OPUS Spectroscopy Software OPUS is the leading spectroscopy software for state-of-the-art measurement, processing and evaluation of IR, NIR and Raman Spectra. Based on decades of experience and driven by the innovative spirit of a technology leader the OPUS software suite combines an unmatched range of functionality with unique ease of use.
'FTIR' redirects here. FTIR may also refer to.Fourier-transform infrared spectroscopy ( FTIR) is a technique used to obtain an of or of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-spectral-resolution data over a wide spectral range. This confers a significant advantage over a spectrometer, which measures intensity over a narrow range of at a time.The term Fourier-transform infrared spectroscopy originates from the fact that a (a mathematical process) is required to convert the raw data into the actual spectrum. An FTIR interferogram. The central peak is at the ZPD position ('zero path difference' or zero retardation), where the maximal amount of light passes through the to the detector.The goal of any (FTIR, etc.) is to measure how much light a sample absorbs at each wavelength. The most straightforward way to do this, the 'dispersive spectroscopy' technique, is to shine a light beam at a sample, measure how much of the light is absorbed, and repeat for each different wavelength.
(This is how some work, for example.)Fourier-transform spectroscopy is a less intuitive way to obtain the same information. Rather than shining a beam of light (a beam composed of only a single wavelength) at the sample, this technique shines a beam containing many frequencies of light at once and measures how much of that beam is absorbed by the sample. Next, the beam is modified to contain a different combination of frequencies, giving a second data point. This process is rapidly repeated many times over a short timespan.
Afterwards, a computer takes all this data and works backward to infer what the absorption is at each wavelength.The beam described above is generated by starting with a light source—one containing the full spectrum of wavelengths to be measured. The light shines into a —a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to. Different wavelengths are modulated at different rates, so that at each moment the beam coming out of the interferometer has a different spectrum.As mentioned, computer processing is required to turn the raw data (light absorption for each mirror position) into the desired result (light absorption for each wavelength). The processing required turns out to be a common algorithm called the.
The Fourier transform converts one domain (in this case displacement of the mirror in cm) into its inverse domain (wavenumbers in cm -1). The raw data is called an 'interferogram'.Developmental background The first low-cost capable of recording an was the Infracord produced in 1957. This instrument covered the wavelength range from 2.5 μm to 15 μm ( range 4000 cm −1 to 660 cm −1). The lower wavelength limit was chosen to encompass the highest known vibration frequency due to a fundamental. The upper limit was imposed by the fact that the was a made from a single crystal of rock-salt , which becomes opaque at wavelengths longer than about 15 μm; this spectral region became known as the rock-salt region. Later instruments used prisms to extend the range to 25 μm (400 cm −1) and 50 μm (200 cm −1).
The region beyond 50 μm (200 cm −1) became known as the far-infrared region; at very long wavelengths it merges into the region. Measurements in the far infrared needed the development of accurately ruled to replace the prisms as dispersing elements, since salt crystals are opaque in this region. More sensitive detectors than the were required because of the low energy of the radiation.
One such was the. An additional issue is the need to exclude atmospheric because water vapour has an intense pure in this region. Far-infrared spectrophotometers were cumbersome, slow and expensive. The advantages of the were well-known, but considerable technical difficulties had to be overcome before a commercial instrument could be built. Also an electronic computer was needed to perform the required Fourier transform, and this only became practicable with the advent of, such as the, which became available in 1965.
Digilab pioneered the world's first commercial FTIR spectrometer (Model FTS-14) in 1969 (Digilab FTIRs are now a part of Agilent technologies's molecular product line after it acquired spectroscopy business from ). Michelson interferometer. Schematic diagram of a Michelson interferometer, configured for FTIRIn a adapted for FTIR, light from the polychromatic infrared source, approximately a radiator, is and directed to a. Ideally 50% of the light is refracted towards the fixed mirror and 50% is transmitted towards the moving mirror. Light is reflected from the two mirrors back to the beam splitter and some fraction of the original light passes into the sample compartment.
There, the light is focused on the sample. On leaving the sample compartment the light is refocused on to the detector. The difference in optical path length between the two arms to the interferometer is known as the retardation or optical path difference (OPD). An interferogram is obtained by varying the retardation and recording the signal from the detector for various values of the retardation. The form of the interferogram when no sample is present depends on factors such as the variation of source intensity and splitter efficiency with wavelength.
