An electrospray ionization (ESI) guided-ion-beam tandem mass spectrometer (GIB MS) was developed for studying gas-phase biological ion-molecule reactions, such as reaction dynamics of amino acids and small peptides with reactive oxygen species. This poster describes the design, construction and operation of this instrument, including the development of a vacuum control system. Biomolecular ions are generated by ESI and introduced into the instrument through a heated capillary and a skimmer. Ions pass into a hexapole ion guide for collisional cooling and focusing, and then are mass-selected by a reactant quadrupole mass filter. Mass-selected ions are guided into an octopole ion guide surrounded by a scattering cell containing the neutral reactant gas. Ions are scattering from the neutral reactant gas at controlled collision energy. Product ions and the remaining reactant ions are collected by the octopole ion guide, and extracted into the product mass filter for mass analysis and detection. The instrument consists of five high vacuum chambers. A vacuum control and interlock system was developed for automatic control of various gates, foreline valves, roughing valves, pumps and pressure gauges. The system also provides various protective safety interlocks and alarm system which respond to failures of cooling water, compressed air, electrical power, etc. Results are presented to illustrate the application of this instrument for ion-molecule reactions of protonated tyrosine cations.