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Introduction to Chemical Ionization & Instrumentation
Advanced Atmospheric chemistry CHEM-5152 Spring 2015 Prof. J.L. Jimenez
2 Chemical Ionization (CI)
Introduced in 1966 by Munson and Field1, it was a direct outgrowth of fundamental studies of ion/molecule interactions.
Where other techniques rely on interaction of molecule and electron, photon, or electric field, ionization of the analyte molecule, M, is achieved through reaction with a reagent ion, R+ CI Method of Munson & Field (Still used)
1. Reagent species is ionized by high-pressure* electron ionization e- + R → R+ + 2 e- 2. Collision of reagent ion with gas-phase analyte (present at <1% abundance of reagent) yields analyte ion + + R + M → M1 + R1 3. Potential fragmentation of M+ by one or more pathways + → + → + M1 M2 + N2 M3 + N3 → + M4 + N4
*: how high P? Remember prob of an electron leading to ionization is ~3 x 10-7 at P = 10-7 Torr
1. Munson and Field, JACS, 2621, 1966. – Suggested reading on course web page
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3 Solvation in IMRs
From Hoffmann
4 Efficiency of IMRs
• Approx. every collision leads to a proton transfer, if G0 < 0
From Hoffmann
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5 CI Ion Source
From Barker
Similar to EI source. •Higher P •Simultaneous introduction of M and R
6 Proton transfer reaction – mass spectrometry
H3O VOC VOCH H2O PA(VOC) > PA(H2O)
Courtesy of Lisa Kaser, NCAR
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Potential Complexity of Reagent Ions
- + - CH + CH4 + e CH4 + 2e 5 + + + CH4 + CH4 CH5 + CH3 C2H5
+ + CH4 CH3 + H + + CH4 CH2 + H2 + C3H5
+ + CH3 + CH4 C2H5 + H2
+ + CH2 + CH4 C2H3 + H2 + H + + C2H3 + CH4 C3H5 + H2
Relevant reaction: + + CH4 + H → CH5 -1 PA(CH4) = -∆H = 131 kcal mol Relevant reaction: + + C2H4 + H → C2H5 PA(C H ) = -∆H = 162.6 kcal mol-1 2 4 7
8 Selected Ion Flow Tube & SYFT MS
1. Generation of ions + + + 2. Ion selection: H3O , NO and O2 3. Sample introduction and reaction 4. Selection of reaction products http://www.syft.com/ 5. Detection.
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9 Atmosperic Pressure CI (APCI)
From Vestal, Chem. Rev., 101, 361, 2001.
• A method for coupling CI to liquid chromatography • Heat and gas flow desolvate nebulizer droplets, yield dry vapor of solvent and analyte molecules. • Corona discharge ionizes solvent, which in turn acts as CI reagent. • Not suitable for very nonvolatile or thermally labile samples. For these, electrospray is the method of choice.
10 - Pictures of NO3 Source (Airmodus)
M. Ehn et al., Nitrate CI and Flow Tube IMR Design, 2012 ToF-CIMS Users Meeting, Boulder, CO R.L. Mauldin, Nitrate Ion Chemical Ionization, 2012 ToF-CIMS Users Meeting, Boulder, CO
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11 - NO3 Source: how it works
M. Ehn et al., Nitrate CI and Flow Tube IMR Design, 2012 ToF-CIMS Users Meeting, Boulder, CO
12 - Aerodyne ToF-CIMS w/ NO3 Source
M. Ehn et al., Nitrate CI and Flow Tube IMR Design, 2012 ToF-CIMS Users Meeting, Boulder, CO
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13 - Properties of NO3 Source • • Optimized for measuring low-volatility compounds – ”Wall-less” design – High sample flow (10 LPM) • Low count rate (~104 cps) but high sensitivity (DLH2SO4 ~ 104 molec cm-3 ~ 10-3 ppt in 15 min) • Highest measurable concentration ~1ppb • Reaction time 200ms • Calibration is tricky, lots of characterization still to be done • Can only operate at ambient pressure • Only ions are mixed into the sample air (in theory) - - • Has been run with NO3 , HSO4 and acetone M. Ehn et al., Nitrate CI and Flow Tube IMR Design, 2012 ToF-CIMS Users Meeting, Boulder, CO
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M. Ehn et al., Nitrate CI and Flow Tube IMR Design, 2012 ToF-CIMS Users Meeting, Boulder, CO
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15 Properties of Flow Tube IMR • Simpler design • Sample flow ~2 LPM • Reaction time ~100ms • High count rate (~106 cps)
•DLCH2O2 ~ 10 ppt for 1s • Can operate at variable pressure (typically ~100mbar)
•N2 through ionizer mixes with sample - + • Run with acetate, I , H3O (H2O)n, benzene
M. Ehn et al., Nitrate CI and Flow Tube IMR Design, 2012 ToF-CIMS Users Meeting, Boulder, CO
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