Untitled Flashcards Set

Voltammetry

• Excitation Signals: Voltage applied to measure current response. Types include linear sweep, square wave, and differential pulse voltammetry.

• Electrochemical Cell:

  • Working Electrode (WE): Where redox reactions occur.

  • Counter Electrode (CE): Completes the circuit by balancing charge.

  • Reference Electrode (RE): Provides a stable potential for comparison.

• Potentiostat: Controls voltage between WE and RE while measuring current at WE.

• Oxygen Removal: Oxygen can interfere with measurements. Removed by purging with inert gases (e.g., nitrogen).

• Cathodic and Anodic Current:

  • Cathodic (reduction): Electrons move into WE.

  • Anodic (oxidation): Electrons leave WE.

• Cyclic Voltammetry: Reversing voltage sweep to analyze redox behavior. Produces characteristic peak-shaped graphs (voltammograms).

Chromatography

Key Terms

• Mobile Phase: Moves analyte through the column (liquid or gas).

• Stationary Phase: Fixed phase where separation occurs.

• Chromatographic Interactions: Adsorption, partitioning, ion exchange, size exclusion.

• Chromatogram: Output graph showing detected compounds.

• Retention Time (tR): Time a compound takes to pass through.

• Dead Time (t0): Time for mobile phase to pass through.

• Retention Factor (k): (tR - t0) / t0; measures how long a compound interacts with the stationary phase.

• Peak Width (W, W1/2): Broadness of the detected signal; affects resolution.

• Column Resolution (Rs): Separation between peaks; higher values indicate better separation.

• Number of Theoretical Plates (N): Indicates column efficiency; calculated as N = (tR/W)².

• Plate Height (H): Column efficiency measure; lower H = better efficiency.

• Partition Coefficient (K): Ratio of analyte concentration in stationary vs. mobile phase.

• Selectivity Factor (α): Ability to separate two compounds.

• Tailing/Fronting Peaks: Peaks should be symmetric; tailing = slow elution, fronting = overload.

• VanDeemeter equation: HETP = A + (B / u) + Cu

Chromatography Classification

• By Physical Contact: Column (stationary phase inside tube) vs. Planar (e.g., TLC).

• By Phase Type: Gas-liquid, liquid-liquid, gas-solid, etc.

Maximizing Resolution (R)

• Increase column length, reduce particle size, optimize flow rate.

Plate Theory vs. Rate Theory

• Plate Theory: Describes chromatographic separation using "theoretical plates."

• Rate Theory: Explains factors affecting peak broadening.

Rate Theory Components

• Eddy Diffusion: Multiple flow paths cause broadening.

• Longitudinal Diffusion: Molecules spread due to concentration differences.

• Mass Transfer: Delays in reaching equilibrium with stationary phase.

• Van Deemter Equation: H = A + B/u + Cu (where u = mobile phase velocity); helps optimize column performance.

Column Efficiency

• Efficiency improves with more plates, smaller plate height, smaller particles, and narrower columns.

Gas Chromatography (GC)

- Types of GC

• Gas-Liquid Chromatography (GLC): Liquid stationary phase.

• Gas-Solid Chromatography (GSC): Solid stationary phase.

Mobile and Stationary Phases

• Mobile Phase: Inert gas (e.g., He, N2, H2); carries analytes.

• Stationary Phase: Coated on the column; determines separation efficiency.

Factors Affecting Separation

• Injection Method: Direct, split/splitless.

• Column Temperature: Higher temp = faster elution, lower resolution.

• Isothermal vs. Temperature Programming:

  • Isothermal: Constant temp; best for simple separations.

  • Temperature Programming: Increasing temp improves separation of complex mixtures.

Columns in GC

• Packed Columns: Short, lower efficiency.

• Capillary Columns:

  • Wall-Coated Open Tubular (WCOT): Liquid phase on inner wall.

  • Support-Coated Open Tubular (SCOT): Support material coated with liquid phase.

  • Fused-Silica Open Tubular (FSOT): More durable, flexible, efficient.

Detectors in GC

• Ideal Detector Characteristics: Sensitive, stable, fast response, wide range.

• Types of Detectors:

  • Concentration-Sensitive: Flame Ionization Detector (FID), Thermal Conductivity Detector (TCD).

  • Mass-Sensitive: Electron Capture Detector (ECD), Mass Spectrometry (MS).