[LECTURE NOTES] Module 5 - CHEM 131.01

Spectroscopy

The study of the interaction of electromagnetic radiation (light) with matter.

Light

The portion of electromagnetic radiation that behaves as both a particle (photon) and a wave.

Dual nature of light (Particle and Wave)

The concept that light exhibits both particle-like (photons) and wave-like properties.

Photon

A discrete packet of light energy.

Light wave

An oscillating electric field and magnetic field that are perpendicular to each other.

Wavelength (λ)

The crest-to-crest distance between successive light waves.

Frequency (ν)

The number of complete wave oscillations passing a point per second.

Speed of light equation (c = νλ)

The relationship stating that the speed of light equals frequency multiplied by wavelength.

Speed of light (c)

The constant speed at which light travels in a vacuum (3.0 × 10⁸ m/s).

Planck's Law (E = hν)

The equation relating photon energy to its frequency.

Photon energy equation (E = hc/λ)

The equation relating photon energy to its wavelength.

Planck's constant (h)

A physical constant equal to 6.626 × 10⁻³⁴ J·s.

Photon energy vs. frequency

Photon energy increases as frequency increases.

Photon energy vs. wavelength

Photon energy decreases as wavelength increases.

Energy states of matter

The discrete energy levels occupied by electrons within atoms.

Excitation

The process in which an electron absorbs energy and moves from a lower to a higher energy level.

Emission

The release of energy as an excited electron returns to a lower energy level.

Absorption of light

The process in which matter takes in light energy, causing excitation.

Order of matter organization (Matter → Atoms → Particles → Electrons → Orbitals → Energy levels) [MAPEOE]

The hierarchy describing where electronic energy transitions occur.

Different wavelength ranges interact with matter in different ways.

Lower frequency and increasing wavelength effect

Transmittance (T)

The fraction of incident light that passes through a sample.

Transmittance equation (T = P/P₀)

The ratio of transmitted light intensity (P) to incident light intensity (P₀).

Incident light intensity (P₀)

The intensity of light entering a sample.

Transmitted light intensity (P)

The intensity of light leaving a sample after passing through it.

Absorbance (A)

The amount of light absorbed by a sample.

Absorbance equation (A = –log T)

The equation relating absorbance to transmittance.

Directly proportional

Relationship of absorbance to concentration of absorbing species

Beer's Law

The principle stating that absorbance is directly proportional to the concentration of the absorbing species.

Beer-Lambert equation (A = abc = εbc = log(P₀/P))

The mathematical relationship between absorbance, absorptivity, path length, concentration, and light intensities.

Absorptivity (a)

A proportionality constant describing how strongly a substance absorbs light.

Molar absorptivity (ε)

The absorptivity of a substance expressed per mole concentration.

Path length (b)

The distance that light travels through the sample.

Concentration (c)

The amount of absorbing species present per unit volume.

Beer's Law mnemonic ("The deeper the glass, the darker the blend, the smaller the light in the end.")

A memory aid emphasizing that higher concentration or longer path length increases absorbance and decreases transmitted light.

The conditions under which Beer-Lambert's linear relationship fails.

• Applicable to dilute concentrations

• Absorbing species does not participate in a concentration-dependent equilibrium

• Behavior is linear under monochromatic light

• Little or no stray radiation due to light scattering

Limitations of Beer's Law

Applicable to dilute concentrations

The condition that solutions must be sufficiently _ for Beer’s Law to remain valid.

At high concentrations, analyte-solvent interactions and light refraction cause deviations from linearity.

High concentration effect on Beer’s Law

Instruments cannot distinguish reflected light from absorbed light.

Instrument limitation at high concentrations

No concentration-dependent equilibrium

The absorbing species should not change chemical form or concentration upon dilution.

Beer’s Law is linear only when measurements are made using a single wavelength.

Monochromatic light requirement

The analyte absorbs most consistently at a single wavelength, maintaining linearity.

Reason for using monochromatic light

Beer’s Law requires little or no scattered or stray light.

Stray radiation limitation

Absorption spectrum

A graph showing how absorbance (A) or molar absorptivity (ε) changes with wavelength.

Observed color

The color perceived because complementary wavelengths are absorbed.

Complementary color

The color opposite the observed color that is absorbed by the substance.

Example of complementary colors (Green observed → Red/Magenta absorbed)

A green sample appears green because it absorbs ___ or __ wavelengths.

Spectrophotometer

An instrument used to measure the transmission or absorption of light.

Five basic components of a spectrophotometer (Light source, Wavelength selector, Sample holder, Detector, Processor/Read-out)

The essential parts required to measure light absorption or transmission.

Using Beer's Law in analysis

The application of Beer’s Law for determining analyte concentration.

Requirements for using Beer's Law in analysis (Compound absorbs EM radiation, Absorption distinguishable from other species, Standard curve prepared)

The conditions necessary for accurate quantitative analysis.

Standard curve

A calibration graph relating absorbance to known concentrations for determining unknown concentrations.

Spectrophotometric titration

A titration method that monitors changes in absorbance throughout the titration.

Light source

The spectrophotometer component that provides electromagnetic radiation.

The light source depends on the required electromagnetic spectrum region.

Light source selection criterion

Ultraviolet light source (Deuterium arc lamp)

The light source used for UV spectroscopy.

Visible light source (Tungsten lamp)

The light source used for visible spectroscopy.

Infrared light source (Silicon carbide rod/Globar)

The light source used for infrared spectroscopy.

Wavelength selector

The component that isolates a specific wavelength of light before it reaches the sample.

Monochromator

Another name for the wavelength selector.

Functions of a monochromator (Disperse light, Select narrow wavelength band)

It separates light into component wavelengths and allows only a narrow range to pass.

Components of a monochromator (Slits, Mirrors, Diffraction grating or Prism)

The optical parts responsible for wavelength selection.

A narrow wavelength band is necessary for Beer’s Law to remain valid.

Importance of a narrow wavelength band

Sample holder

The component that contains the sample during analysis.

Requirements of sample holders (Transparent to specific EM radiation, Cost considered)

The characteristics needed for proper sample measurement.

Detector

The component that measures transmitted light intensity.

Requirements of a detector (Sensitive to faint intensity changes, Converts light to electrical signal)

The characteristics required for accurate detection.

Transducer

A device that converts changes in light intensity into electrical signals.

Photomultiplier tube (PMT)

A highly sensitive detector that uses the photoelectric effect to amplify weak light signals.

Photoelectric effect

The phenomenon in which light causes the emission of electrons from a material, allowing light detection.