INTRODUCTION:

The key parts of a gas chromatograph include: a source of gas as the mobile phase, an inlet to deliver sample to a column, the column where separations occur, an oven as a thermostat for the column, a detector to register the presence of a chemical in the column effluent, and a data system to record and display the chromatogram. In addition, some facility is needed so that temperature of various components can be accurately known and controlled.

These parts of a gas chromatograph have been unchanged in function or purpose for over the last 40 years although technology has been ever improving in design, materials, and methodology. In particular, analog electronics for control of temperature zones and data acquisition were replaced with digital electronics and interfaced with computers in the 1970s and 1980s. The arrangement of these components is shown in a block diagram. And this arrangement is common to virtually all gas chromatographs regardless of age, modeler manufacturer. A modern gas chromatograph. In the discussion below, the general function of each component is provided with comments on the status of the technology. Most descriptions of GC will include a cursory description of instrumentation; few will provide a detailed treatment of the instrumentation or technical details. Some of the best discussions of hardware can be found in publications released by instrument manufacturers^.^1–4

Carrier gas

Fig. 1. Instrumentation of gas chromatography

Instrumentation:

Fig. 2. Flow chart of gas chromatography

1. Sample injection system:

A sample port is necessary for introducing the sample at the head of the column. A calibrated micro syringe is used to transfer a volume of sample through a rubber septum and thus into the vaporization chamber. Most of the separations require only a small fraction of the initial sample volume and a sample splitter is used to direct excess sample to waste. Commercial gas chromatograph involves the use of both split and split less injections when alternating between packed columns and capillary columns. The vaporization chamber is typically heated 50°C above the lowest boiling point of the sample and subsequently mixed with the carrier gas to transport the sample into the column^5,6

The decision of carrier gas (portable stage) is very important. The carrier gas must be chemically inert.

Generally utilized gasses include nitrogen, helium, argon, and carbon dioxide. The decision of carrier gas is regularly depend upon the sort of indicator which is utilized. The carrier gas framework likewise contains a molecular sieve to expel water and different other impurities. So, helium might be more efficient and give the best separation if flow rates are optimized. Helium is non-combustible and works with a more prominent number of detectors. Thus, helium is the most well-known carrier gas utilized. In any case, the cost of helium has gone up significantly over recent years, causing an expanding number of chromatographers to change to hydrogen gas⁷

2. Carrier gas:

The carrier gas or mobile phase in GC is an essential, but limiting, facet in separations. Carrier gas is the means to move constituents of a sample through the column and yet the choice of possible gases is restricted. Moreover, the carrier gas has properties that sometimes can complicate analysis. Unlike liquid chromatography (where a wide selection of mobile phase compositions may be possible,⁸

The carrier gas plays an important role, and varies in the GC used. Carrier gas must be dry, free of oxygen and chemically inert mobile-phase employed in gas chromatography. Helium is most commonly used because it is safer than, but comparable to hydrogen in efficiency, has a larger range of flow rates and is compatible with many detectors. Nitrogen, argon, and hydrogen are also used depending upon the desired performance and the detector being used. Both hydrogen and helium, which are commonly used on most traditional detectors such as Flame Ionization(FID), thermal conductivity (TCD) and Electron capture(ECD), provide a shorter analysis time and lower elution temperatures of the sample due to higher flow rates and low molecular weight^.⁵

3. Column:

A) Packed column:

The first commercial instruments accepted only packed columns, all initial studies of GC were performed on packed columns. Packed columns are typically made of stainless steel and have an outer diameter of 0.64 or0.32 cm and lengths of 0.61–3.05 m. Alternative inert materials have also been used, including glass, nickel, fluorocarbon polymer (Teflon), and steel covered with glass or Teflon. The packing is an inert support impregnated with 5–20% stationary phase. The solid support holds the liquid stationary phase and should have a large surface area (0.5–5m2/g), be chemically inert, have low sportive activity toward common analytic, and have good loading and handling.^9–11

Fig. 3. Packed column

Column

Packed column	Capillary column
Stationary phase is coated directly in the column	Stationary phase is coated with the inner wall of the column
Applicable for both GSC and GLC	Applicable only for GLC
Liquid phase is absorbed on to the surface of the beads in the thin layer or on to the solid inert packing	Liquid stationary phase is immobilized on the capillary tubing walls

B) Capillary column:

