Chapter 5 UV/visible Absorption spectroscopy- Part 2. Apparatus and Instruments used for Uv/Vis measurements

Transcription

1 Chapter 5 UV/visible Absorption spectroscopy- Part 2 Apparatus and Instruments used for Uv/Vis measurements 1

2 2

3 3

4 4

5 5

6 6

7 7

8 8

9 9

10 10

11 11

12 12 EMR

13 13

14 14

15 Deuterium Lamps 15 Used to obtain ultraviolet absorption spectra. Above 350 nm its output is too weak for accurate measurements of absorption. The irradiance is too low for fluorescence measurements. This is a low pressure discharge lamp that produces light by the reaction, D 2 + e D * 2 2D + hn

16 Quartz Halogen Lamps Used to obtain visible absorption spectra. Its output below 300 nm is too weak for accurate measurements of absorption. The irradiance is too low for most fluorescence measurements. Power supply: 12 V dc at 4 A, and highly regulated. 16

17 Xenon Arc Lamps 17 it is difficult to obtain high quality spectra in those regions. This is a high Used to obtain fluorescence excitation, emission spectra, and make quantitative measurements. Below 300 nm the output is too weak to make accurate measurements. The lines between 450 and 500 nm and above 650 nm make pressure discharge lamp.

18 Mercury Arc Lamps Used to perform quantitative fluorescence measurements, or obtain emission spectra. The line output precludes obtaining excitation spectra. Note how the 2537 line is inverted! The UV, blue and green lines emit times more light than a xenon arc lamp. This is a high pressure discharge lamp. 18

19 Calibration Pen Lamps 19 Pen lamps can be purchased with a wide variety of gases that emit sharp line spectra. The line output is commonly used to calibrate wavelength separators, such as grating monochromators. A mercury pen lamp emits intense lines in the ultraviolet through the blue at 2537, 3130, 3650, 4047, and To calibrate in the red use a neon lamp.

20 20

21 21

22 22

23 23

24 Absorption Filters 24 The figure shows the transmission curve for three bandpass filters passes everything except the deep UV. It would be used to reject the mercury nm line passes a band from the near UV to the green, while passes a band from ~280 to 400 nm. These would be used with fluorescence excitation.

25 Absorption filters 25 The figure shows the transmission curve for two high pass filters rejects wavelengths below 500 nm and passes those above performs the same function with a cutoff wavelength of ~580 nm. Note that it is nearly impossible to make a low pass colored glass filter.

26 Interference Filter An interference filter is created when light is multiply reflected between two parallel surfaces. The beams exiting the filter must be in phase. Only specific wavelength can pass through The pass band varies with the order, N. To restrict transmission to only one order, the exit surface is most often a colored glass filter. 26

27 27

28 Interference Filter 28 Center wavelengths can be from the UV to the infrared. Pass bands are narrower than absorption filters, but broader than grating monochromators. The filter above is centered at 415 nm with a 10 nm FWHM and a transmission ~33%. As the bandwidth narrows the transmission drops.

29 29

30 30

31 31

32 32

33 33

34 Prisms Because of a complicated mathematical relationship between wavelength and bend angle, prisms are seldom used in modern instruments For maximum dispersion (angular resolution), visible light is separated with a glass prism and ultraviolet is separated with a quartz prism One common design is the constant deviation, or Pellen-Broca, prism shown in the figure. Constructed from one piece of material, it can be viewed as three separate prisms. For a given orientation, only one wavelength exits at a 90 deviation. To select a different wavelength the prism is rotated about the juncture of the dotted lines, P. P 60

35 35

36 36

37 37

38 38

39 Diffraction Grating A diffraction grating is created by a spatial modulation in phase or amplitude of an incoming plane wave When exiting the grating, the input is separated into several plane waves traveling at angle to the original direction of propagation - these are called diffraction orders. The number and intensity of the orders depends upon the functional form of the modulation. a m = The figure at the top is a transmission grating, while that at the bottom is a reflection grating. The diffraction angle depends upon order, m, wavelength, l, and spatial modulation period, a. a r i 39 sin m ml a

40 Blazed Gratings By forming the bottom of the grooves into a saw tooth shape, the angle of reflection can be made to correspond to one of the diffraction orders. One order then blazes, that is, it has most of the incoming optical power. Show below are three gratings blazed at 300, 400 and 500 nm. Note how the efficiency drops on either side of the blaze wavelength. a i +1 r 40

41 Grating Resolution 7.3 : 9 The angular distribution of a monochromatic beam of light reflecting off a diffraction grating is a sinc function (sinx/x). The node spacing of the sinc function is called the angular resolution, D, 2l D Na cos m where N is the number of grating grooves and a is the groove spacing. The goal is to minimize D, which is accomplished by maximizing the grating width, Na. You can also increase the order, which increases m. Grating spectral resolution is defined as, l R mn Dl where Dl is the range of wavelengths occurring over D, and l is the center wavelength. High resolution means small Dl. The 41 angle at which light leaves a grating leads to an ambiguous wavelength, e.g. 600 nm, m = 1; 300 nm, m = 2;

