Gas Chromatography | working principle and instrumentation lecture

Summary

Gas chromatography (GC) is a vital analytical technique used for separating and analyzing volatile compounds. It involves a carrier gas and a stationary phase within a column, allowing for the identification and quantification of substances based on their retention times. GC is widely applied in various fields, including quality assurance in the chemical industry, environmental monitoring, and forensic science, offering high sensitivity and accuracy for detecting trace amounts of compounds.

Takeaways

• 🌬️ Gas chromatography (GC) is a technique in analytical chemistry used to separate and analyze vaporizable compounds without decomposition.
• 🔍 GC is utilized for purity testing, component separation, and relative quantification in mixtures, and can aid in compound identification.
• 🧪 In preparative chromatography, GC can purify compounds from mixtures, differentiating it from analytical purposes.
• 🌀 The mobile phase in GC is a carrier gas, predominantly helium or hydrogen, which moves the compounds through the system.
• 🛠️ The stationary phase is a thin layer of liquid or polymer on a solid support within a column, where interactions with compounds occur.
• 📊 Retention time, the time it takes for a compound to elute from the column, is a key analytical parameter in GC.
• 🔄 GC is distinct from column chromatography by having a gas mobile phase and a liquid stationary phase, and it involves temperature-controlled conditions.
• 🔬 Similar to fractional distillation, GC separates mixtures based on boiling points or vapor pressures, but operates on a smaller scale.
• 📚 The term GLPC (Gas-Liquid Partition Chromatography) is often preferred for its accuracy in describing the technique.
• 📊 A chromatogram, a graph of detector response versus retention time, is used to identify and quantify analytes in a sample.
• 🔬 Modern GC is often coupled with a mass spectrometer for enhanced identification of analytes through peak pattern analysis.
• 🧪 GC requires samples to be stable up to 300°C, salt-free, and often compared against a reference standard for accurate measurements.
• 🔍 GC's accuracy allows for the measurement of extremely minute quantities, such as picomoles in liquid samples or parts-per-billion in gases.
• 🚒 GC is widely applied in forensic science for the identification and quantification of substances in various evidence types.

Q & A

What is Gas Chromatography (GC) and what are its typical uses?

Gas Chromatography (GC) is an analytical technique used to separate and analyze compounds that can be vaporized without decomposition. It is typically used for testing the purity of substances, separating different components of a mixture, determining the relative amounts of these components, and in some cases, identifying compounds.

What is the role of the mobile phase in GC?

The mobile phase in GC is a carrier gas, usually an inert gas like helium or an unreactive gas like nitrogen, which helps to move the compounds through the column for separation.

Why is helium the most commonly used carrier gas in GC?

Helium is the most commonly used carrier gas in about 90% of instruments because of its inertness and relatively low density, which allows for improved separations compared to other gases.

What is the stationary phase in GC and how does it interact with the compounds?

The stationary phase is a microscopic layer of liquid or polymer coated on an inert solid support inside a column. It interacts with the gaseous compounds being analyzed, causing each compound to elute at a different time, known as the retention time.

How does the gas chromatograph differ from other forms of chromatography like HPLC or TLC?

Gas chromatography differs in that it uses a gas mobile phase and a liquid stationary phase, whereas HPLC and TLC typically use a liquid mobile phase and a solid stationary phase. Additionally, GC allows for temperature control of the gas phase, which is not common in other chromatographic techniques.

What is the significance of retention time in GC analysis?

Retention time is significant in GC analysis as it is used to identify analytes. Each compound elutes at a different time, and by comparing these times, one can determine the identity of the compounds in a mixture.

How is qualitative analysis performed in GC?

Qualitative analysis in GC is performed by analyzing a chromatogram, which is a graph of detector response against retention time. The order in which substances emerge and their retention times are used to identify the analytes.

What is the basis for quantitative analysis in GC?

Quantitative analysis in GC is based on the area under a peak in the chromatogram, which is proportional to the amount of analyte present. This area is calculated using integration, allowing for the determination of the analyte's concentration in the original sample.

