Table of contents
Below you will discover a detailed review of the physical components of a Fourier Transform Infrared (FTIR) Spectrometer. This module focuses on the physical equipments/components which make up the instrument, and not the mathematical aspects of analyzing the resulting data. For the mathematical treatment of FTIR data please see FTIR: Computational.
The history of the FTIR is a twisted and somewhat confusing tale, involving the development of technology, math, and materials. The beginnings of the first commercial FTIR spectrometer have been attributed to the work of M.J. Block and his research team in the small company 'Digilab'. Block's personal memoirs of the experience are both interesting and entertaining, involving highly classified information, money laundering, and fraud charges (follow the link if you wish to discover for yourself http://s-a-s.org/epstein/block/index.htm ). Otherwise let it be enough to say that once the FTIR spectrometer was developed, its impact on the scientific community was paramount. Suddenly it was possible to acquire extremely accurate data in a much shorter amount of time than with traditional IR, as well as allowing for the analysis of exceedingly dilute samples. The device itself is surprisingly simple, with only one moving part. It’s no surprise that the instrument has been growing in popularity ever since its introduction, finding applications in chemistry, biology, materials science, process engineering, pharmaceutical science, and many other professions. FTIR instruments are relatively inexpensive, sturdy, stable, flexible, and fast. Through the years, this instrument have steadily evolved and new applications is continually being developed. Expanded computer power, the trend towards miniaturization, and more sophisticated imaging have all inspired some important new innovations.
FTIR measurements are conducted in the time domain. This is accomplished by directing the radiation from a broadband IR source to a beam splitter, which divides the light into two optical paths. Mirrors in the paths reflect the light back to the beam splitter, where the two beams recombine, and this modulated beam passes through the sample and hits the detector. In a typical interferometer, one mirror remains fixed, and the other retreats from the beam splitter at a constant speed. As the mirror moves, the beams go in and out of phase with each other, which generates a repeating interference pattern—a plot of intensity versus optical path difference—called an interferogram. The interferogram can be converted into the frequency domain via a Fourier transform, which yields the familiar single beam spectrum. The resolution of this spectrum is determined by the distance that the moving mirror traveled. Analyses generally fall into three categories, which are determined by the wavelengths of the radiation. Midrange IR covers the wavenumbers 1400 nm–3000 nm, where strong absorptions from fundamental molecular vibrations are measured. Near-IR (NIR) ranges from 700 nm–1400 nm. Far IR ranges from 3000 nm–1 mm.
Sources of Infrared Radiation
Infrared radiation is a relatively low energy light. All physical objects give of infrared radiation, the wavelength of which is dependent upon the temperature of the object. This phenomenon is known as black body radiation. The ideal IR source would emit radiation across the entire IR spectrum. As this is very difficult, a good compromise is a source which emits continuous mid-infrared radiation. Thankfully this can be achieved by most high temperature black bodies. Black body radiation was studied in depth by Max Planck, and it is through his equations that that the spectral energy density at a given wave number from a blackbody source of a given temperature can be calculated. Not to mention he was the discoverer of the properties of energy quanta. For this Max Planck received the 1918 Nobel Prize in Physics in recognition of the services he rendered to the advancement of Science. Now take a moment to examine the plot of energy density vs. max wave length below.
At first glance it would seem that the source temperature should be as high as possible to maximize the results—this is rarely the case. For example consider a typical incandescent light bulb. The tungsten filament glows at a temperature of 3000k, which would emit massive amounts of IR. The bulb portion of a light bulb is responsible for their lack of use as an IR source. The Bulb is made of glass which seals the tungsten filament in a vacuum. The vacuum is necessary to keep the tungsten from oxidizing at such high temperature, but the glass serves as an IR absorber, blocking its path to the sample. Any source we choose must be in direct contact with the atmosphere, because of this there are drastic limits on the temperature that we may operate an IR source.
There are several other limiting facts that require consideration when choosing an IR source. The material should be thermodynamically stable; otherwise it would quickly break down and need replacing. This would obviously be an expensive and undesired approach. There is also the possibility that the source may produce an excess of IR radiation. This would saturate the detector and possibly over load the analog-to-digital converter.
2. Silicon Carbide Rod (Globar)
The most ubiquitous IR source used in FTIR is a resistively heated silicon carbide rod (see image below). This device is commonly and somewhat simply referred to as a Globar.
An electric current is passed through the bar which become very hot, prducing large amounts of IR raidiation.
A Globar can reach temperatures of 1300K, and in the past required water cooling to keep from damaging the electrical components. Advances in ceramic metal alloys have lead to the production of Globars that no longer require water cooling. However these newer Globars are typically not operated at as high a temperature as 1300K.
3. Nichrome and Kanthanl wire Coils
Nichrome and Kanthanl wire coils where also once popular IR sources. They too did not require water cooling, ran at lower temperatures than a Globar, and possessed lower emissivity.
4. Nerst Glowers
Nernst Glowers are an IR source that is capable of hotter temperatures than a Globar. Nernst Glowers are fabricated from a mixture of refractory oxides. Despite being capable of higher temperature than a globar, the Nernst Glower is not capable of producing IR radiation above 2000 cm-1. As long as the frequency of IR needing to be examined is below 2000 cm-1 The Nernst Glower is an exceptional IR source, but if the entire mid IR range is necessary then using a Nernst Glower would result in low signal to noise ratios.
