Applications of Gas and Thin Layer Chromatography

Applications of Gas and Thin Layer Chromatography

Aim:

The purpose of this lab is to understand the principles and applications of both gas and thin layer chromatography. By the end of this lab, one should be able to calculate the retention factor based on the thin layer chromatography method and determine the ratio of relative compounds, the number of theoretical plates, and resolution based on data acquired from the gas chromatography.

Introduction and Theory:

Chromatography is a common and highly efficient method that is used to analyze and separate compounds from a mixture. This separation technique is comprised of many different methods, two of which are gas chromatography and thin layer chromatography. Both methods’ principle of separation is polarity, where the compounds are separated based on their polar attributes and the interactions between the stationary phase and the mobile phase.

Thin layer chromatography can be used to identify compounds and determining how many compounds are present within a given mixture. Silica gel is commonly utilized as the adsorbent and is on a backing of plastic, glass, or aluminum. This compound serves as the stationary phase. The mobile phase, or eluent is the mixture, which in this experiment was a small amount of solvent. After a thin layer chromatography plate is made and the compound or mixture is introduced to both phases, it can interact with each phase differently based on polarity and separated based on these interactions. Spots are produced by each compound or mixture and retention factor can be measured based on the distance traveled by the spot divided by the total distance traveled by the solvent.

In gas chromatography, the same principle of polarity is utilized to determine attributes of the mixture and its relative compounds, however, the stationary and mobile phases change. For example, the stationary phase is usually a non-volatile liquid or a polymer with a high boiling point. The mobile phase, or the carrier gas is usually nitrogen or helium. When the liquid compound is introduced to gas chromatography machine, it a vaporized and analyzed. The Similar to thin layer chromatography, the compounds are separated based on the way they interact with each phase. Compounds that interact more strongly with the stationary phase are retained longer within the gas chromatography machine and so the peak produced has a higher retention time. A graph of these peaks is produced, and the ratio of compounds, the number of the theoretical plates, and the resolution can be calculated.

Table of Physical Properties and Hazards:

Compound

Physical Properties

Hazards

Acetylferrocene

[Fe(C5H4COCH3)(C5H5)]

Molar Mass: 228.07 g/mol

Melting Point: 81-83°C

Boiling Point: 161°C

Physical State: Solid

Acetylferrocene may cause eye and skin irritation. May be fatal if swallowed and is toxic if absorbed through skin. May also be harmful if inhaled.

Ferrocene

(C10H10Fe)

Molar Mass: 186.04 g/mol

Melting Point: 172.5°C

Boiling Point: 249°C

Physical State: Solid

Ferrocene may cause eye and skin irritation. If inhaled, it may cause irritation of the digestive tract. Inhalation of dust particles may cause respiratory tract irritation.

Silica/ Silicon Dioxide

(SiO2)

Molar Mass: 60.08 g/mol

Melting Point: 1,710°C

Boiling Point: 2,230°C

Physical State: Solid (Powder)

Silica is a very hazardous chemical. These small crystalline particles are classified as a human carcinogen and can cause serious lung disease and lung cancer.

Procedures and Observations:

In this lab, both methods of chromatography (gas chromatography and thin layer chromatography) were carried out. In gas chromatography the mixture labeled GC7 was analyzed. Before this however, the syringe used to inject the compound had to be prepared. Acetone was used to clean the syringe and dispelled on a napkin. Next, about 2 microliters of the GC7 sample was filled in the syringe. It was extremely important to ensure that there were no air bubbles because it would affect the reading carried out by the machine. If there were air bubbles, the sample was ejected, and the syringe was refilled with 2 microliters of the same sample (GC7). The syringe was then checked by the lab instructor. The syringe was then injected into the running gas chromatography machine. The plunger of the syringe was pushed the same time as the “start” button was pressed. As the machine analyzed GC7 and produced a graph depicting the different peaks. The ratio of compounds, the number of the theoretical plates, and the resolution can be calculated.

Using the thin layer chromatography method, the retention factors of the compound acetylferrocene and ferrocene was calculation and a physical separation of both compounds were observed. In this method, a backing with silica gel called the thin layer chromatography plate was created. About an inch from the bottom a line was drawn. Using a capillary, a drop-size amount of each of the compounded separated from the last lab, acetylferrocene and ferrocene, was placed onto the silica gel. A drop of the 1:1 mixture was then placed onto the gel as well. A jar, filled with less than 1 inch of solvent was prepared and the TLC plate was placed into it in a vertical position, with the compounds at the bottom. The plate was observed as the compounds traveled upward and the compounds within the mixture separated with the upward movement of the solvent. This was seen and depicted by the colored spots produced by each compound. Once the solvent traveled to about an inch from the top of the plate, the plate was taken out of the jar and the distance the solvent and both compounds traveled was measured and the retention factor was calculated.

Results:

The Retention factor values of each compound was calculated by dividing the distance traveled by each spot produced by the compounds (ferrocene: 4.75cm, acetylferrocene: 0.9cm) by the distance traveled by the solvent (5.8cm). The retention factor value of acetylferrocene was 0.16 and for ferrocene was 0.81. Next, the graph produced by the GC machine was analyzed. For the mixture GC7, peak 5 (compound A) had an area of 1.52mm^2 while peak 6 (compound B) had an area of 0.82mm^2. The ratio of compound in peak 5 to the compound in peak 6 is 64.96 to 35.04. Calculations are shown in the attached data sheet at the end of this report. Furthermore, the number of theoretical plates for peak 5 is 800.89 and for peak 6 is 852.64. The resolution was 0.225. Again, the calculations are shown at the end of the data report. 

Discussion and Conclusions:

In the final analysis, both gas and thin layer chromatography are efficient and useful methods in determining the composition of a mixture and the identities of the compounds within a given mixture. The retention factor was calculated using the thin layer chromatography method and it was seen that the retention factor of acetylferrocene far lower than that of ferrocene. This shows us that acetylferrocene interacted strongly with the silica stationary phase and did not travel with the mobile phase solvent as much as ferrocene did. Using the gas chromatography method, compound A and B were analyzed from GC7. According to the percent composition, there was more of compound A in the mixture than there was B, since the ratio wasn’t 1 to 1. Furthermore, the resolution was quite low which demonstrates that the degree of separation between the two adjacent compounds is quite small.

References:

National Center for Biotechnology Information. PubChem Compound Database;

CID=11985924, https://pubchem.ncbi.nlm.nih.gov/compound/11985924 

NOAA Office of Response. “Chemical Datasheet: Ferrocene.” NOAA,

cameochemicals.noaa.gov/chemical/20407.

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