Key Concepts in Spectroscopy and Chromatography

Dual nature of light in spectroscopy

Light behaves both as a wave and a particle (wave-particle duality).

Field responsible for spectroscopic effects

The electric field.

Relationship between wavelength, frequency, and speed of light

c = λν

Equation for the energy of a photon

E = hν

Snell's Law in optics

The refraction of light when it passes between two media.

Malus's Law about polarized light

Transmitted intensity = Initial intensity × cos²θ

Double-slit experiment demonstration

It confirms the wave nature of light via interference patterns.

Characteristics of atomic vs. molecular spectra

Atomic: line spectra; Molecular: electronic, vibrational, rotational levels.

Transitions in UV-Vis, IR, and Microwave/NMR regions

UV-Vis: electronic; IR: vibrational; Microwave/NMR: rotational/nuclear spin.

Beer's Law

A = εbc

Uncertainty principle in spectroscopy

Shorter excited-state lifetimes result in broader spectral lines (Δν·Δt ≥ 1).

Phenomena optical spectroscopy relies on

Absorption, fluorescence, phosphorescence, scattering, emission, chemiluminescence.

Types of wavelength selectors

Filters (interference/absorption) and monochromators (prism/grating).

Gratings vs. prisms in monochromators

They offer better resolution, are cheaper, and provide linear dispersion.

Resolving power in spectroscopy

R = λ/Δλ = nN

Main types of detectors

Photon transducers (e.g., PMT) and thermal transducers.

AAS measures

Light absorbed by ground-state atoms.

AES measures

Light emitted by excited atoms.

Hollow cathode lamps in AAS

They emit narrow lines that match absorption lines.

Key benefit of ICP over flame AAS

Higher sensitivity and ability to analyze multiple elements simultaneously.

Plasma in spectroscopy

Ionized gas with free electrons and cations used as an excitation source.

Electronic transition with highest energy

σ → σ*

Conjugation effect on UV/Vis absorption

Causes a red shift (bathochromic) to longer wavelengths.

Auxochrome

A group that modifies absorption of a chromophore, often increasing λ and ε.

Difference between fluorescence and phosphorescence

Fluorescence: singlet state, short-lived; Phosphorescence: triplet state, longer-lived.

Quantum yield (Φ) in fluorescence

Φ = photons emitted / photons absorbed

IR and Raman spectroscopy activation mechanisms

IR: dipole moment change; Raman: polarizability change.

Key component of FTIR instrumentation

Michelson Interferometer.

ATR in IR spectroscopy

Surface analysis using total internal reflections.

Mass analyzer in MS

Separates ions by their mass-to-charge ratio (m/z).

Best ionization method for biomolecules

ESI or MALDI.

Principle of Time-of-Flight (TOF) mass analyzers

Ions are separated based on their time of flight through a drift tube.

Retention factor (k') in chromatography

k' = (tR − tM)/tM, shows time spent in stationary phase.

Higher selectivity factor (α)

Better separation between analytes.

Van Deemter equation

Describes how plate height varies with mobile phase velocity in chromatography.

Common GC detector for organics

Flame Ionization Detector (FID).

Importance of temperature control in GC

It helps separate compounds with wide boiling point differences.

Function of a guard column in HPLC

Protects the analytical column from contamination.

Compounds that elute last in reversed-phase HPLC

Nonpolar compounds.

Gradient elution in HPLC

A method that changes solvent polarity during the run to improve separation.

Number of NMR energy levels

Spin quantum number I → 2I + 1 levels.

Chemical shift in NMR

Indicates local magnetic environment of nuclei relative to TMS.

Cause of spin-spin splitting in NMR

Magnetic coupling with neighboring non-equivalent nuclei.

Measured in FT-NMR

Free induction decay (FID), converted to frequency domain.