What are QMS Cracking Patterns?
Desorbed gases are ionized by accelerated electrons emitted from the filament of a Quadrupole Mass Spectrometer (QMS). Because the energy of these accelerated electrons is inherently high, a portion of the generated ions undergoes dissociation (cracking).
For example, in the case of water (H₂O), the following ions are generated: H₂O⁺, OH⁺, O⁺, and H⁺.
Here, the simply ionized molecule (H₂O⁺) is referred to as the "molecular ion," whereas the ions generated by the cracking of the molecular ion (OH⁺, O⁺, H⁺) are referred to as "fragment ions." The generation ratio between the molecular ions and fragment ions exhibits a constant, reproducible value when ionization occurs under identical operational conditions.

QMS Cracking Patterns Table
The table below shows the cracking patterns for low-molecular-weight species frequently observed in Thermal Desorption Spectroscopy (TDS) experiments.
Please note that cracking patterns may vary slightly from instrument to instrument; the values provided here should be understood as typical baseline averages. These values correspond to a standard ionization voltage of 70 eV.

Note: QMS cracking patterns are also commonly referred to as "fragmentation patterns" across various mass spectrometry and vacuum science literatures. Additionally, these patterns may vary slightly depending on the specific ionization energy, the hardware characteristics of the mass spectrometer utilized, and other subtle analytical variables.