IONIZATION SOURCES UNDER THE GUIDANCE OF MR.RATNAKAR

-: Presented By :- R.RAJAKIRAN REDDY, Dept.of Ph.analysis, 1st year m.pharmacy, GNIP college.

2014.

Mass Spectroscopy

Definition: Mass Spectroscopy is an analytical laboratory technique to separate the components of a sample by their mass by using mass spectrometer.

Also Known As: Mass Spec, mass spectrometry.

Principle: In this technique the sample is vaporized into a gas and then ionized. The ions are then accelerated through a potential difference and focused into a beam. The ion beam passes through a magnetic field which bends the charged stream. Lighter components or components with more ionic charge will deflect in the field more than heavier or less charged components. A detector counts the number of ions at different deflections and the data can be plotted as a 'spectrum' of different masses.

Mass Spectrometry Ionization Sources

Mass specrtromertry ionization sources are:

1.Electro spray Ionization (ESI)2.Nano electro spray Ionization (Nano ESI)3.Atmospheric Pressure Chemical Ionization (APCI)4.Atmospheric pressure photo ionization (APPI)5.Matrix-assisted laser desorption ionization mass

spectrometry (MALDI-MS)6.Fast Atom Bombardment (FAB)7.Electron Ionization (EI)8.Chemical Ionization (CI)9.Thermal ionization (TI)

1. Electro spray Ionization (ESI): The sample solution is sprayed from a region of the strong electric field at the tip of a metal nozzle maintained at a potential of anywhere from 700 V to 5000 V. The nozzle (or needle) to which the potential is applied serves to disperse the solution into a fine spray of charged droplets.

Either dry gas, heat, or both are applied to the droplets at atmospheric pressure thus causing the solvent to evaporate from each droplet.

For example: peptides, proteins, carbohydrates,

smalloligonucleotides, synthetic polymer, and lipids.

2. Nano electrospray Ionization (NAno-ESI):

where the spray needle has been made very small and is positioned close to the entrance to the mass analyzer.The end result of this rather simple adjustment is increased efficiency, which includes a reduction in the amount of sample needed.

Very sensitive Very low flow rates Very small droplet size (~5µ)

3. Atmospheric Pressure Chemical Ionization (APCI): The liquid effluent of APCI is introduced directly into the ionization source. However, the similarity stops there. The droplets are not charged and the APCI source contains a heated vaporizer, which facilitates rapid desolvation/vaporization of the droplets. Vaporized sample molecules are carried through an ion-molecule reaction region at atmospheric pressure.

4. Atmospheric pressure photo ionization (APPI): It generates ions directly from solution with relatively low background and is capable of analyzing relatively non polar compounds.

APPI vaporized sample passes through ultra-violet light. APPI is much more sensitive than ESI or APCI.

5. Matrix-assisted laser desorption/ ionization mass spectrometry (MALDI-MS):

The analyte is first co-crystallized with a large molar excess of a matrix compound, usually a UV-absorbing weak organic acid. Irradiation of this analyte-matrix mixture by a laser results in the vaporization of the matrix, which carries the analyte with it. The matrix plays a key role in this technique. The co-crystallized sample molecules also vaporize, but without having to directly absorb energy from the laser. Molecules sensitive to the laser light are therefore protected from direct UV laser excitation.

6. Fast Atom Bombardment (FAB):

Immobilized matrix is bombarded with a fast beam of Argon or Xenon atoms. Charged sample ions are ejected from the matrix and extracted into the mass analyzers.

Used for Largecompounds with low volatility

(eg peptides, proteins, carbohydrates) Solid or liquid sample is mixed with a non-volatile matrix

(eg glycerol, crown ethers, nitrobenzyl alcohol)

7. Electron Ionization (EI):

Energetic process a heated filament emits electrons which are accelerated by a potential difference of usually 70eV into the sample chamber. Ionization of the sample occurs by removal of an electron from the molecule thus generating a positively charged ion with one unpaired electron.

Produces M+.radical cation giving molecular weight. Produces abundant fragment ions.

8. Chemical Ionization (CI):

This process is initiated with a reagent gas such as methane, isobutane, or ammonia, which is ionized by electron impact.

High gas pressure in the ionization source is required for the reaction between the reagent gas ions and reagent gas neutrals.

possible mechanismReagent (R) + e- →R+ + 2e-R+ + RH →RH+ + RRH+ + Analyte (A) → AH+ + R Biologically important molecules (sugars, amino acids, lipids etc.).

9.Thermal ionization (TI):

In this process samples are deposited on rhenium or tantalum filament and then carefully evaporated and sent to mass analyzer.

used to

quantify toxic trace elements in foods. measurement of stable isotope ratio of inorganic elements.

References:

1. Sharma Y.R. Elementary organic spectroscopy principles and chemical applications. 1st ed. S. Chand and Company ltd; New Delhi :2008.

2. Chatwal G.R, Anand S.K. Instrumental methods of chemical analysis. 1st ed. Himalaya Publishing house; Mumbai: 2004.

3. S. Ravi Shankar. Text book of pharmaceutical analysis. 3rd ed. Rx publication; Tirunelveli: 2006.

4. Skoog DA, West DM. principle of instrumental analysis. 2ed edition