TYPES OF PEAKS IN MASS SPECTROSCOPY.pptx

Types of peaks in mass spectroscopy.
Molecular ion or parent peak.
base peak.
fragment ions.
rearrangement ion.
multiple charged ion.
negative ion.
metastable ion.
isotopes ion.

1. SUBMITTED BY : ANUPAMA.C.P. 1 SEM M-PHARM TYPES OF PEAKS IN MASS SPECTROSCOPY 1

2. MOLECULAR ION OR PARENT PEAK When a sample substances is bombarded with electrons of energies of 9 to15eV, the molecular ion is produced by loss of a single electron, this will give rise to a very simple mass spectrum with essentially all of the ions appearing in one peak called Parent peak. M + ê M + + 2ê M + Molecular ion

3. This M+ ion is not fragmented before collection at the detector and is recorded at M/e value corresponding to the molecular weight of material. Molecular ion peak is usually the peak of highest mass number. The relative height of the peak decreases in the following order aromatic > conjugated olefins > allicyclic > sulphide If a molecule yields the parent peak due to molecular ion ,exact molecular weight can be calculated.

4. BASE PEAK If an electron beam of energy of 70eV is used in a mass spectrometer, the molecular ion is produced by the loss of single electron which undergoes splitting to form many fragments, the parent peak in mass spectrum is called base spectrum. The largest peak in MS or most intense peak in the spectrum whose intensity taken as 100% Sometimes M + ion peak may be base peak . e.g. Toluene

5. FRAGMENT IONS When potential energy is given to the M+ ion during electron impact, further cleavage takes place and ions of lower mass number ie , fragment ions are produced. Many peaks in mass spectrometer are due to fragmentation . These peaks in spectrum give valuble information regarding molecular structure. Fragmentation decreases in fall order branched hydrocarbon > cooH > esters

6. REARRANGEMENT ION In some cases, fragments are observed which are not a part of the original molecule. These are known as rearrangement ions which are formed from the molecular ion by redistribution of atoms or groups of atoms at the moment of decomposition of the molecular ion. Hydrogen transfer rearrangements are very common . In hydrocarbons , the rearrangement is non specific and unpredictable.

7. MULTIPLE CHARGED ION In mass spectrometer, the ions are generally carrying a single positive charge, however sometimes doubly or even triply charged ion are found in the mass spectrum. The formation of these multiple charged ions are common in hetero aromatic atom.

8. NEGATIVE IONS In addition to positive ions negative ions may be formed from electron bombardment of the sample. The formation of negative ions is very rare but these can be produced in three ways. AB + e A + B - AB + e AB - AB +e A + + B - + ē These are not very useful in structural determinations. eg ,. O - , OH -

9. METASTABLE IONS The ions resulting from the decomposition between the source region and magnetic analyser are called metastable ion which appear in the spectrum as broad peaks. The peaks are weaker in intensity but are useful in studying the mechanism of fragmentation. These are not used for the study of structure . The relationship between the apparent m/e of the metastable ion and its parent is given by the following formulation.

10. m 1 + m 2 + + m The metastable ion is observed at a mass m* which is related to m 1 and m 2 by the equation. m* = m 2 2 m 1 m 1= mass of parent ion m 2 = mass metastable ion m o = mass of neutral fragment

11. ISOTOPES IONS In organic compounds, there are generally a small peak appear at one mass unit higher than the parent peak (M+1), Due to presence of C, H, N, O, S Isotopes. If the same sample contains two heavy isotopes like Cl , Br then additional smaller peaks occur at M+2.

12. REFERENCES Instumental method of chemical analysis by Gurudeep.R . Chatwal : 2012: 5 th edition pg.no . 2.285-2.287. Elementary organic spectroscopy principles and chemical applications by Y.R. Sharma:4 th edition ,2009 p.g.no. 288.