A gas analysis system comprising, from upstream to downstream : - a module (SEP) for separating at least some of the species contained in the gas to be analysed, comprising at least one micro-capillary gas chromatography (GC) column, and - a time-of-flight mass spectrometer (TOFMS) coupled to the said separation module, the said spectrometer comprising an ion source (MS1, MS2) adapted to ionise at least some of the said species and to emit a beam of ions, and a region (MS4) for the free flight of the said ions, said mass spectrometer (TOFMS) being arranged in the volume of at least one substrate and comprising a micro-reflectron (R) disposed between the ion source (MS1, MS2) and the free-flight zone (MS4), a wall (R1) of said micro-reflectron comprising a layer of a resistive material, i.e. whose electrical resistivity is at least 1 kOhm.cm, adapted to be polarised between at least two regions of said layer so as to create a continuous electrostatic field gradient (ΔE) to deflect the trajectory of ions in said micro-reflectron, said micro-reflectron being arranged so as to be traversed by the ions in a longitudinal direction parallel to the direction (y) of the ion beam at the output of the source (MS1, MS2) and said micro-reflectron comprising at least two polarisation electrodes of the resistive layer in contact with said regions of the resistive layer, said electrodes extending in the longitudinal direction of the micro-reflectron, so as to generate in said resistive layer a potential gradient in the transverse direction (x) of the micro-reflectron so that the continuous electrostatic field gradient is oriented transversely to the trajectory of the ions in the micro-reflectron, - a module (SEP) for separating at least some of the species contained in the gas to be analysed, comprising at least one micro-capillary gas chromatography (GC) column, and - a time-of-flight mass spectrometer (TOFMS) coupled to the said separation module, the said spectrometer comprising an ion source (MS1, MS2) adapted to ionise at least some of the said species and to emit a beam of ions, and a region (MS4) for the free flight of the said ions, said mass spectrometer (TOFMS) being arranged in the volume of at least one substrate and comprising a micro-reflectron (R) disposed between the ion source (MS1, MS2) and the free-flight zone (MS4), a wall (R1) of said micro-reflectron comprising a layer of a resistive material, i.e. whose electrical resistivity is at least 1 kOhm.cm, adapted to be polarised between at least two regions of said layer so as to create a continuous electrostatic field gradient (ΔE) to deflect the trajectory of ions in said micro-reflectron, said micro-reflectron being arranged so as to be traversed by the ions in a longitudinal direction parallel to the direction (y) of the ion beam at the output of the source (MS1, MS2) and said micro-reflectron comprising at least two polarisation electrodes of the resistive layer in contact with said regions of the resistive layer, said electrodes extending in the longitudinal direction of the micro-reflectron, so as to generate in said resistive layer a potential gradient in the transverse direction (x) of the micro-reflectron so that the continuous electrostatic field gradient is oriented transversely to the trajectory of the ions in the micro-reflectron, <EOS>