Achem Exam 2

Types of Atomic Spectroscopy

Emission (AES): Electrons are excited using a flame and light is emitted from electrons returning to a ground state. More commonly used than AFS

Absorption (AAS): Absorption of sharp lines from a hollow-cathode lamp.

Fluorescence (AFS): Electrons are excited and fluoresce following absorption from an intense light source. 1000x more sensitive than AAS (ppt)

Atomization Methods

Flame Atomization: Temperature not uniform in flame and may not be high enough for AES, sensitivity depends on where in flame the measurement is made

Furnace Atomization: Uses electrical heating, vapor is confined so less sample volume is needed, sensitivity is higher, and solids can be analyzed.

Plasma Atomization: An RF field is used to excite and inert gas which the atomizes the sample. More excited state atoms since temps are higher and more controlled.

Molecular vs Atomic Spectra

Molecular:

• band width 10-100 nm

• Path length ~ 1 cm

Atomic:

• Band width ~.001 nm

• Path length ~ 10 cm bc atoms further apart

• Multiple atoms can be analyzed

Effect of Temperature on Atomic Spectroscopy

N^*/N₀ = (g^*/g₀ ) e^-𐤃E/kT

As temperature increases, the number of atoms in the excited state increases. N^* changes significantly with T but N₀ does not.

• Temperature affects AES more than AAS

• Signal is proportional to concentration of relevant state

  • Excited state for emission

  • Ground state for absorption

Interference types

Spectral: Unwanted signals overlapping analyte signals

• Purify sample, measure at different wavelengths, use higher resolution instrument

Physical: Viscosity or density of solution alters nebulization and transport of analyte.

• Peristaltic pump for more stable flow

Chemical: Chemical reactions decreasing the concentration of analyte atoms

• Releasing agent (La³⁺, EDTA)

Ionization: Ionization of analyte decreases the concentration of neutral atoms.

• 1000 ppm CsCl bc it ionizes more easily so the excess e^- shift equilibrium, decrease temp

Dissociation of Water

H₂O_(l) → OH^- + H⁺

K_w = [OH^-][H⁺]

Acid and Base Hydrolysis/Dissociation

Acid:

HX + H₂O → H₃O⁺ + X^-

K_a = [H⁺][X^-] / [HX]

Base:

B + H₂O → OH^- + HB⁺

K_b = [OH^-][HB⁺] / [B]

Solubility Product

AB_(s) → A⁺ + B^-

K_sp → [A⁺][B^-]

Complex Formation

MX_n-1 + X → MX_n

K_n = [MX_n] / [MX_n-1][X]

Manipulating Equilibrium Constants

Reversing: K^’ = 1/K

Sum: K₃ = K₁K₂

Ionic Strength Impact on Solubility

Ionic Strength (μ) *equation only works if +1/-1

A highly soluble inert salt increases the solubility of a sparingly soluble salt.

• Oppositely charged ions will have a propensity to form a neutral pair. However, an increasingly ionic environment (generated by the inert salt) decreases the attraction between ions, so the ions are less likely to form a pair.

• Smaller, more highly charged ions (e.g Li) bind water molecules more tightly and behave as larger hydrated species (large hydrated radius).

Charge Balance Equations

∑n_i[C] = ∑m_i[A]

Aprotic and Protic Solvents

Aprotic (No reactive H⁺):

• DMSO (CH₃SOCH₃)

• Ketones (molecules with C=O functional groups)

• Ethers

• Nitrile

• Amide (CONH₂)

• Methylene chloride (CH₂Cl₂)

Protic (reactive H⁺):

• H₂O

• Alcohols

• Carboxylic Acids

Stepwise vs Cumulative Formation

Stepwise: MX_n-1 + X → MX_n

Cumulative: M + nX →MX_n

K_D

Distribution constant, larger K_D = better separation

Factors affecting:

• Temperature

• Nature of solute

• Nature of solvent

• pH

Analytical vs Preparative Chromatography

Analytical: identify and characterize a molecule of interest

Preparative: Purify large amounts of a molecule of interest

Separation Factor (α)

Ratio of the adjusted retention times (t^’₂/t^’₁) where t₂ elutes later than 1. Larger α indicates better separation.

• Independent of flow rate so can be used to identify peaks when the flow rate changes.

Factors Affecting Resolution

1. Particle size (smaller is better)

2. Column Length (longer is better)

3. Temperature

4. Mobile phase composition

5. Flow rate

Plate Height

The length of the column required for one equilibration of solute between mobile and stationary phases. Smaller = higher bandwidth

Retention Factor (k)

How far a component moves relative to the stationary front. The larger k indicates greater affinity for the mobile phase, lower indicates greater affinity for stationary.