Instrumental Techniques for Analytical Chemistry

Introduction
Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and
inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a sample
positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the
chemical functional groups in the sample. Different functional groups absorb characteristic frequencies
of IR radiation. Using various sampling accessories, IR spectrometers can accept a wide range of sample
types such as gases, liquids, and solids. Thus, IR spectroscopy is an important and popular tool for
structural elucidation and compound identification.

IR Frequency Range and Spectrum Presentation
Infrared radiation spans a section of the electromagnetic spectrum having wavenumbers from roughly
13,000 to 10 cm–1, or wavelengths from 0.78 to 1000 μm. It is bound by the red end of the visible region
at high frequencies and the microwave region at low frequencies.
IR absorption positions are generally presented as either wavenumbers ( ) or wavelengths (l).
Wavenumber defines the number of waves per unit length. Thus, wavenumbers are directly proportional
to frequency, as well as the energy of the IR absorption. The wavenumber unit (cm–1, reciprocal centimeter)
is more commonly used in modern IR instruments that are linear in the cm–1 scale. In the
contrast, wavelengths are inversely proportional to frequencies and their associated energy. At present,
the recommended unit of wavelength is μm (micrometers), but μ (micron) is used in some older literature.
Wavenumbers and wavelengths can be interconverted using the following equation:
n in cm–1 ( ) 1
l (in μm)
------------------------ 104
(15.1)
IR absorption information is generally presented in the form of a spectrum with wavelength or
wavenumber as the x-axis and absorption intensity or percent transmittance as the y-axis (Fig. 15.1).

INTERPRETING AN INFRA-RED SPECTRUM

This page explains how to use an infra-red spectrum to identify the presence of a few simple bonds in organic compounds.
 
The infra-red spectrum for a simple carboxylic acid
Ethanoic acid
Ethanoic acid has the structure:

You will see that it contains the following bonds:

carbon-oxygen double, C=O carbon-oxygen single, C-O oxygen-hydrogen, O-H carbon-hydrogen, C-H carbon-carbon single, C-C

The carbon-carbon bond has absorptions which occur over a wide range of wavenumbers in the fingerprint region - that makes it very difficult to pick out on an infra-red spectrum.
The carbon-oxygen single bond also has an absorbtion in the fingerprint region, varying between 1000 and 1300 cm^-1 depending on the molecule it is in. You have to be very wary about picking out a particular trough as being due to a C-O bond. The other bonds in ethanoic acid have easily recognised absorptions outside the fingerprint region.
The C-H bond (where the hydrogen is attached to a carbon which is singly-bonded to everything else) absorbs somewhere in the range from 2853 - 2962 cm^-1. Because that bond is present in most organic compounds, that's not terribly useful! What it means is that you can ignore a trough just under 3000 cm^-1, because that is probably just due to C-H bonds.
The carbon-oxygen double bond, C=O, is one of the really useful absorptions, found in the range 1680 - 1750 cm^-1. Its position varies slightly depending on what sort of compound it is in.
The other really useful bond is the O-H bond. This absorbs differently depending on its environment. It is easily recognised in an acid because it produces a very broad trough in the range 2500 - 3300 cm^-1. The infra-red spectrum for ethanoic acid looks like this:

The possible absorption due to the C-O single bond is queried because it lies in the fingerprint region. You couldn't be sure that this trough wasn't caused by something else. The infra-red spectrum for an alcohol
Ethanol

The O-H bond in an alcohol absorbs at a higher wavenumber than it does in an acid - somewhere between 3230 - 3550 cm^-1. In fact this absorption would be at a higher number still if the alcohol isn't hydrogen bonded - for example, in the gas state. All the infra-red spectra on this page are from liquids - so that possibility will never apply.
Notice the absorption due to the C-H bonds just under 3000 cm^-1, and also the troughs between 1000 and 1100 cm^-1 - one of which will be due to the C-O bond.

The infra-red spectrum for an ester
Ethyl ethanoate

This time the O-H absorption is missing completely. Don't confuse it with the C-H trough fractionally less than 3000 cm^-1. The presence of the C=O double bond is seen at about 1740 cm^-1.
The C-O single bond is the absorption at about 1240 cm^-1. Whether or not you could pick that out would depend on the detail given by the table of data which you get in your exam, because C-O single bonds vary anywhere between 1000 and 1300 cm^-1 depending on what sort of compound they are in. Some tables of data fine it down, so that they will tell you that an absorption from 1230 - 1250 is the C-O bond in an ethanoate.
The infra-red spectrum for a ketone
Propanone

