Spectroscopy 1

Introduction to Spectroscopy

  • Spectroscopy is the study of how light interacts with matter and how this interaction can be used to extract information about the molecular composition of food samples.

  • Focus of the course will be on using light to determine:

    • The identity of food samples

    • The concentration of specific compounds present in those samples

Overview of the Course

  • This series of lectures will cover:

    • Basics of spectroscopy

    • Specific examples:

    • UV-Vis absorption spectroscopy

    • Fluorescence spectroscopy

    • Infrared spectroscopy

    • Polarimetry

The Significance of Spectroscopy

  • Humans can be viewed as natural spectroscopists when they observe the world using visible light.

  • Examples of natural spectroscopy:

    • Viewing a rainbow

    • Observing colors in plants

    • Noting the distinct colors and appearances of various food items, e.g., why macaroni and cheese is bright yellow.

Basic Principles of Spectroscopy

  • The interaction of light with a sample can lead to different outcomes:

    • Reflection: Some light is reflected back.

    • Scattering: Some light is diffusely reflected (e.g., milk appears white due to the scattering of light).

    • Absorption: Light is absorbed by the sample, which can be re-emitted at a different wavelength or converted to heat.

    • Transmission: Some light passes through the sample.

  • The color of a substance gives clues about its molecular composition:

    • Example: Cheese appears yellow, which indicates a change in molecular structure during preparation compared to its ingredients (milk, rennet, salt).

Key Questions in Spectroscopy

  1. Why do materials absorb light?

  2. Why do they absorb specific types of light?

  • These questions relate back to the electronic structure of atoms and molecules learned in first-year chemistry.

Light as a Dual Entity: Wave and Particle

  • Light exhibits both wave-like and particle-like properties, known as wave-particle duality.

    • Photon: A single quantum (particle) of light characterized by a specific frequency and energy.

    • The relationship between energy and wavelength is given by the equation:
      E=hcλE = \frac{hc}{\lambda}
      where:

    • $E$ = energy

    • $h$ = Planck's constant

    • $c$ = speed of light

    • $\lambda$ = wavelength.

  • General relationship: As wavelength decreases, energy increases (lower wavelength light has more energy).

Energy Units of Light

  • Different ways to express light's energy include:

    • Wavelength (( \lambda )) in nanometers

    • Frequency (( u )) in Hertz (cycles per second)

    • Wave number (( \bar{\nu} )) in per centimeters (cm-1).

Absorption of Light and Electronic Structure of Molecules

  • Molecules have energy levels:

    • Ground state: Where electrons reside under normal conditions.

    • Excited state: A higher energy level where an electron can move if it gains enough energy.

  • A photon must have energy that corresponds to the gap between these two energy levels for absorption to occur.

  • Absorption of light gives information on molecular structure and composition:

    • Often involves transitions from bonding or nonbonding orbitals to antibonding orbitals.

  • For organic compounds in food, absorption typically occurs in the UV-Vis region, particularly involving pi orbitals.

Colour and Light Absorption

  • The colour we observe from an object is due to the specific wavelengths of light it absorbs and reflects:

    • Example: Leaves appear green because they absorb red and blue light, reflecting green light back to our eyes, thus appearing green.

  • Understanding colour perception and how it translates into spectroscopy:

    • White light contains all colours (a mixture of all wavelengths).

    • If an object absorbs certain wavelengths, it reflects the complementary colour:

    • Absorb red and blue ➜ Reflect green.

    • Absorb green ➜ Reflect magenta (red and blue).

  • Practical application in technology:

    • Computer screens mix colours using individual diodes representing different wavelengths.

Practical Application of Spectroscopy

  • Wide applicability in food chemistry due to sensitivity in detecting low concentrations of compounds.

  • Potential to enhance sensitivity through derivatization (modifying compounds).

  • Used in conjunction with other processes, such as chromatography, to improve detection capabilities.

Presentation of Spectroscopy Data

  • Spectra are typically represented as:

    • Intensity (y-axis): Varies based on the type of spectroscopy.

    • Wavelength (x-axis): Varies based on the type of spectroscopy employed.

  • The process of spectroscopy involves:

    • Using a white light source (e.g., sunlight).

    • Dispersing light into wavelengths through a medium (example: water droplets forming a rainbow).

    • Measuring intensity across different wavelengths to obtain data.

Understanding Light as a Wave

  • Key terms in wave behavior of light:

    • Amplitude: Related to intensity (larger amplitude = higher intensity).

    • Wavelength (( \lambda )): Distance between two peaks of a wave; can be measured in various units (cm, nm).

    • Frequency (( \nu )): Number of cycles per second, can be calculated using speed of light and wavelength:
      ν=cλ\nu = \frac{c}{\lambda}

  • Visibility of different spectral ranges:

    • UV and infrared spectra often complement visible light analysis, helping to understand the energy levels involved in molecular transitions.

Conclusion of Basic Spectroscopy

  • These foundational concepts prepare us for the next lesson, which will delve into practical applications of UV-Vis absorption spectroscopy.

  • Understanding the concepts presented lays the groundwork for further exploration of specific spectroscopy techniques and their applications in food science.