in the sample. Some collisions (those of high enough energy) will cause ionization. In atomic mass spectrometry, these ions are detected. Their mass identifies the type of atoms and their quantity reveals the amount of that element in the sample. Regions The illustrations to the right shows the main regions that may be present in a glow discharge. Regions described as "glows" emit significant light; regions labeled as "dark spaces" do not. As the discharge becomes more extended (i.e., stretched horizontally in the geometry of the illustrations), the positive column may become striated. That is, alternating dark and bright regions may form. Compressing the discharge horizontally will result in fewer regions. The positive column will be compressed while the negative glow will remain the same size, and, with small enough gaps, the positive column will disappear altogether. In an analytical glow discharge, the discharge is primarily a negative glow with dark region above and below it. Cathode layer The cathode layer begins with the Aston dark space, and ends with the negative glow region. The cathode layer shortens with increased gas pressure. The cathode layer has a positive space charge and a strong electric field. Aston dark space Electrons leave the cathode with an energy of about 1 eV, which is not enough to ionize or excite atoms, leaving a thin dark layer next to the cathode. Cathode glow Electrons from the cathode eventually attain enough energy to excite atoms. These excited atoms quickly fall back to the ground state, emitting light at a wavelength corresponding to the difference between the energy bands of the atoms. This glow is seen very near the cathode. Cathode dark space As electrons from the cathode gain more energy, they tend to ionize, rather than excite atoms. Excited atoms quickly fall back to ground level emitting light, however, when atoms are ionized, the opposite charges are separated, and do not immediately recombine. This results in more ions and electrons, but no light. This region is sometimes called Crookes dark space, and sometimes referred to as the cathode fall, because the largest voltage drop in the tube occurs in this region. Negative glow The ionization in the cathode dark space results in a high electron density, but slower electrons, making it easier for the electrons to recombine with positive ions, leading to intense light, through a process called bremsstrahlung radiation. Faraday dark space As the