Introduction to Spectroscopy for B.Sc. Students

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Introduction to Spectroscopy for B.Sc. Students

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Introduction to Spectroscopy
Spectroscopy is a scientific technique that involves the study of the interaction between matter
and electromagnetic radiation. (Electromagnetic radiation is a form of energy that is produced by
oscillating electric and magnetic disturbance, or by the movement of electrically charged particles
traveling through a vacuum or matter.) It serves as one of the most powerful tools in modern
science, offering deep insights into the composition, structure, and dynamics of both organic and
inorganic substances.
The term originates from the Latin spectrum (appearance or image) and the Greek skopia (to look
or observe), essentially meaning "looking at spectra."
Fundamental Principle
At its core, spectroscopy is based on the principle that atoms and molecules absorb and emit
electromagnetic radiation at specific wavelengths. These interactions produce a spectrum—a plot
of intensity versus wavelength or frequency—that acts as a unique fingerprint for a given
substance. By analyzing these spectra, scientists can determine a wide range of physical and
Spectral Regions
Spectroscopy covers a wide range of the electromagnetic spectrum, from high-energy gamma rays
to low-frequency radio waves:
Spectral Region Wavelength Range Typical Applications
Gamma rays < 0.01 nm Nuclear physics, astrophysics
X-rays 0.01 – 10 nm Crystallography, material analysis
Ultraviolet (UV) 10 – 400 nm Electronic transitions, drug analysis
Visible 400 – 700 nm Color analysis, pigment identification
Infrared (IR) 700 nm – 1 mm Molecular vibrations, organic compound ID
Microwave 1 mm – 1 m Rotational spectroscopy, remote sensing
Radio > 1 m Nuclear magnetic resonance (NMR), MRI
Spectroscopy

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Applications
Spectroscopy is employed across virtually every field of science and technology:
• Chemistry: Identification of unknown compounds, reaction monitoring, structure
elucidation.
• Physics: Understanding atomic and molecular energy levels, quantum mechanics.
• Biology and Medicine: Biomolecular analysis, clinical diagnostics, imaging techniques
like MRI.
• Astronomy: Composition and movement of celestial bodies, redshift measurement.
• Environmental Science: Air and water quality monitoring, pollutant detection.
• Forensics and Material Science: Trace evidence analysis, quality control.
Types of Spectroscopy
Spectroscopy is broadly classified based on the type of electromagnetic radiation used and the
nature of the interaction:
1. Absorption Spectroscopy: Measures the amount of light absorbed by a sample at various
wavelengths. Common types include UV-Vis, infrared (IR), and X-ray absorption
spectroscopy.
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2. Emission Spectroscopy: Focuses on the radiation emitted by excited atoms or molecules
as they return to lower energy states. Examples include flame emission and atomic
emission spectroscopy.
3. Scattering Spectroscopy: Involves the study of light that is scattered by particles in a
sample. Raman and Rayleigh scattering are key examples.
4. Fluorescence and Phosphorescence Spectroscopy: Examine the delayed emission of
light following absorption. These techniques are highly sensitive and widely used in
biological and medical diagnostics.
5. Mass Spectrometry (MS): Although not a form of electromagnetic spectroscopy in the
traditional sense, MS is often included in discussions due to its analytical capabilities based
on mass-to-charge ratio.
1. Absorption Spectroscopy-
Absorption spectroscopy is a technique that measures how much electromagnetic radiation
(like light) a sample absorbs at different frequencies or wavelengths.
• Absorption Spectroscopy is a method used to study how substances absorb light (or
other electromagnetic radiation).
• When light passes through a substance, some wavelengths are absorbed.
The amount of light absorbed tells us about the type and amount of substance present.
•
Basic Principle
• "Atoms or molecules absorb light of specific wavelengths. This causes electrons to
move to a higher energy level."
• Each substance has a unique absorption pattern (like a fingerprint).
• The amount of light absorbed depends on:
o Type of substance
o Concentration
o Wavelength of light

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Types of Absorption Spectroscopy
Type Region of Spectrum Example
UV-Visible
Ultraviolet & visible
light
Used for organic compounds, metal ions
IR (Infrared) Infrared region
Used to study functional groups in
molecules
Atomic Absorption
(AAS)
UV-Visible Detects metals like iron, lead, copper
X-ray Absorption X-ray region Used for studying atomic structure

Basic Components of an Absorption Spectrophotometer
1. Light Source – Provides light of different wavelengths
2. Sample Holder – Where the sample is placed
3. Monochromator – Selects a specific wavelength
4. Detector – Measures how much light is absorbed
5. Readout Device – Displays the result (absorbance)
2. Emission Spectroscopy
Emission Spectroscopy is a method used to find out what elements are in a sample by
looking at the light they give off when they are heated or excited.
Emission Spectroscopy is a method used to find out what elements are in a sample by looking
at the light they give off when they are heated or excited.

