Modern Ways to Identify Molecules

This is Analytical Chemistry!

Mass Spectrometry MS
Mass spectrometry means breaking a compound into little pieces then measuring the weights of the pieces to figure out what that compound is. The first step is called ionization, this means zapping the compound with electrons, hot gas, or a laser to make it jump up into a vacuum tube. The next step is detection, this is done by measuring how fast the fragments are moving or how far they are deflected by an electromagnet.


Nuclear Magnetic Resonance NMR
Protons and electrons both have what physicists call spin. If you put a chemical into a really strong magnet all of the protons will spin in the same direction. If you then shoot it with radio waves, you can get all of the protons to flip sideways. If you measure how fast the protons flip back to spinning in their original direction, then do some fancy math called a Fourier transform, you get an NMR spectrum. The NMR spectrum tells you how deshielded the protons are. In other words, it tells you if there are any functional groups in the molecule that withdraw electrons. Groups that deshield a proton by pulling electrons away from it have different strengths. You can tell what they are by how strongly they shift the proton signals to the left.
NMR can be done with any nucleus that has an odd number of protons. Hydrogen NMR is called 1H NMR. The second most common type of NMR is Carbon 13 NMR, which is also called 13C NMR. Since 13C is not the most common isotope of carbon, it takes a long time to get a good carbon spectrum.
When you go to the hospital to get an MRI, what the machine is actually doing is running a 1H NMR spectrum of your whole body.

Fourier Transform Infrared Spectroscopy FTIR
Raman Infrared Spectroscopy
UV-Vis Spectroscopy UV-VIS
By measuring how much light a chemical absorbs at many different wavelengths, that chemical can be identified. Different chemical bonds vibrate at different frequencies. By measuring how much light a chemical absorbs at each wavelength gives information about what types of chemical bonds are present in the molecule.

X-ray Diffraction
First, the scientist grows a crystal. Then she puts the crystal into a machine called an x-ray diffractometer. A beam of x-rays is shot through the crystal. This creates a diffraction pattern. Computers can be used to do some fancy math called a Fourier transform to turn a diffraction pattern into a 3D picture of the crystal.

Immunochemistry
Antibodies are y shaped proteins that will stick strongly to one and only one molecule. These are often used to identify other proteins. ELISA, dot blots, and western blots, are all forms of immunochemical detection. Biochemists often use these methods.

Atomic Absorption
A special machine burns a tiny amount of the chemical then measures exactly wavelengths of light are given off by the flame.

Old Fashioned Ways To Identify Molecules

Thin Layer Chromatography TLC
This is a low tech, old school, way to identify compounds and check their purity. A spot of the compound is put onto some special paper. The bottom of the paper is then dipped into a solvent. Different compounds will travel up the paper at different speeds. If more than one spot shows up, the compound is not pure. The distance the spot traveled divided by the distance the solvent traveled is called the Rf value.
This is an old method, but scientists still use it all the time because it is very fast and cheap. It is often done to see if a chemical reaction is over. If the spot that corresponds to the starting material is still there, then the reaction is not over.

Index of Refraction
How fast light travels in a substance is the index of refraction. The index of refraction is also how far light will bend to the side when it passes through that substance. By coating a glass surface with some of the substance, then measuring the angle that the light passing through that substance bends, you can measure its

Melting Point
The melting point of chemicals is measured by putting them in a tiny sealed glass tube called a capillary. If the substance is not pure, it will melt at a lower temperature. These days, a fancy modern machine called a differential scanning calorimeter is also used to measure melting points. There are also machines that will record a little video clip of the chemical melting in the glass tube.

Specific Gravity
The density of a substance is another clue to the identity. First weigh it, then divide the mass by the volume. Another name for density is specific gravity.

Flame Test
Some elements will turn a flame different colors when burned. By burning a small amount of a chemical, you can get an idea of what elements are in it. This is basically a ghetto version of atomic absorption spectroscopy.

Smell
Smelling an unknown chemical can be very dangerous, but some chemicals are easily identifiable by their smells. Chemicals with a thiol functional group, that means an SH, are very stinky. So are amines and most aldehydes. If you know what a thiol smells like, you can identify them instantly.