Gas Chromatography Overview

A technique used to separate components of a mixture based on their volatility. It involves a mobile phase (carrier gas) and a stationary phase (packed or capillary column).

Gas Chromatography can be categorized as Gas-Solid Chromatography (GSC) and Gas-Liquid Chromatography (GLC).

The mobile phase in GC, which carries the sample through the column. Common choices include Helium (He), Argon (Ar), and Nitrogen (N₂).

The ideal carrier gas is inert, has high purity, is cheap, and doesn't react with the column or detector. Its density and molecular weight affect the separation efficiency.

A device that regulates the flow rate of the carrier gas. Different flow rates are required for different types of columns.

The point where the sample is introduced into the column. It typically contains a septum, which is pierced by a syringe needle.

A device that detects the separated components as they exit the column. Different types of detectors are available, each sensitive to different types of analytes.

A device that records the signals from the detector, producing a chromatogram. The chromatogram shows the peaks of the separated components.

A solid or liquid material in a GC column that interacts with the sample components based on their boiling points and polarity.

An inert carrier gas that moves the sample components through the GC column.

The time it takes for a component to travel through the GC column and reach the detector.

Volatility of the component, polarity of the component, and the size of the molecule all affect how quickly it travels through the GC column.

Identifying the components present in a sample by comparing the retention times of peaks in a chromatogram to known standards.

Determining the amount of each component in a sample by measuring the area under the peak in a chromatogram.

GC is used to analyze and identify organic compounds in various fields, including environmental monitoring, food safety, and pharmaceutical research.

In a homologous series, smaller molecules have shorter retention times than larger molecules because they interact less strongly with the stationary phase and move through the column faster.

The strength of interaction between a sample component and the stationary phase determines its retention time. Stronger interaction leads to longer retention time.

Increasing the amount of stationary phase results in longer retention times for all components because they have more surface to interact with.

Detectors in gas chromatography identify and measure the separated components as they exit the GC column.

The specific detector chosen for GC analysis depends on the type of analyte being analyzed.

A detector that responds to all compounds except the carrier gas.

A detector that responds to a range of compounds with a specific shared physical or chemical property.

TCD detectors are used for detecting various organic and inorganic compounds by measuring changes in the thermal conductivity of the carrier gas.

A universal detector in gas chromatography (GC) that measures changes in the thermal conductivity of the carrier gas (He) as solutes elute from the column.

As the solute elutes from the column, it displaces some of the helium carrier gas, changing the thermal conductivity of the gas. This change in conductivity alters the temperature of a heated wire filament, causing its electrical resistance to change. The change in resistance is measured by a Wheatstone bridge circuit, which produces a voltage signal proportional to the solute concentration.

The TCD is a universal detector that can detect any solute with a different thermal conductivity than helium. It provides a linear response to solute concentrations over a wide range and is non-destructive, meaning the solute is not destroyed in the detection process.

The TCD has a relatively poor detection limit compared to other detectors.

A GC detector specifically designed to detect electronegative functional groups in solutes, making it ideal for detecting compounds like pesticides and polychlorinated biphenyls.

A radioactive source emits beta particles (electrons) that ionize the carrier gas (usually nitrogen). The presence of electronegative atoms in the solute molecule captures some of these electrons, reducing the ionization current. This reduction in current is proportional to the solute concentration.

The ECD is highly sensitive to compounds containing halogen and nitro groups, which are common in pesticides and other environmental contaminants, making it a powerful tool for environmental analysis.

The ECD relies on the capture of electrons by electronegative atoms. This selective interaction makes it highly sensitive towards compounds containing these atoms, primarily halogens and nitro groups.

Electron Capture Detectors are particularly sensitive to compounds containing electron-withdrawing groups like halogens (Cl, Br, F) and nitro groups (NO₂). However, they are less sensitive to amines, alcohols, and hydrocarbons.

When a solute containing electron-withdrawing groups passes through the detector, it captures electrons from a stream of carrier gas (like nitrogen or argon). This capture reduces the electric current, which is measured as a signal.

The Flame Ionization Detector (FID) works by measuring the increase in electric current produced in a hydrogen flame when organic compounds are burned and ionized.

The FID is very sensitive to organic compounds and is commonly used for analyzing atmospheric and aqueous samples.

The FID destroys the sample during the detection process.

Organic compounds are mixed with hydrogen carrier gas and burned in a flame. This creates radicals that are oxidized into ions by oxygen in the flame. These ions are then collected by electrodes, generating an electric current proportional to the concentration of organic compounds.

The FID is particularly sensitive to compounds containing C-C or C-H bonds. It is less sensitive to carbonyl (C=O), alcohols (OH), and halogens or amino groups.

There are two main types of capillary columns: 1. Wall-coated open tubular (WCOT), where the stationary phase is directly coated on the inner wall of the column; and 2. Support-coated open tubular (SCOT), where the stationary phase is coated on a solid support attached to the inner wall.

Capillary columns offer several advantages over packed columns. They are longer, leading to better separation efficiency. They also have larger numbers of theoretical plates, providing greater resolution for complex mixtures.

In isothermal separation, the column temperature remains constant throughout the analysis. In temperature programming, the column temperature is systematically increased during the separation. This allows for better separation of components with a wide range of boiling points.

There are two main types of stationary phases in Gas Chromatography: 1. Solid adsorbents (GSC), such as alumina or silica, where the analytes are adsorbed onto the solid surface; and 2. Liquids coated on solid supports (GLC), where the analytes dissolve into the liquid layer.

The choice of stationary phase is crucial for selectivity in GC. Important criteria include: - Chemical inertness: It should not react with the analytes. - Thermal stability: It should remain stable at the operating temperature. - Polarity match: The polarity of the stationary phase should be suitable for the analytes in the sample.

Temperature is crucial for separation in GC. The separation column is usually placed in a thermostat-controlled oven. Separation can be achieved using two methods: isothermal separation (constant temperature) or temperature programming (increasing temperature).

Temperature programming is frequently used in GC because it allows for the separation of complex mixtures with a wide range of boiling points. It also reduces the analysis time by eluting higher boiling point components more quickly.

The injection port in a gas chromatograph is where the sample is introduced into the system. It is usually heated to vaporize the sample, allowing it to be carried by the carrier gas into the separation column.