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Basic Principle (in simple words):
When atoms are heated or given energy, they get excited.
As they return to normal (ground state), they release light.
This emitted light has specific colors (wavelengths) for each element.
Everyday Example:
• Ever seen different colors in fireworks?
That’s emission! Different metals give different colors when burned:
o Sodium → Yellow
o Copper → Green
o Strontium → Red
How Emission Spectroscopy Works:
1. Energy is added to a sample (heat, electricity, or plasma).
2. Atoms absorb energy and jump to higher energy levels (excited state).
3. They fall back down to their normal state.
4. In doing so, they emit light.
5. This light is analyzed by a device to identify the element and how much is present.
What It Tells Us:
Light Property What It Means
Wavelength (color) Which element is present
Intensity (brightness)
How much of the element is
there

Types of Emission Spectroscopy:
Type How it's excited Use
Flame Emission Spectroscopy Heat from a flame Detect metals like Na, K
Atomic Emission Spectroscopy (AES) Electrical energy Metal detection
ICP-OES (Inductively Coupled Plasma - Optical
Emission Spectroscopy)
Plasma (very high
energy)
Detect trace metals (very
small amounts)

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✅ Advantages:
• Can detect very small amounts of elements.
• Fast and accurate.
• Useful for many types of samples (liquid, solid, gas).
❌ Disadvantages:
• Requires special equipment and trained users.
• Sometimes other elements can interfere with the results.
• Usually limited to metallic elements.
Common Applications:
• Water testing (e.g., heavy metals like lead, arsenic)
• Environmental science
• Mining (checking metal content in ores)
• Astronomy (studying stars by their light)
• Food and agriculture
3. Scattering Spectroscopy
Scattering Spectroscopy is a technique used to study how light interacts with particles or
molecules and how it gets scattered (bent or redirected).
In simple words:
When light hits a sample, some of it bounces off in different directions.
By looking at how the light is scattered, we can learn about the size, structure, or composition
of the sample.
What is Light Scattering?
Light usually travels in straight lines, but when it hits small particles (like dust, molecules, or
cells), it can:
• Be absorbed
• Be transmitted
• Or be scattered
Scattering can happen in different directions and at different intensities.

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Types of Scattering
Type Description Example
Rayleigh
Scattering
Scattering by very small particles (much
smaller than light wavelength)
Blue sky (short wavelengths
scattered more)
Mie Scattering
Scattering by larger particles (similar to light
wavelength)
White clouds, fog
Raman
Scattering
A small portion of light changes its energy
(wavelength)
Used in Raman Spectroscopy

Types of Scattering Spectroscopy
1. Raman Spectroscopy
• Based on Raman Scattering
• Only a very small fraction of light changes its wavelength
• Used to study molecular structure, vibrations, and bonds
2. Dynamic Light Scattering (DLS)
• Measures Brownian motion of particles in a liquid
• Used to determine particle size (like nanoparticles, proteins)
3. Elastic Scattering Techniques

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• Measures how light scatters without changing energy
• Used in imaging, particle analysis

How It Works (Basic Steps):
1. A laser or light source shines on the sample.
2. Light interacts with molecules/particles.
3. Some light is scattered.
4. A detector records the scattered light (angle, intensity, or wavelength).
5. The data tells us about the sample’s size, shape, or structure.


Applications of Scattering Spectroscopy
Field Application
Biology Studying cells, proteins, DNA
Chemistry Analyzing molecular structure
Nanotechnology Measuring nanoparticles
Medicine Detecting diseases, cancer cells
Environmental Air pollution analysis (dust particles)

5 Mass Spectrometry (MS)

Mass Spectroscopy (or Mass Spectrometry) is a technique used to:
• Identify substances,
• Determine molecular weight, and
• Analyze the structure of molecules.
In simple words:
Mass spectroscopy measures the mass of atoms or molecules in a sample by turning them into
charged particles (ions) and analyzing how they move.

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Main Purpose:
• Find out what elements or compounds are present.
• Measure how much of each is there.
• Understand the structure of unknown molecules.

Basic Principle:
A substance is ionized (converted into charged particles), and then these ions are separated
based on their mass-to-charge ratio (m/z).
Lighter ions move faster, heavier ones slower.


Main Parts of a Mass Spectrometer:
1. Sample Inlet – Introduces the sample (solid, liquid, or gas).
2. Ion Source – Converts the sample into ions (e.g., by electron impact).
3. Mass Analyzer – Separates ions based on mass-to-charge ratio.
4. Detector – Detects and records the ions.
5. Computer/Display – Shows a graph (mass spectrum) with results.

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Types of Ionization Methods:
Method Description Used For
Electron Impact (EI)
High-energy electrons knock out
electrons from atoms
Small molecules
Electrospray Ionization (ESI) Produces ions from liquid samples
Large
biomolecules
Matrix-Assisted Laser Desorption
Ionization (MALDI)
Laser is used to ionize large
molecules
Proteins, DNA

Mass Spectrum (Output)
• The result is a graph:
o x-axis: mass-to-charge ratio (m/z)
o y-axis: intensity (how many ions of each m/z)
• The highest peak is called the base peak.
• The molecular ion peak (M⁺) gives the molecular mass of the compound.
Important Terms:
Term Meaning
m/z Mass-to-charge ratio
Ion Charged particle
Molecular Ion (M⁺) Ion formed by removing one electron
Base Peak Most intense (tallest) peak in the spectrum
Fragmentation When the molecular ion breaks into smaller ions

✅ Applications of Mass Spectroscopy:
Field Use
Chemistry Identifying unknown compounds
Biology Protein and DNA analysis
Medicine Drug testing, metabolite analysis
Forensics Identifying poisons or explosives
Environment Detecting pollutants