You will find that this is very similar to the infra-red spectrum for ethyl ethanoate, an ester. Again, there is no trough due to the O-H bond, and again there is a marked absorption at about 1700 cm^-1 due to the C=O.
Confusingly, there are also absorptions which look as if they might be due to C-O single bonds - which, of course, aren't present in propanone. This reinforces the care you have to take in trying to identify any absorptions in the fingerprint region.
Aldehydes will have similar infra-red spectra to ketones.
The infra-red spectrum for a hydroxy-acid
2-hydroxypropanoic acid (lactic acid)

This is interesting because it contains two different sorts of O-H bond - the one in the acid and the simple "alcohol" type in the chain attached to the -COOH group.
The O-H bond in the acid group absorbs between 2500 and 3300, the one in the chain between 3230 and 3550 cm^-1. Taken together, that gives this immense trough covering the whole range from 2500 to 3550 cm^-1. Lost in that trough as well will be absorptions due to the C-H bonds.
Notice also the presence of the strong C=O absorption at about 1730 cm^-1.
The infra-red spectrum for a primary amine
1-aminobutane

Primary amines contain the -NH₂ group, and so have N-H bonds. These absorb somewhere between 3100 and 3500 cm^-1. That double trough (typical of primary amines) can be seen clearly on the spectrum to the left of the C-H absorptions.

Infra Red Waves

Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. Infrared light has a range of wavelengths, just like visible light has wavelengths that range from red light to violet. "Near infrared" light is closest in wavelength to visible light and "far infrared" is closer to the microwave region of the electromagnetic spectrum. The longer, far infrared wavelengths are about the size of a pin head and the shorter, near infrared ones are the size of cells, or are microscopic.
Far infrared waves are thermal. In other words, we experience this type of infrared radiation every day in the form of heat! The heat that we feel from sunlight, a fire, a radiator or a warm sidewalk is infrared. The temperature-sensitive nerve endings in our skin can detect the difference between inside body temperature and outside skin temperature.

Shorter, near infrared waves are not hot at all - in fact you cannot even feel them. These shorter wavelengths are the ones used by your TV's remote control.

How can we "see" using the Infrared?

Since the primary source of infrared radiation is heat or thermal radiation, any object which has a temperature radiates in the infrared. Even objects that we think of as being very cold, such as an ice cube, emit infrared. When an object is not quite hot enough to radiate visible light, it will emit most of its energy in the infrared. For example, hot charcoal may not give off light but it does emit infrared radiation which we feel as heat. The warmer the object, the more infrared radiation it emits.
Humans, at normal body temperature, radiate most strongly in the infrared at a wavelength of about 10 microns. (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter.) This image ( which is courtesy of the Infrared Processing and Analysis Center at CalTech), shows a man holding up a lighted match! Which parts of this image do you think have the warmest temperature? How does the temperature of this man's glasses compare to the temperature of his hand?

To make infrared pictures like the one above, we can use special cameras and film that detect differences in temperature, and then assign different brightnesses or false colors to them. This provides a picture that our eyes can interpret. The image at the left (courtesy of SE-IR Corporation, Goleta, CA) shows a cat in the infrared. The orange areas are the warmest and the white-blue areas are the coldest. This image gives us a different view of a familiar animal as well as information that we could not get from a visible light picture.
Humans may not be able to see infrared light, but did you know that snakes in the pit viper family, like rattlesnakes, have sensory "pits", which are used to image infrared light? This allows the snake to detect warm blooded animals, even in dark burrows! Snakes with 2 sensory pits are even thought to have some depth perception in the infrared! (Thanks to NASA's Infrared Processing and Analysis Center for help with the text in this section.)
Many things besides people and animals emit infrared light - the Earth, the Sun, and far away things like stars and galaxies do also! For a view from Earth orbit, whether we are looking out into space or down at Earth, we can use instruments on board satellites.

Satellites like GOES 6 and Landsat 7 look at the Earth. Special sensors, like those aboard the Landsat 7 satellite, record data about the amount of infrared light reflected or emitted from the Earth's surface.

Other satellites, like the Infrared Astronomy Satellite (IRAS) look up into space and measure the infrared light coming from things like large clouds of dust and gas, stars, and galaxies

What does the Infrared show us?

This is an infrared image of the Earth taken by the GOES 6 satellite in 1986. A scientist used temperatures to determine which parts of the image were from clouds and which were land and sea. Based on these temperature differences, he colored each separately using 256 colors, giving the image a realistic appearance. Why use the infrared to image the Earth? While it is easier to distinguish clouds from land in the visible range, there is more detail in the clouds in the infrared. This is great for studying cloud structure. For instance, note that darker clouds are warmer, while lighter clouds are cooler. Southeast of the Galapagos, just west of the coast of South America, there is a place where you can distinctly see multiple layers of clouds, with the warmer clouds at lower altitudes, closer to the ocean that's warming them.

