Chromatography and Mass Spectroscopy separate molecules according to their size and/or mass and in the process display a spectrum showing the distribution of different molecular species in a compound or mixture.
In Chromatography the substance to be analyzed is a fluid (liquid or gas) that moves through a porous medium designed so that the larger the molecule the more (OR – in GPC – the less) it is impeded by the medium. Multiple interactions like this over the length of the analysis cell cause the originally homogeneous mixture to spread out into a flow of separated molecular components ordered by size / mass.
In Mass Spectroscopy, the process is similar, but the sample is ionized and the ions are separated by electrical or magnetic fields. The resulting spectrum is very high resolution and can distinguish between compounds of the same unit molecular mass (e.g., hexane – C6H14 and diethyl ketone – C2H5COC2H5, which both have mass 86 Da).
Infrared Spectroscopy involves the interaction of Infrared (IR) light with the molecules in the sample. The sample can be solid, liquid or gas. It is a technique mostly based on absorption Spectroscopy. The energy of the infrared photons induces vibrational excitation of covalently bonded atoms and groups. The different vibrational modes of the molecules (bending, stretching, scissoring, rocking and twisting) are characteristic and unique. Infrared Spectroscopy is wildly used to identify organic compounds and unknown polymers. (used a lot in forensic studies & reverse engineering materials). One primary selection rule that influences the infrared absorptions, is that a, change in dipole moment should occur for a vibration to absorb infrared energy. Absorption bands associated with C=O bond stretching are usually very strong because a large change in the dipole takes place in that mode. On the other hand molecular symmetry & spectrometer limitations decrease the intensity of the IR peaks.