Gas chromatography abbreviated as GC; a vitally important chromatographic technique is the forte of this article. Here you will find the basic working principle, a detailed guide on its stationary phase and possible mobile phases as well as the overall scope and applications of GC. In short, everything you need to know about gas chromatography is present in this particular addition in our chromatographic series so let’s begin reading.
What is gas chromatography
Gas chromatography (GC) is a type of column chromatography in which the mobile phase is a gas while the stationary phase could either be solid or liquid coated onto a solid support. In case of a solid stationary phase, the chromatography is called gas-solid chromatography while in case of a liquid mobile phase, the chromatography is called gas-liquid chromatography. Gas-liquid chromatography however is the more preferred type of gas chromatography which is performed in a special column called an open tubular or capillary column.
Mobile phase in gas chromatography
A gaseous mobile phase is the most distinctive feature of gas chromatography. The mobile phase in this case acts as a carrier gas only i.e., it does not chemically interact with the analyte components. Rather, it just sweeps the analyte molecules with it while passing over the stationary phase. The carrier gas used must be non-reactive and/or inert in nature. The analyte components to be separated must be volatile so that they can be converted into a gas while performing gas chromatography. Non-volatile analyte components can be made volatile via derivatization with different functional groups.
Helium, hydrogen, nitrogen and argon are some of the most popular mobile phase choices for gas chromatography. Hydrogen and helium due to their low molecular weights and densities allow a rapid flow through the column thus resulting in faster chromatographic analysis. Helium although expensive but is preferred over hydrogen because of the highly flammable and explosive nature of the later. To read more on the pros and cons of each carrier gas, we recommend: what is mobile phase in chromatography
A rapid mobile phase flow through the capillary column of GC reduces the risk of peak broadening by longitudinal diffusion.
Stationary phase in gas chromatography
As indicated above, gas chromatography can be categorized into two different types on the basis of stationary phase. The table below represents each type with its separation mechanism.
|Type of gas chromatography
|Analyte adsorption directly onto the stationary phase particles
|A non-volatile liquid coated onto the inner walls of the column
|Analyte retention onto the stationary phase through chemical interaction
You must have noticed that in both types of gas chromatography, analyses occur on the basis of analyte interaction with the stationary phase. But there is little interactive role of mobile phase with the analyte components. Separation occurs merely via exchange of components between the two phases. This fact differentiates gas chromatography from liquid chromatographic separations where separation occurs on the basis of differential analyte interaction with the two phases.
Commonly used solid stationary phase materials for gas chromatography includes silica (SiO2), polymers such as polyimide (C35H28N2O7) while liquid stationary phases may include squalene (C30H50), polydimethyl siloxane (CH3)[Si (CH3)2O]nSi(CH3)3 and polyethylene glycol (C2nH4n+2On+1) etc.
The liquid stationary phase can be coated onto the inner walls of a capillary column as a porous layer (porous layer open tubular) or along the walls (wall coated open tubular). It can also be coated onto the column walls through a support (support coated open tubular) and/or as a fused layer (fused silica coated open tubular).
We should be aware that packed columns can also be used in gas chromatography especially gas-solid chromatography, same as those designed for high-performance liquid chromatography (HPLC). But capillary columns are inevitably the most feasible choice for performing a gas chromatographic experiment. Commercially available columns, specifically designed for GC are (15-60) m long capillary columns which are placed in a temperature-controlled oven in a coiled form.
The GC columns can also be classified on the basis of their internal diameter. A wide bore column allows greater sample loading capacity and high mobile phase flow rate thus a rapid separation. A narrow bore column on the other hand, improves separation efficiency thus better resolution of the chromatographic peaks will be achieved.
All the other components of gas chromatography are as discussed in the upcoming section.
Gas chromatography components
- Gas Cylinder
A compressed, high-pressure, gas cylinder supplies the mobile phase (carrier gas) in gas chromatography. The choice of carrier gas depends on the detector used for GC, the desired separation efficiency and also on other factors such as the required speed. Lower the molecular weight of the gas, higher its rate of diffusion and/or flow through the column.
2. Sample Injector
A sharp, needle injector is used to introduce the sample mixture in a gas chromatographic set up. The needle is inserted through a rubber septum. Three different modes of injections can be employed namely:
- Split injection
- Spitless injection
- On-column injection
Split injection is used for large samples, containing targeted analytes > 0.1%. A small proportion of the sample is injected into the column at a time. The sample splits according to a split-ratio which typically ranges from 50:1 to 600:1. Simply putting; 1 particle out of 51 enters the column while 50 stays behind. It is a fast injection mode.
