A temperature distribution was imposed on a solid gas reactor to help accomplish the 15-nitrogen or 13-carbon isotopic determination of biosynthetic materials such as algal protein-N-15. The temperature zonal combustion reactor (TZCR) can operate offline from its mass spectrometer, and this reactor facilitates the stable isotopic determination of highly enriched biomaterials (>95 atom % isotope). The reactor structure is a glass pipe comprising a quartz segment, transition segments, and threaded sockets. Precise positioning of the reactor within a tubular furnace establishes a longitudinal temperature distribution that is used to drive solid gas chemistry in distinct zones: sample, catalyst, and sorbent. The TZCR module typically includes a reactor pipe, gastight valve, sample, reactants, reactant containers, and furnace. An important advantage of TZCR-mass spectrometer is preliminary vacuum heat treatment of the crude sample to remove volatile impurities before sample combustion (enabling isotopic enrichment determination of the analyte). For the determination of N-15 enrichment, sample combustion at 650 degrees C in oxygen was supplemented by Fe3O4 catalyst near 500 degrees C and Li2O at 25 degrees C that concertedly promoted the intrareactor depletion of CO, CO2, and H2O. As a result, the primary gaseous product was dinitrogen. After N-15 sample conversion, the reactor was coupled to a quadrupole MS for product analysis (N-2(+) ions for N-15 determination). Accordingly, uniformly enriched biomaterials were analyzed such as algal protein-N-15 that measured 99.0 +/- 0.12 atom % N-15. By adapting the methodology for C-13 determination, lyophilized algal cells-C-13 were determined to be 99.1 +/- 0.06 atom % C-13. Stable isotope enrichments were computed by minimizing the numeric differences between theoretical and actual MS signals.