Preparation of argon Primary Measurement Standards for the calibration of ion current ratios measured in argon

In this work a procedure is described to prepare SI-traceable argon isotope amount ratios in high purity argon. Following a gravimetric approach, a total of three synthetic isotope mixtures have been prepared, from argon gases enriched in 36Ar, 38Ar and 40Ar, close to the natural isotopic composition of argon. These synthetic mixtures serve as reference gases to calibrate ion current ratio measurements in argon. In this way SI-traceable measurements of argon isotope amount ratios can be performed without any assumptive correction. On a high purity argon gas separated from air the absolute argon isotope amount ratios R36/40 = 0.00334774 (93) and R38/40 = 0.00063529 (57) were measured. From these ratios, the average molar mass of argon gas can be determined with a total relative combined uncertainty of 0.09 × 10−6.

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