Method for measurement of Ar isotopes in He stream (conflo) for K/Ar geochronology

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Budnitskiy, Sergey; Ignatiev, Alexander; Velivetskaya, Tatyana
Holding Date 08 September 2008

We present a new laser GC-MS technique for quantitative measurement of isotopes of argon for K/Ar geochronology. Measurement of argon is conducted by a method of isotope dilution in a continuous flow of helium on dynamically pumped mass-spectrometers.
The system of measurement consists of the following basic units: samples chamber (8-18 samples depending on weight), laser system for fusion of samples, allocated argon clearing and gathering system, system of interface of argon with a stream of helium, Cryo-focuse system, chromatographic column, system of gas input in mass-spectrometer, system of high-vacuum pumps. Dynamically pumped mass-spectrometer MAT-253(Thermo Electron) is used as analyzer.
Mass-spectrometer MAT-253 has three collectors to detect ions that allow making measurements of three isotopes of argon simultaneously. For extraction of argon from samples we use infrared (λ=1.064 µm, CW, 100W) ND-YAG laser that allows to heat it up to 2000oC. Samples are located in a chamber where their fusion occurs. Extracted argon with tracer is cleaned by U-shape trap (T =-196oC) and getter. Then argon gathers on a loop with activated coal at temperature of liquid nitrogen. Then the loop is being fed by helium (He - 99.9999) and heated up to 100oC. The Ar gas in a stream of helium is passed through the capillary chromatographic column (HP-MOLSIV) to separate the Ar from other gases. Argon in a stream of helium is fed into a mass-spectrometer by means of Open Split where its isotope composition is measured.
It takes 20 min to analyze one sample.
The conducted research has shown that present technique allows to analyse 5*10-12 g. 40Ar.