cathode is a negative field, which slows electrons as they are ejected from the surface. Only those electrons with the highest velocity are able to escape this field, and those without enough kinetic energy are pulled back into the cathode. Once outside the negative field, the attraction from the positive field begins to accelerate these electrons toward the anode. During this acceleration electrons are deflected and slowed down by positive ions speeding toward the cathode, which, in turn, produces bright blue-white bremsstrahlung radiation in the negative glow region. Use in analytical chemistry Glow discharges can be used to analyze the elemental, and sometimes molecular, composition of solids, liquids, and gases, but elemental analysis of solids is the most common. In this arrangement, the sample is used as the cathode. As mentioned earlier, gas ions and atoms striking the sample surface knock atoms off of it, a process known as sputtering. The sputtered atoms, now in the gas phase, can be detected by atomic absorption, but this is a comparatively rare strategy. Instead, atomic emission and mass spectrometry are usually used. Collisions between the gas-phase sample atoms and the plasma gas pass energy to the sample atoms. This energy can excite the atoms, after which they can lose their energy through atomic emission. By observing the wavelength of the emitted light, the atom's identity can be determined. By observing the intensity of the emission, the concentration of atoms of that type can be determined. Energy gained through collisions can also ionize the sample atoms. The ions can then be detected by mass spectrometry. In this case, it is the mass of the ions that identify the element and the number of ions that reflect the concentration. This method is referred to as glow discharge mass spectrometry (GDMS) and it has detection limits down to the sub-ppb range for most elements that are nearly matrix-independent. Depth analysis Both bulk and depth analysis of solids may be performed with glow discharge. Bulk analysis assumes that the sample is fairly homogeneous and averages the emission or mass spectrometric signal over time. Depth analysis relies on tracking the signal in time, therefore, is the same as tracking the elemental composition in depth. Depth analysis requires greater control over operational parameters. For example, conditions (current, potential, pressure) need to be adjusted so that the crater produced by sputtering is flat bottom (that is, so that the