COMPLETION OF NIST CALIBRATION OF ARGON SPIKES, AND PLANS FOR INTERLABORATORY CALIBRATION OF MMHB-2

Calibration of several argon spike systems has recently been completed by the National Institute of Standards and Technology (NIST). The argon spike systems are of all-metal construction and are portable. Two different types of argon spike systems were prepared. (1) Two 38Ar spikes that will initially deliver 1.6 and 2.3x10-10 moles of 38Ar respectively per aliquot and (2) Three mixed 40Ar/36Ar spikes with ratios of about 1:1, 30:1, and 300:1 respectively. Each of these three artificial mixtures will deliver about 4x10-10 moles of argon per aliquot. The uncertainties of all of the spike systems are expected to be about 0.1-0.25 percent for both the amount of gas delivered and for the isotopic ratio of the 40Ar/ 36Ar mixtures.

We will use the mixed 40Ar/36Ar spike systems to calibrate the discrimination of the mass spectrometers that we will be using. We will then re-measure the isotopic composition of atmospheric argon. The composition of atmospheric argon was last measured more than 50 years ago (Nier, 1950). Finally we will measure the argon concentration of MMhb-2 using the NIST calibrated 38Ar spike. We will combine the argon data with high precision potassium concentration data from NIST to calculate a high precision K/Ar age for MMhb-2. The argon measurements will be made in several different argon labs. An interesting by-product of this research will be the calculation of a new atomic weight for argon.

Upon completion of the calibration, MMhb-2 will be certified by NIST for potassium and argon concentrations, and NIST will issue a Standard Reference material certificate for it. The distribution of MMhb-2 will be through the NIST Standard Reference Material Program.