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User:Dvnyn/Thin-layer chromatography

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Thin-layer chromatography
Separation of black ink on a TLC plate
AcronymTLC
ClassificationChromatography
Other techniques
Related

Thin-layer chromatography (TLC) is a chromatography technique that separates components in non-volatile mixtures.[1]

It is performed on a TLC plate made up of a non-reactive solid coated with a thin layer of adsorbent material.[2] This is called the stationary phase.[2] The scientist first places a small amount of sample on the plate, then places it into a solvent or solvent mixture known as the mobile phase (or eluent).[3] This solvent then moves up the plate via capillary action.[4] As with all chromatography, some compounds are more attracted to the mobile phase, while others are more attracted to the stationary phase.[5] Therefore, different compounds move up the TLC plate at different speeds and become separated.[6] To visualize colourless compounds, the plate is viewed under UV-light or is stained.[7] Testing different stationary and mobile phases is often necessary to obtain well-defined and separated spots.[8]

TLC is quick, simple, and gives high sensitivity for a relatively low cost.[5] It can monitor reaction progress, identify compounds in a mixture, determine purity, or purify small amounts of compound.[5]

Procedure

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The process for TLC is similar to paper chromatography but provides faster runs, better separations, and the choice between different stationary phases.[5] Plates can be labelled before or after the chromatography process with a pencil or other implement that will not interfere with the process.[9]

There are four main stages to running a thin-layer chromatography plate:[3][9]

Plate preparation: A scientist dissolves the sample in a small amount of solvent. Then, using a capillary tube, they spot a small amount of this solution about 1.5 centimetres from the bottom edge of a TLC plate. They mark this starting position in pencil. To prevent interferences, they let the solvent completely evaporate before the next step. A vacuum chamber may be necessary for non-volatile solvents. To make sure there is enough compound to obtain a visible result, the scientist may repeat this spotting procedure. Depending on the application, they may also place different samples in a row, the same distance from the bottom edge; Each sample will move up the plate in its own "lane."

TLC of three amino acids and a sample (left) with an English translation (right)

Development chamber preparation: The scientist pours solvent or solvent mixture into a transparent container (separation/development chamber) to a depth of less than 1 centimetre. They then place a strip of filter paper (aka "wick") along the container wall. This filter paper should touch the solvent and almost reach the top of the container. The scientist then covers the container with a lid and lets it sit for a few minutes to let the solvent vapours fill the container. Failure to do so results in poor separation and non-reproducible results.

Development: The scientist then places the TLC plate vertically in the container such that the sample spot(s) are not submerged into the mobile phase. The container is covered to prevent solvent evaporation. The solvent moves up the plate by capillary action, meets the sample mixture, and carries it up the plate (elutes the sample). The scientist removes the plate from the container before the solvent reaches the top of the plate. Otherwise, the results will be misleading. Without delay, they mark the solvent front: the furthest extent of solvent up the plate.

Visualization: The scientist lets the solvent evaporate from the plate, then visualizes the spots. Visualization methods include UV-light, staining, and many more.

Separation Process and Principle

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The separation of compounds is due to the differences in their attraction to the stationary phase and because of differences in solubility in the solvent.[10] As a result, the compounds and the mobile phase compete for binding sites on the stationary phase.[10] Different compounds in the sample mixture travel at different rates due to the differences in their partition coefficients.[11] Different solvents, or different solvent mixtures, gives different separation.[5][12] The retardation factor (Rf), or retention factor, quantifies the results. It is the distance travelled by a given substance divided by the distance travelled by the mobile phase.[8]

Development of a TLC plate. A spot that appears purple separates into a red and blue spot.

In normal-phase TLC, the stationary phase is polar. Silica gel is very common in normal-phase TLC. More polar compounds in a sample mixture interact more strongly with the polar stationary phase.[13] As a result, more polar compounds move less (resulting in smaller Rf) while less polar compounds move higher up the plate (higher Rf).[8][11] A more polar mobile phase also binds more strongly to the plate, competing more with the compound for binding sites; A more polar mobile phase also dissolves polar compounds more.[11] As such, all compounds on the TLC plate move higher up the plate in polar solvent mixtures.[8] "Strong" solvents move compounds higher up the plate, whereas "weak" solvents move them less.[14]

If the stationary phase is non-polar, like C18-functionalized silica plates, it is called reverse-phase TLC. In this case, non-polar compounds move less and polar compounds move more.[13] The solvent mixture will also be much more polar than in normal-phase TLC.[14]

