General Introduction: Bruker IFS 120 and 125
Compared to classical spectroscopic techniques, FT spectroscopy is method with a lot of advantages (broadband, high resolution, high accuracy). In our laboratory, the FT spectroscopy is used together with laser techniques for the detection of stable or short time living transient species.
Bruker IFS 120 is full-evacuated instrument acquired from the Bergische Universität Wuppertal in 2004. As in case of every FT spectrometer, the heart of the instrument is the Michelson interferometer. Moveable mirror travels on the INVAR lines in the 170 cm long tunnel. This distance allows resolution (IFS 120) of 0.0035 cm-1. In the case of IFS 125 the tunnel longer length provides the resolution of of 0.002 cm-1. Position of the moving mirror is controlled by the HeNe laser (wavenumber 15802.78 cm-1). The GLOWBAR radiation source (SiC bar heated at 1300K) is used for measurements in the middle infrared region (MIR) from 400 cm-1 to 2000 cm-1 while TUNGSTEN radiator (Wolfram wire, looks like a classical halogen bulb) is used for the near infrared region (NIR) from 2000 cm-1 to 8000 cm-1 (in our configuration). The spectrometer has been recently upgraded also for the operation in the VIS region using Si beam splitter in combination with the Si photodiode or using photomultiplier. In the range of infrared radiation, the HgCdTe detector is used for the MIR spectral range and InSb detector for the NIR spectral range. Detectors are cooled by liquid nitrogen. OPUS software is used to control the spectrometer and also for evaluation of the recorded spectra. Bruker IFS 120 machine has been recently upgraded for the time resolved spectra acquisition. To enhance the detection limit, we have constructed the multipass cell with the 30 m optical path. The cell is used in combination with the new Bruker IFS 125 HR spectrometer.
See also: Poster about FT, Scheme of the FT Spectrometer Function, Poster about Car Emissions, Poster about the analysis of the Cigarette Smoke Composition
From the History of the FT Spectroscopy
Fourier transform algorithm was developed by Jean Baptiste Joseph Fourier (*1768†1830). FT is based on a fact, that each periodical function can be transformed to a sum of sine functions (infinite sum in special cases). This principle is used for an analysis of sounds, wave functions, nuclear magnetic resonance, mass spectrometry and also in IR spectrometry.
FT interferometry was initiated in 1880 when A. Michelson invented the interferometer. Michelson measured very precisely the velocity of light in vacuum, proved that Earth does not move through an ether and although Michelson lacked computers or electronic detectors, he perceived the basic techniques of FT and developed a harmonic analyzer to compute Fourier transforms. This sine and cosine analog mechanical computer could handle about 80 data points. He measured intensity levels with his eye and made crude estimates of the spectrum of the Zeeman effect.
In 50`s P. Jacquinot published idea, that in a lossless optical system, the brightness of an object equals the brightness of the image. FT spectrometry does not use aperture, so throughput is limited only by size of mirrors. This statement is one of crucial ideas of FTS.
Fellgett put forwards his ideas shortly after P. Jacquinott stated the so called throughput advantage. This physician was the first person to transform interferograms numerically. He made a suggestion of the first real FT spectrometer in 1952.
The principle of function is different from a classical absorption spectrometer. The Basic part is an interferometer. A beam of electromagnetic radiation is separated to two fractions. One fraction gets the optical path difference (thanks to moving mirror) and both parts are combined to the one beam again. Each part of this radiation interferes constructively and destructively. Common type of interferometer is the Michelson's interferometer. This instrument is also the basic part of the spectrometer Bruker IFS 120 HR. A Beam travels through the sample chamber to the detector after recombination. The Moveable mirror travels and the instrument records an interferogram. It is a dependence of an intensity on the moveable mirror optical path difference controlled by the HeNe laser. After background measurement it is measured a spectrum of a gas sample. Molecules of gas absorb specific wavenumber parts of radiation and get higher rotation and vibration mode. This fact leads to specific absorption lines in spectrum. The interferogram is calculated to the spectrum by help of Fourier Transformation.
Fellgett got his name to the advantage (the Multiplex advantage), which states, that in one moment all spectral data are available.
