Instrumental Analysis: Spectrophotometry Lecture 2

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Questions and Answers

What is the primary function of an absorption filter in spectrophotometry?

  • It filters light based on wavelength alone.
  • It reflects all incident light.
  • It enhances the intensity of all colors.
  • It absorbs undesired light and transmits desired light. (correct)

In which region of the spectrum is an absorption filter primarily used?

  • Ultraviolet region
  • Visible region (correct)
  • Infrared region
  • Radio frequency region

Which characteristic does NOT describe an absorption filter?

  • It selectively transmits desired wavelengths.
  • It can be used outside the visible region. (correct)
  • It reduces the intensity of undesired light.
  • It is specific to certain wavelengths of light.

What occurs to the light that is absorbed by an absorption filter?

<p>It is transformed into heat. (C)</p> Signup and view all the answers

What would be the outcome if an undesired part of the light spectrum is transmitted instead of absorbed?

<p>The measurement will be inaccurate and contaminated with noise. (A)</p> Signup and view all the answers

What is the primary function of the detector mentioned?

<p>To alternate between the sample and reference beams (B)</p> Signup and view all the answers

How does the detector operate concerning the chopper?

<p>It synchronizes its measurements with the chopper's movements (C)</p> Signup and view all the answers

What does the reference beam allow the detector to do?

<p>It enables background correction of measurements (B)</p> Signup and view all the answers

What is meant by 'synchronism' in the context of the detector's operation?

<p>Coordinating measurements with the chopper (C)</p> Signup and view all the answers

When measuring the beam, what role does the chopper play?

<p>It divides the measurement process into time intervals (D)</p> Signup and view all the answers

What is the primary function of the beam splitter in a double-beam spectrophotometer?

<p>To split the light beam into two separate beams. (C)</p> Signup and view all the answers

What occurs after the light beam passes through the monochromator in a double-beam spectrophotometer?

<p>It is split into two separate beams. (B)</p> Signup and view all the answers

What role does the chopper play in the double-beam spectrophotometer?

<p>It blocks one beam at a time. (A)</p> Signup and view all the answers

Which statement best describes the arrangement of components in the double-beam spectrophotometer?

<p>A single beam is measured against the reference beam sequentially. (A)</p> Signup and view all the answers

What is the significance of using a double-beam system in spectrophotometry?

<p>It helps in reducing fluctuations that may affect measurement accuracy. (A)</p> Signup and view all the answers

What are the two types of cuvettes mentioned?

<p>Glass and Plastic cuvettes (D)</p> Signup and view all the answers

Which type of cuvette is suitable for use in the visible region?

<p>Both Glass and Plastic cuvettes (A)</p> Signup and view all the answers

Which statement best describes the application of the glass cuvette?

<p>Used in the visible region only (B)</p> Signup and view all the answers

What is the primary limitation of plastic cuvettes?

<p>They are less durable than glass cuvettes (B)</p> Signup and view all the answers

In which type of study is a cuvette most commonly used?

<p>Spectrophotometric analysis (B)</p> Signup and view all the answers

What are the main components of the described device?

<p>Photo-emissive cathode and collecting anode (B)</p> Signup and view all the answers

What occurs when the beam of light interacts with the cathode?

<p>It emits electrons in proportion to the light intensity (B)</p> Signup and view all the answers

What is the role of the anode in this context?

<p>To collect emitted electrons from the cathode (A)</p> Signup and view all the answers

Which of the following statements is true regarding the interaction between the cathode and light?

<p>The emitted electrons move towards the anode due to attraction (A)</p> Signup and view all the answers

Which statement correctly describes the relationship between light intensity and electron emission?

<p>Electrons are emitted in the same quantity as light intensity (C)</p> Signup and view all the answers

What role do the dynodes play in a photomultiplier?

<p>They provide increasing positive potential to amplify the signal. (A)</p> Signup and view all the answers

What is the source of electrons in a photomultiplier?

<p>Cathode after exposure to light. (A)</p> Signup and view all the answers

What process is utilized in the generation of electrons in a photomultiplier?

<p>Photoelectric emission. (D)</p> Signup and view all the answers

How are the dynodes configured in a photomultiplier?

