Mass Spectrometry: Ionization Techniques

After the entry of sample stream into the ionization chamber, it has to undergo ionization to get separated and detected further in mass analyzer and detector respectively. Various ionization methods available for the ionization purpose are explained here.

Electron Ionization

As the name indicates, a high energy electron beam is used to convert the molecule species into ions. This method is the commonly used among other available methods.

The filament in the ionization chamber is heated to several thousand degrees Celsius which then emit the high energy electron beam. The electron beam strikes with the molecules of the sample stream coming from the sample inlet. Generally, 70-eV energy electron beam is used for the purpose.

On striking, the electron beam abstracts one electron from the molecules and hence converting them into cations (positively charged molecular ions). The energy required to abstract electron from the molecule is known as its ionization energy or ionization potential.

The cations so formed are repelled toward accelerating plates by a repeller plate. A large potential difference is maintained across accelerating plates so that a rapidly travelling beam of cations could be formed.

Some negative ions produced during the ionization process are absorbed by the repeller plate. Some instruments have the possibility to reverse the polarity of plates and hence to mass analyze the negative ions formed.

The cations formed are directed toward the mass analyzer by passing them through one or more focusing slits to form a uniform beam of ions.  

  

As the high energy electron beam can lead to the fragmentation of molecule further into smaller ions, the method is not much useful for the high molecular weight (HMW) compounds which are relatively less volatile also.

Chemical Ionization

In chemical ionization method, the sample molecules are allowed to collide with the excessively present pre-ionized reagent gas molecules. On collision, the sample molecules get ionized by one of the following mechanisms; electron transfer, proton transfer, adduct formation.

Any available gas or highly volatile liquid can be used as a reagent gas but the most commonly employed are methanol, isobutane, methane and ammonia. The selectivity of ionization and the degree of fragmentation can be varied by varying the reagent gas.

Proton affinity is simply the energy released when an atom bombards with an ion or gaseous molecule. The more is the difference b/w proton affinities of sample molecules and reagent gas, the more energy will be transferred to the sample during ionization process.  The increased amount of this energy can lead to the fragmentation of ion due to the cleavage of covalent bonds in it.

If methane gas is used as a reagent gas, CH₅⁺ and C₂H₅⁺ ions are formed initially.  Then it reacts with the sample M in following manner:

            M + CH₅⁺   → (M+H)⁺ + CH₄

            M + C₂H₅⁺      → (M+C₂H₅)⁺

 

Desorption ionization techniques

In desorption technique, the sample molecules dispersed or dissolved in matrix are exposed to the high energy beam of atoms, electrons or photons. Due to collision b/w sample molecules and beam, some sample molecules gets ionized and ejected from the matrix surface. The ejected ions are then further accelerated towards mass analyzer.

Depending upon the components present in high energy beam, the technique has following types:

1) Fast Atom Bombardment (FAB) : Neutral atoms (Ar, Xe) are present in beam

2) Secondary Ion Mass Spectrometry (SIMS) : Ions are present (Ar+,Cs+)

3) Matrix Assisted Laser Desorption Ionization (MALDI) : High intensity photons (Nitrogen laser is commonly used).

 

The choice of matrix is very important in practical analysis as it should be non volatile, relatively inert and reasonable electrolyte for allowing the formation of ions of sample. The matrix used for FAB and SIMS are given below:

 

Electrospray Ionization(ESI)

In this method, the solution containing sample molecules is filled into a fine capillary across the surface of which a high voltage potential is passed.  The solution is sprayed out of the capillary tip into the heating chamber. The charged droplets of solution get evaporated in the chamber to ions due to the counter flow of drying gas (Nitrogen mostly).

Thermospray ionization(TSI) technique is another technique works by similar mechanism except that a heated capillary is used instead of capillary with electrostatic potential.

ESI is used for studying compounds with high molecular weight or labile samples.