We describe an apparatus for quantitative determination of residual gases in flat panel display vacuum tubes like field emission displays, vacuum fluorescent displays, and plasma displays. This system is based on the principle of expansion of gases from the test device into a reference volume, where the gases are analyzed using a quadrupole mass spectrometer. The system was calibrated using gas expansion from reference gas cells and vacuum envelopes with known pressures. The effectiveness of the system was demonstrated by measuring the residual gases inside flat vacuum tubes, commercial vacuum fluorescent displays modulus, and field emission displays.