Lab Report 1: TLC Guidelines

[ks...@cornell.edu]

Hey everyone! 

Here are some basic guidelines to writing your first lab report in addition to the sample report that was posted on blackboard:

1.) Use the same organization from the sample report. Purpose, balanced equation (if there is one), mechanism (if there is one), results and discussion. There is no experimental section/procedure in the sample report which means you do not have to include one in your report. The sample report doesn't have a conclusion, but it would be a good idea to end with some concluding thoughts on the experiment. This doesn't have to be in its own section, but you should wrap things up in a sentence or two at the end.

2.) You should always discuss the theory behind the experiment. In the case of this lab report, discuss how TLC works and tie that in with your results.

3.) The results section doesn't have to be in paragraph form. I am totally fine with tables/bullet points with the information. This is your call though, I don't mind either way. For future lab reports, tables will be a big help.

4.) You should always discuss possible sources of error in the experiment. For the most part, things don't work perfectly so I want you to discuss the reasons why things could have went wrong. Even if your TLC plates were perfect, you should think about what errors could have affected the experiment.

5.) For this experiment in particular, I want to see a representation of your TLC plate. Ideally, if you can recreate a depiction of it that would be awesome, but I'm also cool with a picture of your TLC plate with the spots clearly labeled. Worst case scenario, you can attach the TLC plate to the report but I would prefer if you could avoid that. 

6.) Share data with your classmates and then share it with me! If you have multiple trials of data, it will strengthen your report. It will be especially valuable if you have data taken with different solvent ratios for the developing solvent. Discussion of the effects of the solvent ratio on Rf value will be important.

7.) If you do use data from other people, CITE THEM! Nothing fancy, just have a works cited page or some footnotes at the bottom saying the person's name, the class and the date. Also, any information you pull from the web to help your lab report must be cited. 

8.) Write these decently well. I appreciate good grammar and spelling and stuff haha. 

I hope this helps. Good luck with the reports and email me if you have any questions!

Best,

Kyle

[Barbara Force]

I thought I would share my lab report for the purpose of comparing and contrasting. I know it's kind of late, but I'm sure at least one person is waiting until the last second to work on the report

Analysis of Peppermint Leaf and Spearmint Leaf Extracts

by Thin Layer Chromatography

 

 

 

By Barbara Force

 

 

 

Lab Instructor: Kyle O’Conner

January 28, 2013

Background:

Chromatography is the separation of a mixture of two or more different compounds or ions by distribution between two phases: stationary and solid. The methods include thin-layer chromatography (TLC), paper chromatography, gas chromatography, and Mass Spectrometry. It can be preferred over distillation because it can be performed on a very small scale on a small sample. There is always a stationary phase and a mobile phase. The methods used depend on the differential absorbatives of the substances to be separated between the two phases. Separation will be achieved if one component of a mixture adheres more to the stationary phase than the other. Doing so allows one to analyze a complex matrix of a mixture.

            TLC consists of a stationary phase immobilized on a glass or plastic plate and a solvent. The sample, either liquid or dissolved in a volatile solvent, is deposited as a spot on the stationary phase. The constituents of a sample can be identified by simultaneously running standards with the unknown. One edge of the plate is then placed in a solvent reservoir and the solvent moves up the plate by capillary action. When the solvent front reaches the other edge of the stationary phase, the plate is removed from the solvent reservoir. The separated spots are then visualized with ultraviolet light or by placing the plate in iodine vapor. [Additionally, a dip into p-Anisaldehyde can give color and obvious spots when ultraviolet light does not do the trick.]

            The different components in the mixture move up the plate at different rates due to differences in their partitioning behavior between the mobile liquid phase and the stationary phase. The tendency of the compound to absorb on the stationary phase relative to its solubility in the mobile phase determines how readily the compound “rises.” In this experiment, we will be using TLC to visualize the components of spearmint and peppermint oil extract. Both spearmint and peppermint are common plants in the United States and Canada. Their oils are obtained from distilment of fresh flowering plants. Peppermint (Mentha piperita) is named for its peppery smell, and spearmint (Mentha spicata) is named for its arrow-shaped leaves, though it also has a distinct smell. They are related plants that are rich in volatile oils called terpenes. Two of the terpenes that this experiment will compare with the extracts are (R)-(-)-carvone and (1R,2S,5R)-(-)-menthol. They have similar structures:

 

 

 

The partitioning of the compounds will depend primarily on the polarity, solubility, hydrogen bonding, and such properties that can alter solubility and the compounds’ affinity for the solvent and stationary phases. A more polar molecule will adhere more strongly to the more polar stationary phase than a less polar molecule. TLC usually uses a thin coating of silica gel (SiO_x) on a plate of glass for the stationary phase. Silica gel is commonly a very fine powder, highly polar, and adheres to glass.

