Gas Chromatography

A gas chromatograph is an integral part of any organic analytical laboratory. Nowadays, gas chromatography (GC) is considered the major technique for the separation and analysis of fugitive compounds. It is used for analyzing liquids, gases, and solids, with the latter usually being dissolved in fugitive solvents. GC can analyze both organic and inorganic materials, molecular weights of which can range from 2 to about1000 Daltons. This paper is devoted to the study of different types of GC machines classified according to the type of detectors they use, of their peculiarities and differences, and of the possible applications of GC method in real life.

General Principle of Gas Chromatography

Chromatographic processes acquire their names due to the physical state of the mobile phase. Consequently, the mobile phase in gas chromatography (GC) is a gas, and the mobile phase in liquid chromatography (LC) is a liquid. The further subdivision is made according to the condition of the stationary phase. In case the stationary phase is a solid, the technique in GC is called gas-solid chromatography (GSC). In case it has a state of a liquid, the GC technique is called gas-liquid chromatography (GLC).

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It is evident that the system should be secluded and hermetic if gas is used in the mobile phase. Moreover, this system is accomplished with a glass or metal tube known as the column. The column is usually named by specifying the stationary phase. These two terms are used interchangeably.

The method of GC consists of the several stages. In the beginning, the sample mixture passes through the inert gas stream. Usually, argon and helium act as the carrier in GC process. The liquid samples are evaporated before they ingress into the carrier stream. Then, the stream of gas passes through the packed column, where the sample components move at the speed dependent on the interaction degree of each component with the stationary permanent phase. The components with the greater interaction with the phases are slowed to a certain degree. As a result, they separate from those components with worse interaction. This chromatographic process is known as elution. After the elution of the component from the GC column, it can be measured by a detector and collected for investigation.

Gas chromatography is considered to stand in range of the most essential methods in chemistry. It is explained by its simple, sensitive, and effective nature of the system during the separation of the analytes. It is frequently used in this field of science for qualitative or quantitative analyses of amalgams and for the purification of compounds. Moreover, it is used for the identification of solution heats and evaporation, vapor pressure, activity coefficients, and other thermochemical constants.

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Types of GC machines According to the Detectors Type 

Gas chromatography machines are designed for different purposes. According to these purposes, a certain type of a detector is chosen. It is an instrument responsible for establishing the identity and concentration of eluting components in the carrier gas stream. It is disposed at the column’s end. Theoretically, any gaseous mixture properties absent in the carrier gas are regarded as detection methods. The mixture properties are divided into two types: general and specific properties. General properties, also called bulk properties, are peculiar both to the carrier gas and to investigated material but to a certain extent. Specific properties, for example, detectors measuring nitrogen-phosphorous content are considered to have limited spheres of usage but they are of increased sensitivity.

Usually, a gas chromatography detector is chosen by several properties. One of the major requirements is the sufficient sensitivity for providing a high peptization sign for all mixture constituents. Modern detectors usually have the sensitivities at the level of 10-8 to 10-15g of solution per second. Another requirement is that the sample quantity must be renewable as many columns deform peaks in case little sample amount is injected. Moreover, the columns should be inert, and the sample should not be altered there in any circumstances. Optimized columns are able to sustain temperatures from -200°C to more than 400°C. Such columns also have a short linear time of response that does not depend on the flow rate. Moreover, they can extend for more than one order of magnitude. Nevertheless, the most important requirement for the detector is its being predictable, effective, and easy in operation.

Of course, it is impossible to combine all necessary features in one detector. That is why the type of detector should be chosen according to the purpose of investigation. Figure 1 shows the most frequently used types of GC detectors and their properties. 

GC_detectors

 

Fig. 1: Most common GC detectors.

Mass Spectrometer

           Mass Spectrometer (MS) detector is the most powerful type of all GC detectors. This type of system uses the mass spectrometer constantly scanning the masses during the process of partition. After leaving the chromatography column, the sample passes over the transfer line to the MS inlet. Then, the ionization and fragmentation of the sample follow, usually by means of the electron-collision source. This process consists of the bombarding of the sample with the help of energetic electrons. By causing the molecule to lose electrons, they ionize them because of the electrostatic repulsion. Afterwards, the bombardment leads to the fragmentation of the ions. After these processes, the ions get to the mass analyzer. There, they are classified due to their mass-to-charge coefficient. The ions are most frequently singly charged. 