Solvent Choice

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An eluotropic series, which orders solvents by how much they move compounds, can help in selecting a mobile phase.[5] Solvents are also divided into solvent selectivity groups.[15][12] Using solvents with different elution strengths or different selectivity groups can often give very different results.[15][12] While single-solvent mobile phases can sometimes give good separation, some cases may require solvent mixtures.[16]

In normal-phase TLC, the most common solvent mixtures include ethyl acetate/Hexanes (EtOAc/Hex) for less polar compounds and methanol/dichloromethane (MeOH/DCM) for more polar compounds.[17] Different solvent mixtures and solvent ratios can help give better separation.[18] In reverse-phase TLC, solvent mixtures are typically water with a less-polar solvent: Typical choices are water with tetrahydrofuran (THF), acetonitrile (ACN), or methanol.[17]

Analysis

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TLC plate visualized with UV-light

As the chemicals being separated may be colourless, several methods exist to visualize the spots:

  • Placing the plate under blacklight (366 nm light) makes fluorescent compounds glow [8]
  • TLC plates containing a small amount of fluorescent compound (usually manganese-activated zinc silicate) in the adsorbent layer allow for visualization of some compounds under UV-C light (254 nm). The adsorbent layer will fluoresce light-green, while spots containing compounds that absorb UV-C light will not.[4]
  • Placing the plate in a container filled with Iodine vapours temporarily stains the spots.[4] They typically become a yellow or brown colour.
  • The TLC plate can either be dipped in or sprayed with a stain. Many stains exist but some examples include: [19][7][20]

Plate Production

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TLC plates are usually commercially available, with standard particle size ranges to improve reproducibility.[21] They are prepared by mixing the adsorbent, such as silica gel, with a small amount of inert binder like calcium sulfate (gypsum) and water.[22] This mixture is spread as a thick slurry on an unreactive carrier sheet, usually glass, thick aluminum foil, or plastic. The resultant plate is dried and activated by heating in an oven for thirty minutes at 110 °C.[22] The thickness of the absorbent layer is typically around 0.1–0.25 mm for analytical purposes and around 0.5–2.0 mm for preparative TLC.[23]Other adsorbent coatings include aluminium oxide (alumina), or cellulose.[22]

Applications

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Reaction Monitoring and Characterization

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TLC is a useful tool for reaction monitoring.[18] For this, the plate normally contains a spot of starting material, a spot from the reaction mixture, and a co-spot (or cross-spot) containing both.[4][17] The analysis will show if the starting material disappeared and if any new products appeared.[17] This provides a quick and easy way to estimate how far a reaction has proceeded. The scientist simply monitors how much starting material spot remains in the reaction mixture. In one study, TLC has been applied in the screening of organic reactions. [24] The researchers react an alcohol and a catalyst directly in the co-spot of a TLC plate before developing it. This provides quick and easy small-scale testing of different reagents.

TLC for reaction monitoring and choosing a purification solvent mixture (left)TLC from the resulting flash column chromatography (right)

Compound characterization with TLC is also possible[8] and is similar to reaction monitoring. However, rather than spotting with starting material and reaction mixture, it is with an unknown and a known compound. They may be the same compound if both spots have the same Rf and look the same under the chosen visualization method.[8] However, co-elution complicates both reaction monitoring and characterization. This is because different compounds will move to the same spot on the plate. In such cases, different solvent mixtures may provide better separation.[25]

Purity and Purification

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TLC helps show the purity of a sample.[8] A pure sample should only contain one spot by TLC. TLC is also useful for small-scale purification.[26] Because the separated compounds will be on different areas of the plate, a scientist can scrape off the stationary phase particles containing the desired compound and dissolve them into an appropriate solvent.[26] Once all the compound dissolves in the solvent, they filter out the silica particles, then evaporate the solvent to isolate the product. Big preparative TLC plates with thick silica gel coatings can separate more than 100 mg of material.[26]

For larger-scale purification and isolation, TLC is useful to quickly test solvent mixtures before running flash column chromatography on a large batch of impure material.[27][16] A compound elutes from a column when the amount of solvent collected is equal to 1/Rf.[28] The eluent from flash column chromatography gets collected across several containers (for example, test tubes) called fractions. TLC helps show which fractions contain impurities and which contain pure compound.[8]

Furthermore, two-dimensional TLC[21] can help check if a compound is stable on a particular stationary phase. This test requires two runs on a square-shaped TLC plate. The plate is rotated by 90º before the second run. If the target compound appears on the diagonal of the square, it is stable on the chosen stationary phase. Otherwise, it is decomposing on the plate. If this is the case, an alternative stationary phase may prevent this decomposition.[29]

