Components of Optical Instruments

1. Components of Optical Instruments 1

2. Spectroscopic methods are based on either: 1. Absorption  2. Emission 3. Scattering X (Inst A. B) • General Designs • Sources • Sample Holders • Wavelength Separators (selector). • Slits • Detectors • Data Collection (Signal Processor). 2

3. Source Sample Cell Wavelength Selector Detector An Absorption Instrumental Setup Detector Source An Emission or Scattering Instrumental Setup Processor Wavelength Selector Sample Cell Processor 3

4. Absorption Florescence, Phosphorescence and Scattering Emission and Chemiluminescenc 4

5. Spectroscopic instruments dependent on any of the above mechanisms encompass common components: 1. A stable source of radiation. 2. A wavelength selector to choose a single wavelength necessary for a certain absorption, emission or scattering process. 3. A radiation detector (transducer) that can measure absorbed, emitted or scattered radiation. 4. A signal processor that can change the electrical signal (current, voltage, or resistance) to a suitable form like absorbance, fluorescence, etc.

6. Properties Sources of Radiation Used in a selected range of wavelength should have the following properties: • It should generate a beam of radiation covering the wavelength range in which to be used. For example, a source to be used in the visible region should generate light in the whole visible region (340-780 nm). 2. The output of the source should have enough radiant power (Intensity) depending on the technique to be used. 3. The output should be stable with time and fluctuations in the intensity should be minimal.This necessitates the use of good regulated power supply. 6

7. Double beam instrument: • Is used to overcome fluctuationsتقلبات in the intensity of the beam with time. • In such instruments, the beam from the source is split into two halves [one goes to the sample while the other travels through a reference(blank)]. • Any fluctuations in the intensity of the beam traversingعبور the sample will be the same as that traversing the reference at that moment. One can thus make excellent correction for fluctuations in the intensity of the beam. 7

8. Classifications of Sources There can be several classifications of sources. • According to where their output is in the electromagnetic spectrum. • According to type: whether the source is a thermal or gas filled lamps, etc. 3) According to spectra needed: continuous or a line source. 4) Other classifications do exist. The easier one: continuous or line sources. 8

9. Continuous Sources(For molecules and compounds) Has an output in a continuum of wavelengths range. Examples: • Deuterium source for ultraviolet (UV) range: The output in the range from 180-350nm. 2)Tungsten lamp for Vis and NIR : The output range from 340-2500 nm The output extends through the whole visible and near infrared (IR) regions. 9

10. Line Sources(for atoms) Has a line output at definite wavelengths, rather than a range of wavelengths. Examples: • Hollow cathode lamp. • Electrodeless discharge lamps. 3) Laser. These lamps produce few sharp lines in the UV and visible (Vis). These will be discussed in details in Chapter 9. 10

11. Lasers The term LASER is an acronymاختصار for Light Amplification by Stimulated Emission of Radiation. تضخيم الضوء بواسطة الانبعاث المستحث للإشعاع. The first laser was introduced in 1960 and since then too many, highly important applications of lasers in chemistry were described. 11

12. Properties of Laser: • Emits very intense, monochromaticlight at high power (intensity). • All wavesin phase(unique), and parallel. • All waves are polarized in one plane. • Used to be expensive. • Not useful for scanningwavelengths.

13. Wavelength Selectors: To give, limited, narrow, continuous groups of wavelengths (band) in order to enhance the sensitivity of absorbance measurements and selectivity of Abs and Em. Ideal: one wave length Practically: band • Filters  • Prisms  • Gratings  • Michelson and other Interferometers X 13

14. Wavelength Selectors Wavelength selectors are important instrumental components that are used to obtain a certain wavelength or a narrow band of wavelengths. Three types of wavelength selectors can be described: I. Filters Filters are wavelength selectors that usually allow the passage of a band of wavelengths and can be divided into three main categories: 14

15. Properties of Filters • Simple, rugged (no moving parts in general) • Relatively inexpensive • Can select some broad range of wavelengths Most often used in • field instruments • simpler instruments • instruments dedicated to monitoring a single wavelength range. 15

16. 1) Absorption Filters This type of filters absorbs most incident wavelengths and transmits a band of wavelengths. Sometimes, they are called transmission filters. Properties: 1- Cheap and can be as simple as colored glasses or plastics. 2- They transmit a band of wavelengths with an effective bandwidth: (The effective band width is the width of the band at half height) in the range from 30-250 nm). 3- Their transmittance is usually low where only about 10-20% of incident beam is transmitted. 16

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18. 2) Cut-off Filters 1-Transmitance of about 100% is observed for a portion of the visible spectrum, which rapidly decreases to zero over the remainder of the spectrum. 2- Usually, cut-off filters are not used as wavelength selectors. 18

19. 3- Used in combination of absorption filters to decrease the bandwidth of the absorption filter or to overcome problems of orders, to be discussed later. 4- Only the combination of the two filters (Absrption and cut-off) (common area) will be transmitted which has much narrower effective bandwidth than absorption filters alone. 19

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21. 3) Interference Filters These filters are sometimes called Fabry-Perot filters and are dependent on the concept of light interference. An interference filter is composed of: 1- A transparent dielectric (غير موصلة):like calcium fluoride, sandwiched between two semitransparent metallic films (Au). 2- The array is further sandwiched between two glass plates to protect the filter. 3- The thickness of the dielectric is carefully controlled, as it is this factor, which defines the resulting wavelength. : 21

