1	Skoog – Chapter 7 Components of Optical Instruments General Design of Optical Instruments Sources of Radiation Wavelength Selectors Sample Containers Radiation Transducers (Detectors) Signal Processors and Readouts Fiber Optics
2	Ultraviolet – Visible – Infrared Instrumentation Focus our attention on measurements in the UV-vis region of the EM spectrum Good instrumentation available Very widely used techniques Longstanding and proven methods IR instrumentation will be considered from time to time particularly when there are similarities to UV-vis
3	"Absorption measurements require:" Absorption measurements require: 1) source of radiation 2) device for dispersing radiation into component wavelengths 3) a means of putting sample into the optical path, i.e., cell 4) Detector to convert the EM to an electrical signal 5) readout device or circuitry, i.e., meter, computer, recorder, integrator, etc.
4	"Block diagram of instrument for..." Block diagram of instrument for absorption
5	"Block diagram of instrument for..." Block diagram of instrument for absorption
6	"Emission measurements require:" Emission measurements require: 1) means of exciting emission i.e., way of populating upper energy level which spontaneously emits 2) device for dispersing radiation into component wavelengths 3) a means of putting sample into the optical path, i.e., cell 4) Detector to convert the EM to an electrical signal 5) readout device or circuitry, i.e., meter, computer, recorder, integrator, etc.
7	"Block diagram of instrument for..." Block diagram of instrument for emission i.e., & fluorescence phosphorescence
8	"The requirements for the various..." The requirements for the various components used in different instruments change with the type of spectroscopy as well as for different kinds of measurements within a type of spectroscopy We will consider the components separately then combine them to make the overall instrument And finally look at the measurements with regard to theory and practice
9	"Sources – important characteristics" Sources – important characteristics 1) Spectral distribution i.e., intensity vs. λ (continuum vs. line sources) 2) Intensity 3) Stability – short term fluctuations (noise), long term drift 4) Cost 5) Lifetime 6) Geometry – match to dispersion device
10	"CONTINUUM SOURCES" CONTINUUM SOURCES Thermal radiation (incandescence) – heated solid emits radiation close to the theoretical “Black Body” radiation i.e., perfect emitter, perfect absorber Behavior of Black Body - Total power ~ T⁴ therefore need constant temperature for stability when using incandescent sources - Spectral distribution follows Planck’s radiation law
11	"Spectral Distribution Curves of a..." Spectral Distribution Curves of a Tungsten (Black Body) Lamp
12	"IR Region thermal sources (..." IR Region thermal sources (Black Body) are: Nernst Glower – fused mixture of ZrO₂, Y₂O₃, and ThO₂ normally operated at 1900 ^oC – better for shorter IR λ’s (near IR) Globar – silicon carbide normally operated at 1200 to 1400 ^oC – better at longer IR λ’s (doesn’t approach Black Body) Incandescent Wire – e.g., nichrome wire – cheapest way
13	"All operated at relatively low..." All operated at relatively low temperature. Good for IR and give some visible emission. Operated in air so will burn up if temp goes too high Advantages Nernst Glower – low power consumption, operates in air, long lifetime Globar – more stable than Nernst Glower, requires more power & must be cooled. Long lifetime, but resistance changes with use
14	"Visible Region sources are:" Visible Region sources are: Glass enclosed Tungsten (W) filament - normally operated at ~3000 ^oK with inert atmosphere to prevent oxidation. Useful from 350 nm to 2000 nm, below 350 nm glass envelope absorbs & emission weak Tungsten-Halogen lamps - can be operated as high as 3500 ^oK. More intense (high flux). Function of halogen is to form volatile tungsten-halide which redeposits W on filament, i.e., keeps filament from burning out. Requires quartz envelope to withstand high temps (which also transmits down to shorter wavelengths). Fingerprints are a problem – also car headlights
15	"Gas Discharge Lamps – two..." Gas Discharge Lamps – two electrodes with a current between them in a gas filled tube. Excitation results from electrons moving through gas. Electrons collide with gas à excitation à emission At high pressure à “smearing” of energy levels à spectrum approaches continuum The higher the pressure, the greater the probability that any given molecule or atom will be perturbed by its neighbor at the moment of emission.
