Introduction to Spectroscopy
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Questions and Answers

What is primarily used for quantitative analysis in spectroscopy?

  • Wavenumber of EMR
  • Wavelength of EMR
  • Absorbance of EMR (correct)
  • Frequency of EMR

    • Electromagnetic radiation can only be classified as visible or invisible.

    False

    The unit of frequency is _______.

    Hertz (Hz)

    Match the type of electromagnetic radiation with its corresponding wavelength range:

    <p>Cosmic Rays = 10-6 nm X-rays = 10-1 nm Infrared Radiation = 102 μm Radio Waves = 1010 μm</p> Signup and view all the answers

    What is the formula relating energy (E), frequency (ν), and Planck's constant (h)?

    <p>E = hv</p> Signup and view all the answers

    Which of the following statements is true regarding the speed of light?

    <p>Speed of light is constant in a vacuum.</p> Signup and view all the answers

    Longer wavelengths correspond to higher frequency.

    <p>False</p> Signup and view all the answers

    What do photons represent in electromagnetic radiation?

    <p>Discrete packets of energy</p> Signup and view all the answers

    What is the wavelength range for visible light?

    <p>400-800 nm</p> Signup and view all the answers

    All forms of electromagnetic radiation are visible to the human eye.

    <p>False</p> Signup and view all the answers

    What is the effect of a molecule absorbing ultraviolet light?

    <p>It raises electrons to a higher energy level and causes an electronic transition.</p> Signup and view all the answers

    Molecules absorb I.R regions to raise the __________ of the nuclei.

    <p>vibration</p> Signup and view all the answers

    Match the type of energy absorption with the corresponding effect:

    <p>Transitional Energy = Raising Electrons to a Higher Energy Level Vibrational Energy = Raising the Vibration of the Nuclei Rotational Energy = Increasing Rotation of the Molecule Around the Axis Electronic Excitation = Displacement of an outer electron</p> Signup and view all the answers

    Which type of transitions occur when a molecule absorbs UV-Vis radiation?

    <p>Electronic, vibrational, and rotational transitions</p> Signup and view all the answers

    F.I.R radiation causes electrons to be ejected from the inner shell.

    <p>False</p> Signup and view all the answers

    What type of energy results when a molecule absorbs far infrared light?

    <p>Rotational Energy</p> Signup and view all the answers

    What is the primary function of colored filters?

    <p>Light absorption</p> Signup and view all the answers

    Interference filters provide better filtering characteristics than colored filters.

    <p>True</p> Signup and view all the answers

    What components are found in a monochromator?

    <p>Entrance slit, collimating lens, dispersing device, focusing lens, exit slit</p> Signup and view all the answers

    A prism monochromator disperses light by __________.

    <p>refraction</p> Signup and view all the answers

    Which of the following is a disadvantage of interference filters?

    <p>Higher cost</p> Signup and view all the answers

    Match the type of monochromator with its dispersing principle:

    <p>Prism monochromator = Dispersion by refraction Grating monochromator = Dispersion by diffraction</p> Signup and view all the answers

    The __________ consists of multiple layers of low and high refractive materials.

    <p>interference filter</p> Signup and view all the answers

    What is the main characteristic of a grating monochromator?

    <p>It disperses light by diffraction through grooves on a surface.</p> Signup and view all the answers

    Why do we carry out measurements at λmax?

    <p>To minimize error and enhance sensitivity</p> Signup and view all the answers

    Saturated hydrocarbons show absorption in the near UV range.

    <p>False</p> Signup and view all the answers

    What is the cut-off wavelength for ethanol?

    <p>210 nm</p> Signup and view all the answers

    Compounds with only σ-electrons absorb wavelengths of less than ______ nm.

    <p>170</p> Signup and view all the answers

    Match the solvent to their respective cut-off wavelengths:

    <p>Water = 205 nm Ethanol = 210 nm Chloroform = 245 nm Acetone = 300 nm</p> Signup and view all the answers

    What is the effect of introducing an auxochrome in terms of absorption?

    <p>Hyperchromic effect</p> Signup and view all the answers

    A bathochromic shift indicates a shift to a longer wavelength.

    <p>True</p> Signup and view all the answers

    What is the typical absorption range for alcohols and ethers?

    <p>Shorter than 185 nm</p> Signup and view all the answers

    What is a characteristic of quartz cuvettes?

    <p>They can be used for both UV and visible ranges.</p> Signup and view all the answers

    Glass cuvettes are equally effective for both UV and visible light.

    <p>False</p> Signup and view all the answers

    What type of detectors are primarily used in the UV/VIS regions?

    <p>Photoelectric detectors</p> Signup and view all the answers

    A __________ cuvette is perfect for the visible range but absorbs UV radiation.

    <p>glass</p> Signup and view all the answers

    Which type of photodetector can suffer from fatigue due to prolonged light exposure?

    <p>Photovoltaic cells</p> Signup and view all the answers

    Match the types of photoelectric detectors with their characteristics:

    <p>Photovoltaic Cells = Durable and cheap, used in visible region only Phototubes = Emit negative electrons when light strikes the cathode Both = Convert radiation to electrical signals</p> Signup and view all the answers

    Phototubes have a vacuum inside their glass envelope.