22. Polychromatic Radiation Glass Plate Metallic Film Dielectric Material Narrow Band of Radiation The structure of the interference filter: A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields. 22

23. Fabry-Perot Filters (Interference Filters) t

24. Incident polychromatic radiation hits the filter at right angles and the transmitted beam will have a very narrow bandwidth (5-30 nm). l = 2thi/n Where: t : Thickness of the dielectric layer. hi : The refractive index of the dielectric layer. n: is the radiation order. (single or more reinforced bands are transmitted)

27. Example 1: An interference filter is constructed with an MgF2 dielectric (index of refraction = 1.36). What are the first, second, and third order wavelengths transmitted by the filter if the dielectric layer is 500 nm thick? λ = 2tτ/n where τ is the refractive index of the dielectric 1st order wavelength: λ = {2*500 * 1.36}/1 = 1360 nm 2nd order wavelength: λ = {2*500 * 1.36}/2 = 680 nm 3rd order wavelength: λ = {2*500 * 1.36}/3 = 453 nm

28. Example 2:Calculate the thickness of the dielectric (τ = 1.38) in an interference filter required for the first order transmission of 580 nm wavelength. What other wavelengths are transmitted? λ = 2 t τ/n 580 = (2*t * 1.38)/1 t = 210 nm Other transmitted wavelengths will be all harmonics derived from the 1st order wavelength. That is: 580/2, 580/3, 580/4, etc

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30. 4) Interference Wedges It is clear from the discussion above that several interference filters are necessary to, for example, cover the visible range of the spectrum. This is not convenient as we would have to interchange filters according to wavelength of interest. To overcome this problem: a wedge machined dielectric was used. The dielectric in this case has different thicknesses and thus can transmit a wide range of wavelengths accordingly. 30

31. Incident Radiation Output Wavelengths Metallic Film Dielectric Glass Plate Wedge Movement Slit 31

33. 2)Prisms A prism is a wavelengths selector that depends on the dispersion ability of the incident radiation by the prism material. Dispersion: The variation of refractive index with wavelength, or frequency. Polychromatic light: is composed of several wavelengths, so dispersion of these wavelengths will be different when they are transmitted through the prism. Dispersion pattern for white light: As  decreases the dispersion increases and well separated. تناسب عكسي مع الطول الموجي 33

35. Red Orange Yellow Incident beam Green Blue 35

36. Two common types of prisms can be identified: • Cornu Prism: It is a 60o prism which is made either from glass or quartz. When quartz is used, two 30oprisms (one should be left handed and the other is right handed) are cemented together in order to get the 60o prism. This is necessary since natural quartz is optically active and will rotate light either to right or left hand. Cementing the left and right handed prisms will correct for light rotation and will transmit the beam in a straight direction. (لتعديل مسار الشعاع بحيث يسير في خط مستقيم) 36

37. Littrow Prism: A littrow prism is a 30o prism which uses the same face for input and dispersed radiation. The beam is reflected at the face perpendicular to base, due to presence of a fixed mirror. A littrow prism would be used when a few optical components are required. 37

38. Mirror Cornu Littrow 38

39. It should be always remembered that glass is nontransparent to UV radiation. • Therefore, when radiation in the ultraviolet is to be dispersed, a quartz prism, rather than a glass, prism should be used. • Quartz serves well in both UV and Vis. • It should also be appreciated that the dispersion of a prism is nonlinear since it is dependent on wavelength. (Dispersion increases for shorter wavelength) 39

40. Prisms are very good wavelength selectors in the range from may be 200-300 nm but are bad ones for wavelength selection above 600 nm. لأنه بزيادة الطول الموجي: يقل التشتت وتبعا لذلك يقل فصل الاطول الموجية عن بعضها البعض • The nonlinear dispersion of prisms also imposes problems on the instrumental designs which will be discussed later.

41. 200 800 250 300 350 500 Wavelength (dispersion ability) 41

42. Reflection Gratings • Widely used in instruments today. • Light reflected off a surface, and not cancelled out by destructive interference, is used for selection of wavelengths • Constructed of various materials…. • Polished glass, silica or polymer substrate • Grooves milled or laser etched into the surface • Coated with a reflective material (silvered) such as a shiny metal • VERY FRAGILE!! • Sealed inside the instrument. DO NOT TOUCH!

43. 3) Gratings Is an optically flat polished surface that has dense parallel grooves. Two types of gratings are usually encountered: • transmission and 2)reflection (diffraction) gratings. Transmission gratings are seldom used in spectroscopic instruments and almost all gratings, which are used in conventional spectroscopic instruments, are of the reflection type. The groove density can be as low as 80 to several thousand (6000) lines/mm. Two common types of reflection gratings can be identified: 43

45. (Reflection or diffraction grating)

48. Echelette Gratings: contain from 300 to 2000 lines/mm but an average line density of about 1200 to 1400 lines/mm is most common. • The echelette grating uses the long face for dispersion of radiation. • It is the grating of choice for molecular spectroscopic instruments. • In contrast to prisms, gratings usually have linear dispersion of radiation. 48

50. 2. Echelle Gratings: These have relatively coarse grooves (~80-300 lines/mm). They use the short face for dispersion of radiation and are characterized by very high dispersion ability.