Chemistry Chapter on Optical Activity and X-ray Analysis

Atomic Spectroscopy

• Emission spectroscopy involves measuring the light emitted by atoms when they are excited, typically by a flame or plasma.
• Absorption spectroscopy involves measuring the light absorbed by atoms when they are illuminated with a known light source.
• Fluorescence spectroscopy measures light emitted by atoms after the absorption of light at a specific wavelength.

Types of Atomic Spectroscopy

• Emission: Light emitted by atoms in a flame or plasma, providing information on the element's presence and concentration.
• Absorption: Atoms in a sample absorb light at specific wavelengths, and the amount of absorption is measured to determine the element's concentration.
• Fluorescence: Light emitted by excited atoms after they absorb light at a specific wavelength determines element presence and relative concentration.

Flames, Furnaces, and Plasmas

• Flames, typically gas-fuel mixtures, are relatively simple and inexpensive.
• Graphite furnaces achieve higher temperatures, allowing for increased atomization and sensitivity.
• Plasmas (ICP) offer the highest temperatures, leading to high sensitivity, multi-element analysis, and less interference from the sample matrix.

Principal Differences

• Emission/Absorption/Fluorescence: Different processes—emission, absorption and fluorescence—for measuring atomic properties.
• Instrumentation: Flame or furnace versus ICP for atomization, and use or non-use of a separate light source/hollow cathode lamp for excitation.
• Sensitivity: Differences in sensitivity that can be attributed to the temperature of the atomization source.

Comparison Flames vs Furnaces

• Sample Volume: Flames take 1-2ml minimum , whereas furnaces use 1µL minimum.
• Sample Injection: Flames use single injections, unlike furnaces with multiple injections for greater sensitivity.
• Nebulization: Flames require a nebulization step, which can dilute the sample, whereas furnaces do not.

ICP-MS

• Advantages: High sensitivity, allowing for the measurement of trace elements; multi-element analysis.
• Disadvantages: Requires high vacuum, which can lead to background interferences and limits sample size. Collision cells or dynamic reaction cells are used to reduce isobaric interference.

X-Ray Spectroscopy

• X-ray fluorescence: Emission of X-rays following the absorption of X-rays by a material, revealing elements present in the sample based on the characteristic X-ray emission peaks.
• X-ray diffraction: Scattering of X-rays by ordered crystal structure identifying crystalline compounds and determining structures, such as steroid, vitamin structures etc.

Instrumentation

• Data Interpretation/Atomic Spectroscopy: Identify elements by analyzing peaks, accounting for artifacts, and appropriate analysis of peak intensity ratios.
• Principal Differences b/t Atomic & Ordinary Molecular Spectroscopy: Key distinctions such as the light source (flame, furnace, or plasma), sample container, and the need to subtract background emission.
• Atomic Linewidths: Linewidths of atomic spectra are crucial for proper analysis according to Beer's Law. Heisenberg's uncertainty principle explains how the excited state's lifetime is inversely related to the width of the spectral line.
• Hollow Cathode Lamps: Precise light sources that produce narrow, near-monochromatic emission lines needed for atomic spectroscopy.
• Multi-element Detection: Simultaneous detection of multiple elements using ICP/microwave plasma spectrometers without the need for separate lamps.
• Background Correction: Separating analyte signal from interfering signals, this improves the accuracy and reliability of the analysis through techniques like scattering correction or Zeeman effect.
• Different Types of Chromatography: Chromatography is used to separate mixtures of compounds: (adsorption, partition, ion exchange, size exclusion, affinity).
• Flow rate: Measurement of the speed of mobile phase flow through the column in chromatography.
• Calibration Standards and Curves: Quantitative analysis requires standards with known concentrations to create a calibration curve, correlating signal with concentration, suitable for both peak height and peak area analyses.
• Internal Standards: Using a known internal standard compound to account for possible errors/variations introduced during sample injection in quantitative analyses.

Temperature Effects on Atomic Spectrometry

• Atomization: Higher temperatures improve atomization but too much heat can cause ionization.
• Ionization: High temperatures lead to ionization interference, which can be reduced or mitigated using suppressors.
• Molecular Absorption/Atomic Transitions: Higher temperatures break down molecules for improved atomization and allow for a higher population of more excited states in emission analyses to produce stronger emission lines.
• Calibration and Sensitivity: Stable temperature is needed for better calibrations and reproducible results.

Detection Limits

• Concentration: Three times the standard deviation of the blank signal.
• Methods: Comparison across various atomic spectrometry techniques to see what has superior/inferior detection limits.

Important Concepts for Low Concentration Work

• Sampling: Proper sampling procedures and devices are essential to avoid contamination at low concentrations.
• Acidification: Acidification of liquid samples is important to stabilize analyte.
• Blank Corrections: Run field blanks repeatedly to compensate for background/interfering signals, typically after every 10 samples.
• Interferences: Four kinds of interferences in atomic analysis (spectral, physical, chemical, ionization) and methods for correcting those include using a high-resolution spectrometer, releasing agents, fuel-rich flames, or ionization suppressors.

Gas Chromatography

• Separation Processes: Vaporization of the analyte, separation by different phases, and detection to generate chromatograms.
• Retention: Retention time is critical for separating compounds; nonpolar compounds elute earlier and more polar ones elute later with increasing order of retention time.
• Instrumentation: Column, detector, injection port, oven.
• Column types: Packed columns or Open tubular columns.
• Chromatograms: Data plots showing the signal response over time, indicating the presence, abundance, and quantity of components.

Solvent Extraction

• Extracting one phase to another: Transferring analytes from one phase to another based on solubility to concentrate analytes of interest.
• pH Effects: Depending on the pH, changing charges/polarity of the analyte, affecting solubility between phases. Some species are better extracted at different pH levels.
• Metal Chelators: For separating metals using ligands, controlling the pH is needed for effective selective extraction.