Space Science and Engineering Center, University of Wisconsin-Madison, Richard Kohrs, designe. 
We know, from looking at an infrared image of a cat, that many things emit infrared light. But many things also reflect infrared light, particularly near infrared light. Near infrared radiation is not related to the temperature of the object being photographed - unless the object is very, very hot.
Infrared film 'sees' the object because the Sun (or some other light source) shines infrared light on it and it is reflected or absorbed by the object. You could say that this reflecting or absorbing of infrared helps to determine the object's 'color' - its color being a combination of red, green, blue, and infrared! 

This is an image of Phoenix, Arizona showing the near infrared data collected by the Landsat 5 satellite. The light areas are areas with high reflectance of near infrared waves. The dark areas show little reflectance. What do you think the black grid lines in the lower right of this image represent?

 This image shows the infrared data (appearing as red) composited with visible light data at the blue and green wavelengths. If near infrared is reflected off of healthy vegetation, what do you think the red square shaped areas are in the lower left of the image?

Instruments on board satellites can also take pictures of things in space. The image below of the center region of our galaxy was taken by IRAS. The hazy, horizontal S-shaped feature that crosses the image is faint heat emitted by dust in the plane of the Solar System.

What is it?

The electromagnetic spectrum is more familiar to you than you might think. The microwave you use to heat your food and the cell phones you use are part of the Electromagnetic Spectrum. The light that our eyes can see is also part of the electromagnetic spectrum. This visible part of the electromagnetic spectrum consists of the colors that we see in a rainbow - from reds and oranges, through blues and purples.

Each of these colors actually corresponds to a different wavelength of light

 

Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom.

 

Infrared Spectroscopy II

Infrared spectroscopy is spectroscopy in the infrared (IR) region of the electromagnetic spectrum. It is a vital part of infrared astronomy, just as it is in visual, or optical, astronomy (and has been since lines were discovered in the spectrum of the Sun, in 1802, though it was a couple of decades before Fraunhofer began to study them systematically).
For the most part, the techniques used in IR spectroscopy, in astronomy, are the same or very similar to those used in the visual waveband; confusingly, then, IR spectroscopy is part of both infrared astronomy and optical astronomy! These techniques involve use of mirrors, lenses, dispersive media such as prisms or gratings, and ‘quantum’ detectors (silicon-based CCDs in the visual waveband, HgCdTe – or InSb or PbSe – arrays in IR); at the long-wavelength end – where the IR overlaps with the submillimeter or terahertz region – there are somewhat different techniques.

As infrared astronomy has a much longer ground-based history than a space-based one, the terms used relate to the windows in the Earth’s atmosphere where lower absorption spectroscopy makes astronomy feasible … so there is the near-IR (NIR), from the end of the visual (~0.7 µm) to ~3 µm, the mid (to ~30 µm), and the far-IR (FIR, to 0.2 mm).
As with spectroscopy in the visual and UV wavebands, IR spectroscopy in astronomy involves detection of both absorption (mostly) and emission (rather less common) lines due to atomic transitions (the hydrogen Paschen, Brackett, Pfund, and Humphreys series are all in the IR, mostly NIR). However, lines and bands due to molecules are found in the spectra of nearly all objects, across the entire IR … and the reason why space-based observatories are needed to study water and carbon dioxide (to take just two examples) in astronomical objects. One of the most important class of molecules (of interest to astronomers) is PAHs – polycyclic aromatic hydrocarbons – whose transitions are most prominent in the mid-IR (see the Spitzer webpage Understanding Polycyclic Aromatic Hydrocarbons for more details).
Looking for more info on how astronomers do IR spectroscopy? Caltech has a brief introduction to IR spectroscopy. The ESO’s Very Large Telescope (VLT) has several dedicated instruments, including VISIR (which is both an imager and spectrometer, working in the mid-IR); CIRPASS, a NIR integrated field unit spectrograph on Gemini; Spitzer’s IRS (a mid-IR spectrograph); and LWS on the ESA’s Infrared Space Observatory (a FIR spectrometer).
Universe Today stories related to IR spectroscopy include Infrared Sensor Could Be Useful on Earth Too, Search for Origins Programs Shortlisted, and Jovian Moon Was Probably Captured.
Infrared spectroscopy is covered in the Astronomy Cast episode Infrared Astronomy.
Sources:
http://en.wikipedia.org/wiki/Infrared_spectroscopy
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/InfraRed/infrared.htm
http://www.chem.ucla.edu/~webspectra/irintro.html