Uses for TLC include establishing two compounds are identical; determining the number of components in a mixture; determining the appropriate solvent for column-chromatographic separation; to monitor a column-chromatographic separation; and to check column-chromatographic effectiveness, recrystallization, or extraction, to monitor the progress of a reaction. All of these can be done with a small amount of sample. Terms used when performing TLC include:

            Injection, or spotting: the sample is applied to the plate before the solvent ascends

            Separation: developing or “running” the plate as solvent ascends the plate and the sample is partitioned between the moving liquid and the stationary phase

            Detection: different methods to visualize separated spots

 

Purpose: The purpose of this experiment is to partition and visualize the components of peppermint and spearmint oil extracts through thin-layer chromatography and then compare them against (R )-Carvone and (1R,2S,5R)-(-)-Menthol standards for identification of components.

 

Results: Injections of

• Spearmint oil 1% in Ethyl Acetate
• Peppermint oil in 1% Ethyl Acetate
• (R )-Carvone 3% in Ethyl Acetate
• And (1R,2S,5R)-(-)-Menthol 3% in Ethyl Acetate on the bottom pencil line were run.

After developing the plate, the solvent line was marked and spots were visualized through ultraviolet light and a dip in p-Anisaldehyde. Rf values are noted below.

 

Rf = (Distance from start to center of the substance spot)/(Distance from start to solvent front)

 

 [I plan on drawing my plate right here.]

 

The letters represent S (Spearmint oil), P (peppermint oil), C (carvone) and M (menthol).

Solvent front was marked at 5.6cm

Spots:

Blue spot above S and C at 1.2cm. The spot is larger above the S.

Grey spots above S, P, and C at 0.0cm. The spot at S is blurred. The spot over P is much darker.

Orange-green spots above P and M at 2.1cm. P is heavier, but the center is the same.

Steel blue spot above S and M at 0.7cm. The blue spot mentioned above blurs the grey spot in S.

Steel blue spot above M at 2.8cm. It is quite faint.

 

Blue spot Rf = 1.2cm/5.6cm = 0.21

Grey spot Rf = 0

Orange-green spot Rf = 2.1cm/5.6cm = 0.38

Steel blue spot (SM) Rf = 0.7cm/5.6cm = 0.13

Steel blue spot (M) Rf = 2.8cm/5.6cm = 0.50

 

While measuring Rf values, I noticed mistakes in several areas. In my first run, I spotted the wrong side of my plate, and nothing ran. The solvent shot right up the other side while my spots did nothing on the plastic/glass. The second time around, I was looking at the wrong side of the plate when I was visualizing my spots, and everything looked disastrous. I thought I had failed the experiment. Then when I turned my plate over, I saw that everything was in order. The third mistake I noticed was that I had used a bit too much spearmint oil and that it creates a very large blue circle that may have smeared my menthol circle if I had added just a little more. I probably should have used the ruler to draw my lines as well, as I had to spend a few moments orienting my spots to see what spot went with which chemical.

 

Conclusion:

Apart with my disorientation between silica and plastic, I did detect both terpenes in spearmint oil, as indicated by the blue dot ((R)-(-)-Carvone)) and the faint steel blue dot ((1R,2S,5R)-(-)-menthol) standards. Comparison with similar data against Don R. Davies of the Journal of Chemical Education suggested that my data was accurate to the components of spearmint oil.

 

 

Bibliography:

            Davies, Don R., and Todd M. Johnson. “Isolation of Three Components from Spearmint Oil: An Exercise in Column and Thin-Layer Chromatography.” Journal of Chemical Education 84:2 (Feb 2007): 318-321. <http://tres.njude.com.cn/rete/eWebEditor/uploadfile/hxyl/Flash/

20090702061052.pdf>

            Padias, Anne B. Making the Connections: A How-To Guide for Organic Chemistry Lab Techniques (Second Edition). Hayden McNeil: 2011.