5. CarbonArcs (an unsuitable IR source)
It should be noted that the carbon IR sources used in many spectrometers today, similar to the Globar discussed above are different then the carbon arcs that you may be familiar with. A carbon arc occurs when an electrical discharge occurs between to carbon electrodes. These sparks are incredibly bright reaching temperature as hot at 6000 K. IR sources capitalizing on the large IR output of these arcs have ultimately shown to possess more draw backs than advantages. Because the carbon electrodes are consumed in the arcing process it would be necessary to continuously feed new rod forward to maintain the arc. The rods would also require an inert atmosphere to avoid combustion of the carbon. These limiting features and add complication of carbon archs makes them unfit as IR sources.
The creation of today’s FTIR would not have been possible had it not been for the existence of the Michelson interferometer. This essential piece of optical equipment was invented by Albert Abraham Michelson. He received the Nobel Prize in 1907 for his accurate measurements of the wavelengths of light. His Nobel winning experiments were made possible by his invention of the interferometer. Albert Michelson was in fact the first member of the United States of America to receive the Nobel Prize, solidifying the U.S as a world leader in science. Michelson did not invent the interferometer to perform infrared spectroscopy; in fact his experiments had nothing to do with any kind of spectroscopy. Michelson’s goal was to discover evidence for luminiferous aether, the material once believed to permeate the universe allowing for the propagation of light waves. Of course it is now known that no such aether exists and that light is capable of propagating in vacuum. For more information on the extraordinary achievement of Michelson and his invention of the interferometer go to http://en.wikipedia.org/wiki/Michels...ley_experiment.
Above is a digram of the basic concepts and components of a Michelson Interferometer.
Each portion of the aboved digram will be discussed in turn and with further detail.
1. Beam Splitter
The beam splitter is made of a special material that transmits half of the radiation striking it and reflects the other half. Radiation from the source strikes the beam splitter and separates into two beams. One beam is transmitted through the beam splitter to the fixed mirror and the second is reflected off the beam splitter to the moving mirror. The fixed and moving mirrors reflect the radiation back to the beamsplitter. Again, half of this reflected radiation is transmitted and half is reflected at the beam splitter, resulting in one beam passing to the detector and the second back to the source.
2. Stationary Mirror
The stationary mirror in an FTIR interferometer is nothing more than a flat highly reflective surface.
3. Moving Mirror
The beauty of the FTIR spectrometers design lies in its simplicity. There is present only one moving part in an FTIR spectrometer, its oscillating mirror. Air bearings are used in FTIR spectrometers because of the higher speed that the oscillating mirror is required to move at. The air bearings eliminate friction that would inevitable cause the moving parts of the mirror to break down, as is the case for the mechanical bearings. The air bearing has nearly replaced the mechanical bearing in all modern FTIR spectrometers. The older mechanical bearings required expensive ruby ball bearings, as they were the only material strong enough to endure the high physical demands of oscillating once every millisecond.
Infrared detectors are classified into two categories; thermal, and quantum models. A thermal detector uses the energy of the infrared beam as heat, while the quantum mechanical detector uses the IR beam as light and provides for a more sensitive detector.
A thermal detector operates by detecting the changes in temperature of an absorbing material. Their output may be in the form of an electromotive force (thermocouples), a change in resistance of a conductor (bolometer) or semiconductor (thermistor bolometer), or the movement of a diaphragm caused by the expansion of a gas (pneumatic detector). There exist major limitations to these forms of IR detectors. Their response time is much slower (several milliseconds) than the vibrational frequency of the oscillating mirror in FTIR. The mirror is moving with a frequency of approximately 1.25 kHz, there for the response time for an IR detector employed in FTIR must have a response time of less than 1ms. A response time of less than one millisecond is obtainable with cryogenically cooled thermo detectors. These detectors are commonly too expensive to be desired over other forms of detectors.
There is one kind of thermo detector that is both inexpensive and possesses a response time fast enough to be appropriate, as well as the additional benefit of operating at room temperature. This detector is the Pyroelectric bolometer detector. These detectors incorporate as their heat sensing element ferroelectric materials that exhibit a large spontaneous electrical polarization at temperatures below their curie point. If the temperature of the ferroelectric material is changed the degree of polarization also changes causing an electric current. Pyroelectric bolometer is based on a Pyroelectric crystal (usually LiTaO3 or PZT) covered by absorbing layer (silver or silver blackened with carbon)
Quantum Well Detector
Because of their higher sensitivity, and faster response times, quantum well detectors are much more ubiquitous to FTIR. The detection mechanism of Quantum Well Infrared Photodetector (QWIP) involves photoexcitation of electrons between ground and first excited states of single or a multiquantum well structure. The parameters are designed so that these photo excited carriers can escape from the well and be collected as photocurrent. These quantum wells can be realized by placing thin layers of two different high bandgap semiconductor materials alternately where the bandgap discontinuity creates potential wells associated with conduction bands and valence bands. When IR photons strick these materials they induce a current that is then transformed into a digital signal via a analog digital converter.
These detector work more effectively (increased sensitivity) when at lower temperature. This is in part due to the higher degree of instrumental noise associated with a higher thermal back ground. Today there are available a wide range of these photo detecting diodes that do not require cooling. The finer details of the detector are numerous and dependent on the parameters of the equipment, there for beyond the scope of this module.
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