Analytical Instrumentation

Working Principle of Gas Chromatograph

Working Principle of Gas Chromatograph

  • A gas chromatograph is an instrument for chemical analysis that separates different chemicals in a complex sample. 
  • The principle of chromatography is the partitioning of analytes between two immiscible phases: a gaseous mobile phase (Carrier gas) and a stationary solid or immobilized liquid phase (packed or hollow capillary column). 

Different forms of Gas Chromatography

  • A method for identifying and separating tiny molecular weight substances from a mixture is gas chromatography, commonly referred to as vapor-phase chromatography (VPC) or gas-liquid partition chromatography (GLPC). 
  • Gas-solid chromatography (GSC) and gas-liquid chromatography (GLC) are the two forms of gas chromatography. 
  • In GLC and GSC, the stationary phases are, respectively, liquid and solid. Compared to gas-solid chromatography, the gas-liquid technique is more commonly utilized.

Gas-solid chromatography 

In the process of gas-solid chromatography, the mixture is separated via the adsorption process. Compared to GLC, it is less frequently employed for the separation of low molecular gases like CO2, H2, H2S2, and CS2

Gas-liquid chromatography

In the process of gas-liquid chromatography, the stationary phase (liquid) is first transformed into vapors. Then, mixtures are separated, which depends on the affinities for the stationary phase and the relative vapor pressure. 

Working Theory of Gas Chromatography

  • The stationary phase and the mobile phase, where the mobile phase serves as the carrier gas, are separated or partitioned in the process of gas chromatography. 
  • Using a GC syringe, a gaseous sample is injected into the injection port, where it is vaporized and conveyed by a carrier gas to pass through the stationary phase (a viscous liquid) by a gas flow regulator. 
  • In the gas chromatography method, inert gases including helium, argon, nitrogen, and hydrogen are typically utilized as the mobile phase/carrier gas. 
  • The detection port then uses appropriate temperature programming to electronically separate this sample. 
  • On recorders and computers, this process can be clearly visualised as peaks.
gas chromatography

Components of Gas Chromatography 

  • A carrier Gas
  • Sample injection port
  • Column
  • Column oven
  • Detector
  • Recorder/ Integrator
Gas chromatography schematic

Carrier gas supply or mobile phase 

  • The mobile phase is made by a constant flow of a carrier gas that holds them at pressures of up to 2500 lb/sq.in.
  • A carrier gas delivery system has a needle valve, a flow rate, a pressure gauge, and a few feet of metal capillary restrictions.
  • Types of gases, how pure gases are, and how fast gases flow
  • Carrier gas can be made from hydrogen, helium, nitrogen, argon, or carbon dioxide.
  • The carrier gas may change how well the column and detection work.
  • When a thermal conductivity detection method is used, helium and hydrogen are most often used.
  • Nitrogen is used when the ability to separate is more important than a quick answer from the detector.
  • Contaminants in the carrier gas can affect how well the column works and how well the detectors work, especially when ionization detectors are used.
  • The column diameter affects the amount of gas flow used in analysis.
  • Usually between 10 and 400 ml/min, which is very low and very high.
  • The flow rate should be kept within 1% to cut down on analysis mistakes.
  • Putting a capillary in front of the column can keep the flow steady.

Sample Injection Port 

  • The goal of the sample injection system is to put a consistent amount of the sample to be analyzed into the stream of carrier gas. 
  •  Samples can be brought in as a gas, a liquid, or a solid. 
  • The type of injection relies on the source of the sample, the type of detector that will be used, and the pressure in the column at the point where the sample will be put in. 
  • The injector is a hollow, hot, glass-lined cylinder that is used to put the sample into the GC. 
  • The temperature of the injector is set so that all of the sample’s parts will evaporate. 

Liquid Samples 

  • Micro syringes are used to inject liquid samples through a silicon rubber valve. 
  • A sample is put into the hot part of the column, which changes the liquid into a gas. 
  • A controlled resistance warmer heats the metal block with the capillary inside. 
  • The sample is turned into a vapor, and a transport gas takes it into the column. 
Liquid Sample

Gas Samples 

  • A gas light needle that can hold 0.1–10 ml of sample is used to inject gas samples. 
  • The bypass system, which is also called a stream splitter, is the other way to inject air samples. 
  • The idea behind the method is to put the sample in a loop whose size is known. 
  • The loop is put right into the carrier gas line by turning a switch. 
  • The valves used to be made out of glass, but now they are made out of polytetrafluoroethylene. 
  • The setup is made up of three stopcocks. In between two of them is a standard volume that holds the gas sample. 
  • A rotating or moving valve brings gas from the bypass capillary loop into the column. This connects the loop to the flow of the carrier gas.
Gas Samples

Solid samples 

  • This can also be shot by putting them on the end of the plunger of a solid injection syringe and pulling the plunger back into the needle. After piercing the injection septum, the plunger is pulled back to put the solid in the hot zone of the column, where it vaporizes.
  • Alternatively, the solid can be deposited in a glass tube from solution. After the solvent evaporates, the sample holder is dropped into the column, making the injection.

