Ionization Methods

Question 1

Electron Impact (EI)

Answer 1

Pros > cheap, tough and reproducible

Cons > rough on molecules, molecular ions often do not survive, molecules need to be volatile, there is an entire class of compounds that doesn’t work

Question 2

Chemical Ionization

Answer 2

Ionizes a carrier gas (CH4 or C4H10) with EI. Then collides carrier gas with an analyte.

Pros > Nice and gentle, molecular ion will survive, it’s cheap

Cons > Molecule must be volatile and fragmentation is not reproducible

Question 3

Fast Atom Bombardment (FAB)

Answer 3

Dissolves the sample in a glycerol matrix. Fires argon atoms at an angle which introduces a proton transfer and ejects the analyte from the surface.

Pros > Molecule does not need to be volatile, it’s gentle, fragments tend to occur at labile bonds. Useful for peptide and sugar sequencing

Cons > costly, there is a high size limit for molecules, often major glycerol adduct peaks

Question 4

Laser desorption techniques (MALDI)

Answer 4

Pros > gentle on molecules, can take huge molecules, great for finding M+H+ or M+Na+

Question 5

Electrospray ionization (ESI)

Answer 5

A solution of an analyte that is already ionized

Pros > very gentle, no volatility requirements, cheap, can be used on large molecules

Cons > Has to be ionized in solution, sodium adducts are present, more expensive than EI and CI

Question 6

High resolution mass spec (HRMS)

Answer 6

Exact mass gives you one or at most three molecular formulas