This results in a maximum at zero retardation, when there is at all wavelengths, followed by series of 'wiggles'. The position of zero retardation is determined accurately by finding the point of maximum intensity in the interferogram. When a sample is present the background interferogram is modulated by the presence of absorption bands in the sample.Commercial spectrometers use Michelson interferometers with a variety of scanning mechanisms to generate the path difference. Common to all these arrangements is the need to ensure that the two beams recombine exactly as the system scans. The simplest systems have a plane mirror that moves linearly to vary the path of one beam.
In this arrangement the moving mirror must not tilt or wobble as this would affect how the beams overlap as they recombine. Some systems incorporate a compensating mechanism that automatically adjusts the orientation of one mirror to maintain the alignment. Arrangements that avoid this problem include using cube corner reflectors instead of plane mirrors as these have the property of returning any incident beam in a parallel direction regardless of orientation.
Interferometer schematics where the path difference is generated by a rotary motion.Systems where the path difference is generated by a rotary movement have proved very successful. One common system incorporates a pair of parallel mirrors in one beam that can be rotated to vary the path without displacing the returning beam. Another is the double pendulum design where the path in one arm of the interferometer increases as the path in the other decreases.A quite different approach involves moving a wedge of an IR-transparent material such as into one of the beams. Increasing the thickness of KBr in the beam increases the optical path because the refractive index is higher than that of air. One limitation of this approach is that the variation of refractive index over the wavelength range limits the accuracy of the wavelength calibration.Measuring and processing the interferogram The interferogram has to be measured from zero path difference to a maximum length that depends on the resolution required. In practice the scan can be on either side of zero resulting in a double-sided interferogram. Mechanical design limitations may mean that for the highest resolution the scan runs to the maximum OPD on one side of zero only.The interferogram is converted to a spectrum by Fourier transformation.
This requires it to be stored in digital form as a series of values at equal intervals of the path difference between the two beams. To measure the path difference a laser beam is sent through the interferometer, generating a sinusoidal signal where the separation between successive maxima is equal to the wavelength. This can trigger an analog-to digital converter to measure the IR signal each time the laser signal passes through zero.
Alternatively the laser and IR signals can be measured synchronously at smaller intervals with the IR signal at points corresponding to the laser signal zero crossing being determined by interpolation. This approach allows the use of analog-to-digital converters that are more accurate and precise than converters that can be triggered, resulting in lower noise. Values of the interferogram at times corresponding to zero crossings of the laser signal are found by interpolation.The result of Fourier transformation is a spectrum of the signal at a series of discrete wavelengths. The range of wavelengths that can be used in the calculation is limited by the separation of the data points in the interferogram. The shortest wavelength that can be recognized is twice the separation between these data points. For example, with one point per wavelength of a helium-neon reference laser at 0.633 μm ( 15 800 cm −1) the shortest wavelength would be 1.266 μm ( 7900 cm −1).
Because of aliasing any energy at shorter wavelengths would be interpreted as coming from longer wavelengths and so has to be minimized optically or electronically. The spectral resolution, i.e. The separation between wavelengths that can be distinguished, is determined by the maximum OPD.
The wavelengths used in calculating the Fourier transform are such that an exact number of wavelengths fit into the length of the interferogram from zero to the maximum OPD as this makes their contributions orthogonal. This results in a spectrum with points separated by equal frequency intervals.For a maximum path difference d adjacent wavelengths λ 1 and λ 2 will have n and (n+1) cycles respectively in the interferogram.
The corresponding frequencies are ν 1 and ν 2:d = nλ 1and d = (n+1)λ 2λ 1 = d/nand λ 2 =d/(n+1)ν 1 = 1/λ 1and ν 2 = 1/λ 2ν 1 = n/dand ν 2 = (n+1)/dν 2 − ν 1 = 1/dThe separation is the inverse of the maximum OPD. For example, a maximum OPD of 2 cm results in a separation of 0.5 cm −1. This is the spectral resolution in the sense that the value at one point is independent of the values at adjacent points. Most instruments can be operated at different resolutions by choosing different OPD's. Instruments for routine analyses typically have a best resolution of around 0.5 cm −1, while spectrometers have been built with resolutions as high as 0.001 cm −1, corresponding to a maximum OPD of 10 m. The point in the interferogram corresponding to zero path difference has to be identified, commonly by assuming it is where the maximum signal occurs. The centerburst is not always symmetrical in real world spectrometers so a phase correction may have to be calculated.