42 42 Commercially Available Gratings The vast majority of commercially available gratings are plastic replicas. The master grating is coated with an epoxy resin to form a negative image. A thin layer of plastic coats the negative and cured. When removed from the negative, the replica is mounted on glass for rigidity and coated with aluminum. Ruled gratings are made with a spacing from 25 mm -1 to 1,800 mm -1. The upper size is cm, with 5 5 cm being typical. Concave ruled gratings can be manufactured with great difficulty. Ruled gratings have a scattered light figure of Holographic gratings are made by coating the surface of glass with a photoresist. Two laser beams irradiate the surface at an angle to each other creating interference fringes. Bright fringes polymerize the photoresist. The surface is washed with an organic solvent to dissolve remaining monomer, then coated with aluminum. Spacings up to 2,400 mm -1 are possible, as well as concave gratings. Holographic gratings have a scattered light figure of 10-4.

43 43

44 44

45 45

46 46

47 47

48 48

49 49

50 50 Reverse bias creates a depletion layer that reduces the conductance of the junction nearly to zero When radiation impinges on the chip holes and electrons are formed in the depletion layer and those provide a current that is proportional to radiant power

51 51

52 52

53 53 IC = Integrated Circuit

54 54

55 55

56 56

57 57

58 58 General Instrument Designs

59 59

60 Spectronic 20 optical diagram 60

61 61

62 1. Double- beam-in-space configuration This requires two detectors that must be perfectly matched 2. Double- beam-in-time configuration Sample and reference measurements are separated in time. Rapidly rotated mirror or chopper is used. Only one detector is used 62

63 63 General instrument designs double beam-in-space

64 64 General instrument designs duoble beam-in-time

65 65

66 66

67 67

68 Multichannel photon detector It consists of an array of tiny photosensitive detectors that are arranged in a pattern that all elements of a beam of radiation that has been dispersed by a grating can be measured simultaneously 68

69 69 Multichannel diode array spectrometer (Multichannel photon detector)

70 Characteristics of UV/Vis Methods: Wide applicability to organic and inorganic systems Sensitivities to 10-4 to 10-7 M Moderate to high selectivity Good accuracy, about 1-3% relative uncertainty Ease and convenient data acquisition 70

71 Derivative and Dual wavelength Spectrohotometery The use of dual dispersing systems are arranged in such a way that two beams of slightly different wavelengths (typically 1 or 2 nm) fall alternatively onto a sample cell and its detector; no reference beam is used. The ordinate parameter is the difference between the alternate signals, which provides a good approximation of the derivative of absorbances as a function of wavelength (DA/ Dl). 71

72 Dual Wavelength spectrophotometer Two beams of differing wavelengths having same intensity are passed alternatively through a single sample cell (previous types used single- wavelength) Two monochromators are used. One monochromator may be used but light from the monochromator is chopped and the monochromator is shifted between the two wavelengths A single covette is used thus, scattering and instrumental stray light are cancelled 72

73 73 Dual-Wavelength Spectrophotometer using two monochromators

74 74

75 Derivative spectroscopy has been mostly used in the ultraviolet and visible regions for qualitative identification of species, in which the enhanced detail of a derivative spectrum makes it possible to distinguish among compounds having overlapping spectra. Dual-wavelength spectrophotometry has proven particularly useful for extracting ultraviolet/visible absorption spectra of analytes present in turbid solutions, where light scattering obliterates the details of an absorption spectrum. 75

76 76 Derivative Spectroscopy

77 77

78 78

79 Photoacoustic Spectroscopy Photoacaustic spectroscopy is based on light absorption effect. This effect is observed when a gas in a closed cell is irradiated with a chopped beam of radiation of a wavelength that is absorbed by the gas. The absorbed radiation causes periodic heating of the gas, which in turn results in regular pressure fluctuations within the chamber. If the chopping rate lies in the acoustical frequency range, these pulses of pressure can be detected by a sensitive microphone. Photoacoustic or optoacoustic spectroscopy, which was developed in the early 1970s, provides a means for obtaining ultraviolet and visible absorption spectra of solids, semisolids, or turbid liquids. Acquisition of spectra for these kinds of samples by ordinary methods is usually difficult at best and often impossible because of light scattering and reflection. 79

80 80 In photoacoustic studies of solids, the sample is placed in a closed cell containing air or some other nonabsorbing gas and a sensitive microphone. The solid is then irradiated with a chopped beam from a monochromator. The photoacoustic effect is observed provided the radiation is absorbed by the solid; the power of the resulting sound is directly related to the extent of absorption. Radiation reflected or scattered by the sample has no effect on the microphone and thus does not interfere. This latter property is perhaps the most important characteristic of the method.

81 81

82 Photoacoustic Spectrometer Light source is modulated at an audio frequency The sample absorbs the radiation and becomes a heat source producing alternating regions of compressions and refractions in the enclosed gas that is acoustic or sound wave The acoustic signal is converted into electrical signal by microphone Solid sample is directly irradiated with the modulated source resulting in production of acoustic waves in the surrounding gas 82

83 83

84 84

85 85

86 86