Introduction of Analytical Techniques Used in Forensic Analysis

Physical and Chemical methods of chemistry play a vital role in the characterization, identification, and quantification of chemical substances. These techniques provide information about the composition, structure, bonding, purity, and molecular interactions of materials.
They are broadly classified into spectroscopic, chromatographic, thermal, and electrochemical methods.

2. Objectives of the Lecture

• To understand the working principles of major physical and instrumental methods used for characterization.
• To differentiate between qualitative and quantitative applications.
• To appreciate the role of these methods in research, industry, and forensic analysis.

3. Major Physical Methods of Chemical Characterization

A. Spectroscopic Methods

Absorption:
Absorption occurs when atoms or molecules take in energy, usually in the form of light, causing electrons to move from a lower to a higher energy level. In absorption spectroscopy, the amount of light absorbed at specific wavelengths is measured to determine the nature and concentration of substances. The resulting absorption spectrum displays dark lines or bands where light has been absorbed. This process depends on the ability of molecules to interact with and absorb photons of certain energies. Techniques such as UV-Visible and Infrared (IR) spectroscopy are common examples of absorption-based methods used for qualitative and quantitative analysis.

Emission:
Emission occurs when excited electrons in atoms or molecules return from a higher to a lower energy state, releasing energy as light. In emission spectroscopy, the intensity of this emitted light is measured to identify and quantify substances. The emission spectrum shows bright lines or bands corresponding to the wavelengths of light emitted by the excited species. This process depends on the relaxation of excited particles and is highly useful in detecting elements at trace levels. Common emission-based techniques include flame photometry, fluorescence spectroscopy, and atomic emission spectroscopy.

Emission Spectroscopic Techniques of Analysis

1. Introduction

Emission spectroscopy involves the study of light emitted by atoms or molecules when they transition from an excited energy state to a lower energy state. The wavelength and intensity of the emitted radiation provide valuable information about the qualitative (what elements are present) and quantitative (how much of each element) composition of a sample.

When a substance is excited by heat, electrical energy, or electromagnetic radiation, its atoms or ions emit characteristic wavelengths of light. Measuring these emissions forms the basis of emission spectroscopic analysis.

2. Principle

• When atoms absorb sufficient energy, their electrons are promoted to higher energy levels.
• Upon returning to the ground state, these electrons release energy in the form of electromagnetic radiation (light).
• Each element emits radiation at specific wavelengths — producing a unique emission spectrum that acts as a “fingerprint” for that element.
• The intensity of the emitted light is directly proportional to the concentration of the element in the sample.

3. Major Emission Spectroscopic Techniques

A. Flame Emission Spectroscopy (Flame Photometry)

• Principle: Metallic elements are excited in a flame; emitted light intensity at specific wavelengths is measured.
• Excitation Source: Flame (usually air-acetylene).
• Applications:
• Determination of alkali and alkaline earth metals (Na, K, Ca, Li).
• Analysis of biological fluids, fertilizers, and environmental samples.

B. Atomic Emission Spectroscopy (AES)

• Principle: Atoms in a high-temperature plasma or arc are excited, and the emitted light is analyzed spectrally.
• Excitation Source: Arc, spark, or plasma.
• Applications:
• Multi-elemental analysis of metals and alloys.
• Environmental monitoring and geochemical analysis.
• Trace metal determination in water and soil samples.

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C. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES or ICP-OES)

• Principle: A high-temperature argon plasma excites atoms and ions, which emit radiation at characteristic wavelengths.
• Excitation Source: Inductively coupled argon plasma (~10,000 K).
• Applications:
• Simultaneous detection of multiple elements with high precision.
• Trace metal analysis in environmental, pharmaceutical, and food samples.
• Quality control in metallurgical industries.

D. Fluorescence and Phosphorescence Spectroscopy (Molecular Emission)

• Principle: Certain molecules emit light (fluorescence or phosphorescence) when excited by UV or visible radiation.
• Difference: Fluorescence occurs almost immediately after excitation, while phosphorescence is delayed.
• Applications:
• Detection of trace organic compounds.
• Biochemical and pharmaceutical analysis (proteins, vitamins, drugs).
• Environmental and forensic analysis of pollutants

2. Major Absorption Spectroscopic Techniques

1. UV-Visible Spectroscopy

• Principle: Based on the absorption of ultraviolet or visible light, which causes electronic transitions in molecules.
• Applications:
• Qualitative: Identification of conjugated systems, chromophores.
• Quantitative: Determination of concentration using Beer-Lambert’s law.

2. Infrared (IR) Spectroscopy

• Principle: Molecules absorb infrared radiation causing vibrations (stretching and bending) of chemical bonds.
• Applications:
• Qualitative: Identification of functional groups.
• Quantitative: Measurement of bond strengths or impurity levels.

3. Nuclear Magnetic Resonance (NMR) Spectroscopy

• Principle: Nuclei in a magnetic field absorb radiofrequency radiation depending on their environment.
• Applications:
• Qualitative: Determination of molecular structure and environment of atoms.
• Quantitative: Determination of purity and molecular ratio.

4. Mass Spectrometry (MS)

• Principle: Ionization of molecules followed by separation of ions based on mass-to-charge ratio (m/z).
• Applications:
• Qualitative: Molecular weight determination and structure elucidation.
• Quantitative: Trace-level detection of compounds.

5. Atomic Absorption Spectroscopy (AAS)

• Principle: Ground-state atoms absorb light of specific wavelengths, which is proportional to their concentration.
• Applications:
• Qualitative: Identification of metals in a sample.
• Quantitative: Measurement of metal concentration in environmental and biological samples.

B. Chromatographic Methods

1. Gas Chromatography (GC)

• Principle: Separation of volatile components based on differential partitioning between stationary and mobile phases.
• Applications:
• Qualitative: Identification of volatile organic compounds.
• Quantitative: Determination of concentration of components.

2. High-Performance Liquid Chromatography (HPLC)

• Principle: Components are separated due to differences in interactions with stationary and mobile phases under high pressure.
• Applications:
• Qualitative: Identification of non-volatile or thermally unstable compounds.
• Quantitative: Pharmaceutical purity and concentration analysis.

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C. Thermal Analysis

1. Thermogravimetric Analysis (TGA)

• Principle: Measurement of weight changes in a substance as a function of temperature or time.
• Applications:
• Qualitative: Determination of decomposition patterns.
• Quantitative: Calculation of moisture or ash content.

2. Differential Scanning Calorimetry (DSC)

• Principle: Measures the heat flow associated with phase transitions (melting, crystallization).
• Applications: