Fourier Transform Infrared Spectroscopy

Introduction The range of Infrared region Is 12800- 10 cm-l. It bear be divided into near- infrared radiation region (12800 4000 crn-ll mid-infrared region (4000 200 crnl ) and far-infrared region (50 1000 cm-l). scientists kick in established various ways to utilize infrared light. Infrared compactness spectroscopy is the manner which scientists use to determine the structures of molecules with the molecules characteristic ingress of infrared radiation. Infrared spectrum is molecular vibrational spectrum.When exposed to Infrared radiation, sample molecules selectively absorb radiation of pecific wavelengths which causes the change of dipole moment of sample molecules. Consequently, the vibrational zipper levels of sample molecules transfer from ground state to excited state. The frequency of the absorption peak is determined by the vibrational energy gap. The payoff of absorption peaks is link to the number of vibrational freedom of the molecule. The intensity of absorpti on peaks is related to the change of dipole moment and the possibility of the transition of energy levels.Therefore, by analyzing the infrared spectrum, one can readily obtain luxuriant structure information of a molecule. Most molecules are infrared active agent except for several homonuclear diatomic molecules such as 02, N2 and C12 collect to the cryptograph dipole change in the vibration and rotation of these molecules Concept Fourier alter spectroscopy Is a less Intuitive way to obtain the kindred Information. Rather than shining a monochromatic beam of light at the sample, this technique shines a beam containing many frequencies of light at once, and measures how untold of that beam Is absorbed by the sample.Next, the beam Is modified to contain a diametric combination of frequencies, giving a second data point. This performance is repeated many eons. Afterwards, a computer takes all these data and works backwards to Infer what the absorption Is at each wavelength T he beam exposit above is generated by starting with a broadband light character reference one containing the full spectrum of wavelengths to be metric. The light shines into a Michelson interferometera authentic configuration of reflects, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked. ransmitted, blocked, transmitted. by the Interferometer, due to wave interference. Different wavelengths are modulated at different rates, so that at each moment, the beam coming out of the interferometer has a different spectrum. Fourier Transform of Interferogram to Spectrum The interferogram is a scarper of time and the values outputted by this function of time are said to make up the time domain. The time domain Is Fourier transformed to get a frequency domain, which is deconvoluted to product a spectrum Step 1 The first footstep is sample preparation. The standard method to prepare solid sample for FTIR spectrometer is t o use KBr.About 2 mg of sample and 200 mg KBr re dried and ground. The particle sizing should be unified and less than devil micrometers. indeed, the mixture is squeezed to form right-down pellets which can be measured directly. For perspicuouss with high boiling point or viscous ancestor, it can be added in between two NaCl pellets. Then the sample is fixed in the cell by skews and measured. For volatile liquid sample, it is dissolved in CS2 or CC14 to form 10% solution. Then the solution is injected into a liquid cell for measurement. Gas sample needs to be measured in a gas cell with two KBr windows on each side. The gas cell should first be vacuumed.Then the sample can be introduced to the gas cell for measurement. Step 2 The second step is getting a emphasise spectrum by collecting an interferogram and its subsequent changeover to frequency data by inverse Fourier transform. We obtain the background spectrum because the resolving in which we place our sample depart hav e traces of dissolved gases as well as solvent molecules that contribute information that are non our sample. The background spectrum will contain information about the species of gases and solvent molecules, which whitethorn then be subtracted away from our sample spectrum in order to accomplish nformation about Just the sample.Figure 6 shows an example of an FTIR background spectrum. Figure 6. primer coat IR spectrum The background spectrum also takes into account several other factors related to the instrument performance, which includes information about the source, interferometer, detector, and the contribution of ambient water (note the two irregular groups of lines at about 3600 cm-l and about 1600 cm-l in Figure 6) and carbon copy dioxide (note the doublet at 2360 cm-l and sharp spike at 667 cm-l in Figure 6) dedicate in the optical bench.Step 3 Next, we collect a item-by-item-beam spectrum of he sample, which will contain absorption bands from the sample as well as th e background (gaseous or solvent). Step 4 The ratio between the single-beam sample spectrum and the single beam background spectrum gives the spectrum of the sample (Figure 7). Advantages Speed Because all of the frequencies are measured simultaneously, most measurements by FT-IR are made in a yield of seconds rather than several minutes.This is sometimes referred to as the Felgett Advantage. Sensitivity Sensitivity is dramatically improved with FT-IR for many reasons. The detectors employed are uch more sensitive, the optical throughput is overmuch higher (referred to as the enable the coaddition of several scans in order to sign the random measurement noise to any desired level (referred to as signal averaging). ? Mechanical Simplicity The moving mirror in the interferometer is the just continuously moving part in the instrument. Thus, there is very minor possibility of mechanical breakdown. Internally Calibrated These instruments employ a HeNe laser as an internal wavelength calibration standard (referred to as the Connes Advantage). These instruments are self-calibratingand neer need to be calibrated by the user.