The interferogram signal decays as the path difference increases, the rate of decay being inversely related to the width of features in the spectrum. If the OPD is not large enough to allow the interferogram signal to decay to a negligible level there will be unwanted oscillations or sidelobes associated with the features in the resulting spectrum. To reduce these sidelobes the interferogram is usually multiplied by a function that approaches zero at the maximum OPD.
This so-called apodization reduces the amplitude of any sidelobes and also the noise level at the expense some reduction in resolution.For the number of points in the interferogram has to equal a power of two. A string of zeroes may be added to the measured interferogram to achieve this. More zeroes may be added in a process called zero filling to improve the appearance of the final spectrum although there is no improvement in resolution.
Alternatively interpolation after the Fourier transform gives a similar result.Advantages There are three principal advantages for an FT spectrometer compared to a scanning (dispersive) spectrometer. The multiplex. This arises from the fact that information from all wavelengths is collected simultaneously. It results in a higher for a given scan-time for observations limited by a fixed detector noise contribution (typically in the thermal infrared spectral region where a is limited by ).
For a spectrum with m resolution elements, this increase is equal to the square root of m. Alternatively, it allows a shorter scan-time for a given resolution. In practice multiple scans are often averaged, increasing the signal-to-noise ratio by the square root of the number of scans. The throughput or Jacquinot's advantage. This results from the fact that in a dispersive instrument, the has entrance and exit slits which restrict the amount of light that passes through it. The interferometer throughput is determined only by the diameter of the collimated beam coming from the source. Although no slits are needed, FTIR spectrometers do require an aperture to restrict the convergence of the collimated beam in the interferometer.
This is because convergent rays are modulated at different frequencies as the path difference is varied. Such an aperture is called a Jacquinot stop. For a given resolution and wavelength this circular aperture allows more light through than a slit, resulting in a higher signal-to-noise ratio. The wavelength accuracy or Connes' advantage. The wavelength scale is calibrated by a laser beam of known wavelength that passes through the interferometer. This is much more stable and accurate than in dispersive instruments where the scale depends on the mechanical movement of diffraction gratings. In practice, the accuracy is limited by the divergence of the beam in the interferometer which depends on the resolution.Another minor advantage is less sensitivity to stray light, that is radiation of one wavelength appearing at another wavelength in the spectrum.
In dispersive instruments, this is the result of imperfections in the diffraction gratings and accidental reflections. In FT instruments there is no direct equivalent as the apparent wavelength is determined by the modulation frequency in the interferometer.Resolution The interferogram belongs in the length dimension. (FT) inverts the dimension, so the FT of the interferogram belongs in the reciprocal length dimension(L−1), that is the dimension of. The in cm −1 is equal to the reciprocal of the maximal retardation in cm. Thus a 4 cm −1 resolution will be obtained if the maximal retardation is 0.25 cm; this is typical of the cheaper FTIR instruments.
Much higher resolution can be obtained by increasing the maximal retardation. This is not easy, as the moving mirror must travel in a near-perfect straight line. The use of mirrors in place of the flat mirrors is helpful, as an outgoing ray from a corner-cube mirror is parallel to the incoming ray, regardless of the orientation of the mirror about axes perpendicular to the axis of the light beam. In 1966 Connes measured the temperature of the atmosphere of by recording the of Venusian CO 2 at 0.1 cm −1 resolution. Himself attempted to resolve the hydrogen in the spectrum of a atom into its two components by using his interferometer. P25 A spectrometer with 0.001 cm −1 resolution is now available commercially.
The throughput advantage is important for high-resolution FTIR, as the monochromator in a dispersive instrument with the same resolution would have very narrow.Motivation FTIR is a method of measuring infrared absorption and emission spectra. For a discussion of why people measure infrared absorption and emission spectra, i.e. Why and how substances absorb and emit infrared light, see the article:.Components IR sources FTIR spectrometers are mostly used for measurements in the mid and near IR regions. For the mid-IR region, 2−25 µm (5000–400 cm −1), the most common source is a silicon carbide element heated to about 1200 K.