Spitless injection on the other hand is preferred for trace analysis of analytes containing <0.01% of the targeted analyte. A large volume of the sample is injected at once but at a slow pace. On-column injection however is the most favorable choice for gas chromatography in the modern scientific world. In this, the samples are directly injected onto the columns with an extremely fine microliter syringe needle. On-column sample injections are specifically favoured for mixtures containing analytes prone to thermal degradation.
3. Vaporization chamber
Prior to injection, the analyte mixture is dissolved in a volatile solvent. The solvent gets evaporated by high temperature conditions provided in the vaporization chamber. Solid samples break down by a process called pyrolysis while liquid samples directly vaporize. In both cases, the goal is to obtain gaseous analyte components so that they can be transported into the chromatographic column by the carrier gas.
As discussed above, the column used in gas chromatography could either be a packed column or the more preferable choice i.e., the open tubular/capillary column. The temperature of the column is maintained approximately 25°C above the boiling point of the highest boiling component in order to maintain the gaseous state of the analyte components. Situating the GC column in an oven, allows this temperature maintenance.
Based on their polarity/difference in boiling point as well as on the affinity of the analyte components towards the stationary phase, the main analytical separation occurs inside the column. Generally, the most volatile (lowest boiling point) components elute out of the column first while the least volatile (highest boiling point) components are the last to get eluted out of the GC column. As a general matter of fact, the most volatile components are also the most polar ones.
The analyte components eluting out of the column finally reaches the detector. The detector response is obtained as an electrical signal which is then amplified and recorded as a chromatogram. Mass spectroscopy is the most popular choice for detection in gas chromatography. The two techniques coupled together form a widely popular research tool known as GC-MS. The mass spectrometer (MS) operates on the principle of fluctuating electric and magnetic fields, with the final outcome recorded as relative abundance versus mass-to-charge (m/e) ratio for different isotopes of an element present. You may read this for more details on GC-MS.
Other than MS, electron capture detector, fluorescence detector, charged aerosol detector, thermal conductivity detector and flame ionization detector are other popular choices to be coupled with GC. A chromatographic interface however is needed in all cases, which connects the exit of the analytical column with the entrance of the detector mechanically.
In accordance with mass spectrometer being the best choice for a majority of gas chromatographic analysis, a GC chromatogram usually is displayed as a plot of abundance (detector response) versus retention time. The volatile components separated by gas chromatography can then be identified by matching their retention times displayed onto the chromatogram with the already present data in the GC-MS library. In this way, gas chromatography allows both qualitative as well as quantitative chromatographic analyses.
It is a plus-point of GC over HPLC that GC does not necessarily requires plotting standard calibration curves, rather it comes with a computerized, data-fed library which can help in component identification. There are also negligible chances of peak broadening and poor chromatographic resolution due to multiple path effect in capillary columns of GC as opposed to that witnessed in HPLC.
Greater chances of longitudinal diffusion and the compulsory volatility requirement of samples to be analyzed in GC are some of the shortcomings associated with this technique in comparison to HPLC. Both column chromatographic techniques however hold their special significance depending upon the type of analysis and the separation efficiency required.
Where do we need gas chromatography
Gas chromatography is very useful in different research and development sectors; practically everywhere in the scientific world where there is a need to analyze volatile organic compounds (VOCs).
- Flavor and Fragrance Industry: Essential oils, alcohols and esters make up the basic chemical composition of perfumes, air-fresheners and the scented candles that we all love and use on a daily basis. Natural extracts such as these essential oils and other aromatic compounds are also fundamentally present in therapeutic teas. Gas chromatography plays a significant role in the analysis of all such compounds as they are highly volatile in nature.
- Environmental Sector: Gaseous air pollutants and toxins present in the atmosphere can be identified via GC. Gas chromatography makes it easier for the scientists and the environmentalists to maintain a healthy ambient air quality index (AQI) and/or to be aware of any underlying climatic threats.
- Petrochemical Industry: Gas chromatography can also help detect and quantify VOCs released during product refining in the petrochemical industry.
For more chromatographic applications regarding all the different types of chromatography, refer to our article: What are the uses of chromatography?.
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