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Sensitivities and fragmentation patterns

Sensitivities and fragmentation patterns. RTOF Tutorial 4. Fragmentation patterns. Molecules are dissociated/ionized by electron impact Fragmentation pattern depends on electron energy

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Sensitivities and fragmentation patterns

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  1. Sensitivities and fragmentation patterns RTOF Tutorial 4

  2. Fragmentation patterns • Molecules are dissociated/ionized by electron impact • Fragmentation pattern depends on electron energy • Fragmentation patterns are instrument dependent. It is not permissible to take patterns for example from the NIST! • For RTOF the nominal electron energy is 75eV • The fragmentation table contains the parent and all non-zero daughters • The numbers are normalized to the highest peak (100%) which is not necessarily the parent (analog to the NIST tables)

  3. Uncertainties of cross sections

  4. Sensitivities Ii (A) = Ie (A) Sj (mbar-1) Qijpj (mbar) Ii (A) = Ie (A) Sj (cm3) Qijnj (cm-3) Sj: Sensitivity for gas j Qij : Fragmentation pattern for ion species i of gas j e.g. O+ from CO2 p=nkT Imain (A) = Ie (A) Sparent (cm-3) n(cm-3)

  5. Sensitivities Transfer function Detector yield Ionization cross section and fragmentation

  6. Definition for RTOF S =Cs (Ie)*tau*yield With Cs(Ie): cross section (function of electron current, electron energy and species) tau: transfer function (function of energy and resolution) yield: detector yield relative to N2, 3000V (function of energy and species)

  7. Sensitivity table So far only 100 uA/10 kHz/SS data are available

  8. Instrumental effects: It’s not always as easy as it seems! • t0, C variations • Clock frequency disturbance (FM only) • Additional noise /FS only) • Chemical reactions in the storage source • ML modes • Extraction frequency dependence of sensitivity • GCU stability Literature: PhD thesis Stephan Scherer (SébastienGasc)

  9. C, (t0) variation C follows closely the pressure -> T-dependence

  10. Chemical reactions and storage of ions • In the SSource ions are stored up to 100 ms • This may lead to chemical reactions, mostly charge transfer in the ion source • May influence relative intensities • Leads to different behavior depending on the extraction frequency. Sensitivities are therefore not proportional to extraction frequencies • Not yet fully studied

  11. Mass Line modes • The upper limit for the registration of ions per extraction is given by the number of mass lines • For high intensities heavy masses may therefore be discriminated

  12. GCU Long term stability Short term stability Short term stability not a problem for RTOF due to long integration time Long term stability is a problem!

  13. …and then there are the other modes (ions, HIRM, DTS,….) For advanced students only, after you have mastered the normal modes!

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