The output is similar to a blackbody. Shorter wavelengths of the near-IR, 1−2.5 µm ( cm −1), require a higher temperature source, typically a tungsten-halogen lamp. The long wavelength output of these is limited to about 5 µm (2000 cm −1) by the absorption of the quartz envelope. For the far-IR, especially at wavelengths beyond 50 µm (200 cm −1) a mercury discharge lamp gives higher output than a thermal source. Detectors Mid-IR spectrometers commonly use pyroelectric detectors that respond to changes in temperature as the intensity of IR radiation falling on them varies. The sensitive elements in these detectors are either deuterated triglycine sulfate (DTGS) or lithium tantalate (LiTaO 3). These detectors operate at ambient temperatures and provide adequate sensitivity for most routine applications.
To achieve the best sensitivity the time for a scan is typically a few seconds. Cooled photoelectric detectors are employed for situations requiring higher sensitivity or faster response. Liquid nitrogen cooled mercury cadmium telluride (MCT) detectors are the most widely used in the mid-IR. With these detectors an interferogram can be measured in as little as 10 milliseconds.
Uncooled indium gallium arsenide photodiodes or DTGS are the usual choices in near-IR systems. Very sensitive liquid-helium-cooled silicon or germanium bolometers are used in the far-IR where both sources and beamsplitters are inefficient.Beam splitter. Simple interferometer with a beam-splitter and compensator plateAn ideal beam-splitter transmits and reflects 50% of the incident radiation. However, as any material has a limited range of optical transmittance, several beam-splitters may be used interchangeably to cover a wide spectral range. For the mid-IR region the beamsplitter is usually made of KBr with a germanium-based coating that makes it semi-reflective. KBr absorbs strongly at wavelengths beyond 25 μm (400 cm −1) so CsI is sometimes used to extend the range to about 50 μm (200 cm −1). ZnSe is an alternative where moisture vapor can be a problem but is limited to about 20μm (500 cm −1).
CaF 2 is the usual material for the near-IR, being both harder and less sensitive to moisture than KBr but cannot be used beyond about 8 μm (1200 cm −1). In a simple Michelson interferometer one beam passes twice through the beamsplitter but the other passes through only once. To correct for this an additional compensator plate of equal thickness is incorporated. Far-IR beamsplitters are mostly based on polymer films and cover a limited wavelength range. Attenuated total reflectance. Main article:(ATR) is one accessory of FTIR spectrophotometer to measure surface properties of solid or thin film samples rather than their bulk properties. Generally, ATR has a penetration depth of around 1 or 2 micrometers depending on your sample conditions.Fourier transform The interferogram in practice consists of a set of intensities measured for discrete values of retardation.
The difference between successive retardation values is constant. Thus, a is needed.
The (FFT) algorithm is used.Spectral range Far-infrared The first FTIR spectrometers were developed for far-infrared range. The reason for this has to do with the mechanical tolerance needed for good optical performance, which is related to the wavelength of the light being used. For the relatively long wavelengths of the far infrared, 10 μm tolerances are adequate, whereas for the rock-salt region tolerances have to be better than 1 μm. A typical instrument was the cube interferometer developed at the and marketed. It used a stepper motor to drive the moving mirror, recording the detector response after each step was completed.Mid-infrared With the advent of cheap it became possible to have a computer dedicated to controlling the spectrometer, collecting the data, doing the Fourier transform and presenting the spectrum. This provided the impetus for the development of FTIR spectrometers for the rock-salt region. The problems of manufacturing ultra-high precision optical and mechanical components had to be solved.
A wide range of instruments are now available commercially. Although instrument design has become more sophisticated, the basic principles remain the same. Nowadays, the moving mirror of the interferometer moves at a constant velocity, and sampling of the interferogram is triggered by finding zero-crossings in the fringes of a secondary interferometer lit by a. In modern FTIR systems the constant mirror velocity is not strictly required, as long as the laser fringes and the original interferogram are recorded simultaneously with higher sampling rate and then re-interpolated on a constant grid, as pioneered.
This confers very high wavenumber accuracy on the resulting infrared spectrum and avoids wavenumber errors.Near-infrared. Main article:The near-infrared region spans the wavelength range between the rock-salt region and the start of the region at about 750 nm. Of fundamental vibrations can be observed in this region. It is used mainly in industrial applications such as and.Applications FTIR can be used in all applications where a dispersive spectrometer was used in the past (see external links).
In addition, the improved sensitivity and speed have opened up new areas of application. Spectra can be measured in situations where very little energy reaches the detector and scan rates can exceed 50 spectra a second.
