Gravimetric methods for the preparation of standard gas mixtures

Abstract

The most widely used method for the preparation of primary standard gas mixtures involves weighing the individual components into a cylinder. We present a new mathematical description of the method and its uncertainties. We use this to demonstrate how strategies for serial dilution can be identified that minimize the uncertainty in the final mixture and show how they can be implemented practically. We review published reports of high accuracy gravimetry and give examples of relative uncertainties in the composition of standards approaching 1 part-per-million in the best cases and in the range of 100 to 1000 parts-per-million more typically.

Estimation of Mass Discrimination Factor for a Wide Range of m/z by Argon Artificial Isotope Mixtures and NF3Gas

“…In gas metrology, gravimetry is used as the primary method to produce a reference gas mixture. 3,4 Hence if isotope enriched gases are mixed using this method, very precise isotope gas mixtures can be prepared. The molar mass and weight of two isotope enriched gases (A and B) are used to calculate the mole fraction of i isotope (x i ) of the gravimetrically prepared gas mixture in Eq.…”

“…Nowadays, needs for more precise atomic weight are growing in variety of scientific research fields, such as for preparation of reference gas mixtures by gravimetric method, 3,4 for correction of the buoyancy effect on the comparison of mass standards in air, 5,6 for precise determination of the triple point temperature of neon (Ne) and the Avogadro constant using silicon single crystals, 7,8 and for re-defining the International System (SI) unit of temperature, the Kelvin, by acoustic technique. 9 In gas metrology, the molecular weight of a source gas is crucial constituent in gravimetric technique.…”

“…Gravimetry is the primary method for preparation of reference gases. 3,4 In the international comparison on gravimetry organized by Gas Analysis Working Group of Consultative Committee for Amount of Substance (CCQM GAWG), it is found that isotope variation of reference gases can remarkably affect to the measurement value. 10,11 For this reason, it is recommended to check or verify the isotopic composition of source gases.…”

“…4 If isotope enriched gases are available, isotope mixtures can be prepared and employed to determine the f MD . In the real experimental fields, isotopic enriched gases with required component are not readily available, and not many laboratories have a gravimetric preparation system for isotope references.…”

Gas weighing challenge

“…As the containers cannot be weighed with open valves, the density of the entire container is needed. Since the determination is a complex task, it is common practice to compare the mass of an empty reference container with the mass of the filled target container: the difference of these two masses is the mass of the added gas [3,4]. This is a simple approach but no buoyancy correction is applied, and from the point of view of a metrologist, this is very unpleasant.…”