<p>In a series with gradually increasing positive potential. (C)</p> Signup and view all the answers

What occurs at the dynodes when electrons are present in the photomultiplier?

<p>Electrons cause the release of additional electrons. (C)</p> Signup and view all the answers

Flashcards

Absorption Filter

An optical filter that selectively absorbs specific wavelengths of light while transmitting others.

Visible Region

The portion of the electromagnetic spectrum that is visible to the human eye.

Spectrophotometry

A method used to measure the absorbance and transmittance of light through a solution or material.

Double-beam spectrophotometer

A spectrophotometer that splits the light beam into two separate beams.

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Beam splitter

A device that divides the light beam into two separate beams.

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Monochromator

A device that isolates a specific wavelength of light.

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Chopper

A device that alternately blocks one beam at a time.

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Light beam splitting

The process of dividing the light beam into two.

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Beam measurement

A method of measuring a beam where a single detector alternates between measuring the sample beam and the reference beam, synchronized with a chopper.

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Sample beam

The beam of radiation that passes through the sample being measured.

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Reference beam

A beam of radiation used as a standard for comparison during measurement.

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Synchronization with chopper

The coordinated timing between the detector's measurements and the switching of the sample/reference beams by the chopper.

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Sample Cuvette Types

Glass or plastic cuvettes are used in visible light spectrophotometry.

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Visible Light Region

The part of the electromagnetic spectrum that human eyes can see.

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Spectrophotometer Sample Compartments

The part of the spectrophotometer holding the sample cuvette(s).

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Cuvette

A small container holding the sample solution for analysis.

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Photomultiplier

A device that amplifies light signals using secondary emission processes.

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Dynodes

Anodes in a photomultiplier with progressively increasing positive potential.

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Secondary emission

Process where electrons are released from a material upon impact by other electrons.

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Cathode

The component in the photomultiplier that releases electrons when light hits it.

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Photoelectric effect device

A device that converts light into an electric current, using a photo-emissive cathode and a collecting anode.

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Cathode

The photo-emissive part of the device that emits electrons when light hits it.

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Anode

The collecting part of the device that attracts the emitted electrons.

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Electron emission

The release of electrons when light falls upon a photosensitive material.

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Light beam

The light source that stimulates electron emission.

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Study Notes

Spectrophotometry Lecture Notes

  • Course: Instrumental Analysis (PMC-205)
  • Level: II
  • Semester: 3
  • Topic: Spectrophotometry (Lecture 2)
  • Instructor: Dr. Rania El-Shaheny

Key Learning Objectives

  • Identify the basic components of a spectrophotometer.
  • Describe the types and properties of each spectrophotometer component.
  • Compare and contrast UV and visible spectrophotometers.

Spectrophotometer Components

  • Light Source: Necessary for stable and intense light over a broad wavelength range. Two main types:

    • Tungsten lamp: Used for visible light.
    • Deuterium lamp: Used for UV light.

  • Wavelength Selector (Monochromator): Isolates a specific wavelength of light from a broad spectrum. Three types:

    • Filters: Selection based on filtering undesirable wavelengths. Two types:
      • Absorption filters: Absorb unwanted wavelengths, transmit desired ones (used in visible region only).
      • Interference filters: Reflect unwanted wavelengths, transmit desired ones (used in both visible and UV regions).
    • Prisms: Separate light into beams of different wavelengths using refraction. Two types:
      • Glass prisms: Used in the visible region only (opaque to UV light).
      • Quartz prisms: Used in both visible and UV regions.
    • Gratings: Highly polished surface with parallel, equally spaced grooves. Two types:
      • Reflection gratings: Reflect light at different angles based on wavelength (most common).
      • Transmission gratings: Transmit light at different angles based on wavelength.

  • Sample Compartment & Cuvette: Holds the sample to be analyzed. Two types:

    • Glass or plastic cuvettes: Used for visible light only.
    • Quartz cuvettes: Used for both visible and UV light.

  • Detector: Measures the amount of light passing through to generate an electrical signal. Two common types:

    • Phototubes: Emit an electric current when exposed to light.
    • Photomultipliers: Multiply the initial electrons emitted from a photocathode, allowing for detection of very weak light.

  • Readout device: Converted the signal from the detector to a readable form, either transmittance or absorbance.

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