Fourier transform infrared spectroscopy is used in, chemistry, materials and biology research fields.Biological materials FTIR is used to investigate proteins in hydrophobic membrane environments. Studies show the ability of FTIR to directly determine the polarity at a given site along the backbone of a transmembrane protein. Microscopy and imaging An infrared microscope allows samples to be observed and spectra measured from regions as small as 5 microns across. Images can be generated by combining a microscope with linear or 2-D array detectors. The spatial resolution can approach 5 microns with tens of thousands of pixels. The images contain a spectrum for each pixel and can be viewed as maps showing the intensity at any wavelength or combination of wavelengths.
This allows the distribution of different chemical species within the sample to be seen. Typical studies include analysing tissue sections as an alternative to conventional histopathology and examining the homogeneity of pharmaceutical tablets.Nanoscale and spectroscopy below the diffraction limit The spatial resolution of FTIR can be further improved below the micrometer scale by integrating it into platform. The corresponding technique is called and allows for performing broadband spectroscopy on materials in ultra-small quantities (single viruses and protein complexes) and with 10 to 20 nm spatial resolution. FTIR as detector in chromatography The speed of FTIR allows spectra to be obtained from compounds as they are separated by a gas chromatograph.
However this technique is little used compared to GC-MS (gas chromatography-mass spectrometry) which is more sensitive. The GC-IR method is particularly useful for identifying isomers, which by their nature have identical masses. Liquid chromatography fractions are more difficult because of the solvent present. One notable exception is to measure chain branching as a function of molecular size in polyethylene using, which is possible using chlorinated solvents that have no absorption in the area in question.TG-IR (thermogravimetric analysis-infrared spectrometry) Measuring the gas evolved as a material is heated allows qualitative identification of the species to complement the purely quantitative information provided by measuring the weight loss.See also.References. ^ Griffiths, P.; de Hasseth, J.
(18 May 2007). (2nd ed.). 'The Infracord double-beam spectrophotometer'. Clinical Science.
July 27, 2009. Brault, James W. 'New Approach to high-precision Fourier transform spectrometer design'. Applied Optics.
35 (16): 2891–2896. Connes, J.; Connes, P. 'Near-Infrared Planetary Spectra by Fourier Spectroscopy. Instruments and Results'. Journal of the Optical Society of America. 56 (7): 896–910. Smith, D.R.; Morgan, R.L.; Loewenstein, E.V.
'Comparison of the Radiance of Far-Infrared Sources'. 58 (3): 433–434. Griffiths, P.R.; Holmes, C (2002).
Handbook of Vibrational Spectroscopy, Vol 1. Chichester:.
Chamberain, J.; Gibbs, J.E.; Gebbie, H.E. 'The determination of refractive index spectra by fourier spectrometry'. Infrared Physics. 9 (4): 189–209.
Manor, Joshua; Feldblum, Esther S.; Arkin, Isaiah T. The Journal of Physical Chemistry Letters.
3 (7): 939–944. Brielle, Esther S.; Arkin, Isaiah T. 'Site-Specific Hydrogen Exchange in a Membrane Environment Analyzed by Infrared Spectroscopy'. The Journal of Physical Chemistry Letters. 9 (14): 4059–4065.
Amenabar, Iban; Poly, Simon; Nuansing, Wiwat; Hubrich, Elmar H.; Govyadinov, Alexander A.; Huth, Florian; Krutokhvostov, Roman; Zhang, Lianbing; Knez, Mato (2013-12-04). Nature Communications. 4: 2890.External links. photograph.
The Grubb-Parsons-NPL cube interferometer. Properties of many salt crystals and useful links. from University of Bristol.
OVIRS instrument of the Osiris-REx probe is a visible and infrared spectrometerInfrared spectroscopy ( IR spectroscopy or vibrational spectroscopy) involves the interaction of radiation with. It covers a range of techniques, mostly based on. As with all spectroscopic techniques, it can be used to identify and study. Samples may be solid, liquid, or gas. The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer (or spectrophotometer) to produce an infrared spectrum. An IR spectrum can be visualized in a graph of infrared light (or ) on the vertical axis vs.
