixture accuracy, in turn, is largely | | | | responsible for assigning the concentration |
| dependent upon its analytical traceability to | | | | and accuracy, based solely on their analysis. |
| a reference material. For analytical | | | | |
| traceability to exist, there must be an | | | | Gas mixtures that are produced using GMISs do |
| unbroken link of comparison between the | | | | not provide direct NIST-traceability, and so |
| measured value of the gas mixture and a | | | | may not be suitable for some applications. |
| reference that is recognized in the national | | | | For example, the EPA requires that a |
| or international measurement system as being | | | | Continuous Emission Monitor be calibrated |
| a true value. Thus, analytical traceability | | | | using a protocol that is at least as accurate |
| is the link between the analyzed | | | | to within ±2%. Use of a protocol that was |
| concentration and the true concentration of a | | | | prepared using a GMIS may not guarantee such |
| gas mixture. The National Institute of | | | | accuracy. |
| Standards and Technology (NIST) provides | | | | |
| Standard Reference Materials (SRMs) for this | | | | When selecting any specialty gas mixture, be |
| purpose. Another such metrological | | | | sure you know what type of standard the |
| organization, the Netherlands Measurement | | | | mixture you are purchasing will be analyzed |
| Institute (NMi), provides Primary Reference | | | | against. To ensure accuracy, insist on |
| Materials (PRMs). | | | | traceability to an SRM, PRM or NTRM. Also |
| | | | remember that, as in the case of emission |
| Analytical traceability is achieved by | | | | monitoring, traceability and subsequent |
| laboratory analysis of a mixture using an | | | | accuracy may be governed by law. |
| instrument that has been directly calibrated | | | | |
| using either an SRM or a PRM. A mixture | | | | You can ensure the integrity of your |
| analyzed in this manner is said to be | | | | laboratory calibrations by obtaining gas |
| "directly traceable." SRMs and PRMs are at | | | | mixtures from reliable companies such as |
| the top of the traceability hierarchy and add | | | | Scott Specialty Gas, which has been the |
| the least amount of uncertainty to a mixture | | | | leading supplier of precursor standards for |
| analysis because they are individually | | | | NIST SRMs for more than 25 years, Praxair, |
| analyzed and certified by NIST or NMi | | | | the second largest supplier of industrial and |
| laboratories. | | | | specialty gases to chemical companies |
| | | | worldwide, and Airgas, which offers an |
| Since 1995, NIST and NMi have maintained an | | | | extensive line of high purity specialty gases |
| equivalence agreement in those cases where | | | | as well as a full range of specialty gas |
| comparative analyses have been completed by | | | | equipment specifically designed for use with |
| the two labs. Because of this NIST/NMi | | | | highest purity gases and high tolerance gas |
| equivalency, traceability to a PRM is | | | | mixtures. Your dedication to accurate |
| considered the legal equivalent of an SRM. | | | | calibrations will be rewarded many times over |
| | | | in the precision of your measurements, and |
| Direct analytical traceability can also be | | | | thus the quality of your products, which can |
| established using what are known as NIST | | | | only make your customers smile. |
| Traceable Reference Materials (NTRMs). NTRMs | | | | |
| are produced by gas suppliers and then batch | | | | For more information about EPA protocol gas |
| analyzed and audited by NIST against the same | | | | accuracy, download a copy of Accuracy Counts |
| internal standards as SRMs. NTRM values are | | | | from |
| assigned by NIST and offer nearly the same | | | | |
| accuracy confidence level as SRMs and PRMs in | | | | This article is provided by Scott Specialty |
| most cases. | | | | Gases. Scott Specialty Gases, a leading |
| | | | global manufacturer of specialty gases |
| The high cost of producing SRMs and PRMs make | | | | located in Plumsteadville, PA. More |
| it economical for some specialty gas | | | | information on the company can be found at |
| suppliers to blend intermediate standards | | | | |
| called Gas Manufacturer Intermediate | | | | This article is copyrighted by Scott Gases. |