Unit-1: Instrumentation

1. Fundamentals of Chromatography

Chromatography is a physical method of separation in which the components to be separated are distributed between two phases: a stationary phase and a mobile phase that moves in a definite direction.

Definition: Chromatography
A bi-phasic separation technique based on the differential distribution of analyte molecules between a stationary phase (solid or liquid supported on a solid) and a mobile phase (liquid or gas).

Key Concepts

• Stationary Phase: The phase that remains fixed in place inside a column or on a planar surface (e.g., silica gel layer, column packing, liquid film).
• Mobile Phase: The phase that moves over or through the stationary phase, carrying the analyte mixture with it (e.g., organic solvents, inert gas).
• Distribution Coefficient (K): The equilibrium ratio of analyte concentration in the stationary phase to that in the mobile phase. Components with higher affinity for the stationary phase move slower, while components with higher affinity for the mobile phase move faster.

2. Thin Layer Chromatography (TLC)

Fundamental Principles

Thin Layer Chromatography (TLC) is a planar chromatographic technique based primarily on the principle of adsorption (or partition, depending on the stationary phase layer). A thin layer of stationary phase (such as silica gel or alumina) is coated uniformly on an inert backing sheet made of glass, aluminum foil, or plastic.

When the bottom edge of the plate is placed in a liquid mobile phase (developing solvent) inside a sealed chamber, the solvent moves up the plate by capillary action. As the mobile phase travels over the analyte spot, individual components migrate at different rates based on their relative polarities and differential affinities for the stationary and mobile phases.

Key Parameter: Retention Factor (Rf)
Rf = Distance traveled by solute / Distance traveled by solvent front

The Rf value is a constant for a specific compound under identical experimental conditions (temperature, solvent system, stationary phase type).

Step-by-Step Methodology

1. Sample Application: Using a micro-capillary tube, a tiny spot of the sample solution is applied near the baseline of the TLC plate.
2. Development: The plate is placed vertically into a developing chamber saturated with mobile phase vapors, ensuring the solvent level is below the applied sample spot.
3. Visualization: Once the solvent front reaches near the top, the plate is removed and dried. Colorless spots are visualized using UV light (254 nm or 365 nm) or chemical reagents (e.g., iodine vapor, ninhydrin spray).

Forensic Applications of TLC

• Document and Ink Analysis: Rapid comparison and separation of dye components in questioned signatures, altered documents, or counterfeit notes.
• Illicit Drug Screening: Preliminary identification and screening of controlled substances such as heroin, cocaine, amphetamines, and cannabis extracts from seized samples.
• Explosive Residue Analysis: Detection of unreacted explosive components (e.g., TNT, RDX, PETN) from post-blast residues.
• Toxicological Screening: Screening stomach contents, urine, or tissue extracts for suspected poisons or toxic substances.

Exam Notes & Common Mistakes

• Important Observation: Polar compounds interact strongly with polar stationary phases (e.g., silica gel) and exhibit lower Rf values, while non-polar compounds move faster and exhibit higher Rf values.
• Common Mistake: Immersing the sample spot directly into the mobile phase solvent liquid cause the sample to dissolve into the solvent reservoir instead of migrating up the plate.

3. Gas Chromatography (GC)

Fundamental Principles

Gas Chromatography (GC) is a separation technique where the mobile phase is a carrier gas (inert gas such as Helium, Nitrogen, or Hydrogen) and the stationary phase is either a solid adsorbent (Gas-Solid Chromatography) or a high-boiling liquid coated on an inert solid support or inner capillary wall (Gas-Liquid Chromatography).

The sample is injected into a heated vaporizing port where it instantly volatilizes. The gaseous mobile phase carries the vaporized mixture through a thermostated column. Separation occurs based on the boiling points of the components and their differential partition coefficients between the gas phase and the liquid stationary phase.

Key Parameter: Retention Time (Rt)
The time elapsed between sample injection and the emergence of the peak maximum at the detector.

Components of a Gas Chromatograph

• Carrier Gas Supply: High-purity inert gas supplying a constant flow rate.
• Injection Port: Vaporizes the liquid sample instantly at high temperatures.
• Column Oven: Houses capillary or packed columns and controls separation temperature accurately.
• Detectors: Flame Ionization Detector (FID) for hydrocarbons, Electron Capture Detector (ECD) for halogenated compounds, or Mass Spectrometer (MS) for definitive structural identification.

