RTOF Tutorial 4

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)

Uncertainties of cross sections

Sensitivities I i (  A) = I e (  A) S j (mbar -1 ) Q ij p j (mbar) I main (  A) = I e (  A) S parent (cm -3 ) n(cm -3 ) Sj:Sensitivity for gas j Q ij : Fragmentation pattern for ion species i of gas j e.g. O + from CO 2 p=nkT I i (  A) = I e (  A) S j (cm 3 ) Q ij n j (cm -3 )

Sensitivities Transfer function Detector yield Ionization cross section and fragmentation

Definition for RTOF With Cs(I e ): 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) S =Cs (I e )*tau*yield

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

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 (Sébastien Gasc)

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

Chemical reactions and storage of ions In the SSource ions are stored up to 100  s 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

Mass Line modes Extraction frequency (kHz) Number of mass lines 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

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!

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