Mass Spectroscopy ionization techniques

1. Mass Spectroscopy
Ionization Techniques
(EI, CI, DART, ICP, MALDI, FAB, ESI, DESI)
By

2. In order for a sample to be analyzed by MS, it must however, first be ionized. There are a number of
ways of achieving sample ionization depending on the sample type, target analyte and desired workflow.
Gas phase methods
Electron Ionization (EI),
Chemical Ionization (CI),
Direct Analysis in Real Time (DART)
Inductively Coupled Plasma (ICP)
Desorption Methods
Matrix Assisted Laser Desorption Ionization (MALDI),
Fast Atom Bombardment (FAB),
thermal ionization sources,
plasma ionization sources and liquid metal ion sources (LMIS)) and
Spray methods
Electrospray Ionization (ESI)
Desorption Electrospray Ionization (DESI).

3. Electron ionization :
 It is also known as electron bombardment ionization.
Main function of this ionization technique is to convert the gaseous sample into molecular ions.
It is a hard ionization technique because it will produce 70eV.
Ionization potential of organic compounds are approximately 8-15 eV, and direct bombardment
take place, so due to high energy this technique will produce large no.of fragment ions from
molecular ions
Operating pressure is 10-6 to 10-5 M+e- M++ 2e- M+, M+
1, M+
2, M+
3…………

4. CHEMICAL IONIZATION (CI):
 Chemical ionization is a gaseous phase ionization method it is very important soft ionization technique.
Fragmentation is less and give intense peak of molecule ion.
Some molecules like alcohols, ethers, amines, esters, amino acids are highly fragmented in electron ionization,
so molecular ion peaks will not be detected.
To get proper molecular ion peaks we are using chemical ionization.
Steps of chemical ionization:
A carrier gas /reagent gas is introduced into the ionization source at slightly higher pressure(1torr)
Normally used carrier gases are : Methane , Ammonia, Isobutene
Step:1 : Carrier gas will be ionized due to electron impact from the ionization source.
Step 2 : Primary ions (CH4
+ ) will react with excess CH4 and it will produce different
type of secondary ions.
Step 3 : Secondary ions (CH5
+ , C2 H5
+ ) will react with analyte molecule and form ions
by 3 ways
1. Proton transfer 2. Hydride Transfer 3. Electrophilic addition

5. DIRECT ANALYSIS IN REAL TIME (DART):
 In the DART source a plasma from nitrogen or helium is first created, producing ions, electrons and excited-state species.
 The gas flows from the plasma chamber to a second chamber where ions are removed from the gas.
 The gas then passes to a third section of the source where it can be heated if desired, which aids desorption of some
materials.
 The gas exits the source through a grid electrode which serves several purposes to enhance signal and can be directed
towards a liquid, solid or vapor phase sample located in front of an inlet to the mass spectrometer.
 The interaction of the excited state species is responsible for the ionization of the analyte molecule.

6. INDUCTIVELY COUPLED PLASMA (ICP):
 ICP is typically used for liquid samples (which may be acid digestions of solid materials).
 In its simplest form, ICP takes a pre-prepared liquid containing the analyte and pumps it through a nebulizer to create
an aerosol which is introduced into an argon gas plasma.
 As the liquid droplets enter the high temperature plasma (~ 5500-6500 K), they are converted to the gaseous state.
 Upon absorbing more energy, they will eventually release an electron to form a single, positively charged ion.

7. MATRIX ASSISTED LASER DESORPTION IONIZATION (MALDI)
 It is a soft ionization technique under desorption ionization methods which uses pulsed LASER ( Light Amplification by
Stimulated Emission of Radiation) beam.
 It is used to determine the molecular weight of peptides, antibodies, protein molecules etc.., upto the size of 300K Da.
 Sample is dissolved in solvent - ( solution A)
 Matrix is dissolved in solvent - ( solution B)
+
 Ratio 1 : 1000 : 10000 ( sample : Matrix : solvent)
 Matrix material : Nicotinic Acid , Dihydroxic benzoic acid (DHB) , Urea, Cinnamic acid derivatives.
 LASER beam will hit the sample : Matrix mixture and analyte/ sample will convert into the form of gas.
 Protonation and Deprotonation.
LASER
Solution A
Solvent +
sample
Solution B
Solvent + Matrix
co-crystallize crystallized
Sample + Matrix
MIxture
MALDI plate
Sample + Matrix

8. FAST ATOM BOMBARDMENT (FAB):
 FAB is another soft ionization mechanism where a beam of accelerated atoms, typically inert gases such as argon or
xenon, is focused onto the sample to be analyzed.
 In order to create the “fast” beam, the atoms must first themselves be ionized such that they can be accelerated by
applications of high electric fields and focused using electrostatic lenses.
 By directing this beam of ions into a collision cell containing the same inert gas, charge exchange reactions will
occur, neutralizing the inert gas ions.

9. ELECTROSPRAY IONIZATION (ESI)
 ESI is another soft ionization technique suitable for analysis of large molecules and macromolecules. The sample is
delivered into a capillary held at high voltage (a few kV).
 This produces a mist of charged droplets of the same polarity.
 By using a drying gas or elevated temperatures, the charged droplets move through the source and are gradually
reduced in size through evaporation of the solvent, leading to an increased surface charge density.
 At a certain point, the electric field strength within the droplet will be large enough for ions at the surface of the
droplet to eject into the gaseous phase.

10. DESORPTION ELECTROSPRAY IONIZATION (DESI)
 As the name might suggest, DESI is very similar to ESI except now the charged droplets initially formed in the
ESI source are used as the source and directed to a sample held at ambient pressure (which is rather similar to
DART). The reflected or “splashed” droplets from the sample carry desorbed and ionized analytes that can be
subsequently directed into a mass spectrometer. The ionization mechanisms can be complex.