FTIR Basics
Barbara Kanegsberg and Ed Kanegsberg, January 2006
TO TRACK DOWN A CONTAMINANT, an immediate response is: run an FTIR. FTIR
(Fourier Transform Infrared Spectroscopy), is especially useful in identifying
organic (carbon-containing) molecules. Relatively speaking, FTIR is cost-effective
in terms of labor and instrumentation, easy to perform, rapid, and provides
fairly definitive or good circumstantial identification of contaminants.
FTIR is used so routinely that non-chemists and even chemists tend to
forget the basic principles.
Spectroscopy is the study of chemical composition by observation of the
interaction of molecules with electromagnetic radiation. Infrared
(IR) spectroscopy uses
light of about 2,500 to 16,000 nm, light that is of longer wavelengths,
beyond the visible range. Some spectroscopic techniques analyze radiation
emitted
by the molecule— infrared spectroscopy is an absorption technique.
The Right Bonds
FTIR is useful for identifying organic contamination because organic
molecules are held together by covalent bonds. In contrast with ionic
bounds (as
in salts), covalent bonds are not rigid. Instead, they move or vibrate
in a
characteristic manner such as stretching or twisting. Particular portions
of molecules absorb infrared radiation at specific frequencies. A plot
of per cent of transmitted IR light on the y axis and the wavelength
or the
wave number [1], on the x axis, yields a series of valleys. The specific
pattern of absorption can be related back to the molecular structure.
Detective Work
To identify a molecule by infrared spectroscopy, the chemist engages
in detective work, or what might be termed “chemical profiling.” The complex
chemical IR fingerprint is interpreted by comparison with an infrared library
of patterns of specific molecules (a kind of “mug shot” book).
For example, it is relatively simple to distinguish ketones (containing carbon
double-bonded to oxygen, like acetone or MEK) from amines (containing carbon
and nitrogen). Ketones show a strong absorption peak with a wave number at
1705 cm-1 to 1725 cm-1. Amines have a have a characteristic absorption pattern
at 3300 cm-1 to 3500 cm-1. By looking at other wavelengths, it is possible
to distinguish various ketones, such as acetone versus methyl ethyl ketone.
In the era prior to computerized libraries, the analyst had to peer
at the scan, and then look through books of scans in an attempt to
find
a match.
In such a situation, it was easy to think of oneself as an ancient
Greek soothsayer, rummaging through the entrails of animals in an
attempt to
divine the future.
Data Handling
The popularity of FTIR is based not only on the physical measurement
but on efficient data management. Specifically, Fourier Transform
(FT) and
computerized evaluation relative to standard IR libraries allow
reliable and routine analysis.
Fourier Transform, a mathematical technique for converting a signal
in the time domain into the frequency domain, enhances IR measurements
by
providing
lightning speed and high sensitivity. A short pulse of broad-spectrum
IR radiation is applied to the sample; the Fourier Transform analysis
breaks
the response down to individual frequencies. It is like taking
a recording of a violin chord and analyzing what frequencies, including
harmonics,
are present and at what amplitudes. Computerized IR libraries eliminate
much
of the analytical variability in compound identification.
A Silver Bullet?
Reasonable analytical skills and a modicum of common sense are
needed to take full advantage of the power of FTIR. It is preferable
to
have a pure
compound to obtain clear identification of the contaminant. Sometimes,
time-consuming extractions are necessary. With a standard library,
and a partial knowledge
of the contents of the extract being analyzed, it is sometimes
possible to use FTIR to identify contaminants in a mixture.
References:
1 Wave number (wavelengths per centimeter) rather than wavelength
is typically used. For example, visible light of 5000 Angstroms
corresponds to a wave
number of 20,000 cm-1. Wave number is frequency in a spatial
dimension rather than in time. J. D. Roberts, M.C. Caserio. “Spectroscopy of Organic
Molecules” in Basic Principles of Organic Chemistry, W.A. Benjamin,
Inc., (1965) P. 27.
Introduction to Spectroscopy, University of Michigan
http: //www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm
Barbara Kanegsberg and Ed Kanegsberg
are independent consultants in critical cleaning, precision cleaning, surface
preparation, and contamination control. They are the editors of “Handbook
for Critical Cleaning,” CRC Press. Contact them at BFK Solutions
LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