In the late 50`s and early 60`s several developers and users of FTS came into prominence. Pierre and Janine Connes measured excellent spectra of planets and published their data in "Atlas des Spectres dans le Proche Infrarouge de Venus, Mars, Jupiter, et Saturne." They have contributed many improvements to data handling and computational techniques, including practical real-time analysis equipment for 30 000 data points and quick computer techniques for the large machines so that 5 million samples can be transformed in 9 min. Spectra of planets agreed with spectra of samples in terrestrial laboratories with high accuracy. This advantage - high wavenumber accuracy of FTS - is called the Connes advantage. Important step in the slow acceptance of Fourier spectrometry by the chemical community was the discovery of the fast Fourier transform algorithm (FFT) by Cooley and Tukey in 1964. Using of HeNe control lasers and minicomputers (and microcomputers in the late 70`s) was also very important.
In England J. E. Chamberlain, J. E. Gibbs and H. A. Gebbie concentrated on developing equipment by conducting experiments in molecular spectroscopy and making atmospheric studies like experiments on ozone. Special techniques of double - beam FTS was developed by Rendall. Atmospheric studies of Randall and Dowling on NO and another gases constituted contributions to molecular spectroscopy. R. Hanel et al. packaged a Michelson interferometer for the Nimbus III satellite and obtained data from different parts of the Earth.
In the 70`s Loewenstein, Vanasse, Sakai, Murphy and Stair Jr. made a number measurements on atmospheric gases in the laboratory and from airplanes. FT spectrometer was placed on the board of probe Viking, which made excellent exploration of Mars.
FT equipment spanned spectral region from the UV or the visible to the millimeter wavelengths in the 70`s.
Commercial instruments were available (e.g. Bomem - 1981 or Nicolet) in the 80`s.
Development of time resolved FT was very important in the early 90`s. Observation of high resolution spectra of instable radicals in chemical reactions is possible at present.
Study of the Isotopic Exchange
The spontaneous exchange of 18O between carbon dioxide and water in water solutions was thoroughly experimentally studied (Ref. 1 – 4) Long before it was found that it makes a major contribution to the global isotope cycle of oxygen, together with the exchange catalyzed by the presence of rock and soil and the exchange which takes place during photosynthesis. On the other hand, the fact that the spontaneous exchange of 18O takes place also in the gaseous mixtures of CO2 and H2O, i.e. also directly in the atmosphere, has been recently discovered and its possible effect on the global isotope cycle has not been explored (Ref. 5 – 8). The possible isotope exchange was first indicated incidentally when regular measurements of isotope atmosphere composition started. For reference, each atmospheric sample is kept in multiple receptacles and the composition of atmospheric samples stored for a long time in different receptacles showed suspiciously large discrepancies (Ref. 9) This phenomenon has been studied and it has been found that during storage, the gaseous mixture clearly changes its composition and that the velocity of the process depends not only on the initial composition of gases but also on the amount of liquid phase (condensed water) and on the „personality“of the storage receptacle. (This is the reason why from this date the stored samples are freeze-dried before storage).
The spontaneous exchange of 18O between H2O and CO2 in the gaseous mixture was exactly confirmed and its velocity constant was measured in our laboratory during experiments which took place between 2006 and 2009. This important result was published in (Ref. 10, 11). The published experiment consisted of the preparation of gaseous mixtures of saturated water vapor H218O and carbon dioxide C16,16O2 with an inert component (CO and N2 gases were used) at atmospheric pressure. Apart from the saturated water vapor, the mixtures also contained liquid water which, however, had a minimal effect on the spontaneous exchange of 18O. We assumed this from the knowledge of the velocity constants of a solution. The mixtures were kept in the reaction receptacle for the period of two months, while very small samples of the mixture were gradually taken. Immediately after their taking, the samples were measured with high-resolution FTIR spectroscopy.
