DEFINITION OF INFRAREDSPECTROSCOPHYThe absorption of light, as it passes through a medium, varies linearly with the distance the light travels and with concentration of the absorbing medium. Where a is the absorbance, the Greek lower-case letter epsilon is a characteristic constant for each material at a given wavelength (known as the extinction coefficient or absorption coefficient), c is concentration, and l is the length of the light path, the absorption of light may be expressed by the simple equation a= epsilon times c times l.
INFRARED SPECTROSCOPHYInfrared spectroscopy is the measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample. Mid-infrared is energetic enough to excite molecular vibrations to higher energy levels.The wavelength of infrared absorption bands is characteristic of specific types of chemical bonds, and infrared spectroscopy finds its greatest utility for identification of organic and organometallic molecules. The high selectivity of the method makes the estimation of an analyte in a complex matrix possible
THEORY OF INFRAREDABSORPTION SPECTROSCOPHYFor a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation (remember that electromagnetic radiation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule.If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration
MOLECULAR ROTATIONSRotational transitions are of little use to the spectroscopist. Rotational levels are quantized, and absorption of IR by gases yields line spectra.However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions
INTRODUCTION TO FOURIER TRANSFORM OF INFRARED SPECTROSCOPHY
WHAT IS FTIR SPECTROMETERA spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, 5 microns to 20 microns.FTIR (Fourier Transform InfraRed) spectrometer is a obtains an infrared spectra by first collecting an interferogram of a sample signal using an interferometer, then performs a Fourier Transform on the interferogram to obtain the spectrum.An interferometer is an instrument that uses the technique of superimposing (interfering) two or more waves, to detect differences between them. The FTIR spectrometer uses a Michelson interferometer.
FOURIER TRANSFORMSFourier transform defines a relationship between a signal in time domain and its representation in frequency domain.Being a transform, no information is created or lost in the process, so the original signal can be recovered from the Fourier transform and vice versa.The Fourier transform of a signal is a continuous complex valued signal capable of representing real valued or complex valued continuous time signals
SAMPLE ANALYSIS PROCESS 1. The Source: Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which controls the amount of energy presented to the sample (and, ultimately, to the detector). 2. The Interferometer: The beam enters the interferometer where the “spectral encoding” takes place. The resulting interferogram signal then exits the interferometer. 3. The Sample: The beam enters the sample compartment where it is transmitted through or reflected off of the surface of the sample, depending on the type of analysis being accomplished. This is where specific frequencies of energy, which are uniquely characteristic of the sample, are absorbed. 4. The Detector: The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal. 5. The Computer: The measured signal is digitized and sent to the computer where the Fourier transformation takes place. The final infrared spectrum is then presented to the user for interpretation and any further manipulation.
FTIR THEORYThe spectrometer described here is a modified Bomem MB-100 FTIR.The heart of the FTIR is a Michelson interferometer .The mirror moves at a fixed rate. Its position is determined accurately by counting the interference fringes of a collocated Helium-Neon laser.The Michelson interferometer splits a beam of radiation into two paths having different lengths, and then recombines them.A detector measures the intensity variations of the exit beam as a function of path difference.A monochromatic source would show a simple sine wave of intensity at the detector due to constructive and destructive interference as the path length changes.
In the general case, a superposition of wavelengths enter spectrometer, and the detector indicates the sum of the sine waves added together. shows some idealized light sources, and the interferograms that they would theoretically produce.The difference in path length for the radiation is known as the retardation d (OM = OF + d) . When the retardation is zero, the detector sees a maximum because all wavenumbers of radiation add constructively.When the retardation is l/2, the detector sees a minimum for the wavelength l. An interferogram is the sum of all of the wavenumber intensities
FTIR INSTUMENTATIONIn a conventional IR spectrophotomer, a sample IR beam is directed through the sample chamber and measured against a reference beam at each wavelength of the spectrum. The entire spectral region must be scanned slowly to produce good quality spectrum. In 5.32, we will be using a Nicolet FTIR Spectrophotometer (Nicolet was heavily involved in the design of the Hubble telescope!). IR spectroscopy has been dramatically improved by the development of the Fourier Transform method in much the same way as NMR has been revolutionized by this method.
