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Chapter 13 An Introduction to Ultraviolet/Visible Molecular Absorption Spectrometry.

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Presentation on theme: "Chapter 13 An Introduction to Ultraviolet/Visible Molecular Absorption Spectrometry."— Presentation transcript:

1 Chapter 13 An Introduction to Ultraviolet/Visible Molecular Absorption Spectrometry

2 Terms & Symbols

3 Beers Law log P o /P = ebc = A The derivation of this law assumes a) That the incident radiation is monochromatic. b) The absorption occurs in a volume of uniform cross-section. c) The absorbing substances behave independently of each other in the absorbing process.

4 Beers Law Limitations to Beers Law Real Limitations to Beers Law Apparent Chemical Deviations Apparent Instrumental Deviations with Polychromatic Radiation Instrumental Deviations in the Presence of Stray Radiation

5 The Effects of Instrumental Noise on Spectrophotometric Analyses Types of Noise Shot noise – This noise is generated by current flowing across a P-N junction and is a function of the bias current and the electron charge. The impulse of charge q depicted as a single shot event in the time domain can be Fourier transformed into the frequency domain as a wideband noise. Thermal noise – In any object with electrical resistance the thermal fluctuations of the electrons in the object will generate noise. White noise- The spectral density of thermal noise is flat with frequency. Burst noise – Occurs in semiconductor devices, especially monolithic amplifiers and manifests as a noise crackle.

6 The Effects of Instrumental Noise on Spectrophotometric Analyses Types of Noise Avalanche noise – Occurs in Zener diodes are reversed biased P-N junctions at breakdown. This noise is considerably larger than shot noise, so if zeners have to be used as part of a bias circuit then they need to be RF decoupled. Flicker noise – This noise occurs in almost all electronic devices at low frequencies. Flicker noise is usually defined by the corner frequency FL. Sources of Noise Case I: s T = k 1 Case II: s T = k 2 (T 2 + T) ½ Case III: s T = k 3 T

7 The Effects of Instrumental Noise on Spectrophotometric Analyses Effect of Slit Width on Absorbance Measurements

8 Instrumentation Instrument Components Sources Wavelengths selectors Sample containers Radiation detectors Signal processors and readout devices

9 Instrumentation Sources- Light sources Deuterium and Hydrogen Lamps Tungsten Filament Lamps

10 Instrumentation Types of Instruments Single-beam Double-beam in space Double-beam in time Multichannel

11 Instrumentation Single-Beam Instruments

12 Instrumentation Double-Beam Instruments

13 Instrumentation Typical Instruments Photometers Visible Photometers Probe-type Photometers Ultraviolet Absorption Photometers Spectrophotometers

14 Instrumentation Most common spectrophotometer: Spectronic On/Off switch and zero transmission adjustment knob 2.Wavelength selector/Readout 3.Sample chamber 4.Blank adjustment knob 5.Absorbance/Transmittanc e scale

15 Instrumentation Visible Region

16 Instrumentation Single-Beam Instruments for the Ultraviolet/Visible Region

17 Instrumentation Single-Beam Computerized Spectrophotometers Inside of a single-beam spectrophot ometer connected to a computer.

18 Instrumentation Double-Beam Instrumetents Double-Dispersing Instruments Diode Array Instruments

19 References html df


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