Controlling isotopic effects in the resonance ionisation mass spectrometry of krypton

We have investigated and characterised possible sources of variation in measured isotope ratios in the resonance ionisation mass spectrometry of krypton using resonant transitions at 116.48 and 558.04 nm and ionisation at 1064 nm. Variation arises when the spectral width of the hyperfine structure of the odd isotopes is comparable to the laser bandwidth and from the alignment of the first excited state, which depends on the relative polarisations of the laser pulses exciting the resonant transitions. These effects can be eliminated when the transitions are strongly saturated and the relative polarisation controlled. The precision of measurements of ratios involving an odd mass isotope is then consistent with that of ratios among the even isotopes, so such spectroscopic effects will not affect the reliability of measurements of samples.