The heart of an FTIR Spectrophotometer is a Michelson Interferometer built around the sample chamber. Radiation from an IR source is directed through the sample cell to a beam splitter. Half of the radiation is reflected from a fixed mirror while the other half is reflected from a mirror which moved continuously over a distance of about 2.5 micrometers. When the two beams are recombined at the detector, an interference pattern is produced. A single scan of the entire distance takes about 2 seconds and is stored in the computer. In order that several scans may be added, they must coincide exactly. Obviously, this would be impossible considering the thermal fluctuations and vibrations in the laboratory. In order to solve this problem, a helium-neon laser is simultaneously directed through the Michelson Interferometer and the interference pattern of the laser is used as a frequency reference. The performance of an FTIR is dramatically superior to that of conventional instruments. Generally, only a small amount of sample will produce an excellent spectrum in a fraction of the time.
PREPARATION OF SAMPLEDue to the sensitivity of the FTIR instrument, the most convenient and satisfactory method involves simple evaporation of a solution of the sample (chloroform, ether, dichloromethane; or even a CDCl3 NMR sample may be used) onto a KBr salt plate and acquisition of the spectrum from the thin film remaining. This method provides excellent spectra with flat baseline unless the thin film is too powdery in which caseexcessive scattering of the light leads to an irregular baseline. The sample may alternatively be prepared as a nujol mull (mull accessories: agate mortar and pestle, nujol and NaCl discs may be obtained from LS).
PREPARATION OF INSTRUMENTIf the instrument has just been turned on, then it is necessary to runa TEST ( F10 ) to be sure that all components are ON. If the instrument is not turned on or does not check out when the TEST is performed, then ask the instrument TA for help. In addition, it is important that N2 is flowing through the chamber so that most of the CO2 andH2O are flushed from the chamber and from inside the instrument. F4 SCAN BACKGROUND is performed with a blank IR plate in the chamber. F8 then F4 DISPLAY BACKGROUND will show the spectrum of CO2 and H2O that remain in the chamber. If the background shows excessive CO2 and H2O, then be sure the N2 is flowing briskly, wait a minute or two and try again. Once a good background has been obtained, several students in succession can use the same background.
SCANNING OF SAMPLE: Place the sample plate in the FTIR and wait for N2 to purge out the air.F5 SCAN SAMPLE. Wait until the scan and Fourier transform are completed.F8 then F1 DISPLAY SPECTRUM will automatically subtract the stored background and display the spectrum.F7 PRINT. Important: Make sure that the printer is on- line before pressing F7.Type PEAKPICK S 4000 600 to find the peaks in the spectrum. This data is printed by pressing F7 . If no one else is using the instrument next, please turn off the nitrogen purge
FTIR METHODS• EPA Method 318 - Extractive FTIR Method for Measurement of Emissions from Mineral Wool and Wool Fiberglass Industries• EPA Performance Specification 15 for Extractive FTIR CEMS in Stationary Sources• EPA Method 320 -Vapor Phase Organic and Inorganic Emissions by FTIR (extractive)• EPA Method 321 - Determination of HCl for Portland Cement Industries• EPA Protocol for Extractive FTIR for Analysis of Gas Emissions• NIOSH Method 3800 - Organic and Inorganic gases by Extractive FTIR Spectrometry
FTIR BENEFITSReal-time measurement results.• Simultaneous analysis of multiple gaseous compounds.• Measures a wide variety of volatile compounds (Inorganic and Organic).• Sensitivity from very low parts per million to high percent levels.• Provides a precise measurement method which requires no rigorous external calibration.• Speed. Measurements take only seconds
ADVANTAGES OF FTIR Some of the major advantages of FT-IR over the dispersive technique include: Speed: Because all of the frequencies are measured simultaneously, most measurements by FT-IR are made in a matter of seconds rather than several minutes. This is sometimes referred to as theFelgett Advantage.• Sensitivity: Sensitivity is dramatically improved with FT-IR for many reasons. The detectors employed are much more sensitive, the optical throughput is much higher (referred to as the Jacquinot Advantage) which results in much lower noise levels, and the fast scans enable the coaddition of several scans in order to reduce the random measurement noise to any desired level (referred to as signal averaging).• Mechanical Simplicity: The moving mirror in the interferometer is the only continuouslymoving part in the instrument. Thus, there is very little possibility of mechanical breakdown
APPLICATIONS OF FTIRIdentification of inorganic compounds and organic compounds Identification of components of an unknown mixtureAnalysis of solids, liquids, and gassesIn remote sensingIn measurement and analysis of Atmospheric Spectra - Solar irradiance at any point on earth - Longwave/terrestrial radiation spectraCan also be used on satellites to probe the space