Chemistry Chapter on Optical Activity and X-ray Analysis

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Questions and Answers

What characteristic is true for enantiomers?

They rotate polarized light in the same direction.

They have different boiling points.

They have different melting points.

They possess identical physical properties. (correct)

What does a racemate consist of?

Chiral compounds only.

Equal amounts of two enantiomers. (correct)

A single enantiomer.

A mixture of different compounds.

Which symbol indicates a compound that rotates light to the right?

(r or s)

(+ or d) (correct)

(±)

(- or l)

Which factor does NOT affect optical rotation?

Molecular weight Signup and view all the answers

Which statement accurately describes specific rotation?

It varies with light wavelength and temperature. Signup and view all the answers

What is one of the advantages of using disperse X-ray fluorescence over EDS?

It provides greater spectral resolution. Signup and view all the answers

Which of the following is an artifact that can complicate spectrum interpretation?

Bremsstrahlung Signup and view all the answers

What happens when two photons with the same energy strike the detector at the same time?

A sum peak is formed. Signup and view all the answers

What type of structures can X-ray diffraction help identify qualitatively?

Crystalline compounds Signup and view all the answers

What is a characteristic of escape peaks in X-ray detection?

They occur from photon absorption by the detector material. Signup and view all the answers

What is the primary role of the device discussed in enhancing atomization efficiency?

To control heat distribution and prevent sample loss Signup and view all the answers

At what temperature is the drying process of the samples conducted in the furnace?

125°C Signup and view all the answers

What happens during the charring process in the furnace?

Organic matter is destroyed Signup and view all the answers

Which gas is used in the furnace to prevent oxidation?

Argon (Ar) Signup and view all the answers

What is meant by 'memory effect' in the context of this furnace?

Interference from previously analyzed samples Signup and view all the answers

How does a matrix modifier function in the analysis process?

It modifies the matrix to enhance volatility or decrease analyte volatility Signup and view all the answers

What advantage does a transversely heated furnace have over a longitudinally heated furnace?

It provides a uniform temperature across the furnace Signup and view all the answers

What is a key characteristic of the Inductively Coupled Plasma (ICP) compared to flames?

It is more stable and twice as hot Signup and view all the answers

What is the main purpose of scattering correction in analytical measurements?

To correct for unwanted scattered light Signup and view all the answers

What could be a consequence of not correcting for scattering in a sample analysis?

Overestimation of the analyte's concentration Signup and view all the answers

What is the role of deuterium lamps in background correction?

To produce light unaffected by analyte absorption Signup and view all the answers

During the correction process with a deuterium lamp, what happens when light from the hollow cathode lamp is analyzed?

It is absorbed by the analyte and scattered by the background Signup and view all the answers

How does the Zeeman correction enhance the accuracy of absorption measurements?

By pulsing the magnetic field on and off Signup and view all the answers

In Zeeman correction, what occurs when the magnetic field is on?

Only background absorption is observed Signup and view all the answers

What does the difference observed in the Zeeman correction indicate?

The corrected signal of the analyte Signup and view all the answers

Which of the following particles could potentially cause scattering during analytical measurements?

Water droplets in a flame or furnace Signup and view all the answers

What is the primary reason for band spreading in chromatography?

Injection volume and detection volume Signup and view all the answers

Which statement about the Van Deemter equation is true?

It indicates that the type of column affects the constants A, B, and C. Signup and view all the answers

What effect does higher linear flow have on longitudinal diffusion?

It decreases longitudinal diffusion. Signup and view all the answers

In gas chromatography, what type of stationary phase is used in gas-liquid partition chromatography?

A nonvolatile liquid-like polymer Signup and view all the answers

What typically causes fronting in chromatographic peaks?

Increased sample solubility in the stationary phase Signup and view all the answers

How does the type of column affect its separation efficiency?

Open tubular columns generally provide higher resolution. Signup and view all the answers

What is the consequence of a skewed chromatographic peak?

It reveals issues with concentration variations inside the column. Signup and view all the answers

Which component does NOT directly affect plate height in chromatography?

Identification of the mobile phase Signup and view all the answers

What is the primary mechanism of mass transfer in chromatography?

Movement of solute between stationary and mobile phases Signup and view all the answers

Which of the following is a disadvantage of packed columns?

Higher analysis time Signup and view all the answers

What is the primary role of the carrier gas in a gas chromatograph?

To collect the vapor and transport it to the column Signup and view all the answers

Which type of column is typically favored for higher resolution in gas chromatography?

Open tubular column Signup and view all the answers

What impact does the increased temperature have on the elution of analytes during gas chromatography?

It reduces retention time for late-eluting components Signup and view all the answers

Which principle best describes how stationary phases in gas chromatography interact with solutes?

Like dissolves like Signup and view all the answers

What is a significant disadvantage of packed columns compared to open tubular columns?

Longer analysis time Signup and view all the answers

How is the retention index (I) for linear alkanes calculated in gas chromatography?

100 times the number of carbon atoms Signup and view all the answers

What is the main purpose of temperature programming in gas chromatography?

To improve separation efficiency and decrease retention times Signup and view all the answers

What happens if the initial column temperature is set too low during a gas chromatography run?

Analytes will have high retention times Signup and view all the answers

Which of the following gases is most commonly used as a carrier gas in gas chromatography?

Helium Signup and view all the answers

What is one consequence of increasing the inlet pressure in gas chromatography?

Increased flow of mobile phase and decreased retention time Signup and view all the answers

Study Notes

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.

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Description

This quiz tests your knowledge on enantiomers, racemates, and their specific optical properties. It also covers aspects of X-ray diffraction and fluorescence techniques in material characterization. Answer questions relating to their use and the phenomena observed during analysis.