Ionization II: Chemical Ionization

CU- Boulder

CHEM 5181

Mass Spectrometry & Chromatography

J. Kimmel

Fall 2007

“The development of mass spectrometry can be seen, from one perspective, to be based on the invention and utilization of ion sources of ever greater power and more general applicability.”

- R. G. Cooks et al., J. Mass Spec, 2005, 1261

•Early stages of MS: Precise determination of atomic masses and isotope abundances.

•Last 50 years: Shift towards analytical applications involving molecules of greater complexity

•Last 15 years: Explosion of biological applications

Recommend: Vestal, Chem. Rev., 101, 361, 2001.

Creating the gas-phase ion ….

In theory, a mass spectrometer is capable of measuring any gas-phase molecule that carries a charge.

Need an energetic process. Must convert to ion. Prefer to preserve properties of sample that are of interest.

Physical states of sampleChromatographic couplingSample/matrix complexity

Destruction of matrixSelectivity

Ionization efficiency Coupling / transmission to MS

Molecular mass and/or structural elucidationDegree of Fragmentation

Relevant considerations (brainstorm)?

Sources to be Discussed

Molecular Analysis

1. Chemical Ionization (CI)2. Atmospheric Pressure CI (APCI)3. Electrospray (ESI)4. Nanospray 5. Secondary Ion (SIMS) / Fast Atom Bombardment (FAB)6. Matrix-Assisted Laser Desorption/Ionization (MALDI)7. Desorption Electrospray Ionization (DESI)

Elemental Analysis

1. Thermal Ionization2. Spark Source3. Glow Discharge4. Inductively-Coupled Plasma (ICP-MS)

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+

1. Munson and Field, JACS, 2621, 1966.

GENERAL STEPS

1. Reagent species is ionized by high-pressure electron ionization

e + R → R±

2. Collision of reagent ion with gas-phase analyte (present at <1% abundance of reagent) yields analyte ion

R± + M → M1

± + N1

3. Potential fragmentation of M ± by one or more pathways

M1 ± → M2

± + N2

→ M3 ± + N3

→ M4 ± + N4

CI Ion Source

From Barker

Similar to EI source.

•Higher P

•Simultaneous introduction of M and R

CI Reactions

Many types of reactions can account for ionization in Step 2; Proton transfer is the most common.

Proton transfer: M +RH+ (M+H)+ + R

Charge Transfer: M + R·+ M·+ + R

Electron capture: R + e- R-

Adduct formation (slow): M + RH+ (M-RH)+

M + MH+ (M-MH)+

In step 1, maintaining a large excess of R compared to M ensures preferential ionized

Step 2 requires collision, therefore source is held at higher pressure than typical EI

L = 4.95/pmTorr

0.01 cm = 4.95/ pmTorr

p = 495 mTorr = 65 Pa

CI: Reduced Fragmentation

• As you will see in a later lecture, EI produces an assembly of molecular ions with internal energies between ~0 and 10 eV. As a result, spectra are dominated by fragment ions.

• For the proton transfer reaction RH+ + M → MH+ + R

The degree of fragmentation of MH+ will depend on the internal energy of the products, which in turn depends on ΔH of the reaction.

• ΔH depends on the relative proton affinities (PA) of the reactants (Recall that PA equals the negative of ΔH for the protonation reaction).

ΔH = PA(R) - PA(M)

• Observation of MH+ implies that PA(M) > PA(R)

• Choice of reagent gas systems can be tailored to the problem to be solved.

ΔH = PA(R) - PA(M)

If ΔH is POSITIVE: ____________

If ΔH is NEGATIVE: ____________

If ΔH is VERY, VERY NEGATIVE: ____________

Example: Methane as Reagent Species

CH4 + e- CH4+ + 2e-

CH4 + + CH4 CH5

+ + CH3

CH4+ CH3

+ + H CH4

+ CH2+ + H2

CH3 + + CH4 C2H5

+ + H2

CH2+ + CH4 C2H3

+ + H2 + H

C2H3+ + CH4 C3H5

+ + H2

CH5+

C2H5+

C3H5+

Relevant reaction:

CH4 + H+ → CH5+

PA(CH4) = -ΔH = 131 kcal mol-1

Relevant reaction:

C2H4 + H+ → C2H5+

PA(C2H4 ) = -ΔH = 162.6 kcal mol-1

Question?

From the text: PA(methane) = 5.7 eV; PA(isobutane) = 8.5 eV

The analyte molecule M can is known to ionize by proton a transfer mechanism with either methane or isobutane. Which is true?

(A) PA(M) > 8.5 eV(B) 5.7 eV < PA(M) < 8.5 eV(C) PA(M) < 5.7 eV(D) I don’t know

Question?

From the text: PA(methane) = 5.7 eV; PA(isobutane) = 8.5 eV

The analyte molecule M can is known to ionize by proton a transfer mechanism with either methane or isobutane. Which is true?

(A) PA(M) > 8.5 eV(B) 5.7 eV < PA(M) < 8.5 eV(C) PA(M) < 5.7 eV(D) I don’t know

Answer: (A) ΔH = PA(R) - PA(M)

To be spontaneous, ΔH must be negative. Therefore, PA(M) must be greater than PA(isobutane) and PA(methane)

Question?

From the text: PA(methane) = 5.7 eV; PA(isobutane) = 8.5 eV

Which is reagant gas is more likely to yield fragmenation of M?

(A) Isobutane(B) Methane(C) Depends on structure of M

Question?

From the text: PA(methane) = 5.7 eV; PA(isobutane) = 8.5 eV

Which is reagant gas is more likely to yield fragmenation of M?

(A) Isobutane(B) Methane(C) Depends on structure of M

Answer: (B) ΔH = PA(R) - PA(M)

ΔH will be more negative when methane is used

Fragmentation

From de Hoffmann

EI

CI, R= Methane (PA=5.7 eV)

CI, R= Isobutane (PA=8.5 eV)

NOTE

•Many instruments include dual sources: CI for molecular weight; EI for ID by fragmenation

•The unpredictable nature of CI fragmentation prevents development of spectral libraries.

Selective Detection

From Hoffmann

Hydrocarbons have lower proton affinity than Butyl

methacrylate

EI CI: CH4 CI: Isobutane

Atmosperic Pressure CI (APCI)

From Vestal, Chem. Rev., 101, 361, 2001.

A method for coupling CI to liquid chromatrography

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.