Abstract

Interpretation of Mass Spectra

R Srinivas, Chief Scientist & HOD, Indian Institute of Chemical Technology

Mass Spectrometry has been established as a powerful analytical technique for structure elucidation apart from determination of molecular weight of organic and bio molecules. Both qualitative and quantitative analysis can be performed using mass spectrometry. It is a complementary tool for other spectroscopic techniques such as UV, IR and NMR. It is also useful for determination of fundamental properties of molecules in the gas phase. Among several ionization techniques, only electron ionization (EI) provides sufficient structural information and this technique is limited to volatile, semi volatile and thermally stable molecules of molecular weights less than 1000 u. Compounds which do not give molecular weight information can be analyzed using softer ionization techniques such as chemical ionization (CI), fast atom bombardment ionization (liquid secondary ion mass spectrometry), atmospheric pressure ionization (ESI, APCI, APPI) and matrix assisted laser desorption ionization techniques. These ionization methods provide mass spectra which contain mainly [M+H]+ ion without much fragmentation. Then it becomes essential to perform MS/MS experiments or tandem mass spectrometry of protonated molecules to derive structural information. Tandem mass spectrometry mainly involves two mass spectrometry systems; the first system (MS-1) performs the mass selection of a desired target ion from a stream of ions produced in the ion source. This mass-selected ion undergoes either unimolecular fragmentation or collision induced dissociation in the intermediate region to produce product ions. The second MS system (MS-2) performs the mass analysis of the product ions that are formed in the intermediate step. In the present lecture, some basic aspects of mass spectral interpretation including general fragmentation behavior of various types of organic and bio molecules will be discussed.