How are very minute amounts of substances measured in GC?

Very minute amounts of substances can be measured in GC by comparing the sample to a reference standard containing the pure suspected substance. Modern GC systems often use computer software to integrate peaks and match mass spectra for accurate measurements.

What are some practical applications of GC in various industries?

GC has practical applications in various industries such as the chemical industry for quality assurance, environmental analysis for measuring toxic substances in soil, air, or water, and forensic science for identifying and quantifying substances in crime scene evidence.

How does GC handle the analysis of light gases, especially when hydrogen is involved?

When analyzing light gases, including hydrogen, dual TCD instruments with a separate channel for hydrogen using nitrogen as a carrier are common due to helium's similar thermal conductivity to hydrogen. Argon is also used in some cases for simplicity in the gas supply, even though it may offer less sensitivity.

Outlines

00:00

🌬️ Gas Chromatography: Separation and Analysis of Vaporizable Compounds

The first paragraph introduces gas chromatography (GC) as a prevalent analytical technique for separating and analyzing compounds that can be vaporized without decomposition. It discusses the various applications of GC, including purity testing, component separation, and compound identification. The mobile phase in GC is typically an inert carrier gas like helium or nitrogen, with helium being the most common. The stationary phase is a thin layer of liquid or polymer on a solid support within a column. A gas chromatograph is the instrument used for this process. The interaction of compounds with the column walls, coated with the stationary phase, results in different elution times, which is the basis for GC's analytical utility. The paragraph also distinguishes GC from other chromatographic methods like column chromatography and HPLC, highlighting the differences in phases and temperature control. It concludes with the mention of alternative names for GC, such as vapor-phase chromatography (VPC) and gas-liquid partition chromatography (GLPC), and describes the process of analyzing a sample using a gas chromatograph.

05:13

📊 Analytical Techniques in Gas Chromatography: Identification and Quantification

The second paragraph delves into the qualitative and quantitative analysis methods used in GC. It explains how substances are identified by their elution order and retention time, which remain constant under unchanged method conditions. Modern GC systems often interface with a mass spectrometer for analyte identification. For quantitative analysis, the area under the peak in a chromatogram, which represents the detector response over retention time, is proportional to the analyte's amount. This area is calculated using integration, and the analyte's concentration is determined through calibration curves or relative response factors. The paragraph also touches on the requirements for sample preparation, such as being salt-free and stable up to 300°C, and the necessity of using reference standards for accurate measurements. It mentions the high sensitivity and accuracy of GC, capable of measuring extremely minute amounts of substances, and its applications in various fields, including education and professional settings.

10:27

🔬 Applications of Gas Chromatography in Experiments and Forensic Science

The third paragraph discusses the application of GC in analyzing hydrocarbons and other substances in experiments, such as studying the effects of artificially injuring plant leaves. It describes the use of packed columns for light gas separation and the detection methods like Thermal Conductivity Detectors (TCD) and Flame Ionization Detectors (FID). The paragraph addresses the challenges of analyzing light gases, especially hydrogen, due to its thermal conductivity similarity with helium, and the common use of dual TCD instruments with nitrogen as a carrier for hydrogen. It also mentions the use of argon as a carrier gas in gas phase chemistry reactions for simplicity. The paragraph concludes with the extensive use of GC in forensic science for various disciplines, including drug identification, arson investigation, and toxicology.

15:27

🚔 Forensic Applications of Gas Chromatography in Criminal Investigations

The fourth paragraph focuses on the role of GC in forensic science, detailing its use in identifying and quantifying substances found in biological specimens and crime scene evidence. It highlights the versatility of GC in various forensic disciplines, such as drug analysis, arson investigations, paint chip analysis, and toxicology cases. The paragraph emphasizes the importance of GC in providing crucial evidence for criminal investigations, contributing to the resolution of legal cases.

