THROUGHPUT OF AN OPTICAL INSTRUMENT CHEM 314

Transcription

1 THROUGHPUT OF AN OPTICAL INSTRUMENT CHEM 314

2 OBJECTIVES Calculate the number of photons present in a single beam UV- Vis: At the source Entering the monochromator Incident on the diffracgon gragng Emerging from the monochromator Incident on the sample Emerging from the sample Incident on the detector Calculate the number of electrons generated by the photons on the detector. Examine detector current to voltage converter electronics. Calculate the voltage measured from the detector.

3 BUILDING A SPECTROSCOPIC INSTRUMENT Components 1. Stable radiagon source 2. Wavelength isolagon- monochromator 3. Transparent sample holder/ opgcs 4. Detector 5. Signal processing

4 BUILDING A SPECTROSCOPIC INSTRUMENT

5 BUILDING A SPECTROSCOPIC INSTRUMENT A

6 SOURCES ConGnuum sources with constant output as a funcgon of wavelength required for molecular spectroscopy

7 Output spectrum at 2870 K TUNGSTEN FILAMENT

8 BLACKBODY RADIATION Wein s Displacement Law λ max = max output λ T= temp (K) L= luminosity A= surface area α=constant h\ps://docs.kde.org/stable/en/kdeedu/kstars/ai- blackbody.html

9 OpGcs

10 TRANSMISSION At a given frequency i: n i refracgve index v i velocity of propagagon c speed of light in a vacuum

11 REFRACTION Snell s law Less dense Light bends toward normal more dense

12 REFLECTION h\p://maxgrace.wordpress.com/2010/07/14/the- love- chapter- one- more- Gme/

13 PLOT OF % REFLECTION VS ANGLE OF INCIDENCE

14 TASKS 1. Measure distances (page 2) on the spectrometer. 2. Cut and weigh the output spectrum to determine fracfon of emission in nm range.

15

16 LEARNING CHECK Skoog n Holler, pg 142

17 LEARNING CHECK Diagram a monochromator Explain how it works

18 LEARNING CHECK Is λ1 or λ2 larger wavelength light? How do you know?

19 THROUGHPUT OF AN OPTICAL INSTRUMENT CHEM 314

20 OBJECTIVES Calculate the number of photons present in a single beam UV- Vis: At the source Entering the monochromator Incident on the diffracgon gragng Emerging from the monochromator Incident on the sample Emerging from the sample Incident on the detector Calculate the number of electrons generated by the photons on the detector. Examine detector current to voltage converter electronics. Calculate the voltage measured from the detector.

21 BUILDING A SPECTROSCOPIC INSTRUMENT Components 1. Stable radiagon source 2. Wavelength isolagon- monochromator 3. Transparent sample holder/ opgcs 4. Detector 5. Signal processing

22 BUILDING A SPECTROSCOPIC INSTRUMENT A

23 WAVELENGTH SELECTION

24 MONOCHROMATORS 1. Entrance slit- provides rectangular opgcal image 2. CollimaGng lens or mirror- makes light beams parallel 3. Dispersive element- disperses light into component wavelengths 4. Focusing element- reforms rectangular opgcal image focused on focal plane 5. Exit slit- on focal plane, selects desired bandwidth

25 MONOCHROMATORS: PRISMS VS GRATINGS When might a prism be be\er than a diffracgon mono?

26 Dispersion

27 Prisms Snell s law RefracGve index

28 PRISMS Bunsen Prism Cornu Prism Li\row Prism

29 REFLECTION GRATING MONOCHROMATOR λ 1 >λ 2 h\ps://encrypted- tbn0.gstagc.com/images?q=tbn:and9gcs53if5b18udb7pvw7texat3q63kqm1qmwvo1pbt5r- uv1axefg0- T4hL0

30 ECHELLETTE- DIFFRACTION LONG EDGE

31 MONOCHROMATOR CALCULATION An echelleoe grafng that contains 1200 blazes per mm was irradiated with a polychromafc beam at an incident angle of 29 to the grafng normal. Calculate the wavelengths of radiafon that would appear at an angle of reflecfon of +20, +10, and 0. Determine the angle at which 500 nm light will appear.