TLC is also an analytical method for the direct separation of enantiomers and the control of enantiomeric purity, e.g. active pharmaceutical ingredients (APIs) that are chiral.[30]

  • Separation of green plant matter in spinach (note that images from steps 1-6 are zoomed into the bottom of the plate)
  • Step 1
    Step 1
  • Step 2
    Step 2
  • Step 3
    Step 3
  • Step 4
    Step 4
  • Step 5
    Step 5
  • Step 6
    Step 6
  • Step 7
    Step 7

See also

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References

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  1. ^ Harry W. Lewis & Christopher J. Moody (13 Jun 1989). Experimental Organic Chemistry: Principles and Practice (Illustrated ed.). WileyBlackwell. pp. 159–173. ISBN 9780632020171.
  2. ^ a b Zhang, Meizhen; Yu, Qian; Guo, Jiaqi; Wu, Bo; Kong, Xianming (2022-11-28). "Review of Thin-Layer Chromatography Tandem with Surface-Enhanced Raman Spectroscopy for Detection of Analytes in Mixture Samples". Biosensors. 12 (11): 937. doi:10.3390/bios12110937. ISSN 2079-6374. PMC 9687685. PMID 36354446.{{cite journal}}: CS1 maint: PMC format (link) CS1 maint: unflagged free DOI (link)
  3. ^ a b Silver, Jack (2020-12-08). "Let Us Teach Proper Thin Layer Chromatography Technique!". Journal of Chemical Education. 97 (12): 4217–4219. doi:10.1021/acs.jchemed.0c00437. ISSN 0021-9584.
  4. ^ a b c d e Stahl, Egon (1969), Stahl, Egon (ed.), "Apparatus and General Techniques in TLC", Thin-Layer Chromatography: A Laboratory Handbook, Berlin, Heidelberg: Springer, pp. 52–86, doi:10.1007/978-3-642-88488-7_3, ISBN 978-3-642-88488-7, retrieved 2023-03-29
  5. ^ a b c d e f Santiago, Marina; Strobel, Scott (2013-01-01), Lorsch, Jon (ed.), "Chapter Twenty-Four - Thin Layer Chromatography", Methods in Enzymology, Laboratory Methods in Enzymology: Cell, Lipid and Carbohydrate, vol. 533, Academic Press, pp. 303–324, doi:10.1016/b978-0-12-420067-8.00024-6, retrieved 2023-03-29
  6. ^ A.I. Vogel; A.R. Tatchell; B.S. Furnis; A.J. Hannaford & P.W.G. Smith (1989). Vogel's Textbook of Practical Organic Chemistry (5th ed.). ISBN 978-0-582-46236-6.
  7. ^ a b Jork, H., Funk, W., Fischer, W., Wimmer, H. (1990): Thin-Layer Chromatography: Reagents and Detection Methods, Volume 1a, VCH, Weinheim, ISBN 3-527-278834
  8. ^ a b c d e f g h i "AN OVERVIEW ON THIN LAYER CHROMATOGRAPHY | INTERNATIONAL JOURNAL OF PHARMACEUTICAL SCIENCES AND RESEARCH". 2011-02-01. doi:10.13040/ijpsr.0975-8232.2(2).256-67. Retrieved 2023-03-29.
  9. ^ a b Thin Layer Chromatography: How To http://www.reachdevices.com/TLC.html
  10. ^ a b Reich, E.; Schibli A. (2007). High-performance thin-layer chromatography for the analysis of medicinal plants (Illustrated ed.). New York: Thieme. ISBN 978-3-13-141601-8.
  11. ^ a b c Thin Layer Chromatography (TLC): Principle with animation
  12. ^ a b c Bickler, Bob (1 March 2015). "Using TLC to Scout Flash Chromatography Solvents". Biotage. Retrieved 30 March 2023.
  13. ^ a b Lade, Bipin D; Patil, Anita S; Paikrao, Hariprasad M; Kale, Ankit S; Hire, Kushal K (2014). "A Comprehensive Working, Principles and Applications of Thin Layer Chromatography". Research Journal of Pharmaceutical, Biological and Chemical Sciences. 5 (4): 489–495 – via Semantic Scholar.
  14. ^ a b Dolan, John W. (31 May 2006). "The Power of Mobile Phase Strength". Retrieved 30 March 2023.
