Gas Chromatography Principles Quiz

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Questions and Answers

What effect does decreased thickness of the stationary phase have in gas chromatography?

Increased retention times

Decreased retention times (correct)

Increased sample capacity

Decreased resolution

Which gas provides better resolution in gas chromatography?

Carbon Dioxide (CO2)

Argon (Ar)

Nitrogen (N2)

Hydrogen (H2) (correct)

What is a common disadvantage of using hydrogen as a carrier gas?

Forms explosive mixtures (correct)

High viscosity

Incompatibility with detectors

High cost

What is the purpose of a split injection in gas chromatography?

To avoid column overload Signup and view all the answers

What happens to the stationary phase over time due to aging?

Exposure of silanol groups Signup and view all the answers

What can accelerate the degradation of the stationary phase?

High temperature oxygen exposure Signup and view all the answers

What factor primarily determines the resolution in open tubular columns?

Narrowness of the column Signup and view all the answers

What mechanism does split injection utilize to deliver the sample to the column?

Fast vaporization of the sample Signup and view all the answers

What is the primary purpose of qualitative analysis in chromatography?

To compare retention times with standards Signup and view all the answers

How does the Flame Ionization Detector (FID) generate ions during the detection process?

By burning the eluate in a hydrogen-oxygen mixture Signup and view all the answers

What happens to the density of a supercritical fluid when pressure increases?

Density increases with increasing pressure Signup and view all the answers

Which of the following characteristics describes the response of the FID?

Response is proportional to the number of carbon atoms entering the flame Signup and view all the answers

What does the term 'co-chromatography' refer to in qualitative analysis?

Adding an authentic compound to the unknown sample Signup and view all the answers

Why is mass spectrometric detection used in gas chromatography?

To provide molecular weights and fragmentation patterns of analytes Signup and view all the answers

What is the impact of a more dense mobile phase in supercritical fluid chromatography?

It decreases the amount of solute in the stationary phase Signup and view all the answers

In gas chromatography, what is the relationship between peak area and analyte concentration?

Peak area is directly proportional to concentration Signup and view all the answers

What is one advantage of using supercritical fluids as a mobile phase in chromatography?

They possess lower surface tension than liquids. Signup and view all the answers

Which statement about supercritical fluids is incorrect?

They cannot be detected by common detectors. Signup and view all the answers

Compared to High-Performance Liquid Chromatography (HPLC), how does the minimum plate height in Supercritical Fluid Chromatography (SFC) compare?

It is one-third compared to HPLC. Signup and view all the answers

In Supercritical Fluid Chromatography (SFC), what gradient is primarily utilized?

Pressure gradient Signup and view all the answers

Which of the following is NOT a characteristic of supercritical fluids?

They can only operate at high temperatures. Signup and view all the answers

What is one disadvantage of liquid mobile phases in chromatography compared to supercritical fluids?

Slower diffusion inside the column. Signup and view all the answers

Which of the following detectors is commonly compatible with supercritical fluid chromatography?

Flame ionization detector Signup and view all the answers

What is the main focus of Capillary Electrophoresis in the context of biotechnology?

Analysis of nucleic acids and proteins. Signup and view all the answers

What is the primary purpose of the carrier gas in Gas Chromatography?

To transport the gaseous analyte through the column Signup and view all the answers

Which type of injection technique is characterized by the continuous introduction of samples into the column?

On-column injection Signup and view all the answers

Which detector is commonly used in Gas Chromatography to measure the amount of analyte?

Flame Ionization Detector (FID) Signup and view all the answers

In Gas Chromatography, what does the term 'stationary phase' refer to?

The liquid or solid that the analytes interact with Signup and view all the answers

Which of the following statements accurately describes supercritical fluid chromatography (SFC)?

SFC combines properties of both liquid and gas chromatography. Signup and view all the answers

What is one reason that Helium (He) is commonly used as a carrier gas in Gas Chromatography?

It is inert and does not react with analytes. Signup and view all the answers

Which factor is essential for obtaining efficient separation in a Gas Chromatography column?

Sufficient vapor pressure of analytes Signup and view all the answers

What is a disadvantage of quantitative analysis mentioned in the content?

The split ratio is not reproducible from run to run. Signup and view all the answers

What distinguishes Gas-Liquid Chromatography (GLC) from Gas-Solid Chromatography (GSC)?

GLC relies on partitioning between liquid and vapor phases. Signup and view all the answers

What sample characteristic is associated with splitless injection?

Approximately 80% of the sample is delivered to the column. Signup and view all the answers

How can band broadening due to slow injection be avoided?

By setting the column temperature lower than the boiling point of the solvent. Signup and view all the answers

What is the preferred injection method for samples that decompose above their boiling point?

On-column injection. Signup and view all the answers

What effect does temperature programming have during separation?

It sharpens the peaks of analytes. Signup and view all the answers

What is one advantage of using a pressure gradient over temperature programming?

