Infrared radiation interacts with the sample resulting to molecular vibrations such as bending, stretching, and contraction particularly of the functional groups (Hsu 249). A functional group absorbs infrared energy of specific frequency or wave number regardless of molecular structure. For instance, the carbon-oxygen stretching of the carbonyl group in various molecular types is observed at 1700cm-1 (Hsu 266). Moreover, infrared absorption of functional group is found to be consistent amidst the changes in intra-molecular structure, pressure, sampling, and temperature (Hsu 266).
Hence, the identification of a specific functional group is possibly done by correlation between the chemical structure and the wave number (Hsu 249). As infrared absorption generates molecular vibrations and rotations, these motions are detected as absorption bands. Generally, the total number of absorption bands is reduced to infrared active modes to determine the number of molecular vibrations (Hsu 251). This is done because a specific frequency causes more than one molecular vibrations.
Also, additional bands can be produced through coupling interactions, appearance of overtones, and combination of or difference in fundamental frequencies (Hsu 251). On the other hand, the intensities of difference, overtone, and combinations are always less than those of fundamental bands (Hsu 251). Then, the blending of all these factors produces an infrared spectrum unique for a specific compound (Hsu 251). Fundamental Operational Principles Figure 1. FTIR Sample Analysis Process (ThermoNicolet 5).
A glowing black-body source produces infrared energy that passes through an aperture which regulates the energy that absorbs by the sample (ThermoNicolet 5). As the beam reaches the interferometer, spectral encoding occurs resulting to interferogram which is reflected off or transmitted through the sample compartment (ThermoNicolet 5). At this point, a characteristic energy is absorbed by the sample. Then, the energy left after absorption is measured by the detector (ThermoNicolet 5).
Finally, the detected interferogram undergoes fourier transformation and printed as infrared spectrum through computer technology (ThermoNicolet 5). Meanwhile, in terms of sensitivity, solids, liquids or gaseous samples can be analyzed through FTIR in microgram level (Hsu 248). Also, impurities in as low as 1% to 0. 01% levels can be traced. Further, analysis per sample can be done form one to ten minutes depending on the type on FTIR instrument and the required resolution (Hsu 248). Transmission FTIR and HATR-FTIR
In transmission FTIR, infrared energy directly passes through the sample giving a high signal-to-noise ratio resulting to a clear transmission spectrum (Smith 87). In addition, this technique is applicable to solid, liquid, gaseous, and polymer samples and requires inexpensive sample preparation (Smith 87). However, this technique may only work best for one to twenty micron-thick samples. Samples thicker than twenty microns absorbs high amount of infrared energy while less than one micron-thin sample hardly absorb infrared energy (Smith 87).
This conditions leads to poor detection of the spectrometer. Furthermore, solvents used in sample preparation may absorb infrared radiation in the sensitive region of analysis (Grdadolnik 631). The infrared absorption of the solvent generates bands overlapped with the spectrum region of the target analyte (Grdadolnik 631). This condition then leads to spectrum region saturation. Thus, transmission FTIR necessitates laborious sample preparation which most often prone to contamination (Smith 87).
Meanwhile, in order to address the disadvantages of the transmission FTIR, Horizontal Attenuated Total Reflectance or HATR FTIR was introduced (PIKE Technologies 1). This technique has replaced KBr pellets, transmission cells, and salt plates in FTIR analysis (PIKE Technologies 1). Generally, HATR places the sample into the HATR crystal, hence, requires no preparation (PIKE Technologies 1). Works Cited Grdadolnik, Joze. “ATR-FTIR Spectroscopy: Its Advantages and Limitations. ” Acta Chimica Slovenia 49 (2002): 631-642.
Hsu, C. -P. Sherman. “Infrared Spectroscopy. ” Handbook of Instrumental Techniques for Analytical Chemistry. Ed. Settle, Frank A. New Jersey: Prentice Hall, 1997. PIKE Technologies. Multiple Reflection HATR –Maximum Sensitivity and Highly Versatile FTIR Sampling. Madison, WI: Pike Technologies, 2008. Smith, Brian C. Fundamentals of Fourier Transform Spectroscopy. United Kingdom: CRC Press, 1996. ThermoNicolet. Introduction to Fourier Transform Infrared Spectrometry. Wiscosin: Thermo Electron Business, 2001.