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Soft Ionization Methods

! Electrospray Ionization (ESI)

A solution is nebulized under atmospheric pressure and exposed to a high electrical field which creates a charge on the surface of the droplet. Droplets rapidly become much smaller through vaporization of solvent and into an analyzer.

By producing multiply charged ions,electrospray is extremely useful for accurate mass measurement,particularly for thermally labile, high molecular mass substances (ie. proteins, oligonucleotides, synthetic polymers, etc.)

! Matrix-Assisted Laser Desorption/Ionization (MALDI)

Laser evaporation from a crystallized sample/matrix mixture. The matrix material must have an which has an absorption spectrum that matches the laser wavelength of energy.

Matrix acts as a receptical for the laser energy and facilitaes ionization while minimizing ablation of the sample and analyte ion that would otherwise occur from direct laser desorption.

More tolerant of salts and complex mixture analysis than ESI.

analyte / matrix

laser

analyte ions

matrix ions

cation

sample plate

Comparing Ionization Methods

ionization

type of

sample

mass

method

method

ion formed

analytes

intro

limits

type

EI

CI

M+, M

[M +H]+, [M + X]

+

small volatiles

GC, liquid or solid probe

small volatiles

GC, liquid or solid probe

FI: volatiles FD: nonvolatiles

GC, liquid or solid probe

peptides, proteins nonvolatile

LC or syringe

carbohydrates organometallics peptides, nonvolatile

in viscous matrix

peptides, proteins nucleotides

in solid matrix

ES

n+ n–

[M + nH] , [M – nX]

FAB

[M+H]+, [M-H]

APCI

FI/FD

solid probe

MALDI

[M +H]+, [M + X]

+

[ M + H ] + , [ M + X ] + , [M – H]

[M +H]+, [M + X]

+

small volatiles (less polar species)

LC or syringe

103

hard method structural info

103

soft method

2x103

soft method

2x103

soft method

2x105

soft method multiply charged ions

6x103

soft but harder than ESI or MALDI

5x105

soft

4

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