Atomic Spectroscopy Notes

1.5 Atomic Spectroscopy: Identifying Unknown Elements

Flame Tests and Atomic Absorption Spectroscopy (AAS)

• Analytical techniques used to identify elements.
• Based on electron transfer between atomic energy levels.
• Unique wavelengths of radiation are emitted and absorbed by each element, allowing for its identification in a sample.

Electron Configuration

• Electron configuration can be written using subshell notation for atoms or monatomic ions of the first 38 elements.
• The absorption or emission of radiation affects the electron configuration of electrons in atoms or ions.
• Unique wavelengths emitted and absorbed by an element due to its specific electron configuration.

The Science of Firework Color

• The color of fireworks is related to specific metal ions.

Metal Ion Flame Tests

• Flame test: Analytical procedure to detect the presence of particular metal ions based on the flame color produced.
• When heated, electrons in the metal ion gain energy and jump to higher energy levels.
• Electrons fall back to their original energy levels, releasing energy as light.
• The color of the light is unique to each metal ion because the transitions vary.
• Examples of metal ions and their flame colors:
  • Lithium (Li)
  • Sodium (Na)
  • Potassium (K)
  • Rubidium (Rb)
  • Cesium (Cs)
  • Calcium ($Ca^{2+}$)
  • Strontium ($Sr^{2+}$)
  • Barium (Ba)
  • Radium (Ra)
  • Copper ($Cu^{2+}$)
  • Iron (Fe)
  • Boron ($B^{3+}$)
  • Indium ($In^{3+}$)
  • Lead (Pb)
  • Arsenic ($As^{3+}$)
  • Antimony (Sb)
  • Selenium (Se)
  • Zinc ($Zn^{2+}$)

The Electromagnetic Spectrum

• The electromagnetic spectrum includes:
  • Gamma rays
  • X-rays
  • Ultraviolet
  • Visible light (400-750 nm)
  • Infrared
  • Microwaves
  • Radio waves
• Energy increases with frequency, wavelength decreases with frequency.

Atomic Emission Spectra

• When metals or salts containing metal ions receive energy (heat, light, electricity), electrons become excited and move to a higher energy level.
• Atoms absorb specific amounts of energy to allow electrons to move to higher energy levels.
• The atom returns to its ground state by emitting the energy as electromagnetic radiation.

Atomic Absorption Spectra

• When white light passes through a prism, it produces a continuous ROYGBIV spectrum.
• When light passes through a vapor of metal atoms or ions, electrons in the atoms absorb specific frequencies of light.
• These frequencies correspond to the energy difference between ground and excited energy levels.
• The resulting absorption spectrum has dark lines corresponding to the absorbed wavelengths.

Emission vs. Absorption Spectra

• Emission spectra: Hot gas emits light at specific wavelengths.
• Absorption spectra: Light passes through a cold gas, which absorbs specific wavelengths.

Line Absorption Spectrum

• Line absorption spectrum of the Sun and emission spectra of selected elements (hydrogen, helium, and mercury).

Example Question

• Arsenic levels are increasing in the environment due to industrial pollution.
• Rice is efficient at taking arsenic out of soils in anaerobic environments.
• Flame emission spectroscopy is used to determine the presence of arsenic and other harmful elements in a rice sample.
• If the line emission spectra of arsenic matches a sample, arsenic is present in the sample.

Atomic Absorption Spectroscopy (AAS) Principles

• AAS is used for quantitative analysis.
• Used to identify elements in a sample.
• Calibration graphs are used to determine the concentration of an element in a sample.

AAS Applications

• Qualitative and quantitative analysis of metal ions.
• Determines the concentration of metal ions in aqueous solution.
• Useful for ions present in ppm concentrations and lower.
• Used to quantify metal ions in drinking water, biological fluids, wine, and industrial waste.
• Trace amounts of metals can be detected in solid and gaseous mixtures (e.g., heavy metals in soil, cigarette smoke).

AAS Procedure

1. Light Source: A hollow cathode lamp specific to the element being tested provides radiation through stimulated atomic emission.
2. Vaporize the sample: The sample is aspirated into a flame, decomposing metallic compounds into metal atoms and ions using a nebulizer to create small droplets.
3. Monochromator: Energies of radiation from the lamp that match the energy transitions in the metal atoms are absorbed. Unabsorbed radiation is transmitted through a monochromator to select one frequency for analysis.
4. Detector: Measures how much of the light source has been absorbed, and an absorbance value is provided.

Calibration Curves

• Calibration curves are plotted using absorbance readings of solutions with known concentrations.
• The concentration of an unknown sample can be inferred from the graph.

Example Exam Questions and Answers

• Question: Write the electron configuration of $Mn^{2+}$ using subshell notation.
  • Answer: $1s^22s^22p^63s^23p^63d^5$
• Question: Explain why AAS determination of $Mn^{2+}$ concentration is unaffected by other metal cations.
  • Answer: A single wavelength is selected at the detector that is unique to manganese; hence, this is not a wavelength that is absorbed by other metals present in the analysis.
• Question: Write the electron configuration of $Ca^{2+}$ using subshell notation.
  • Answer: $1s^22s^22p^63s^23p^6$
• Question: Explain why a magnesium lamp is needed as the AAS light source when determining $Mg^{2+}$ concentration.
  • Answer: Atomic emission from the lamp results in wavelengths characteristic of magnesium. The detector measures absorbances for one wavelength unique to Mg, unaffected by other elements.
• Question: How to construct an AAS calibration graph.
  • Answer: A calcium lamp is chosen, and a unique wavelength is selected for detection. Standard solutions of varying concentrations are prepared for Ca, plus a control of distilled water. Each solution is introduced to the AAS, and absorbance values are obtained.
• Question: Identify the correct electron configuration of $Fe^{3+}$ in its lowest energy state.
  • Answer: A: $1s^2 2s^2 2p^6 3s^2 3p^6 3d^5$
• Question: What is the full name of the analytical technique used to determine $Fe^{3+}$ concentration?
  • Answer: Atomic Absorption Spectroscopy
• Question: Determine the concentration of $Fe^{3+}$ in a tap water sample with an absorbance of 0.1 using a calibration graph.
  • Answer: 0.25 ppm