Forensic Applications of GC

• Blood Alcohol Content (BAC) Analysis: Quantitative determination of ethanol concentration in biological fluids (blood, urine) using Headspace-GC.
• Arson and Accelerant Investigation: Identification of volatile ignitable liquid residues (e.g., gasoline, diesel, kerosene) extracted from fire debris samples.
• Toxicological Analysis: Quantification of volatile poisons (e.g., methanol, isopropanol, cyanide, carbon monoxide derivatives) in blood and tissues.
• Seized Drug Analysis: Quantitative and qualitative profiling of illicit drugs and their cutting agents.

Exam Notes & Common Mistakes

• Important Observation: GC is limited strictly to volatile and thermally stable compounds. Non-volatile or heat-sensitive analytes require chemical derivatization before injection.
• Common Mistake: Confusing carrier gas selection with retention control; carrier gas flow rate affects resolution, but column temperature is the primary factor controlling volatility and retention time.

4. Liquid Chromatography (LC)

Fundamental Principles

Liquid Chromatography (LC), particularly High-Performance Liquid Chromatography (HPLC), uses a liquid mobile phase pumped under high pressure through a column packed with fine, solid stationary phase particles (typically modified silica particles).

Separation depends on interactions between the solute, the liquid stationary phase, and the liquid mobile phase. Modes include:

• Normal-Phase LC: Polar stationary phase (silica) paired with non-polar mobile phase (hexane). Polar compounds retain longer.
• Reversed-Phase LC (RP-HPLC): Non-polar stationary phase (C18 / octadecylsilane) paired with polar mobile phase (water-methanol/acetonitrile mixtures). Non-polar compounds retain longer. RP-HPLC is the most widely applied mode.

Forensic Applications of LC

• Heat-Sensitive and Non-Volatile Drug Analysis: Analysis of thermally unstable drugs, steroids, proteins, synthetic opioids (e.g., fentanyl analogs), and benzodiazepines that cannot be analyzed by GC without decomposition.
• Toxin and Poison Profiling: Detection of thermolabile natural toxins (e.g., aconitine, strychnine, ricin components, mushroom toxins) in biological fluids.
• Dye and Pigment Analysis: Examination of non-volatile dyes in textile fibers, inks, lipsticks, and automotive paints.
• Explosives Analysis: Quantitation of heat-sensitive high explosives like HMX, RDX, and PETN.

5. Comparison of Chromatographic Techniques

6. Ultraviolet-Visible (UV-Vis) Spectroscopy

Fundamental Principles

Ultraviolet-Visible spectroscopy measures the absorption of electromagnetic radiation in the UV (200-400 nm) and visible (400-800 nm) regions of the spectrum. Absorption of UV-Vis photon energy promotes valence electrons from ground state molecular orbitals to higher energy excited molecular orbitals.

Principle: Electronic Transitions
When molecules absorb UV-Vis radiation, bonding (π, σ) or non-bonding (n) electrons absorb energy and jump to anti-bonding orbitals (π*, σ*). The most common low-energy transitions observed are n -> π* and π -> π*.

Key Concepts

• Chromophore: A functional group containing unsaturated bonds capable of absorbing UV-Vis radiation (e.g., C=C, C=O, N=N, aromatic rings).
• Auxochrome: A saturated group containing non-bonding electrons attached directly to a chromophore that alters the wavelength and intensity of absorption (e.g., -OH, -NH2, -Cl).
• Bathochromic Shift (Red Shift): Shift of absorption maximum (λmax) to a longer wavelength.
• Hypsochromic Shift (Blue Shift): Shift of absorption maximum (λmax) to a shorter wavelength.

Forensic Applications

• Presumptive Drug Quantitation: Measuring concentration of pure drug samples (e.g., morphine, paracetamol, barbiturates).
• Colorant and Dye Comparison: Spectral comparison of colored evidence such as inks, fibers, and liquid poisons.
• Biochemical Analysis: Spectrophotometric assays for hemoglobin derivatives, carboxyhemoglobin, and methemoglobin in forensic pathology.

7. Infrared (IR) Spectroscopy

Fundamental Principles

Infrared spectroscopy deals with the absorption of radiation in the IR region of the spectrum (mid-IR range: 4000 cm⁻¹ to 400 cm⁻¹). Absorbing IR light causes changes in the vibrational energy states of covalent chemical bonds within a molecule.

