Paper, Liquid, And Gas And Thin-layer Chromatography: Research Paper

Chromatography research task:

Many materials appear to have the same relation, relative position, or structure but actually they are just a combination of materials; a main example being black ink which is found in pens and it is a mixture of different coloured substances. On multiple occasions, these materials are allowed to separate by dissolving them in an appropriate liquid and by letting them be moved through an absorbent matrix.

Chromatography is a technique where the components that a mixture is made up of is separated, however, this depends on the distribution of these components between the mobile phase and the adjacent stationary phase. The mobile phase can either be in a liquid or a gas state and as for the stationary phase, it can be in a solid or a liquid state. (Giddings et al, 2009). Furthermore, chromatography is used by scientists to separate organic and inorganic compounds so they can be analysed and studied. Consequently, by analysing a compound, scientists are able to figure out exactly what the compound is made up of.

This analytical method began at about the same time as mass spectrometry, as the beginning practice of chromatography comprised of the applying liquid samples to short man made columns of different absorbents or directly on absorbent paper. (Sparkman et al, 2011)

Chromatography is made up of four types:

1. Paper Chromatography
2. Liquid Chromatography
3. Gas Chromatography
4. Thin-layer Chromatography

1. Paper Chromatography:

According to analytical chemistry, it is a technique which separates chemical substances that are dissolved in terms of the rate it travels at across the sheets of paper. This is an economical yet powerful tool for analysis which only requires small amounts of material. (The Editors of Encyclopaedia Britannica, 2018)

Some of its uses include:

• separating amino acids and anions
• RNA fingerprinting
• separating and testing histamines, antibiotics

Method:

Supposing that there are four black pens and you are interested in finding out which pen was used to write a message, samples of each ink are then spotted on to a pencil line which is drawn on a sheet of chromatography paper. Some ink from the message is then dissolved in the smallest amount possible of an appropriate solvent, and that is also spotted onto the same line. When creating a visual of the results, the pens and the message are all labelled.

The paper is hanged in a container with a thin layer of a suitable solvent or a mixture which has the solvents in it. the solvent level should be below the line with the spots on it.

There is a covering on the container to ensure that the atmosphere in the beaker is saturated with solvent vapour as the solvent stops evaporating when it makes its way up the paper.

Additionally, whilst the solvent slowly travels up the paper, the different substances of the ink mixtures travel at different rates and the mixtures are separated into different coloured spots. (Clark, 2007)

2. Liquid Chromatography

Liquid chromatography is an analytical technique that is useful for separating ions or molecules which are dissolved in a solvent. If the solution that you are sampling, is in contact with another liquid or solid phase then the different solutes will interact with the other phase to various degrees because of the differences in absorption, ion-exchange, partitioning, or size. Consequently, the differing components of the mixture are separated from each other by using these differences to determine the transport time of the solutes through a column. (Brian M. Tissue, 2000)

Some of its uses include:

• test water samples to look for pollution
• analyses metal ions and organic compounds in solutions.
• uses liquids which may incorporate hydrophilic, insoluble molecules.

Method:

Components within a mixture are separated in a column based on each component’s likeliness to be attracted to the mobile phase. So, if the components have different polarities and a mobile phase of a similar polarity is passed through the column then, one component will migrate through the column faster than the other. Due to molecules of the same compound will generally move in groups, the compounds are separated into clear bands within the column. If the components being separated are coloured, their corresponding bands are visible. Or else, as in high performance liquid chromatography (HPLC), to see if the bands are present other instrumental analysis techniques are used.

In the first step, the mixture of components sits above the wet column. As the mobile phase passes through the column, the two components begin to separate into bands. As each component is eluted from the column, each can be separately collected and analysed by whichever method is preferred. The relative polarities of these two compounds are based on the polarities of the stationary and mobile phases. (Betancourt et al, 2019)

3. Gas Chromatography:

Gas chromatography is used to separate and measure different types of gases. It is a sensitive technique that can analyse small amounts, and can be automated, but is also fairly expensive and requires technical knowledge. The analysis of lowly concentrated gases is only possible with gas chromatography equipment, making it the most precise method for analytical research. (M. Yahia et al, 2019)

Some of its uses include:

• detect bombs in airports
• identify and quantify such drugs as alcohol
• used in forensics to compare fibres found on a victim
• Used to analyse volatile gases. Helium is used to move the gaseous mixture through a column of absorbent material.

