Introduction to Spectroscopy: Concepts & Units (Analytical Chemistry 1)

Introduction to Spectroscopy

Overview

Spectroscopy is a fundamental analytical technique in chemistry that involves the study of the interaction between electromagnetic radiation and matter. This introductory section covers the essential concepts and units relevant to understanding how light is used to analyze chemical substances.

Properties of Light (Electromagnetic Radiation)

Wave and Particle Nature of Light

• Electromagnetic radiation (EMR) exhibits both wave-like and particle-like properties.

• As a wave, EMR is characterized by oscillating electric and magnetic fields perpendicular to each other and to the direction of propagation.

• As particles (photons), EMR can be described as discrete packets of energy with zero rest mass.

Key Terms:

• Wavelength (λ): The distance between two consecutive peaks (or troughs) of a wave. Measured in nanometers (nm) or meters (m).

• Frequency (ν): The number of wave cycles passing a point per second. Measured in hertz (Hz).

• Amplitude (A): The height of the wave, related to the intensity of the radiation.

• Speed of Light (c): The velocity at which light travels in a vacuum, $c = 3.00 \times 10^8$ m/s.

Relationship between wavelength, frequency, and speed of light:

$c = \lambda \nu$

Where $c$ is the speed of light, $\lambda$ is the wavelength, and $\nu$ is the frequency.

Refractive Index (n)

• The refractive index of a material is a dimensionless number that describes how light propagates through that medium.

• Defined as:

$n = \frac{\text{speed of light in vacuum}}{\text{speed of light in medium}}$

• For example, the refractive index of water is 1.333, meaning light travels 1.333 times faster in vacuum than in water.

• When light enters a denser medium, its velocity and wavelength decrease, but its frequency remains unchanged.

Electromagnetic Spectrum

Regions of the Electromagnetic Spectrum

The electromagnetic spectrum encompasses all types of electromagnetic radiation, classified by wavelength or frequency. Analytical techniques are often categorized by the region of the spectrum they utilize.

• Radio waves: Used in NMR spectroscopy.

• Microwaves: Used in rotational spectroscopy.

• Infrared (IR): Used in vibrational spectroscopy.

• Visible and Ultraviolet (UV-Vis): Used in electronic spectroscopy.

• X-rays: Used in X-ray crystallography and absorption studies.

Visible Spectrum: The portion of the EM spectrum visible to the human eye, ranging from approximately 400 nm (violet) to 780 nm (red).

Color	Wavelength Range (nm)
Violet	400 – 420
Indigo	420 – 440
Blue	440 – 490
Green	490 – 570
Yellow	570 – 585
Orange	585 – 620
Red	620 – 780

Relationship Between Color, Wavelength, and Energy

• Isaac Newton and Joseph von Fraunhofer contributed to the understanding of the relationship between color and energy.

• The color of light corresponds to its wavelength, and thus to its energy.

• For example, the color of heated steel changes with temperature, indicating a change in the energy (and wavelength) of emitted light.

Color of Light	Approximate Wavelength (nm)	Temperature of Steel (°C)
Bright yellow	~590	1037
Orange	~620	927
Red	~700	746
Blue	~450	302
Brown/Purple	~500	249
Light Straw	~580	219

Wave and Particulate Properties of Electromagnetic Radiation

Wave Properties

• EM radiation can be described as sinusoidal waves with parameters such as amplitude, frequency, and wavelength.

• Does not require a medium for transmission.

• Consists of perpendicular electric and magnetic fields.

Particulate (Photon) Properties

• Some interactions with matter (e.g., absorption, emission) are best described by considering EM radiation as particles called photons.

• The energy of a photon is given by:

$E = h \nu = \frac{hc}{\lambda}$

• Where $E$ is energy, $h$ is Planck's constant ($6.626 \times 10^{-34}$ J·s), $\nu$ is frequency, $c$ is the speed of light, and $\lambda$ is wavelength.

Example Calculation

Problem: By how many kilojoules per mole is the energy of a molecule increased when it absorbs ultraviolet light with a wavelength of 150 nm?

• Convert wavelength to meters: $150\ \text{nm} = 1.5 \times 10^{-7}\ \text{m}$

• Calculate frequency: $\nu = \frac{c}{\lambda} = \frac{3.00 \times 10^8}{1.5 \times 10^{-7}} = 2.0 \times 10^{15}\ \text{s}^{-1}$

• Calculate energy per photon: $E = h\nu = 6.626 \times 10^{-34} \times 2.0 \times 10^{15} = 1.33 \times 10^{-18}\ \text{J}$

• Convert to kJ/mol: $1.33 \times 10^{-18}\ \text{J} \times 6.022 \times 10^{23}\ \text{mol}^{-1} / 1000 = 800\ \text{kJ/mol}$

Types of Spectra

Emission Spectrum

• An emission spectrum is produced when atoms or molecules emit light at specific wavelengths after being excited by energy (thermal, electrical, or radiant).

• The emission spectrum of hydrogen, for example, consists of discrete, narrow lines corresponding to specific energy transitions.

Absorption Spectrum

• An absorption spectrum is observed when a substance absorbs certain wavelengths of light, resulting in a reduction in intensity at those wavelengths.

• Absorption methods are based on the attenuation of EM radiation as it passes through a sample.

Applications in Analytical Chemistry

Spectroscopic Methods of Analysis

• Spectroscopic techniques are classified based on the region of the electromagnetic spectrum they use and the type of interaction (emission or absorption).

• Common applications include qualitative and quantitative analysis of elements and compounds.

• Examples: UV-Vis spectrophotometry, IR spectroscopy, atomic absorption/emission spectroscopy.

Spectrophotometry Experiment (General Principle)

• Measures the intensity of light entering and emerging from a sample solution.

• The difference in intensity is used to determine the concentration of the analyte.

Summary Table: Color, Absorbed Wavelength, and Observed Color

Observed Color of Compound	Color of Light Absorbed	Approximate Wavelength Absorbed (nm)
Red	Green	~500
Yellow	Violet	~400
Yellow-green	Purple	~530
Blue	Orange	~600
Orange	Blue	~450
Purple	Yellow-green	~550

Additional info: The above tables and explanations are expanded for clarity and completeness, based on standard analytical chemistry textbooks and the provided lecture content.