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Analysis of Analgesics by Thin Layer Chromatography

Analysis of Analgesics by Thin Layer Chromatography

Abstract:

The chemical compositions of four analgesics (Anacin, Excedrin, Nuprin, and Tylenol) were determined by thin layer chromatography in this experiment using ethyl acetate as both the solvent and the elluent. Anacin, Excedrin, Nuprin, and Tylenol were pulverized, dissolved in ethyl acetate, and run plated on a silicon gel to determine their Rf factors. These Rf factors were then compared to the Rf factors of the pure standard samples: acetaminophen, aspirin, caffeine, and ibuprofen. The TLC plates indicated that Anacin contains both aspirin and caffeine; Excedrin contains acetaminophen, aspirin, and caffeine; Nuprin contains only ibuprofen; and Tylenol contains only acetaminophen.

Introduction:

Thin layer chromatography (TLC) is a method used in chemistry and biochemistry for the separation and analysis of a wide variety of inorganic ions and organic molecules. The TLC plate typically consists of a thin layer of gel (fluorescent silicon gel was used in this experiment) bonded to a glass or plastic backing. A line is drawn 1 cm from the bottom of the gel and a small volume of sample (dissolved in a solvent, in this case ethyl acetate) is applied to the silicon with a micropipette and allowed to dry. The plate is then placed in a chamber containing a small volume of the appropriate solvent designated the eluent (in this experiment, ethyl acetate). Only the edge of the plate nearest the samples is in contact with the eluent. The eluent is drawn into the silicon by capillary action and travels up the plate through the samples. The migration rate of the sample components over the silicon depends on their chemical structure.

TLC is generally very sensitive to small differences in the chemical structure of the sample. The structure affects the strength and type of interactions between the sample and the gel. In addition, different samples have different solubilities in a given solvent which is also dependent upon chemical structure. Thus, the general rule of thumb is the more polar the compound, the less the distance from the origin it migrates. Nonpolar samples tend to migrate the furthest away from the origin because it is unable to interact readily with the silicon gel (see chart below).

The Rf value is the ratio of distance traveled by the spot divided by the distance traveled by the solvent. Since the solvent front will always be ahead of the sample spots, the Rf value will always be less than 1. Each sample has its own distinct Rf value for a particular solvent. For a given sample, the Rf value (and the distance moved by the spot) will increase as the polarity of the solvent is increased (see chart below). This is because the solvent begins to compete more and more with the silica gel for the polar parts of the molecule, and so the sample is eluted up the plate more effectively. It is important to note that no solvent is more polar than the silica gel surface; for this reason non-polar samples prefer the polar solvent over the surface of the silica gel causing it to migrate the furthest from the origin. Thus, thin layer chromatography is principally a competition between the solvent and the silica gel for the polar parts of the molecule. Using a common eluent while cmparing and contrasting the Rf values of known pure samples with that of an unknown is the way to identify the unknown by TLC and the main purpose of this experiment.

Relative polarity of organic samples:

Alkanes

Alkenes

INCREASING Aromatic hydrocarbons

POLARITY Ethers, alkyl halides

Aldehydes, ketones, esters

Amines

Alcohols

Carboxylic acids

Common TLC developing solvents:

Hexane, cyclohexane, petroleum ether

Toluene

Dichloromethane

Diethyl ether

INCREASING Chloroform

POLARITY Ethyl acetate

Isopropyl alcohol

Acetone

Ethanol

Methanol

Acetonitrile

Water

Table of Physical Properties:

Compounds and Solvents

Aspirin

(compound)

Acetaminophen

(compound)

Caffeine

(compound)

Ibuprofen

(compound)

Ethyl acetate

(solvent)

Structural Formula

Molecular Formula

C9H8O4

C8H9NO2

C8H10N4O2

C13H18O2

C4H8O2

Molec. Wt. g/mol

180.16

151.16

194.19

206.284

88.106

Density g/cm3

1.35

1.293

1.23

0.5 – 0.75

0.8945

Melting pt. oC

135

169

238

75-77

-83.6

Boiling pt. oC

140

77.1


Experiment:

This experiment involved the use of the technique: thin-layer chromatography. Ethyl acetate was used as the elluent to run the TLC plates in. This elluent was placed in a small jar lined with filter paper to keep the vapors high up in the vessel. TLC plates were cut and marked so that the elluent would have 5 cm to run with 1 extra cm at the top and 1 cm at the bottom.

Each TLC plate was spotted with two known analgesics in an ethyl acetate solution and one unknown analgesic in an ethyl acetate solution. The four known solutions used were of caffeine, aspirin, acetaminophen and ibuprofen. The unknown solutions used were of Anacin, Nuprin, Tylenol, and Excedrin. It was given that the unkown analgesics contained one or more of the known analgesics.

The elluent used for the TLCs run was ethyl acetate. After running the TLC plates for the full 5 cm, they were allowed to dry and were placed under ultraviolet light. While being viewed in ultraviolet light, the movement of the spots placed on the TLC plates could be determined, and thusly the Rf values for each spot could be determined. Once the Rf values had been determined, the unkowns were compared with the knowns to determine their composition.

Results:

Compound

Dist of Solvent

Dist (a)

Dist (b)

Dist (c)

Rf (a)

Rf (b)

Rf (c)

Acetaminophen

5 cm

1.55 cm

0.31

Aspirin

5 cm

2.35 cm

0.47

Caffeine

5 cm

0.55 cm

0.11

Ibuprofen

5 cm

2.86 cm

0.57

Anacin

5 cm

0.53 cm

2.35 cm

0.106

0.47

Excedrin

5 cm

0.53 cm

1.54 cm

2.37 cm

0.016

0.308

0.474

Nuprin

5 cm

2.85 cm

0.57

Tylenol

5 cm

1.52 cm

0.304

Composition of Unknowns:

Acetaminophen

Aspirin

Caffeine

Ibuprofen

Anacin

No

Yes

Yes

No

Excedrin

Yes

Yes

Yes

No

Nuprin

No

No

No

Yes

Tylenol

Yes

No

No

No

Anacin = Aspirin + Caffeine

Excedrin = Acetaminophen + Aspirin + Caffeine

Nuprin = Ibuprofen

Tylenol = Acetaminophen

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