Keywords

💡Gas Chromatography (GC)

Gas Chromatography, often abbreviated as GC, is a technique used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. It is central to the video's theme, as it discusses the various applications and principles of GC. The script mentions that GC is used for testing purity, separating components of a mixture, and identifying compounds, illustrating its versatility in chemical analysis.

💡Mobile Phase

The mobile phase in GC refers to the carrier gas that moves through the system. It is usually an inert gas like helium or nitrogen, which serves to transport the sample through the column. The script specifies that helium is used in about 90% of instruments, although hydrogen may be preferred for certain separations, highlighting the importance of the mobile phase in the chromatographic process.

💡Stationary Phase

The stationary phase in GC is a microscopic layer of liquid or polymer coated on an inert solid support within a column. It plays a crucial role in the separation process by interacting with the compounds in the sample, causing different compounds to elute at different times. The script describes the stationary phase as being coated on the walls of the column, which is key to the retention times and separation of compounds.

💡Retention Time

Retention time is the time it takes for a compound to pass through the GC column and is a critical parameter in identifying and analyzing compounds. The script explains that each compound elutes at a different time, known as its retention time, which is used to compare and identify the substances being analyzed.

💡Gas Chromatograph

A gas chromatograph is the instrument used to perform GC. The script mentions it as the device that carries out the analysis by separating chemicals in a complex sample through a flow-through narrow tube known as the column. It is the central piece of equipment in the GC process.

💡Qualitative Analysis

Qualitative analysis in GC involves identifying the substances in a sample based on their retention times and the order in which they elute from the column. The script describes how modern GC is often connected to a mass spectrometer for more accurate identification of analytes, enhancing the qualitative aspect of the analysis.

💡Quantitative Analysis

Quantitative analysis uses the area under the peaks in a chromatogram to determine the concentration of analytes in the sample. The script explains that this is done by using a calibration curve or by determining the relative response factor, which is essential for measuring the amount of a substance in a sample.

💡Carrier Gas Flow Rate

The carrier gas flow rate is one of the parameters that can alter the retention time and separation efficiency in GC. The script mentions it as a factor that influences how the analyte molecules move through the column, affecting the separation process.

💡Column Length

The length of the GC column is another parameter that affects the separation of compounds. The script notes that longer columns can provide better separation but at the cost of longer analysis times, illustrating the trade-off between resolution and analysis duration.

💡Temperature Programming

Temperature programming in GC is the method of changing the oven temperature during the analysis to optimize the separation of compounds. The script suggests that various temperature programs can be used to make readings more meaningful, such as differentiating between substances with similar behaviors.

💡Forensic Science

In the context of the script, forensic science utilizes GC for the identification and quantification of various substances in legal investigations. The script mentions its use in diverse disciplines, such as drug dose identification, arson investigation, and toxicology, demonstrating the broad applicability of GC in analyzing evidence.

Highlights

Gas chromatography (GC) is a common analytical technique for separating and analyzing vaporizable compounds without decomposition.

Typical GC uses include testing substance purity, separating mixture components, and determining their relative amounts.

GC can aid in compound identification and is used in preparative chromatography to purify compounds from mixtures.

The mobile phase in GC is a carrier gas, commonly helium or nitrogen, with helium used in about 90% of instruments.

The stationary phase consists of a microscopic layer of liquid or polymer on an inert solid support within a column.

A gas chromatograph, or 'aerograph', performs the analysis by interacting compounds with a stationary phase-coated column.

Retention time, the time compounds elute from the column, is key to GC's analytical usefulness.

GC differs from column chromatography primarily in the phases used for separation and temperature control capabilities.

GC is similar to fractional distillation, but operates on a smaller scale and is known as vapor-phase or gas-liquid partition chromatography.

A gas chromatograph separates chemicals in a sample using a flow-through column and carrier gas, with detection and identification of components at the outlet.

The stationary phase in the column separates components based on chemical and physical properties and interaction with the phase material.

Qualitative GC analysis identifies substances by their retention time and order of emergence from the column.