  15. ^ a b Johnson, Andrew R.; Vitha, Mark F. (2011-01-28). "Chromatographic selectivity triangles". Journal of Chromatography A. EDITORS' CHOICE V. 1218 (4): 556–586. doi:10.1016/j.chroma.2010.09.046. ISSN 0021-9673.
  16. ^ a b Snyder, Lloyd R.; Kirkland, Joseph J.; Dolan, John W. (2009-11-11). Introduction to Modern Liquid Chromatography. Hoboken, NJ, USA: John Wiley & Sons, Inc. doi:10.1002/9780470508183.app1. ISBN 978-0-470-50818-3.
  17. ^ a b c d "SiliaPlate - TLC Practical Guide". SiliCycle. March 28, 2023.
  18. ^ a b Dickson, Hamilton; Kittredge, Kevin W.; Sarquis, Arlyne M. (1 July 2004). "Thin-Layer Chromatography: The "Eyes" of the Organic Chemist". Journal of Chemical Education. 81 (7): 1023. doi:10.1021/ed081p1023. ISSN 0021-9584.
  19. ^ Thin Layer Chromatography stains http://www.reachdevices.com/TLC_stains.html
  20. ^ Jork, H., Funk, W., Fischer, W., Wimmer, H. (1994): Thin-Layer Chromatography: Reagents and Detection Methods, Volume 1b, VCH, Weinheim
  21. ^ a b Stahl, Egon (1969), Stahl, Egon (ed.), "Apparatus and General Techniques in TLC", Thin-Layer Chromatography: A Laboratory Handbook, Berlin, Heidelberg: Springer, pp. 52–86, doi:10.1007/978-3-642-88488-7_3, ISBN 978-3-642-88488-7, retrieved 2023-03-29
  22. ^ a b c O. Kaynar; Y. Berktas (2010-12-01). "How To Choose The Right Plate For Thin-Layer Chromatography?". Reviews in Analytical Chemistry. 29 (3–4): 129–148. doi:10.1515/REVAC.2010.29.3-4.129. ISSN 2191-0189.
  23. ^ Tables showing the thickness value of commercial regular and preparative Thin Layer Chromatography plates
  24. ^ TLC plates as a convenient platform for solvent-free reactions Jonathan M. Stoddard, Lien Nguyen, Hector Mata-Chavez and Kelly Nguyen Chem. Commun., 2007, 1240–1241, doi:10.1039/b616311d
  25. ^ Bickler, Bob (22 November 2022). "Why is Solvent Evaluation by TLC Important for Good Flash Chromatography Results?". Biotage. Retrieved 1 April 2022.
  26. ^ a b c Wing, R. E.; Bemiller, J. N. (1972-01-01), Whistler, Roy L.; BeMiller, James N. (eds.), "[8] - Preparative Thin-Layer Chromatography", General Carbohydrate Method, Academic Press, pp. 60–64, doi:10.1016/b978-0-12-746206-6.50015-1, ISBN 978-0-12-746206-6, retrieved 2023-04-01
  27. ^ Bickler, Bob (13 November 2020). "Invest 10 minutes on TLC and save a day of grief". Retrieved 30 March 2023.
  28. ^ Fair, Justin D.; Kormos, Chad M. (2008-11-21). "Flash column chromatograms estimated from thin-layer chromatography data". Journal of Chromatography A. 1211 (1): 49–54. doi:10.1016/j.chroma.2008.09.085. ISSN 0021-9673.
  29. ^ "Thin Layer Chromatography: A Complete Guide to TLC". Chemistry Hall. 2020-01-02. Retrieved 2020-01-30.
  30. ^ Bhushan, R.; Tanwar, S. J. Chromatogr. A 2010, 1217, 1395–1398. (doi:10.1016/j.chroma.2009.12.071)

Bibliography

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  • F. Geiss (1987): Fundamentals of thin layer chromatography planar chromatography, Heidelberg, Hüthig, ISBN 3-7785-0854-7
  • Justus G. Kirchner (1978): Thin-layer chromatography, 2nd edition, Wiley
  • Joseph Sherma, Bernard Fried (1991): Handbook of Thin-Layer Chromatography (= Chromatographic Science. Bd. 55). Marcel Dekker, New York NY, ISBN 0-8247-8335-2.
  • Elke Hahn-Deinstorp: Applied Thin-Layer Chromatography. Best Practice and Avoidance of Mistakes. Wiley-VCH, Weinheim u. a. 2000, ISBN 3-527-29839-8
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