Frequency or wavelength on the horizontal axis. Typical of frequency used in IR spectra are (sometimes called ), with the symbol cm −1. Units of IR wavelength are commonly given in (formerly called 'microns'), symbol μm, which are related to wave numbers in a way. A common laboratory instrument that uses this technique is a (FTIR). Two-dimensional IR is also possible as discussed.The infrared portion of the is usually divided into three regions; the, mid- and infrared, named for their relation to the visible spectrum. The higher-energy near-IR, approximately cm −1 (0.7–2.5 μm wavelength) can excite.
The mid-infrared, approximately 4000–400 cm −1 (2.5–25 μm) may be used to study the fundamental vibrations and associated structure. The far-infrared, approximately 400–10 cm −1 (25–1000 μm), lying adjacent to the region, has low energy and may be used for. The names and classifications of these subregions are conventions, and are only loosely based on the relative molecular or electromagnetic properties. 3D animation of the symmetric stretching of the C–H bonds ofIn particular, in the and harmonic approximations, i.e. When the corresponding to the electronic can be approximated by a in the neighborhood of the equilibrium, the resonant frequencies are associated with the corresponding to the molecular electronic ground state potential energy surface.
The resonant frequencies are also related to the strength of the bond and the at either end of it. Thus, the frequency of the vibrations are associated with a particular normal mode of motion and a particular bond type.Number of vibrational modes In order for a vibrational mode in a sample to be 'IR active', it must be associated with changes in the dipole moment. A permanent dipole is not necessary, as the rule requires only a change in dipole moment.A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with N number of atoms, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.Simple have only one bond and only one vibrational band. If the molecule is symmetrical, e.g.
N 2, the band is not observed in the IR spectrum, but only in the. Asymmetrical diatomic molecules, e.g., absorb in the IR spectrum.
More complex molecules have many bonds, and their vibrational spectra are correspondingly more complex, i.e. Big molecules have many peaks in their IR spectra.The atoms in a CH 2X 2 group, commonly found in and where X can represent any other atom, can vibrate in nine different ways. Six of these vibrations involve only the portion: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting, as shown below. Structures that do not have the two additional X groups attached have fewer modes because some modes are defined by specific relationships to those other attached groups. For example, in water, the rocking, wagging, and twisting modes do not exist because these types of motions of the H atoms represent simple rotation of the whole molecule rather than vibrations within it. DirectionSymmetricAntisymmetricRadialSymmetric stretchingAntisymmetric stretchingLatitudinalScissoringRockingLongitudalWaggingTwistingThese figures do not represent the ' of the atoms, which, though necessarily present to balance the overall movements of the molecule, are much smaller than the movements of the lighter atoms.The simplest and most important or fundamental IR bands arise from the excitations of normal modes, the simplest distortions of the molecule, from the with v = 0 to the first with vibrational quantum number v = 1. In some cases, are observed.
An overtone band arises from the absorption of a photon leading to a direct transition from the ground state to the second excited vibrational state (v = 2). Such a band appears at approximately twice the energy of the fundamental band for the same normal mode. Some excitations, so-called combination modes, involve simultaneous excitation of more than one normal mode.
The phenomenon of can arise when two modes are similar in energy; Fermi resonance results in an unexpected shift in energy and intensity of the bands etc.Practical IR spectroscopy The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR is the same as the vibrational frequency of a bond or collection of bonds, absorption occurs. Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This measurement can be achieved by scanning the wavelength range using a. Alternatively, the entire wavelength range is measured using a instrument and then a or spectrum is generated using a dedicated procedure.This technique is commonly used for analyzing samples with. Simple spectra are obtained from samples with few IR active bonds and high levels of purity.
More complex molecular structures lead to more absorption bands and more complex spectra. Typical IR solution cell. The windows are CaF 2.
Sample preparation Gaseous samples require a sample cell with a long to compensate for the diluteness. The pathlength of the sample cell depends on the concentration of the compound of interest. A simple glass tube with length of 5 to 10 cm equipped with infrared-transparent windows at the both ends of the tube can be used for concentrations down to several hundred ppm. Sample gas concentrations well below ppm can be measured with a in which the infrared light is guided with mirrors to travel through the gas. White's cells are available with optical pathlength starting from 0.5 m up to hundred meters.Liquid samples can be sandwiched between two plates of a salt (commonly, or common salt, although a number of other salts such as or are also used).The plates are transparent to the infrared light and do not introduce any lines onto the spectra.Solid samples can be prepared in a variety of ways. One common method is to crush the sample with an oily mulling agent (usually mineral oil ).