Quantitative GC analysis calculates analyte concentration through peak area integration and calibration curves or relative response factors.

Modern GC systems often integrate with mass spectrometers for analyte identification and use computer software for peak integration and spectral matching.

GC can measure very minute amounts of substances, requiring samples to be salt-free and stable up to 300°C.

GC is highly accurate and can detect picomole levels in liquid samples or parts-per-billion concentrations in gases.

GC has practical applications in education, such as analyzing Lavender oil or measuring ethylene in plants.

In forensic science, GC is extensively used for drug identification, arson investigation, and toxicology cases.

Dual TCD instruments with separate hydrogen channels using nitrogen as a carrier are common for light gas analyses including H2.

Argon is often used as a carrier gas in gas phase chemistry reactions for simplicity, despite lower sensitivity.

Transcripts

Gas chromatography (GC) is a common type of chromatography used in analytical chemistry

for separating and analyzing compounds that can be vaporized without decomposition.

Typical uses of GC include testing the purity of a particular substance, or separating the

different components of a mixture (the relative amounts of such components can also be determined).

In some situations, GC may help in identifying a compound.

In preparative chromatography, GC can be used to prepare pure compounds from a mixture.[1][2]

In gas chromatography, the mobile phase (or "moving phase") is a carrier gas, usually

an inert gas such as helium or an unreactive gas such as nitrogen.

Helium remains the most commonly used carrier gas in about 90% of instruments although hydrogen

is preferred for improved separations.[3] The stationary phase is a microscopic layer

of liquid or polymer on an inert solid support, inside a piece of glass or metal tubing called

a column (an homage to the fractionating column used in distillation).

The instrument used to perform gas chromatography is called a gas chromatograph (or "aerograph",

"gas separator").

The gaseous compounds being analyzed interact with the walls of the column, which is coated

with a stationary phase.

This causes each compound to elute at a different time, known as the retention time of the compound.

The comparison of retention times is what gives GC its analytical usefulness.

Gas chromatography is in principle similar to column chromatography (as well as other

forms of chromatography, such as HPLC, TLC), but has several notable differences.

First, the process of separating the compounds in a mixture is carried out between a liquid

stationary phase and a gas mobile phase, whereas in column chromatography the stationary phase

is a solid and the mobile phase is a liquid.

(Hence the full name of the procedure is "Gas�liquid chromatography", referring to the mobile and

stationary phases, respectively.)

Second, the column through which the gas phase passes is located in an oven where the temperature

of the gas can be controlled, whereas column chromatography (typically) has no such temperature

control.

Finally, the concentration of a compound in the gas phase is solely a function of the

vapor pressure of the gas.[1]

Gas chromatography is also similar to fractional distillation, since both processes separate

the components of a mixture primarily based on boiling point (or vapor pressure) differences.

However, fractional distillation is typically used to separate components of a mixture on

a large scale, whereas GC can be used on a much smaller scale (i.e. microscale).

Gas chromatography is also sometimes known as vapor-phase chromatography (VPC), or gas�liquid

partition chromatography (GLPC).

These alternative names, as well as their respective abbreviations, are frequently used

in scientific literature.

Strictly speaking, GLPC is the most correct terminology, and is thus preferred by many

authors.

A gas chromatograph is a chemical analysis instrument for separating chemicals in a complex

sample.

A gas chromatograph uses a flow-through narrow tube known as the column, through which different

chemical constituents of a sample pass in a gas stream (carrier gas, mobile phase) at

different rates depending on their various chemical and physical properties and their

interaction with a specific column filling, called the stationary phase.

As the chemicals exit the end of the column, they are detected and identified electronically.

The function of the stationary phase in the column is to separate different components,

causing each one to exit the column at a different time (retention time).

Other parameters that can be used to alter the order or time of retention are the carrier

gas flow rate, column length and the temperature.

In a GC analysis, a known volume of gaseous or liquid analyte is injected into the "entrance"

(head) of the column, usually using a microsyringe (or, solid phase microextraction fibers, or

a gas source switching system).

