Alkali metaborate ions are usually monitored for the isotopic measurement of boron by positive thermal ionization mass spectrometry. Large variations in the of ¹⁰B/¹¹B isotopic ratios are observed with the change in the mole ratios of B/Na when sodium carbonate solutions are added when compared with solution of neutral salt of sodium (NaCl). To understand the reason for the variations observed in the isotopic ratio of ¹⁰B/¹¹B with the change in the mole ratio of B/Na, various sodium containing buffers effective in the pH range 3–9 were employed in the present studies instead of the conventionally used sodium carbonate for formation of Na₂BO₂⁺ ions in the ion source. NIST SRM-951 having certified ¹⁰B/¹¹B ratio 0.2473 ± 0.0002 was used for all isotopic measurements by TIMS. It could be concluded that irrespective of the pH, the foremost reason for variations in the isotopic ratio of ¹⁰B/¹¹B is the amount of Na present as Na₂O on the filament.