Principle: Molecular Vibrations
For a molecule to absorb IR radiation, the vibration must result in a net change in the dipole moment of the molecule.

Types of Molecular Vibrations

• Stretching Vibrations: Change in interatomic distance along the bond axis (Symmetric and Asymmetric stretching).
• Bending Vibrations: Change in bond angle between two bonds (In-plane: Scissoring, Rocking; Out-of-plane: Wagging, Twisting).

Spectral Regions

• Functional Group Region (4000 cm⁻¹ to 1500 cm⁻¹): Characteristic absorption bands for specific functional groups like -OH, -NH, C=O, C≡N.
• Fingerprint Region (1500 cm⁻¹ to 400 cm⁻¹): Highly complex absorption pattern unique to the molecule as a whole. No two compounds (except enantiomers) share an identical fingerprint region.

Forensic Applications

• Confirmatory Identification of Controlled Substances: Provides definitive identification of seized drugs (e.g., cocaine base vs. cocaine HCl salt forms).
• Automotive Paint Analysis: Characterization of organic polymers, binders, and pigments in hit-and-run paint chips.
• Synthetic Fiber Identification: Distinguishing generic classes of synthetic fibers (e.g., nylon, polyester, acrylic).
• Document Examination: Nondestructive characterization of organic toner and ink formulations using FTIR-ATR.

8. Colorimetric Analysis

Fundamental Principles

Colorimetric analysis involves determining the concentration of a chemical compound in a solution by measuring the intensity of color produced when the analyte reacts with specific chemical reagents. The depth of color is directly proportional to the concentration of the colored substance formed.

Definition: Colorimetry
A quantitative or semi-quantitative analytical technique based on comparing the intensity of color developed by an unknown analyte solution against standard reference solutions of known concentration.

Instruments Used

• Visual Colorimeters: Color intensity is compared visually using standard tubes (e.g., Nessler tubes).
• Photoelectric Colorimeters: Uses optical filters to select specific wavelength ranges and photocells to measure light transmittance accurately.

Forensic Applications

• Field Spot Tests: Rapid preliminary testing for illicit drugs (e.g., Marquis test turning purple for opiates; Scott test turning blue for cocaine).
• Forensic Serology: Color tests for biological fluids (e.g., Kastle-Meyer phenolphthalein test for blood, acid phosphatase test for seminal fluid).
• Gunshot Residue (GSR) Testing: Modified Griess test for detecting nitrites and dermal nitrate tests in shooting investigations.

9. Lambert-Beer Law

Fundamental Principles

The Lambert-Beer Law (or Beer-Lambert Law) forms the quantitative foundation of colorimetric and spectrophotometric analysis. It combines two distinct physical laws governing light absorption:

• Lambert's Law: The fraction of light absorbed by a homogeneous medium is directly proportional to the thickness of the absorbing medium (path length).
• Beer's Law: The fraction of light absorbed by a solution is directly proportional to the concentration of the absorbing solute.

Mathematical Formulation

Beer-Lambert Law Equation
A = ε * c * l

Where:

• A: Absorbance (unitless quantity, also called Optical Density)
• ε (epsilon): Molar absorptivity or extinction coefficient (L · mol⁻¹ · cm⁻¹)
• c: Concentration of the absorbing solute (mol / L)
• l: Path length of the light absorbing cell/cuvette (cm)

Absorbance and Transmittance Relationship

Transmittance (T) is the ratio of transmitted light intensity (I) to incident light intensity (I0):

T = I / I0

Absorbance is logarithmically related to Transmittance:

A = -log10(T) = log10(I0 / I)

Deviations and Limitations

• Chemical Deviations: Changes in chemical structure, ionization, association, or dissociation of the solute with varying concentrations.
• Instrumental Deviations: Use of polychromatic light instead of strictly monochromatic light, presence of stray light reaching the detector.
• Real / Physical Deviations: High concentration solutions (> 0.01 M) where electrostatic interactions between neighboring absorbing species alter molar absorptivity.

Exam Notes & Step-by-Step Calculation Guide

• Step 1: Identify given variables (Ensure concentration is in mol/L and path length in cm).
• Step 2: Use equation A = ε * c * l to calculate unknown concentration or molar absorptivity.
• Step 3: Convert percent transmittance (%T) to Absorbance using formula A = 2 - log10(%T) if required.

10. Spectroscopic Methods Comparison