The resulting data were thoroughly analyzed. High-resolution FTIR is one of the suitable analytical instruments for measurements of isotopomer concentrations. The rovibrational lines of the individual isotopomers are sufficiently separated, the absorption power therefore allows the deduction of the concentration of the particular compound. We have developed a very reliable method determining the molar concentrations from the measured spectra, which uses tens of rovibrational lines and a whole scale of absorption coefficients for each isotopomer. In this way, we eliminate errors which can occur when using only a single line, e.g. errors caused by deviations of the apparatus calibration curve from linearity. In this way we measured concentrations of all the isotopomers involved in the studied chemical reaction ( H218O, C16,16O2, C16,18O2 and C18,18O2) apart from H216O, whose amounts in the samples could not be specified due to residual traces of air moisture in the evacuated reference cell and had to be calculated from concentrations of other isotopomers, while due to the presence of liquid phase we had to consider the distribution of H216O between the gas and liquid phase, which was considerably influenced by small fluctuations in temperature.
The newest work is focused on the isotopic exchange between solid material and the gas phase. The light–induced oxygen-isotope exchange between gaseous CO2 and solid Ti18O2 (anatase) and the spontaneous thermal isotope exchange that takes place between the vacuum-calcined solid Ti18O2 and CO2 were studied by gas-phase high-resolution Fourier transform infrared absorption spectroscopy over a period of several days. The absorption ro-vibrational spectra of all the measured carbon dioxide isotopologues were assigned and served as the quantification of the time-dependent isotope exchange between the oxygen atoms from the Ti18O2 solid and the oxygen related to the gaseous CO2. The C18O2 was formed as the dominating final product with a minor content of C16O18O. The rate of oxygen-isotope exchange is highly sensitive to the conditions of the titania pretreatment; vacuum-annealed Ti18O2 at 450 deg. C exhibited a very high spontaneous oxygen exchange activity with gaseous C16O2. A mechanism for the 18O/16O exchange process is discussed at the molecular level. The photocatalytic formation of methane, acetylene and C16O released from the Ti18O2 surface was observed after irradiation by an excimer laser.
Detection of Molecules Produced by the Burning Processes .
Infrared spectroscopy is a method simply applicable for an analysis of gas mixture composition. High resolution spectrometer Bruker IFS 120 was applied for a detection of main burning products.
We studied burning products of a wide range of materials. Automobile emissions have been the first the first subject of our interest. We have analyzed the composition of gases exhausted by the modern automobile Renault Scénic and old Czechoslovak automobile Tatra 613 using the Bruker IFS 120 infrared spectrometer. All the results have been compared with the optoacoustic measurements. We have simulated so called cold-starts which are typical for a traffic in a city. The crucial problem was preparation of spectral database intended for the identification of gases in our spectra. Selected compounds were also synthesized (ozone, cyanogen). In the second part of our research we analyzed a cigarette smoke and products of some materials burning mainly using the FT spectrometer but selected samples have been also measured using mass spectrometers GC-MS and SIFT-MS.
See also: Poster about car exhaust and cigarette smoke.
The basic Principle of the Time Resolved Spectroscopy
Methods for the aquisition of the time resolved spectra were developed in 60`s and 70`s. Time intervals about milliseconds, microseconds and nanoseconds were gradually available. Time resolved spectroscopy using FT is useful because of multiplex advantage, Jacquinott advantage and high signal to noise ratio. Data acquisition speed is limited by a time range of an observed process, time response of a detector and by a capacity of electronic components. The main deal of time resolved measurement is to reconstruct a set of time shifted interferograms. For this must be developed a special set-up of classical FT spectrometer.
All electronic components are used for the synchronization of data acquisition (signal from the detector with fast response), the discharge pulse generation and for the spectrometer control.
Our method is conformable to step-scan system but our instrument is able to cumulate data (scan) continously. Studied reactions must be done in pulses. Electric discharge (positive column discharge cell and hollow cathode) and laser ablation is used.
A time-resolved Fourier transform spectroscopic method was developed with a FPGA microcontroller and a high resolution spectrometer Bruker IFS 120 HR. During a cycle of the HeNe laser fringe digital signal 30 or 64 data points are recorded with a present time interval of units µs. This new system which uses recently available electronic facilities has advantage in flexibility with easy set-up. The system is applied for absorption and also for emission measurements from a pulsed discharge or pulsed laser ablation of some materials.
See also Special Chapter about the Time resolved spectroscopy
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