Laser ablation molecular isotopic spectrometry (LAMIS) recently has been investigated for analysis of the boron isotopic composition in ambient air at atmospheric pressure. The initial precision of 100–400‰ (2σ) for the ¹⁰B/¹¹B isotopic ratio was less than results obtained from other analytical methods like TIMS or ICP–MS. This paper describes how accuracy and precision for boron isotopic ratio using LAMIS can be improved to the few per mil level. Several optimization procedures, viz., spectral region of analysis, effect of flicker noise from matrix, spectral normalization and pre-treatment procedures, were studied. This paper reports a precision of 9‰ (2σ) for the ¹⁰B/¹¹B ratio using alternative spectral normalization and pretreatment procedures.