    <p>True</p> Signup and view all the answers

    What must the quality of a cuvette be in order to avoid unwanted absorption?

    <p>High quality with an exact known internal pathlength</p> Signup and view all the answers

    Study Notes

    Introduction to Spectroscopy

    • Spectroscopy is the study of the interaction between electromagnetic radiation (EMR) and matter.
    • EMR is used to identify and/or quantify an analyte.
    • Quantitative analysis uses the analyte's absorbance of EMR.
    • Qualitative analysis identifies the analyte by its characteristic EMR absorption spectrum.

    Electromagnetic Radiation

    • EMR can be described as both particles and waves.
    • Light waves are made of perpendicular oscillating electric and magnetic fields.

    Characterizing Light Waves

    • Wavelength (λ) is the distance between two wave peaks.
    • Frequency (ν) is the number of wave peaks passing a point per second.
    • Wavenumber is the number of waves per centimeter.
    • Velocity of Light (c) is the speed of light (c= ν.λ).

    The Particle Nature of Light

    • EMR consists of discrete packets of energy called photons.
    • Each photon carries energy (E), which is related to frequency and wavelength by the equation E=hv=hc/λ.
    • Higher energy photons have higher frequencies and shorter wavelengths.

    Classifying Electromagnetic Radiation

    • EMR is classified into different types based on its wavelength.
    • Visible light is a small portion of the electromagnetic spectrum, ranging from 400-800 nm.
    • Visible light is polychromatic, meaning it contains multiple wavelengths.
    • Different colors of visible light have different wavelengths.
    • Invisible radiation includes ultraviolet (UV) and infrared (IR) radiation.

    Interaction of a Substance with EMR

    • When a substance absorbs EMR, it can experience three types of energy changes: electronic, vibrational, and rotational.
    • Electronic transitions occur when a molecule absorbs visible or UV radiation, causing an electron to move to a higher energy level.
    • Vibrational transitions occur when a molecule absorbs IR radiation, causing the molecule to vibrate more strongly.
    • Rotational transitions occur when a molecule absorbs far-infrared (FIR) radiation, causing the molecule to rotate faster.

    Ultraviolet-Visible Spectrophotometry

    • UV-Vis spectrophotometry is commonly used for quantitative analysis.
    • When a molecule absorbs UV-Vis radiation, it undergoes both electronic and vibrational transitions.
    • The wavelength of maximum absorption (λmax) is used to determine the concentration of the analyte.

    σ-Absorption

    • Saturated hydrocarbons absorb EMR in the far UV region (<170 nm).
    • These compounds are often used as solvents for UV-Vis spectroscopy because they are transparent in the near UV region (200-300 nm).

    n-electron Absorption

    • Saturated compounds containing heteroatoms or halogens do not absorb EMR in the near UV region.
    • Examples include methanol, triethylamine, and chloroform.
    • Alcohols and ethers absorb UV radiation at wavelengths below 185 nm.

    Solvent Cut-off

    • Most solvents absorb some UV radiation.
    • The solvent cut-off wavelength is the minimum wavelength at which a solvent becomes transparent.
    • The cut-off wavelength is typically between 200-300 nm for common solvents.
    • Quantitative measurements should be done after the solvent cut-off to avoid interference.

    Important Terms

    • Bathochromic shift (or red shift): A shift in λmax to a longer wavelength due to substitution of certain functional groups or conjugation.
    • Hypsochromic shift (or blue shift): A shift in λmax to a shorter wavelength due to the removal of conjugation.
    • Hyperchromic effect (or shift): An increase in the intensity of absorption, often caused by the introduction of an auxochrome.
    • Hypochromic effect (or shift): A decrease in the intensity of absorption.

    Filters

    • Colored filters: Absorb light but do not create monochromatic light.
    • Interference filters: Use constructive and destructive interference to create a narrow band of monochromatic light.

    Monochromators

    • Monochromators convert polychromatic light into monochromatic light.
    • Components of a monochromator include an entrance slit, collimating lens, dispersing element, focusing lens, and exit slit.
    • The dispersing element can be a prism or a grating.
    • A prism disperses light by refraction.
    • A grating disperses light by diffraction.

    Sample Cell Compartment

    • Sample cells (cuvettes) hold the sample being analyzed.
    • Glass cuvettes are used for visible light.
    • Quartz cuvettes are used for both UV and visible light.

    Detectors

    • Detectors convert light into electrical signals.
    • Photoelectric detectors are used in UV-Vis spectroscopy.
    • Some common photoelectric detectors include photovoltaic cells, phototubes, and photomultiplier tubes.
    • Photovoltaic cells are cheap and durable, but only function in the visible region.
    • Phototubes are used in both the visible and UV regions.
    • Photomultiplier tubes are highly sensitive and amplify the signal, but are more expensive.

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    Description

    This quiz explores the fundamentals of spectroscopy and the interaction between electromagnetic radiation and matter. It covers key concepts such as the nature of light waves, quantitative and qualitative analysis, and the particle nature of light.

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