Pyrolysis

  • Pyrolysis is a method for injecting certain kinds of low- or non-volatile materials that can be thermally decomposed in an inert atmosphere to produce a mixture of volatile fragments that is both qualitatively and quantitatively repeatable. The pyrolysis products are then transferred to a chromatographic column and separated.

Chromatograph column Oven

  • The columns, valves, and detectors that make up the gas chromatograph’s analytical section are housed inside of a heated oven compartment.
  • The chromatograph columns and detectors’ performance and response are particularly sensitive to temperature fluctuations, so the oven is built to shield these parts from the effects of external temperature variations and to maintain an extremely stable internal temperature.
  • For reproducible results, the column must be operated at a steady temperature. To ensure appropriate component separation, temperature can be changed throughout the process. 
  • Low boiling point compounds can be resolved well at a moderate temperature. However, materials with high boiling points have a tendency to be retained and elute slowly, which causes peak broadening. 
  • Programming the temperature can solve this issue. When the temperature of the column is raised after the initial elution of low-boiling components, the energy of high-boiling components increases and they elute more quickly.
oven

Chromatographic Column (CC) 

  • The column is the core part the gas chromatograph.
  • When a sample is given to a column, molecular diffusion causes the sample to spread, resulting in a concentration profile. 
  • The degree of peak broadening with respect to time and column length is a measure of column efficiency
  • Columns can be divided into two categories: packed columns and capillary or open tubular columns.

Packed columns

packed column
  • Packed columns are thick columns made of glass or stainless-steel tubes, and their separation performance is low. 
  • They are immune to contamination and are able to handle huge sample quantities. 

Capillary Column

  • The packed solid support is not present in these columns. 
  • A thin layer of solid support is provided to the column’s inner wall before the mobile phase is added. The main goals of this technique are to improve the column’s surface area and its capacity to hold stationary phases of the phase diagram.
  • Capillary columns consist of a thin, fused silica glass tube with a thin, internal liquid phase coating. 

Detectors 

  • There must be a detector to track the signal at the exit. The detector’s job is to keep an eye on the carrier gas as it leaves the column. 
  • The following characteristics are necessary for the perfect detector. prompt reaction extremely sensitive (up to ppb) broad linear response range. ought to be steady under operating circumstances ought to be quiet. simple, affordable, and durable.

Basic types of detector  

According to time response, there are two primary types of detectors. 

Integral type Detectors and Differential type Detectors

Integral detectors: Integral detectors depict the accumulation of a physical quantity over time, meaning that the signal they produce pertains to the entire amount of material flowing through the detector at any one time. The employment of integral detectors in ordinary chromatographic analysis is less frequent.

Differential type detectors: A differential detector displays the variation of a physical quantity over time by displaying a signal that corresponds to the amount passing through the detector at each instant in a given time. In general chromatography analysis, differential detectors are frequently used as detectors.

Types of Detectors

  • Flame Ionization Detector
  • Thermal conductivity detectors,
  • Refractive index(RI)detectors, 
  • Ultraviolet(UV),
  • Radiochemical, 
  • Electrochemical, 
  • Infrared( IR),  
  • Fluorescent detectors, 
  • Mass spectroscopy(MS)

are some of the types of detectors that can be used in Gas Chromatograph

Flame Ionization Detector

  • One of the most often utilized detectors is this one. H2 gas is combined with the effluent gas and burned. 
  • The burner serves as one of the DC cell’s electrodes as well. 
  • The second electrode is located above the burner and is shaped like a loop. As the sample approaches the flame, it is ionized. 
  • The quantity of organic molecules in the flame that contain carbon directly affects how much ion current is generated. 
  • All organic compounds—aside from formic acid—trigger the FID. To begin with, a derivative is created for inorganic substances. such as CO2 to CH4

Thermal Conductivity Detector

Thermal conductivity detector
  • A heated filament with an applied current is present in a detector cell. The filament current alters as a carrier gas carrying solutes moves through the cell. 
  • A reference cell’s current and the current change are compared. 
  • A signal is produced when the difference has been measured 
  • Sensitivity: 5–20 ng
  • Temperature: 150-250 °C

Mass spectrometer

  • Mass spectrometer is obtained by converting components of a sample into rapidly moving gaseous ions and resolving them on the basis of their mass- to-charge ratios.
  • In Gas chromatography, mass spectrometer receives effluent and transforms the constituents into ions.
Mass Spectrometer

Application of Gas Chromatography

  • The GC is capable of analyzing any collection of organic and inorganic substances having a low boiling point.
  • It is possible to identify the beverages’ alcohol content. 
  • Gas chromatography is the finest tool for fuel gas analysis. 
  • The best method for analyzing auto-exhaust is GC. 
  • Aromatic and flavoring substances tend to be volatile.

What is meant by retention times?

A gas chromatograph has a narrow tube called the “column” that a gas stream flows through. As the chemicals come out of the end of the column, they are electronically found and named. The stationary phase in the column is there to split the different parts so that they leave the column at different times called “retention times”. The carrier gas flow rate, column length, and temperature are also factors that can be changed to change the order or timing of retention.

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