A thin film of the mull is applied onto salt plates and measured. The second method is to grind a quantity of the sample with a specially purified salt (usually ) finely (to remove scattering effects from large crystals).
This powder mixture is then pressed in a mechanical to form a translucent pellet through which the beam of the spectrometer can pass. A third technique is the 'cast film' technique, which is used mainly for polymeric materials. The sample is first dissolved in a suitable, non hygroscopic solvent. A drop of this solution is deposited on surface of KBr or NaCl cell. The solution is then evaporated to dryness and the film formed on the cell is analysed directly. Care is important to ensure that the film is not too thick otherwise light cannot pass through. This technique is suitable for qualitative analysis.
The final method is to use to cut a thin (20–100 µm) film from a solid sample. This is one of the most important ways of analysing failed plastic products for example because the integrity of the solid is preserved.In the need for sample treatment is minimal. The sample, liquid or solid, is placed into the sample cup which is inserted into the photoacoustic cell which is then sealed for the measurement. The sample may be one solid piece, powder or basically in any form for the measurement. For example, a piece of rock can be inserted into the sample cup and the spectrum measured from it.Comparing to a reference. Schematics of a two-beam absorption spectrometer. A beam of infrared light is produced, passed through an (not shown), and then split into two separate beams.
![Infrared Infrared](http://jasco.hu/4tgrdvxgv/uploads/2011/12/JASCO_FTIR__SMII.png)
One is passed through the sample, the other passed through a reference. The beams are both reflected back towards a detector, however first they pass through a splitter, which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained. This 'two-beam' setup gives accurate spectra even if the intensity of the light source drifts over time.It is typical to record spectrum of both the sample and a 'reference'. This step controls for a number of variables, e.g., which may affect the spectrum. The reference measurement makes it possible to eliminate the instrument influence.The appropriate 'reference' depends on the measurement and its goal.
The simplest reference measurement is to simply remove the sample (replacing it by air). However, sometimes a different reference is more useful. For example, if the sample is a dilute solute dissolved in water in a beaker, then a good reference measurement might be to measure pure water in the same beaker. Then the reference measurement would cancel out not only all the instrumental properties (like what light source is used), but also the light-absorbing and light-reflecting properties of the water and beaker, and the final result would just show the properties of the solute (at least approximately).A common way to compare to a reference is sequentially: first measure the reference, then replace the reference by the sample and measure the sample. This technique is not perfectly reliable; if the infrared lamp is a bit brighter during the reference measurement, then a bit dimmer during the sample measurement, the measurement will be distorted. More elaborate methods, such as a 'two-beam' setup (see figure), can correct for these types of effects to give very accurate results.
The method can be used to statistically cancel these errors.Nevertheless, among different absorption based techniques which are used for gaseous species detection, (CRDS) can be used as a calibration free method. The fact that CRDS is based on the measurements of photon life-times (and not the laser intensity) makes it needless for any calibration and comparison with a reference FTIR. An interferogram from an measurement. The horizontal axis is the position of the mirror, and the vertical axis is the amount of light detected.
This is the 'raw data' which can be to get the actual spectrum.infrared (FTIR) spectroscopy is a measurement technique that allows one to record infrared spectra. Infrared light is guided through an and then through the sample (or vice versa). A moving mirror inside the apparatus alters the distribution of infrared light that passes through the interferometer. The signal directly recorded, called an 'interferogram', represents light output as a function of mirror position. A data-processing technique called turns this raw data into the desired result (the sample's spectrum): Light output as a function of infrared (or equivalently, ). As described above, the sample's spectrum is always compared to a reference.An alternate method for acquiring spectra is the 'dispersive' or 'scanning ' method.
In this approach, the sample is irradiated sequentially with various single wavelengths. The dispersive method is more common in, but is less practical in the infrared than the FTIR method. One reason that FTIR is favored is called ' or the 'multiplex advantage': The information at all frequencies is collected simultaneously, improving both speed. Another is called 'Jacquinot's Throughput Advantage': A dispersive measurement requires detecting much lower light levels than an FTIR measurement. There are other advantages, as well as some disadvantages, but virtually all modern infrared spectrometers are FTIR instruments.Absorption bands IR spectroscopy is often used to identify structures because give rise to characteristic bands both in terms of intensity and position (frequency). The positions of these bands are summarized in correlation tables as shown below.