The capillary columns were introduced in 1959; they did not gain popularity until 1980. At present, it is estimated that more than 80% of all applications are run on capillary columns due to the fast and efficient separation the afford. Capillary chromatographic columns are not filled with packing material; instead, a thin film of liquid phase coats the inner wall. Because the tube is open, its resistance to flow is very low, and it is thus referred to as an open tubular column. Open tubular columns can be divided into three groups and are described in the next sections.⁹

Fig. 4. Capillary column

4) Detection System:

Detectors: Most common types of detectors used in GC are: Mass Spectrometer, Flame ionization detector (FID), Electron capture detector (ECD), Thermal conductivity detector (TCD),Atomic emission detector (AED), Photo ionization detector (PID), and . Detector is present at the end of the column & gives the quantitative measurement of the components of the mixture as they elute in combination with the carrier gas.^12–14

Types of Detectors:

Types of Detectors	Applicable Samples	Detection Limit
Mass Spectrometer	Tunable for any sample	0.25 to 100pg
Flame Ionization	Hydrocarbons	1pg/s
Thermal Conductivity	Universal	500pg/ml
Electron capture	Halogenated Hydrocarbons	5fg/s
Atomic Emission	Element selective	1pg
Photo ionization	Vapor and gaseous Compounds	0.002 to 0.02 μg/L

1. Mass Spectrometer:

Mass spectrometer is an analytical device which used to detect the separated charged ions. The journey of ions starts from the inlet system- source- analyzer and ends at detector by striking on it. Mass detector mainly detects the current signal generated from the passes or incident ions which are absolute or relative concentration of each analyte.The mass spectrometer is a universal detector for gas chromatographs since any compound that can pass through a gas chromatograph is converted into ions in mass spectrometer. The MS principle consists of ionizing produces compounds to generate charged molecules or molecule fragments and measuring there mass to charge ratio. Mass spectrometry data analysis is a complicated subject that is very specific to the type of experiment producing the data. Mass spectrometry is one of the most versatile and sensitive analytical methods which is employed in many studies, ranging from medical to technological science. This method allows the molecular mass/structure determination of unknown compounds, and to quantify small molecules to bimolecular such as proteins, lipids and oligonucleotides^15–18

2. Flame Ionization:

Flame Ionization Detector (FID) Description. Ill though the FID tagged four >ears behind the TCD an ‘acceptance’ detector, the number of FID’-. Going into service at present is greater than the number of TCD’s. The FID’s popularity is primary ongoing to its detection of small sample concentrations (about 1000 times lower than the TCD) Of all the ionization detectors, the FID has the best record of reliable performance Figure 2show a schematic representation of the essential components of an FID. The carrier gas must be inert and may not be absorbed by the column material. Helium or hydrogen or nitrogen is normally used as carrier gas with GC FID burner jet collector electrode and appropriate plumbing for column effluent, hydrogen, and air. hydrogen flame detector1 and a thermal conductivity device for gas chromatography were recently described in This journal flame ionization detector presented here is very easy to construct, extremely sensitive, and readily visible for demonstration purposes.^19–21

Fig. 5. Flame Ioinization Detector

3. Thermal Conductivity:

This method of detection made it possible to apply the GC technique to almost any mixture of volatile compounds, and papers concerning the development and application of the technique appeared in the scientific literature at an unprecedented rate. The person now entering the field, sixteen years later, is faced with a weighty and disperses GC literature from which he must choose in order to trace the available information concerning any one facet of the technique. The TC detector is included in the class whose signal is dependent upon the concentration of sample in this class are non-destructive to the sample and include photometric systems. The other types are dependent upon the mass flow rates and are called flow sensitive detectors. Typical of this class are the mass spectrometer, which are both destructive to the sample^22,23

Fig. 6. Thermal conductivity Detector

4. Electron Capture:

The electrons are constantly accelerated toward the anode in the DC mode and their energy is consequently higher. Thus, the choice of the DC potentials critical since it will affect the absorption process. This is in contrast to the pulse technique where the pulse potential merely serves to collect all electrons not consumed during the relatively long interval between the short pulses. Detector dimensions and compound type determine the choice of DC potential, alongside the day-to-day influence of detector contamination and carrier gas composition. The ECD is a specific detector wisitive primarily to halogenated hydrocarbons and certain other classes of organics such as conjugated carbonyls and nitrates which have the ability to accept a negative charge.^20,24

