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Boron carbide (B4C) is widely used as a control rod material in fast nuclear reactors. The absorption cross-sections of 10 B and 11 B for thermal neutrons are 3837 b and 5 mb, respectively, and for fast neutrons, the cross-section is about 1 b for 10 B and a much smaller value for 11 B. The natural abundances of B isotopes are: 19.89% of 10 B and 80.11% 11 B 1 . Hence, B4C pellets enriched in 10 B are used in the fabrication of control rods; the extent of enrichment varies from 60%, for a commercial fast reactor, to as high as 90% for small core test reactors. Generally, the enriched 10 B is obtained as boric acid using an ion-exchange method. The concentration of boric acid used in the enrichment plant is ∼0.1 M. It is essential to determine the isotopic enrichment obtained at different stages of plant operation and a large number of samples need to be analyzed. In addition to the above applications, there is also a need to determine the isotopic ratio of B in heavy water, the moderator used in thermal nuclear reactors. Boric oxide is added to the moderator as a neutron poison during the start-up of a thermal reactor to control the nuclear reactivity.Thermal ionization mass spectrometry (TIMS) is the conventional method used to determine the isotopic ratio. Though this is the most accurate method, 2 it is relatively timeconsuming and the boric acid sample has to be converted to sodium borate. 2 Another method used is based on the use of inductively coupled plasma mass spectrometry (ICP-MS).3 This method requires comparatively more expensive instrumentation because of the need for differential pumping and the need to sustain plasma. The laser desorption (vaporization)/ionization source coupled to a time of flight mass spectrometer for mass analysis, provides a fast, less expensive method without the need for extensive sample preparation. Present day solid-state lasers are relatively inexpensive, rugged and compact. 4 A reflectron time-of-flight mass spectrometer (RTOFMS) is easy to construct, operate and maintain. 5 By the use of time of flight measurement, the ion currents due to different masses can all be recorded for each laser pulse. In ICP-MS systems using a quadrupole mass spectrometer, such simultaneous recording of all masses is not possible, and either the value of the mass selected has to be scanned or one must go from one to another. In TIMS systems using magnetic sectors, this is only possible if a multi-collector detector is used.In this work, we use such a laser mass spectrometry (LMS) method to determine the isotopic ratio of boron in boric acid samples as obtained from the enrichment plants or thermal reactors. The volume of sample needed for analysis is very small (25 -50 µL). The laser desorption (vaporization) removes only a few picograms of the material from the irradiated part of the surface. This LMS method is simpler than to other similar LMS methods used in the analysis of solid or liquid samples. 6 In this method, the ions are generated from the sample surface by the use of a single las...