32 ECHELLE MONOCHROMATOR

33 MONOCHROMATOR CALCULATION An echelle grafng that contains 1400 blazes per mm was irradiated with a polychromafc beam at an incident angle of 20 to the grafng normal. Calculate the angles at which 488, 514, 600 nm light would appear at the exit slit.

34

35 MONOCHROMATOR PERFORMANCE CHARACTERISTICS 1. Spectral purity 2. Dispersion of grafng (D) Reciprocal linear dispersion (D - 1 ) 3. Resolving power (R= λ/δλ) 4. EffecFve bandwidth (Δλ eff ) 5. Light gathering power (F) Focal length (f)

36 EFFECTIVE BANDWIDTH

37 SLIT WIDTH

38 MONOCHROMATOR PERFORMANCE CHARACTERISTICS 1. Spectral purity 2. Dispersion of grafng (D) Reciprocal linear dispersion (D - 1 ) 3. Resolving power (R= λ/δλ) 4. EffecFve bandwidth (Δλ eff ) 5. Light gathering power (F) Focal length (f)

39 OPTICS AND SAMPLE HOLDERS Absorbance Quartz PlasGc Wavelength (nm)

40 Sources of Nonlinearity of Beer s law 1. SoluGon factors 2. Non- monochromagc light 3. Not analyzing at l max 4. Stray light 5. Mismatched cuve\es 6. Instrument noise Too much or too li\le absorpgon

41 1. SoluGon factors High analyte concentragons (>0.01M) High electrolyte concentragons RefracGve index of medium ReacGons within the solugon HIn ó H + + In - Red Cl - 40 Na Red + Cl Red - Na 40 + Na + 40 Cl - Na+ Na + Cl Red - Cl - Cl - Na + 40 Cl - Na + Na + Cl - Cl - Na Red + Cl - Na + Cl - Red Na 40 + Na + 40 Na + Cl Red - Na + Cl - Cl - Na 40 + Cl - Incident Light P 0 Emergent Light P

42 1. SoluGon factors ReacGons within the solugon HIn ó H + + In -

43 2. Non monochromagc source λ and λ are different wavelengths

44 2. Non monochromagc source Mono slit width determines spread of λ incident on sample (bandwidth) Image incident on mono exit plane Wide slits allow More light (higher throughput) More λ (larger bandwidth) No such thing as a free lunch

45 2. Non monochromagc source And mono slit width What slit width should you choose?

46 2. Non monochromagc source and slit width

47 3. Not analyzing at λmax

48 3. Not analyzing at λmax

49 4. Stray light

50 5. Mismatched cuve\es Differences in: Path length OpGcal characterisgcs Most likely to affect calibragon curve intercept SoluGon: Double beam: Use matched cuve\es Single beam; Use the same cuve\e

51 6. Instrumental noise in Transmission measurements

52 Instrumental noise in Transmission measurements

53

54 Linear Range of Beer s Law Absorbance (arb) [Kool-aid]

55 DerivaGon of Beer s Law

56

57 THROUGHPUT OF AN OPTICAL INSTRUMENT CHEM 314

58 BUILDING A SPECTROSCOPIC INSTRUMENT A

59 DETECTORS

60 DETECTORS Figure 7-27 CdS PMT Se/SeO CdSe GaS PbS Si photodiode Thermocouple Golay cell

61 PHOTOMULTIPLIER TUBE (PMT)

62 OPERATIONAL AMPLIFIER Many resistors and transistors on a single chip

63 UV- VIS ELECTRONIC CIRCUIT

64 THE REAL ELECTRONIC CIRCUIT

65 OPERATIONAL AMPLIFIERS Originally applied to analog computers to do simple math funcfons. Large open loop gains ( )- large amplificafon. High input impedance ( Ω)- doesn t mess with current from PMT. Low output impedance ( Ω)- doesn t mess with reading voltage. Very low dark current- which can be easily offset.

66 CURRENT TO VOLTAGE CONVERTER Low resistance to source current Prevents loading error during current measurement High amplificafon of signal

67 PARTS OF A CIRCUIT

68 PARTS OF A CIRCUIT Current to voltage converter Capacitor stabilizes output voltage Current shunt to ground Voltage readout Dark current adjustment