The certification of a nominal 20 μmol/mol H 2 S/N 2 standard reference material using two independent methods

To support federal, state and local regulations on the amount of hydrogen sulfide (H 2 S) released into the atmosphere, the National Institute of Standards and Technology (NIST) has certified standard reference material (SRM) 2731 Hydrogen Sulfide in Nitrogen (nominal amount-of-substance fraction 20 μmol/mol H 2 S). Since it was first produced and certified in 1989, NIST has certified four separate lots of this SRM. In each case, the value assignment of the lot concentration was accomplished by comparison to a permeation tube-generated calibration curve. For the certification of the new lot, two independent methods were used to value-assign the concentration. In addition to the permeation tube method, a second method involving the dynamic dilution of a high-concentration gravimetrically prepared primary standard mixture containing H 2 S/N 2 was also used. Agreement between the two methods was 0.10% and the total uncertainty assigned to the lot concentration was 1.35% (relative).