As the carrier gas sweeps the analyte molecules through the column, this motion is inhibited

by the adsorption of the analyte molecules either onto the column walls or onto packing

materials in the column.

The rate at which the molecules progress along the column depends on the strength of adsorption,

which in turn depends on the type of molecule and on the stationary phase materials.

Since each type of molecule has a different rate of progression, the various components

of the analyte mixture are separated as they progress along the column and reach the end

of the column at different times (retention time).

A detector is used to monitor the outlet stream from the column; thus, the time at which each

component reaches the outlet and the amount of that component can be determined.

Generally, substances are identified (qualitatively) by the order in which they emerge (elute)

from the column and by the retention time of the analyte in the column.

Qualitative analysis[edit] Generally chromatographic data is presented

as a graph of detector response (y-axis) against retention time (x-axis), which is called a

chromatogram.

This provides a spectrum of peaks for a sample representing the analytes present in a sample

eluting from the column at different times.

Retention time can be used to identify analytes if the method conditions are constant.

Also, the pattern of peaks will be constant for a sample under constant conditions and

can identify complex mixtures of analytes.

However, in most modern applications, the GC is connected to a mass spectrometer or

similar detector that is capable of identifying the analytes represented by the peaks.

Quantitative analysis[edit] The area under a peak is proportional to the

amount of analyte present in the chromatogram.

By calculating the area of the peak using the mathematical function of integration,

the concentration of an analyte in the original sample can be determined.

Concentration can be calculated using a calibration curve created by finding the response for

a series of concentrations of analyte, or by determining the relative response factor

of an analyte.

The relative response factor is the expected ratio of an analyte to an internal standard

(or external standard) and is calculated by finding the response of a known amount of

analyte and a constant amount of internal standard (a

chemical added to the sample at a constant concentration, with a distinct retention time

to the analyte).

In most modern GC-MS systems, computer software is used to draw and integrate peaks, and match

MS spectra to library spectra.

In general, substances that vaporize below 300 �C (and therefore are stable up to that

temperature) can be measured quantitatively.

The samples are also required to be salt-free; they should not contain ions.

Very minute amounts of a substance can be measured, but it is often required that

the sample must be measured in comparison to a sample containing the pure, suspected

substance known as a reference standard.

Various temperature programs can be used to make the readings more meaningful; for example

to differentiate between substances that behave similarly during the GC process.

Professionals working with GC analyze the content of a chemical product, for example

in assuring the quality of products in the chemical industry; or measuring toxic substances

in soil, air or water.

GC is very accurate if used properly and can measure picomoles of a substance in a 1 ml

liquid sample, or parts-per-billion concentrations in gaseous samples.

In practical courses at colleges, students sometimes get acquainted to the GC by studying

the contents of Lavender oil or measuring the ethylene that is secreted by Nicotiana

benthamiana plants after artificially injuring their leaves.

These GC analyse hydrocarbons (C2-C40+).

In a typical experiment, a packed column is used to separate the light gases, which are

then detected with

a TCD.

The hydrocarbons are separated using a capillary column and detected

with a FID.

A complication with light gas analyses that include H2 is that He, which is the most common

and most sensitive inert carrier (sensitivity is proportional to molecular mass) has an

almost identical thermal conductivity to hydrogen (it is the difference in thermal conductivity

between two separate filaments in a Wheatstone Bridge type arrangement that shows when a

component has been eluted).

For this reason, dual TCD instruments used with a separate channel for hydrogen that

uses nitrogen as a carrier are common.

Argon is often used when analysing gas phase chemistry reactions such as F-T synthesis

so that a single carrier gas

can be used rather than two separate ones.

The sensitivity is less, but this is a trade off for simplicity in the gas supply.

Gas Chromatography is used extensively in forensic science.

Disciplines as diverse as solid drug dose (pre-consumption form) identification and

quantification, arson investigation, paint chip analysis, and toxicology cases, employ

GC to identify and quantify various biological specimens and crime-scene evidence.