A sensitive laser spectroscopic method has been applied to the quantitative
determination of the isotope ratio of helium at the level of 3He/4He = 10^-7 -
10^-5. The resonant absorption of 1083 nm laser light by the metastable 3He
atoms in a discharge cell was measured with the frequency modulation saturation
spectroscopy technique while the abundance of 4He was measured by a direct
absorption technique. The results on three different samples extracted from the
atmosphere and commercial helium gas were in good agreement with values
obtained with mass spectrometry. The achieved 3-sigma detection limit of 3He in
helium is 4 x 10^-9. This demonstration required a 200 micro-L STP sample of
He. The sensitivity can be further improved, and the required sample size
reduced, by several orders of magnitude with the addition of cavity enhanced
spectroscopy