The most popular type of MS mass analyzer is the quadrupole analyzer functioning as an ion trap. It allows electric and magnetic fields to hold gasiform cations and anions for a long time. Usually, the packaging of the quadrupole ion trap is as follows: a cored ring an electrode, and two grounded cork electrodes. An array in the upper cork allows ions to the cavity. Different radio frequencies are used with the ring electrode and the ions having an applicable mass-to-charge coefficient orbit about the cavity. The radio frequency increases linearly, which is why the ions having a stable mass-to-charge coefficient are extruded with the help of mass-selective outburst due to the mass. The ring electrode wall neutralizes the charge of the destabilized ions having too big or too small mass. The released ions come to the electron multiplier, transforming the discovered ions to the electrical signals. Then, various computer programs pick up the electrical signal and produce a chromatogram that represents the mass-to-charge ratio as compared to the sample’s abundance ratio. 

The units of MS are favorable because they can perpetrate the immediate mass identification of the investigated material. Moreover, they can be applied to the identification of the incomplete separations components. They are strict and easy in operation, and they make the analysis of the sample almost immediately after its elution. Nevertheless, MS also has some disadvantages - detectors have the tendency for thermal degradation of samples before the identification and the destruction of a sample by the process of fragmentation.

The most typical scheme of MS is presented in Figure 2.

 ms system

Fig. 2. Scheme of the MS system

 

Flame Ionization Detectors 

            Flame ionization detector (FID) is also one of the most frequently used types of detectors. In this system, the sample after leaving the column gets to the air-hydrogen flame. This flame has high temperature that leads to the process of pyrolysis, also called chemical decomposition, in the sample. This procedure releases the electrons and ions carrying the current. The current is measured by a picoammeter with high impedance for monitoring the elution of the sample.

The advantage of using FID lies in the fact that the flow coefficient, water, and noncombustible gases do not affect the detector. This feature of FID explains its low noise and high sensitivity. It is also quite effective and relatively easy in operation. The disadvantage of this technique is the necessity of using flammable gas that leads to the destruction of the sample.

The scheme of FID is shown in Figure 3.

 scheme_of_FID

Fig. 3.  Scheme of a simple flame ionization detector

 

Thermal Conductivity Detectors 

          Thermal conductivity detector (TCD) is one the first detectors created for the usage in gas chromatography analyses. The principle of TCD’s work is the measurement of the thermal conductivity change of the carrier gas due to the sample presence, the thermal conductivity of which is different from the carrier gas conductivity. This device has a relatively simple design. It consists of a source heated by electrics working on the constant power. Thermal conductivities possessed by surrounding gases influence the source’s temperature. Usually, the source has a form of a thin golden or platinic wire. The temperature dependent on the gas heat conductivity in its turn influences the resistance inside the wire.

Two detectors are usually used in TCDs – the first one functions as the carrier gas reference and the second one controls the heat conductivity of the carrier gas and the sample as well. Even the small admixture of the sample is immediately detected because such carrier gases as helium or hydrogen have very high heat conductivities. 

The main advantages of TCD are the simplicity in operation, the possibility to apply it to organic and inorganic mixtures, and the possibility to collect the analyte after separation and identification. TCD’s major disadvantage is low sensitivity in comparison with other methods. Moreover, it is dependent on the flow rate and on concentration.

The scheme of TCD is presented in Figure 4.

 thermal_conductivity_detector

Fig. 4.  Scheme of thermal conductivity detector 

Electron-Capture detectors 

Electron-capture detector (ECD) is a highly selective detector frequently used for the detection of environmental samples. These devices selectively detect organic solutions with such moieties as peroxides and halogens as well as nitro groups. They do not give response for other types of solutions. Consequently, the ECD method is advantageous in applications, during which pesticides and other chemical trace quantities need to be identified and when no other chromatographic method is possible.   

The primitive ECD form consists of gaseous electrons used in the electric field in the radioactive emitter. After the analyte exits the gas chromatography column, it passes through the emitter, usually consisting of tritium or nickle-63. The nitrogen carrier gas is ionized by the electrons exiting from the emitter. They lead to the release of electrons burst in the carrier gas. When there are no organic solutions, a permanent standing current appears between the electrodes. When the organic solutions having electronegative functional groups are added, the functional groups absorb the electrons and the current decreases greatly.

