Soft ionization methods, specially the ionization with UV light is prefered. The actual technology is based on one-photon ionization, based on an innovative vacuum ultraviolet light source. The advantage of this ionization method is that substances can be measured directly in complex mixtures. Time-consuming separation techniques combined with MS (e.g. GC) are not needed. That is why MS containing soft ionization is ideally suited for process monitoring and quality control. Another selective online UV-ionization technique is the Resonance Enhanced Multi-Photon Ionization (REMPI) which is especially suitable for the detection of Polycyclic Aromatic Hydrocarbons (PAH).