It prevents longer cooling times for the column. Signup and view all the answers

What happens to the retention time of analytes as the temperature of the column is increased?

Retention time decreases and peaks become sharper. Signup and view all the answers

In splitless injection, why is the temperature set at 220 °C?

To avoid decomposition of the sample. Signup and view all the answers

Study Notes

Lecture 7: Gas Chromatography and Supercritical Fluid Chromatography

Gas chromatography (GC) transports a gaseous analyte through a column using a carrier gas.
The stationary phase is typically a non-volatile liquid or solid.
Analytes are gases or volatile liquids.
Gas chromatography can use partition or adsorption stationary phases (GLC and GSC).

Learning Outcomes

Define supercritical fluid chromatography (SFC).
Compare HPLC and SFC.
Describe gas chromatography (GC).
Compare GSC and GLC.
Describe GC column types and stationary phases, and their uses.
Compare injection techniques: split, split-less, and on-column.
Describe the function and use of FID in GC.
Explain temperature/pressure programming.

Gas Chromatography Set-Up

A volatile liquid or gaseous sample is injected into a heated port.
The vapor is swept through the column by a carrier gas (e.g., H₂, He, N₂).
The column oven must be hot enough to vaporize the sample and elute the analytes.
Separated analytes pass through a detector.
Detector response is displayed on a computer chromatogram.
The detector is typically held at a higher temperature than the column.

GC Columns

Packed columns are typically used in HPLC, less common in GC. They provide high capacity but low resolution.
Open tubular columns (OTCs) are most common in GC.
OTCs are coated with a stationary liquid phase.
Using thinner stationary phases and smaller column diameters improves resolution in GC.

Open Tubular Columns (OTCs)

Mostly made of fused silica (SiO₂).
Coated with polyimide for protection from atmospheric moisture.
Smaller inner diameter leads to higher resolution but higher pressure is needed.
Degradation can occur due to aging (stationary phase degradation) or oxygen exposure at high temperatures.

Effect of Decreased Stationary Phase Thickness

Decreasing stationary phase thickness in GC leads to decreased retention times.
There is decreased sample capacity with reduced stationary phase thickness.
Resolution increases with reduced thickness (less diffusion).

Open Tubular vs. Packed Columns

Open tubular columns generally have a higher number of theoretical plates than packed columns, leading to better resolution.
Open tubular columns have a higher linear gas velocity.

Carrier Gases

Hydrogen (H₂) offers better resolution (smaller plate height).
Helium (He) is the most common due to compatibility with all detectors.
Nitrogen (N₂) is also used.

Types of Injection

Split Injection:
- Used for samples representing more than 0.1% of the sample content.
- Delivers only a small percentage of the sample to the column.
- Disadvantage: quantitative analysis can be inaccurate if the split ratio varies.
Splitless Injection:
- Used for trace analysis (<0.01% of the sample).
- Delivering approximately 80% of the sample to the column.
- Often used with lower injection temperatures for volatile samples.
On-Column Injection:
- Injects samples directly into the column.
- Suitable for samples that decompose above their boiling point.
- Lowest possible temperature for separation is used.

Temperature and Pressure Gradient

Temperature gradient: The column temperature is raised during separation to increase solute vapor pressure.
As temperature increases, retention times decrease, leading to sharper peaks.
Pressure gradient: Less common than temperature programming. Used for temperature-sensitive analytes that don't tolerate high temperatures. It increases flow rate and reduces retention times.

Detection

Quantification: Peak area is proportional to analyte concentration.
Qualitative analysis: Based on retention time comparisons with standards to identify unknown compounds and mass spectrometry for molecular weight determination.
- "Co-chromatography" involves comparing the unknown analyte's retention time to accurately identify components.

Detectors in Gas Chromatography

Various detectors are available, each with different detection limits and linear ranges.
Thermal conductivity, flame ionization (FID), electron capture, flame photometric are some common detectors.

Supercritical Fluid Chromatography (SFC)

SFC uses a supercritical fluid (e.g., CO₂) as the mobile phase.
Supercritical fluids have properties intermediate between gases and liquids.
Density increases with pressure.
Compared to HPLC or GC, SFC has a faster equilibration rate for better resolution with a similar minimum plate height.

Use of Supercritical Fluids in SFC

Lower surface tension compared to liquids and higher density.
Non-volatile materials can be dissolved in the supercritical fluid.
Analytes separate and evaporate upon pressure reduction, making the method highly adaptable.
Lower critical temperature than typical liquids and is non-toxic.

Effect of Flow Rate in SFC

Lower minimum plate height at the same flow rate compared to HPLC for better resolution.
SFC allows for faster and more efficient separation.

5% Presentation: Capillary Electrophoresis

The presentation is about capillary electrophoresis principles and applications in biotechnology.

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Description

Test your knowledge on key concepts of gas chromatography with this quiz. Discover the effects of stationary phase thickness, the choice of carrier gas, and the mechanics of split injection. Perfect for students and professionals looking to refresh their understanding of analytical techniques.