16	"Hydrogen Lamp - most common..." Hydrogen Lamp - most common source for UV absorption measurements H₂ emission is from 180 nm to 370 nm limited by jacket Line spectrum from à 100 watt Hydrogen Lamp at low pressure in Pyrex
17	"Deuterium Lamp – same λ..." Deuterium Lamp – same λ distribution as H₂ but with higher intensity (3 to 5 times) - D₂ is a heavier molecule & moves slower so there is less loss of energy by collisions High pressure D₂à with quartz jacket
18	"For higher intensity" For higher intensity Xenon Lamp – Xe at high pressure (10-20 atm) - high pressure needed to get lots of collisions for broadening leading to continuum - short life relatively - arc wander (stabilize) - need jolt to start - output = f(time)
19	"d)" d) High Pressure Mercury Lamp – can’t completely eliminate bands associated with particular electronic transitions even at very high pressures (e.g., 100 atm)
20	"For UV-vis absorption spectrophotometry usually..." For UV-vis absorption spectrophotometry usually use H₂ for UV and tungsten for visible region (switching mid scan) Sometimes use D₂ instead of H₂ For fluorescence spectrophotometry use xenon arc lamp in scanning instruments Can use He below 200 nm Hg at low pressure is used in fixed wavelength (non scanning) fluorometers Can use mixture of Hg and Xe
21	"LINE SOURCES" LINE SOURCES Gas (Vapor) Discharge Lamps at low pressure (i.e., few torr) – minimize collisional interaction so get line spectrum - most common are Hg and Na - often used for λ calibration - Hg pen lamp - fluorescent lights are another example - also used UV detectors for HPLC Hollow Cathode Lamps (HCL) – for AA Electrodeless Discharge Lamps (EDL) - AA
22	"4)" 4) Lasers (Light Amplification by Stimulated Emission of Radiation) – start with material that will exhibit stimulated emission and populate upper states typically using another light source
23	"Stimulated Emission – photon strikes..." Stimulated Emission – photon strikes excited state causing it to emit a burst of photons Pumping source used to populate upper states can be flashlamp, another laser or electrical Often use prism to select pumping wavelength Advantages of lasers 1) Intense 2) Monochromatic – very narrow band 3) Coherent – all radiation at same phase angle
24	"4)" 4) Directional – full intensity emitted as beam Limitations of lasers High cost in many cases Wavelength range is somewhat limited Many operate in pulsed mode – some are continuous wave (CW) Pulsed mode lasers are not always problematic as light sources, can use pulse frequency with gated detection
25	"Types of Lasers:" Types of Lasers: 1) Solid State Lasers a) Ruby laser – Al₂O₃ + Cr(III) - 694.3 nm pumped with Xe arc flashlamp – pulsed (can be continuous) b) Nd/YAG laser – yittrium aluminum garnet + Nd - 1064 nm 2) Gas Lasers a) Neutral atom – He-Ne – 632.8 nm continuous b) Ion lasers – Ar⁺ or Kr⁺514.5 nm
26	"c)" c) Molecular lasers – CO₂ (10,000 nm = 1000 cm^-1) or N₂ (337.1 nm) pulsed d) Eximer lasers – inert gas + fluorine creates eximers ArF⁺(193 nm), KrF⁺(248 nm), XeF⁺ (351) pulsed Dye Lasers – tunable over 20 – 50 nm many dyes available for wide range of λ’s Semiconductor Diode Lasers – wide range of λ’s available, continuous
27	Wavelength Selection Three main approaches: 1) Block off unwanted radiation – optical filters 2) Disperse radiation & select desired band – monochromator 3) Modulate wavelengths at different frequencies - interferometer FILTERS 1) Absorption – colored glass, colored film, colored solutions – cheapest way
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30	"Absorption filters are also known..." Absorption filters are also known as bandpass filters Usually exhibit low peak transmittance Typically have a broad peak profile Can use two or more absorption filters together to produce desired transmittance characteristics Generic filters are 2 x 2 inch glass or quartz Relatively inexpensive
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35	"Condition for constructive interference" Condition for constructive interference mλ 2d = ------ η If distance (d) is multiple (m) of wavelength (λ) then it won’t be interfered with Concept of Order – constructive & destructive interference causes waves with different phase angles to be eliminated except if they are multiples of each other
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37	"Transmittance vs." Transmittance vs. wavelength for typical Fabrey-Perot Interference filter showing first and second order λ’s (m = 1 & m = 2)
38	"3)" 3) Neutral density filters – reduces intensity without any λ discrimination
39	"MONOCHROMATORS" MONOCHROMATORS
40	"Optical Materials – need optically..." Optical Materials – need optically transparent materials for lenses, prisms & sample cells In visible region – can use glass down to 350 nm In the UV region – quartz is material of choice In the IR region – NaCl, KBr, etc. The heavier the atoms of the salt, the farther into the IR region (i.e., longer λ) before significant absorption occurs Problem – sensitivity to moisture