Chapter 4 textbook

4.1 Introduction

• Importance of analyzing chemical compounds and confirming identities in practical chemistry.

• Applications in:

  • Research laboratories

  • Pharmaceutical industry

  • Food and drink quality control

  • Environmental monitoring

  • Forensics

• Overview of analytical techniques offered in modern laboratories.

• Focus on identifying compounds and spectroscopic properties rather than quantifying trace elements.

• Classification of analytical methods:

  • Compositional analysis and formula determination

  • Investigating bonding, connectivity of atoms, and oxidation states

  • Determining molecular structure

4.2 Separation and Purification Techniques

• Importance of ensuring the purity of a compound before analysis.

Gas Chromatography (GC)

• Mobile phase: gas

• Stationary phase: packed in a capillary/microbore column.

• Principles:

• Separates volatile components based on interactions with the stationary and mobile phases.

• Components are vaporized and eluted through the column at controlled temperatures.

• Characteristic retention times for components, visible as chromatograms.

• Interaction strength affects elution speed:

  • Least interaction leads to shortest retention time.

  • Strongly adsorbed components move slower.

• GC advantages for volatile compounds; liquid chromatography (LC) applied for broader scenarios in inorganic chemistry.

Liquid Chromatography (LC)

• Mobile phase: liquid

• Stationary phase: inside a column or on a plate.

• Column LC often purifies products post-synthesis.

• Prior test separations done with thin-layer chromatography (TLC).

• Retention Factor (Rf value): Ratio of distance traveled by analyte to that of solvent front.

• Chemicals separated through their solubility and interactions with the stationary phase determined by an equilibrium constant K.

• Solvent flow: Gravity or pressure is used during elution.

• Detection methods may include UV absorption or mass spectrometry.

High-Performance Liquid Chromatography (HPLC)

• A variant of LC where the mobile phase is pushed under pressure.

• Stationary phase: very small particles (3-10 mm).

• All processes are computer-controlled:

  • Solvent composition controlled by preset flow rates.

  • Sample monitored using various detectors (UV-VIS, fluorescence).

  • Data recorded as absorbance versus retention time.

4.3 Elemental Analysis

CHN Analysis by Combustion

• Quantitative analysis for C, H, N simultaneously using a CHN analyser.

• Process:

• Sample combusted, products measured.

• Sample weighed (2-5 mg) in a capsule.

• Automated analysis process exhaustively oxidizes elements producing CO2, H2O, or nitrogen oxides.

• Detection employs specialized gas chromatography (frontal chromatography).

Atomic Absorption Spectroscopy (AAS)

• Metal quantification via atomic absorption spectrum observed from gaseous atoms of the metal.

• The process involves:

• Digestion and decomposition of sample.

• Calibration curve preparation using standards.

• Absorption measured at specific wavelengths corresponding to element.

• Deviations occur at high absorbance values (>0.5), necessitating dilution.

4.4 Compositional Analysis: Thermogravimetry (TAG)

• Monitors mass changes of sample upon heating.

• Investigates thermal degradation, solvate presence, or gas uptake.

• TGA Instrument: Heats sample while recording mass.

4.5 Mass Spectrometry

Techniques Overview

• Separation of ions based on the mass-to-charge ratio (m/z).

• Electron Ionization (EI) Mass Spectrometry:

• Produce ions by bombarding gases with electrons; causes fragmentation—considered a 'hard' technique.

• Spectrum indicates mass of ions, fragmentation patterns arise from complex connectivity.

Fast Atom Bombardment Mass Spectrometry (FAB)

• Soft technique: minimal fragmentation.

• Ions generated by bombarding neutral molecules.

Matrix-Assisted Laser Desorption Ionization (MALDI)

• UV laser produces ions from a sample; suitable for large molecular weights.

• Mixed with a matrix for standardized outcomes.

Electrospray Ionization (ESI)

• Converts sample solution into a fine spray under electrical potential; forms both singly and multiply charged ions.

4.6 Infrared and Raman Spectroscopies

• Examination of vibrational modes.

• IR activity: increase in dipole moment.

• Raman activity: change in molecular polarizability during vibrations.

• Application in various fields, like forensics and analytical chemistry.

4.7 Electronic Spectroscopy

• Involves transitions between electron energy levels (absorption and emission types).

• Absorption transitions generally observed in UV or visible spectra.

4.8 Nuclear Magnetic Resonance (NMR) Spectroscopy

• Measures transitions between energy levels of nuclear spins induced by radiofrequency.

4.9 Electron Paramagnetic Resonance (EPR) Spectroscopy

• Focused on systems with unpaired electrons; detects energy levels via microwave transitions.

4.10 Mo¨ssbauer Spectroscopy

• Analyzes interactions of gamma radiation with nuclei in the rigid lattice of solids.

• Provides valuable insights into electronic environment and oxidation states.

4.11 Structure Determination: Diffraction Methods

X-ray Diffraction (XRD)

• Utilized for determining structures in molecular and non-molecular solids.

• Bragg's Law assists in structural determination from diffraction data.

Powder X-ray Diffraction

• Useful for bulk characterization and distinguishing different molecular phases.

4.12 Photoelectron Spectroscopy (PES)

• Studies energy states of atomic or molecular orbitals, useful for chemical analysis.

4.13 Computational Methods

• Includes various theoretical strategies, such as density functional theory, for analyzing complex systems.

KEY TERMS AND ACRONYMS

• Gas Chromatography (GC)

• Liquid Chromatography (LC)

• Column Chromatography (CC)

• High-Performance Liquid Chromatography (HPLC)

• Thermogravimetric Analysis (TGA)

• Mass Spectrometry (MS)

• Atomic Absorption Spectroscopy (AAS)

• Infrared Spectroscopy (IR)

• Ultraviolet-Visible Spectroscopy (UV-VIS)

• Nuclear Magnetic Resonance (NMR)

• Electron Paramagnetic Resonance (EPR)

• Photoelectron Spectroscopy (PES)