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 usedso 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 emittedby 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 , on the x axis, yields a series of valleys. The specificpattern of absorption can be related back to the molecular structure.
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 possibleto 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.
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 muchof 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.
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
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and 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; firstname.lastname@example.org; www.bfksolutions.com.