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PART 3 1 2 Absorption Spectrometer Source Sample Wavelength Selector Dr. S. M. Condren Detector Signal Processor Readout 4 (a) Construction materials Dr. S. M. Condren (b) wavelength selectors for spectroscopic instruments. (c) Sources. Dr. S. M. Condren (d) Detectors for spectroscopic instruments. Dr. S. M. Condren IR Region Nernst glower - rare earth oxides globar - silicon carbide rod incandescent wire - nichrome wire Dr. S. M. Condren Filters interference filters interference wedges absorption filters Dr. S. M. Condren Monochromators Components entrance slit collimating element (lens or mirror) prism or grating as dispersing element focusing element (lens or mirror) exit slit Dr. S. M. Condren “Two types of monochromators: (a) CzerneyTurner grating monochromator (b) Bunsen prism monochromator." Dr. S. M. Condren UV-Visible-Near IR IR NaCl Cornu type Littrow type Quartz dq dq dn --- = ----- ----dl dn dl where q => angle l => wavelength n => refractive index Dr. S. M. Condren R => resolving power l dn R = ------ = b ----dl dl where b=> length of prism base Dr. S. M. Condren Diffraction Monochromators Diffraction If l is large compared to the aperture, the waves will spread out at large angles into the region beyond the obstruction. Video 1 Video 2 Diffraction increases as aperture size l Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998. Diffraction Pattern From a Single Slit Ingle and Crouch, Spectrochemical Analysis Diffraction Pattern From a Single Slit For Destructive Interference: x = l/2 W sin q = l Ingle and Crouch, Spectrochemical Analysis W x sinq 2 Diffraction Pattern From a Single Slit For Destructive Interference: x = l/2 W sin q= 2 l Ingle and Crouch, Spectrochemical Analysis x W sinq 4 Diffraction Pattern From a Single Slit For Destructive Interference: W sin q = m l m = ±1, ±2, ±3, … Ingle and Crouch, Spectrochemical Analysis Diffraction Gratings Plane or convex plate ruled with closely spaced grooves (3002400 grooves/mm). Eugene Hecht, Optics, 1998. http://www.olympusmicro.com/primer/java/imageformation/gratingdiffraction/index.html Grating Equation Two parallel monochromatic rays strike adjacent grooves and are diffracted at the same angle (b). Difference in optical pathlength is AC + AD. For constructive interference: ml = (AC + AD) m = 0, 1, 2, 3, … Ingle and Crouch, Spectrochemical Analysis Grating Equation ml = (AC + AD) AC = d sin a AD = d sin b Combine to give Grating Equation: d(sin a + sin b) = ml Grating Equation only applies if: d > l/2 Ingle and Crouch, Spectrochemical Analysis Are you getting the concept? At what angle would you collect the 1st order diffracted light with l = 500 nm if a broad spectrum beam is incident on a 600 groove/mm grating at qi = 10°? For l = 225 nm? For l = 750 nm? Modern infrared spectrometers are very different from the early instruments that were introduced in the 1940s. Most instruments today use a Fourier Transform infrared (FT-IR) system. In early experiments infrared light was passed through the sample to be studied and the absorption measured. This approach has been superseded by Fourier transform methods. A beam of light is split in two with only half of the light going through the sample. The difference in phase of the two waves creates constructive and/or destructive interference and is a measure of the sample absorbance. The waves are rapidly scanned over a specific wavelength of the spectra and multiple scans are averaged to create the final spectrum. This method is much more sensitive than the earlier dispersion approach. A Fourier transform is a mathematical operation used to translate a complex curve into its component curves. In a Fourier transform infrared instrument, the complex curve is an interferogram, or the sum of the constructive and destructive interferences generated by overlapping light waves, and the component curves are the infrared spectrum. An interferogram is generated because of the unique optics of an FT-IR instrument. The key components are a moveable mirror and beam splitter. The moveable mirror is responsible for the quality of the interferogram, and it is very important to move the mirror at constant speed. For this reason, the moveable mirror is often the most expensive component of an FTIR spectrometer. The beam splitter is just a piece of semireflective material, usually mylar film sandwiched between two pieces of IRtransparent material. The beam splitter splits the IR beam 50/50 to the fixed and moveable mirrors, and then recombines the beams after being reflected at each mirror. Michelson Interferometer "Schematic of a Michelson interferometer illuminated by a monochromatic source." Dr. S. M. Condren "Illustration s of time doamin plots (a) and (b); frequency domain plots (c), (d), and (e)." Dr. S. M. Condren “Comparison of interferograms and optical spectra.” Dr. S. M. Condren Dr. S. M. Condren 37