HR fitting and elemental analysis
Harald Stark
“Standard” CIMS fitting procedure
• Assign HR peaks by unique ions, do elemental
analysis
Data source: acetate CIMS BEACHON 2011
Data Analysis challenges
• Goal: multi-peak fitting of all (high-resolution ) spectra
with one single peak list
• Problem: increasing number of compounds at higher
mass-to-charge ratios (m/Q, unit “Thomson”,
“Th”), harder to assign peaks correctly
3
Challenge
• Too many possible peaks at high m/Q
Peaks within <
1/2 FWHM
(130 ppm)
Peak Assignment Method(s)
• Free fit: numerical algorithm varying peak positions
• Free compounds: all possible (organic) compounds
• Final list: manual (0-100 Th), semi-automatic (100-200 Th) , or automatic (200500 Th) assignment of free compounds to free fits within m/Q tolerance
5
Peak Assignment Evaluation
• >98% of total area fit
•  1800 ions assigned
Example elemental parameter
approximate oxidation state (carbon):
OSC = 2×O:C – H:C (Kroll et al., 2011)
Key assumption: peak assignment is representative
of elemental composition, average elemental
parameters are correct (within uncertainties)
6
Mass defect
• Mass defect = m/Qexact – m/Qunit
Idea: Elemental Parameters related to mass defect?
• Elemental mass defects of organic elements:
Element
C
H
O
Mass Defect (Th)
0
+0.0078
-0.0051
Kendrick mass
defect (CH2)
-0.013
+0.0067
-0.0229
Less O / More H
More O / Less H
Mass defect as proxy for elemental information
Key assumption: peak assignment is
representative of elemental composition,
average elemental parameters are
correct (within uncertainties)
Oxidation state is best correlated with mass defect
Comparison with AMS, Oxidation State
• CIMS measures more highly oxidized
compounds (subset of AMS)
• Comparable range in oxidation state
Excellent relative agreement:
• Confirms peak assigment method
10
Comparison with AMS,
Carbon
Number
Excellent relative agreement:
• AMS ionization method
fragments more than CIMS
(not a surprise)
• Confirms peak assigment method
New:
• AMS ionization method retains some
information on carbon chain length
• carbon number from AMS?
11
Oxidation state without peak assignment
1. multi-variate fit to known peak list
Oxidation state without peak assignment
2. “Mass defect tile plot” gives oxidation state
Avg. bulk oxidation state (weighted signal average) from tile plot: 0.12
Comparison with exact mass list (weighted elemental signals): 0.15
Mass defect tile plots
Slope = 0.18
Slope = 0.23
Slope = 0.07-0.27
Slope = 0.45