Fig. 7. Electron capture detector

5. Atomic Emission:

AED measures the energy emitted at characteristic wavelengths by sample atoms present in the helium plasma cavity to quantify their number in a chromatographic peak. Combining this data with GC analyte separation, the amount of the substance can be quantitatively determined. Atomic spectroscopy the best element selective method available to the analyst. Since its first analytical use — the visual identification of salts by introducing a sample into a flame —for a long time, atomic spectrometry remained in the domain of the inorganic chemist. Atomic emission detectors (AED), one of the newest additions to the gas chromatographer's arsenal, are element selective detectors that utilize plasma, which is a partially ionized gas, to atomize all of the elements of a sample and excite their characteristic atomic emission spectra. AED measures the energy emitted at characteristic wavelengths by sample atoms present in the helium plasma cavity to quantify their number in a chromatographic peak. Combining this data with GC analyte separation, the amount of the substance can be quantitatively determined AED is an extremely powerful alternative that has a wider applicability due to its based on the detection of atomic emissions .There are three ways of generating plasma: microwave-induced plasma (MIP), inductively coupled plasma (ICP) or direct current plasma (DCP). MIP is the most commonly employed form and is used with a position able diode array to simultaneously monitor the atomic emission spectra of several elements.^25–27

Fig. 8. Atomic Emission Detector

6. Photo Ionization:

Fig. 9. Photo Ionization Detector

Photo ionization, as a means of detection, has been with us for about 25 years. Robinson. First reported on the development of a photo ionization detector in 1957. A t the same time, groups in various parts of the world. We’re working on the development of flame ionization techniques. This latter technique became very popular and was rather quickly licensed to a number of commercial gas chromatography (GC) manufacturers since the detector was very sensitive and easy to build. Lovelock. Became interested in the photo ionization technique and published a review of ionization techniques in 1961. This included the flame ionization detector (F I D), the photo ionization detector (P I D), cross section, and electron capture detection (ECD).The PID was found to be easier to use, more sensitive, and have a wider dynamic range than the flame photometric detector FPD.²⁸

Types of Gas Chromatography:

Fig. 10 Types of gas chromatography

Application of Gas Chromatography^29–31

1) Residual solvent analysis;

2) Analysis of various functional groups;

3) Percentage of purity of pharmaceutical compounds；

4) For the analysis of drugs of abuse;

5) In pharmaceutical R & D’s to determine the identity of natural products this contains complex mixture of similar compounds;

6) In the metabolomics studies.

7) GC can be used for the direct separation and analysis of gaseous samples, liquid solutions, and volatile solids

8) Gas chromatography has qualitative and quantitative analysis.

Advantages Of Gas Chromatography^30,32–34

1) Good resolution, as shown by sharp and symmetric peaks;

2) High repeatability and reproducibility of retention times;

3) High precision and accuracy in quantization based on peak area measurements, i.e. no discrimination of components through volatility, polarity or concentration

4) Minimum thermal and catalytic decomposition of sensitive sample components

5) The use of fused-silica capillary columns with improved surface inertness, thermal stability and resolution.

6) The identification is based on retention time matching that may be inaccurate or misleading

7) The major advantage of gas chromatography is its high sensitivity, resolution, and separation ability, which allows it to separate a wide range of volatile compounds.

Disadvantages of Gas Chromatography:

1) Limited to volatile sample.

2) Not suitable for thermally labile samples.

3) Samples be soluble and do not react with column.

4) During injection of the gaseous sample proper attention is required.

CONCLUSION:

GC is the most widely used analytical technique available for separations & identifications of compounds or complex mixtures. Gas chromatography (GC) is a common kind of chromatography used as a piece of analytical science for segregating and investigating exacerbates that can be vaporized without disintegration. Various types of detectors are used for analysis of the product based on the retention time. An expansive variety of tests can be analyzed the until the compounds are adequately thermally steady and reasonably volatile. Gas chromatography is an important analytical technique for qualitative and quantitative analysis in a wide range of application areas. It is fast, provides a high peak capacity, is sensitive and allows combination with a wide range of selective detection methods including mass spectrometry.

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Received on 12.10.2022 Modified on 11.11.2022

Asian J. Pharm. Ana. 2023; 13(1):47-52.

DOI: 10.52711/2231-5675.2023.00008