Method:

All forms of chromatography involve a stationary phase and a mobile phase. In gas chromatography, the mobile phase is a gas and the stationary phase is a high boiling point liquid which is absorbed onto a solid. The rate at which a compound travels through the machine will depend on how much time it spends moving with the gas in comparison to being attached to the liquid in a certain way. Very small amounts of the sample are injected into the machine using a small syringe. The syringe needle passes through a thick rubber disc (known as a septum) which reseals itself again after the syringe is pulled out.

The injector is contained in an oven so the temperature can be controlled. It is hot enough so that all the sample boils and is carried into the column as a gas. There are two main types of column in gas chromatography. One of them being a long thin tube packed with the stationary phase whilst the other is even more shallow and the stationary phase is bonded to its inner surface. Three possible things might happen to a particular molecule in the mixture injected into the column: (Clark, 2007)

1. It may condense on the stationary phase.
2. It may dissolve in the liquid on the surface of the stationary phase.
3. It may remain in the gas phase.
4. Thin-layer Chromatography:

Thin-layer Chromatography uses an absorbent material on a flat glass or plastic surface. It is a simple and quick method to check the refinement of an organic compound. It is used to detect pesticide or insecticide residues in food.

Some of its uses include:

• used in forensics to analyse the dye composition of fibres
• used to detect pesticide or insecticide residues in food

Method:

A complex mixture is applied to the plate, in a suitable solvent as a spot, and then is dried and the chromatogram is developed by allowing the solvent to spread along the plate as a sharp front from one edge towards the other side of the plate. A selection of solvents can be used in no particular order as long as the previous solvent is dried off before the next one is applied. Components of the sample usually have different affinities for the separating medium. This technique is especially useful with samples that’s contain heavy material. Larger and more polar molecules in the first sample mixture may be deserted due to the lack of mobility after smaller molecules are removed by suitable solvents. Smaller evaporative components may be lost during solvent evaporation. (Kandiyoti et al, 2017)

Bibliography:

1. Brian M. Tissue. (2000). Definition of Liquid Chromatography (LC). Available: https://www.chemicool.com/definition/liquid_chromatography_lc.html. Last accessed 31st January, 2020.
2. Elhadi M. Yahia, Jeffrey K. Brecht. (2019). Controlled Atmosphere Storage. Available: https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/gas-chromatography. Last accessed 31st January 2020.
3. Jim Clark. (2007 (modified July, 2016)). PAPER CHROMATOGRAPHY. Available: https://www.chemguide.co.uk/analysis/chromatography/paper.html. Last accessed 31st January, 2020.
4. Jim Clark. (2016). GAS-LIQUID CHROMATOGRAPHY. Available: https://www.chemguide.co.uk/analysis/chromatography/gas.html. Last accessed 31st January, 2020.
5. J. Calvin Giddings Roy A. Keller. (Oct 08, 2009). Chromatography. Available: https://www.britannica.com/science/chromatography. Last accessed 31st January 2020.
6. Jennifer Betancourt, Sean Gottlieb . (2019). Liquid Chromatography. Available: https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Instrumental_Analysis/Chromatography/Liquid_Chromatography. Last accessed 31st January, 2020.
7. O. David Sparkman,Fulton G. Kitson, . (2011). Gas Chromatography and Mass Spectrometry (Second Edition), . Available: https://www.sciencedirect.com/topics/chemistry/chromatography. Last accessed 31st jan 2020.
8. Rafael Kandiyoti, Trevor Morgan. (2017). Analytical techniques for low mass materials. Available: https://www.sciencedirect.com/topics/engineering/thin-layer-chromatography. Last accessed 31st January, 2020.
9. The Editors of Encyclopaedia Britannica. (June 01, 2018). Paper chromatography. Available: https://www.britannica.com/science/paper-chromatography. Last accessed 31st Jan 2020.