FTIR - Introduction
FTIR is most useful for identifying chemicals that are either organic or inorganic. It can be utilized to quantitate some components of an unknown mixture. It can be applied to the analysis of solids, liquids, and gasses. The term Fourier Transform Infrared Spectroscopy (FTIR) refers to a fairly recent development in the manner in which the data is collected and converted from an interference pattern to a spectrum. Today's FTIR instruments are computerized which makes them faster and more sensitive than the older dispersive instruments.
FTIR can be used to identify chemicals from spills, paints, polymers, coatings, drugs, and contaminants. FTIR is perhaps the most powerful tool for
identifying types of chemical bonds (functional groups). The wavelength of light absorbed is characteristic of the chemical bond as can be seen in this annotated spectrum.
By interpreting the infrared absorption spectrum, the chemical bonds in a molecule can be determined. FTIR spectra of pure compounds are generally so unique that they are like a molecular "fingerprint". While organic compounds have very rich, detailed spectra, inorganic compounds are usually much simpler. For most common materials, the spectrum of an unknown can be identified by comparison to a library of known compounds. We have several infrared spectral libraries including on-line computer libraries. To identify less common materials, IR will need to be combined with nuclear magnetic resonance, mass spectrometry, emission spectroscopy, X-ray diffraction, and/or other techniques.
Because the strength of the absorption is proportional to the concentration, FTIR can be used for some quantitative analyses. Usually these are rather simple types of tests in the concentration range of a few ppm up to the percent level. For example, EPA test methods 418.1 and 413.2 measure the C-H absorption for either petroleum or total hydrocarbons. The amount of silica trapped on an industrial hygiene filter is determined by FTIR using NIOSH method 7602.
Molecular bonds vibrate at various frequencies depending on the elements and the type of bonds. For any given bond, there are several specific
frequencies at which it can vibrate. According to quantum mechanics,
these frequencies correspond to the ground state (lowest frequency) and several excited states
(higher frequencies). One way to cause the frequency of a molecular vibration
to increase is to excite the bond by having it absorb light energy. For any
given transition between two states the light energy (determined by the wavelength)
must exactly equal the difference in the energy between the two states [usually
ground state (E0) and the first excited state
The energy corresponding to these transitions between molecular vibrational states is generally 1-10 kilocalories/mole which corresponds to the infrared portion of the electromagnetic spectrum.
E1 - E0
h c / l Where h = Planks constant c = speed of light, and l = the wavelength of light.
Samples for FTIR can be prepared in a number of ways. For liquid samples, the easiest is to place one drop of sample between two plates of sodium chloride (salt). Salt is transparent to infrared light. The drop forms a thin film between the plates. Solid samples can be milled with potassium bromide (KBr) to form a very fine powder. This powder is then compressed into a thin pellet which can be analyzed. KBr is also transparent in the IR. Alternatively, solid samples can be dissolved in a solvent such as methylene chloride, and the solution placed onto a single salt plate. The solvent is then evaporated off, leaving a thin film of the original material on the plate. This is called a cast film, and is frequently used for polymer identification.
Solutions can also be analyzed in a liquid cell. This is a small
container made from NaCl (or other IR-transparent material) which can be filled with liquid, such
as the extract for EPA 418.1 analysis. This creates a longer path length for
the sample, which leads to increased sensitivity. Sampling methods include
making a mull of a powder with a hydrocarbon oil (Nujol) or pyrolyzing insoluble
polymers and using the distilled pyrolyzate to cast a film.
Matrials can be placed
in an Attenuated Total Reflectance (ATR) cell and gases in gas cells.
For small samples or samples with surface contamination visible to the naked eye, a microscope attachment can be used to obtain reflectance or transmitance spectra on particles as small as 20 microns.
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