By A.Venu M.Pharm PharmaceuticsSri Krupa Institute of Pharmaceutical Sciences Under the Guidance of Dr. S. Y. MANJUNATH
MASS SPECTROMETRY Introduction Principle Instrumentation applications
INTRODUCTION Mass spectrometry is one of the most generally applicable tools providing both qualitative and quantitative information about the atomic and molecular materials. Here the compound under the investigation is bombarded with a beam of electrons which produce an ionic molecule or ionic fragments of the original species. The resulting charging particle is then seperated according their masses.
Mass Spectrometry Analytical method to measure the molecular or atomic weight of samples
PRINCIPLE Organic molecules are bombarded with electron and are converted to high energetic positively charged ions(molecular ions or parent ions). Which will break up in to smaller ions (fragment ions or daughter ions). The loss of electron from a molecule leads to a radical cation. M+e- M++2e-
This molecular commonly M+ decomposes to a pair of fragments which may be either radical +anion or a small molecule + radical cation. These molecular ions, are isolated in the electric field at an voltage V, these charging particles which are isolated then made to enter into an magnetic field H. Here the field attracts the particles and move in a circle around it. Here the radius of the ionised molecule depends on m, its mass. This forms the basis of separation of particles according to their masses. These ions are made to impinge upon the collector inturn thus giving rise to a spectrum. The ion source, ion path and collector of the mass spectrometer must be under high vacuum(107 mm Hg) for optimum operation.
INSTRUMENTATION SAMPLE INLET SYSTEM ION SOURCE(OR IONISATION CHAMBER) THE ELECTROSTATIC ACCELERATING SYSTEM THE MAGNETIC FIELD THE ION SEPERATOR THE ION COLLECTOR THE VACUUM SYSTEM
SAMPLE INLET SYSTEM1. Handling gas samples: It involves transfer of samples from small containers of known volume coupled to a mercury manometer. The sample is then expanded into a reservoir immediately a head of the sample inlet2.Introduction of liquids: The sample is converted into gaseous state then injected by using a a. micropipet to a sintered glass disk under a layer of molten gallium,or b. by hypodermic needle injection through a silicone rubber septum.
3. Handling of solids: Solids with a very low pressure can be introduced directly into an entrance to the ion chamber on a silicon platinum Volatilised by gently heating until sufficient vapour pressure is indicated by the total ion currrent indicator.
Gas Ionization techniques Technique Means of ionization Fast atom bombardment(FAB) Impact of high velocity atoms on a sample dissolved in a liquid matrix. Secondary ion MS(SIMS) Impact of high velocity on a thin film of sample on a metal substrate (or dissolved in a liquid matrix. Impact of nuclear fission fragments., e.g. Using Plasma desorption isotope on solid sample deposited on a metal foil. Impact of high energy photon on a sample Matrix Assisted Laser Desorption /Ionization embedded in a solid organic matrix (MALDI) Imposition of high electric field gradient on Field Desorption sample deposited on a special solid support Electrospray Formation of charged liquid droplets from which ions are desolvated or desorbed
Electrostatic AccelerationSystem The positive ions formed in the ionisation chamber are withdrawn by the electric field which exists between the first accelerator plate and the second repeller plate. A strong electrostatic field between accelerator and repeller plate of 400-4000 accelerates the ions of masses m1 m2 m3... to their final velocities. The ions which escape through slit having velocities and kinetic energies give eV=1/2m1v12=1/2m2v22=1/2m3v32...... when ever the mass spectrometer is started to record the spectrum, the second accelerator is charged to an a potential of 400V
Magnetic field As the accelerated particles from the electrical field enter magnetic field, the force of magnetic field requires to move in a curved path The radius of this curvature, r, is dependent upon the mass m, the accelerating voltage, V, the electron charge, e, and the strength of the magnetic field, H. It is the two properties m/e and r upon which mass spectroscopy is based.
Ion seperators or analysers Double focussing analyser Cycloidal focussing analyser Quadrupole mass spectrometer Time of flight Radio frequency analysers
Quadrupole Mass Analyser Quadrupole Mass spectrometer, initially devised to separate uranium isotopes. Focusing of ions after accelerating from the ion source is affected by a quadrupole mass filter where they are separated according to mass and detected by an electron multiplier. The mass filter consists of a quadrant of four parallel circular tungsten rods which foce ions by means of an oscillating and variable radiofrequency filed.
Ion collector The ion beam currents are of the order of 10-15 to 10-19 ampere. The generally employed ion collectors are photographic plates, Faraday cylinders, electron multipliers and electrometers.
Vacuum system A high vacuum is to be maintained. The inlet system is generally maintained at 0.015 torr, the ion source at 10-15 torr and analyzer tube at 10-7 torr or as low as possible. Oil diffusion and mercury diffusion pumps are commonly used in different types of combination
Applications Structure elucidation Detection of impurities Quantitative analysis Drug metabolism studies Clinical, toxicological and forensic applications GC MS
Reference1. Instrumental methods of chemical analysis by Gurdeep R. Chatwal, Sham k. Anand, first edition page no 2.273-2.2852. Organic spectroscopy by William Kemp, Third edition page no 285-290.3. Spectrometric identification of organic compounds by Silverstein. Sixth edition page no 2-54. Instrumental methods of analysis by Willard, seventh edition page no -466