Sensitivities and fragmentation patterns

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

Parent daughter 1 daughter 2 daughter 3 daughter 4 daughter 5 daughter 6 daughter 7N2 N

O2 O

H2O OH O18.0100161 17.0021911 15.9943661

100 21.1 2.4CO O C

27.9943661 15.9943661 11.9994514100 2 5

CO2 CO O C43.9892807 27.9943661 15.9943661 11.9994514

100 10 10 10H2CO HCO CO O CH C

30.0100161 29.0021911 27.9943661 15.9943661 13.0072765 11.999451458 100 25 1 1 1

CH4 CH3 CH2 CH C16.0307516 15.0229265 14.0151015 13.0072765 11.9994514

100 95 20 15 5C2H2 C2H C2 CH C

100 20 8 2 126.0151015 25.0072765 23.9994514 13.0072765 11.9994514

C2H4 C2H3 C2H2 C2H C2 CH2 CH C100 65 55 10 1 1 0.5 0.5

28.0307516 27.0229265 26.0151015 25.0072765 23.9994514 14.0151015 13.0072765 11.9994514C2H6 C2H5 C2H4 C2H3 C2H2 C2H C2 CH3

30 25 100 30 20 5 0.5 330.0464016 29.0385766 28.0307516 27.0229265 26.0151015 25.0072765 23.9994514 15.0229265

C3H8 C3H7 C3H6 C3H5 C3H4 C3H3 C3H2 C3H44.0620517 43.0542267 42.0464016 41.0385766 40.0307516 39.0229265 38.0151015 37.0072765

30 25 11 15 4 20 8 3

SensitivitiesIi (A) = Ie (A) Sj (mbar-1) Qij pj (mbar)

Imain (A) = Ie (A) Sparent (cm-3) n(cm-3)

Sj: Sensitivity for gas jQij : Fragmentation pattern for ion species i of gas j e.g. O+ from CO2

p=nkT

Ii (A) = Ie (A) Sj (cm3) Qij nj (cm-3)

Sensitivities

Transfer function

Detector yield

Ionization cross section and fragmentation

Definition for RTOF

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)

S =Cs (Ie)*tau*yield

Sensitivity tableParent 20mA 100mA (*10-20) 200mA

Source OS SS OS SS OS SSExtr. Freq. 10 5 2 10 5 2 10 5 2 10 5 2 10 5 2 10 5 2

H2O 0.01 0.001 0.04 0. 3 0.12 0.1 0.1 0.01

CO 0.001 0.0002 0.02 0.26 0.003 0.004

CO2 0.28

H2CO 0.32

CH4 0.16

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 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

Mass Line modes

Extraction frequency (kHz)

Number of mass lines

10 31

5 63

2 255

•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

GCULong term stability

Short term stability

Short term stability not a problem for RTOF due to long integration timeLong 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!