Mass spectroscopy complements the information made available by other spectroscopic techniques. However, it is different in several respects from other optical spectroscopic methods of analysis and such differences will be highlighted in the present article.
Optical Spectroscopic Techniques
Optical Spectroscopic techniques provide qualitative and quantitative information on light absorbing or emitting species present in samples. The basis of such spectroscopic techniques is interaction of electromagnetic radiation with matter. The interactions are specific for each species and vary with the wavelength of the incident electromagnetic radiation. Useful details are, however, provided only at wavelengths characteristic for each molecule or ionic species. Before proceeding to Mass spectroscopic techniques it would not be out of place to make a brief mention of the typical information provided by some popular optical spectroscopic techniques:
UV – VIS Spectroscopy
Qualitative and quantitative estimation of molecules containing chromophoric groups and those having different degrees of conjugation. Trace metals quantification through metal-ligand complex reactions is another major application.
FT – IR Spectroscopy
Identification and structural conformation of compounds having different functional groups. By and large it is mainly used for qualitative conformation of components for material characterization.
Emission intensity measurements are recorded at a wavelength different from the excitation wavelength. Light of higher wavelength is emitted by the sample after absorption after a time gap on exposure to incident light of lower wavelength.
Atomic Absorption Spectroscopy
Quantitative determination of trace metals in different sample matrices by absorption of light by atoms in ground state in the atomic flame or graphite furnace at specific wavelengths generated by same metals present in the light source.
ICP – OES
Emission of light at specific wavelength by sample elements introduced in the high temperature plasma source
Mass Spectroscopy is distinctly different from optical spectroscopy and it provides more details on the sample molecules.
The sample molecules in the vapour phase are ionized by impact with high energy electrons. The charged molecules on passing through magnetic and/or electrostatic field are deflected along a circular path in relation to their mass to charge ratio (m/z). Ionization by displacement of a single electron results in a molecular ion. Due to its instability the molecular ion can disintegrate to even smaller mass fragments. The deflected ionic beams get accelerated along different circular paths according to the m/z ratios of the ionized species. Scanning of the magnetic and or electrostatic field will result in different paths for different fragmented ions which will reach the detector at different points and generate detector signals.
The main differences of mass spectroscopic from optical spectroscopic techniques are:
- Molecular species are characterized by their mass to charge ratios whereas the wavelengths of absorption and emission characterize molecular or ionic species in optical techniques.
- Accurate determination of molecular weight of the molecule or ionic mass of ions of interest in addition to their identification.
- Structural elucidation and molecular formula of a compound
- Isotopic abundance of different isotopes constituting the molecule
It can be seen that Mass Spectroscopy provides a wealth of information which is not possible from any single spectroscopic technique. The scope of applications is further extended through hyphenation with other techniques such as GC, LC or FT-IR.