CHM 312 Fall 2008

Special Topics in MS

Dr. Ralph Mead

Outline

• Introduction

• Chemical Ionization

• Electrospray Ioniszation (ESI)

• Atmopsheric Pressure Chemical Ionization (APCI)

• Meta-stable ions

• MS/MS

IONS are Key!!!

Hard and Soft Ionization Techniques for the Organic Chemist

“Hard” Ionization

• Electron Impact (usually no M+ but rich in fragments)

“Soft” Ionization

•Chemical Ionization•Electrospray •APPI

Chemical Ionization“ Soft”

•Alternative to EI.

•Provides less fragments and usually M+

• This is due to the ionization of the reagent gas which transfers as proton and energy to the molecule of interest ( more on this latter)

• Done in a regular EI source but with a reagent gas present (0.5-2 Torr compared to EI 10-6 Torr)

• General Mechanism (GC/MS)

Methane Reagent Gas for GC/MS

Methane:

CH4 + e -----> CH4+· + 2e ------> CH3

+· + H

CH4+· + CH4 -----> CH5

+· + CH3·

CH4+· + CH4 -----> C2H5

+ + H2 + H·

Acetone with Methane CI:

Other Reagent Gases

CI GC/MS Spectra (Which Gas is for you?)

Simple Answer: Depends…

Long Answer:

•Amount of excess energy imparted on [M+H]+ ion upon formation depends on proton affinities of the conjugate base of the reactant ion and M.

• Proton affinity of conjugate base decreases (i.e. acidity of reactant gas ion increases)…fragmentation of protonated molecules increases since more energy is transferred upon formation

• Acidity increases from NH4

+<C4H9+<C2H5

+,CH3+<H3

+

Degree of fragmentation follows this as seen to the left

What about molecules that are not amenable to GC/MS?

The trick is making ions in a solvent rich mobile presentAND keeping the MS under vacuum

Electrospray Ionization

Uses:Small and Large Polar Molecules (Typically proteins and other bio-molecules)Drugs, pesticides etc.

No fragments form…quasi molecular ions…multiple charged fragments[M+ zH]z+ for basic compounds and [M – zH]z- for acidic compoundsCharge formation

ESI Mass SpectraPositive ion ESI spectrum of protein Horse heart myoglobin

Should be one peak for molecular ion but multiple charges since many hydrogens thus multiple peaks.

Either positive mode or negative mode ionization

Bases as in positive mode as conjugate acids

Acids in negative mode as conjugate basesMW???

MW Determination from ESI

Atmospheric Pressure Chemical Ionization

•Compared to ESI-LC/MS can accept flow rates up to 2ml/min •More suitable for smaller molecules (up to Mr=1500)• Spectrum are similar to ESI

Meta-Stable ions…Putting them to work for you

Continued Meta-Stable ions…Putting them to work for youDescription

MS/MS …Putting it to work for you

Instrument schematic and example

Continued MS/MS …Putting it to work for you

QQQ example:

Continued MS/MS …Putting it to work for you

Continued MS/MS …Putting it to work for you