The main advantages of ECD are the high level of selectivity and sensitivity to some organic compounds, the functional groups of which are electronegative. Nevertheless, these detectors have a confined signal bound. Moreover, they are quite dangerous due to their radioactive nature. Another disadvantage is that the radioactive decay and the O2 presence in the detector limit the signal-to-noise coefficient.

The scheme of ECD detector is shown in Figure 5.

 scheme_of_ecd_detector

Fig. 5. Scheme of ECD detector

Atomic Emission Detectors 

            Atomic emission detector (AED) is one of the most modern types of the GC detectors. These detectors are element-selective. They use a partly ionized gas called plasma. It is used for atomizing all the sample elements and stimulating the spectra of their peculiar atomic emission. AED is a very powerful alternative to other types of detectors because it can be applied to a wider range of cases as it works on the principle of atomic emissions detection. Plasma can be produced in three ways: inductively coupled plasma (ICP), microwave-induced plasma (MIP), and direct current plasma (DCP). It is stated that MIP is used most frequently in GC. It uses a moving diode array for the simultaneous monitoring of the spectra of atomic emission of numerous elements.

            The scheme of AED is presented in Figure 6.

 AED_detector_scheme

Fig. 6.  Scheme of AED detector

Chemiluminescence Spectroscopy

A process that allows determining both quantitative and qualitative properties is called chemiluminescence spectroscopy (CS). Such detectors use the optical emission appeared from the stimulated chemical species. They utilize the light created by the energized molecules. Moreover, CS can appear in both the solution and the gas phase. The reaction of the chemicals producing light energy is considered to be the light source for chemiluminescence. Instead of using a separate light source, for example, light beams, the light band is used.

CS, like any other method, has its disadvantages. The main disadvantage is the CS’ detection limits connected with the photomultiplier tube (PMT) use. Moreover, the dark current is required in PMT for the detection of the light appeared from the investigated material.

The scheme of CS detector is represented in Figure 7.

 Scheme_of_CS_Detector

Fig. 7. Scheme of a CS Detector

Photoionization Detector

Photoionization detector (PID) is another kind of GC detector that employs the features of CS. PID is a hand-held gas and vapor detector, making the selective identification of the aromatic hydrocarbons, some inorganic species, organo-heteroatom, and different organic components. An ultraviolet lamp is used in PID for the emission of photons that are absorbed in the ionization chamber by the compounds leaving the GC column. Photoionization detectors can also be used in the portable pick-and-go models as well as in numerous lamp configurations. 

The advantage of PID is that it gives almost immediate results. This type of detectors is frequently used for the detection of volatile compounds in sediment, air, soil, and water as well as for the detection of contaminants in atmospheric air and in soil. The major disadvantage of PID is that it is unable to identify some hydrocarbons having small molecular weight - ethane and methane, for example.

The scheme of PID is shown in Figure 8.

scheme_of_PID_detector

Fig. 8. Scheme of PID detector

Application of GC Method in Real Life

Nowadays, GC is widely used in various fields – pharmaceutical industry, cosmetics industry and even during the detection of environmental toxins. It plays an important role in the detection and quantification of hazardous pollutants in the environment. It can help to detect the organic pollutant groups such as volatile organic compounds (VOCs), pesticides, polycyclic aromatic hydrocarbons (PAHs), and halogenated compounds.

Moreover, gas chromatography is applied widely to food analysis. Usually, these are the quantitative and qualitative analysis of food components. Most frequently, the analysis concerns food additives, a variety of transformation products, and contaminants such as pesticides, flavor and aroma components, fumigants, environmental pollutants, veterinary drugs, natural toxins, and packaging materials. 

GC is also used for the determination of suspected allergens in cosmetic products. Among them are benzyl alcohol, limonene, linalool, methyl 2-octynoate, beta-citronellol, geraniol, citral, 7-hydroxycitronellal, anisyl alcohol, cinnamal, cinnamyl alcohol, eugenol, and many others.

In other words, GC method is used for the detection of hazardous components in the products and in environment. Timely detection can protect people’s health and even save their lives.

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Conclusion

To sum it up, gas chromatography is a method of physical separation assigned to separate volatile mixtures. The samples for analysis should be volatile, that is why human saliva, blood, breath, and other types of secretion containing organic light constituents can be regarded as a perfect material for investigation in GC. The knowledge about the compound amount, which is present in a sample, is helpful during the study of its effects on the human health or on the environment.

 

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