Forensic Examination of Drugs by Thin Layer Chromatography.

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But Forensic Science

Skills for the Laboratory

Experiment Title:        Forensic Examination of Drugs by Thin Layer Chromatography

Aims:                        To determine the constituents of four unknown drugs.

To establish good safe & organised working practice.

To encourage consistent & meticulous keeping of data & records.

To develop consistent & accurate laboratory procedures.

To establish team work & investigative approaches.

Background Info:        The majority of evidence submitted to crime labs comes from drug-related crimes. Often, this evidence includes unidentified powders that may be illegal drugs. In order to prosecute individuals for possession of illegal substances, it is necessary for forensic scientists to positively identify any suspected drugs submitted to the laboratory. In addition, forensic toxicologists must determine the identity of drugs found in the bodies of drug-overdose victims. Although illegal substances can cause overdose, people also overdose on common over-the-counter (OTC) drugs, like aspirin, when attempting to take their own lives. Thin-layer chromatography (TLC) is one technique used to identify unknown drugs.

        Thin-Layer Chromatography (TLC) is a simple & inexpensive technique that is often used to judge the purity of a synthesized compound or to indicate the extent of progress of a chemical reaction. In this technique, a small quantity of a solution of the mixture to be analyzed is deposited as a small spot on a TLC plate, which consists of a thin layer of silica gel (SiO2) or alumina (Al2O3) coated on an aluminium or plastic sheet. The plate constitutes the stationary phase. The sheet is then placed in a chamber containing a small amount of solvent, which is the mobile phase. The solvent gradually moves up the plate via capillary action, & it carries the deposited substances along with it at different rates due to the differential solubility of each of its components. The desired result is that each component of the deposited mixture is moved a different distance up the plate by the solvent. The components then appear as a series of spots at different locations up the plate. Substances can be identified from their so-called Rf values. The Rf value for a substance is the ratio of the distance that the substance travels to the distance that the solvent travels up the plate as you can see from the formula below:

        Rf        =        Distance travelled by compound

                        Distance travelled by solvent

 For example, an Rf value of 0.5 means that the spot corresponding to the substance travels exactly half as far as the solvent travels along the plate.

As a forensic application TLC can be found to be very useful to many aspects one of which being to determine the composition of unknown drugs. In this experiment we investigate this forensic application of thin-layer chromatography. Control samples must be used that provide Rf & colour reactions to compare the unknown samples to. It is essential that a control is used on every plate as Rf values are rarely identical between operators. Since in this experiment a team effort is employed each member should test each unknown with one of the three drugs supplied (Aspirin, Parecetemol & Caffeine). Since these compounds have different molecular structures they interact variably with the mobile & stationary phases in thin layer chromatography. The result is that each component travels at differing rates resulting in a separation of each component, hence each individual spot on the TLC plate represents a compound within the mixture. The unknown samples in this case consist of a mixture of the three compounds provided. Since this is the case by comparing each of the unknown samples to the tested known samples it is possible to determine the composition of each of the unknowns. In the forensic lab further examination of the samples would be carried out to determine the composition of the questioned powders, remember the TLC is only a screening test & would usually be carried out after a presumptive test has been used. Such tests would only be employed if there is sufficient sample & are used because they are cheep & quick. Presumptive tests are usually used to provide the analyst with an indication as to the class of drug involved.  If the amount of sample is limited then only instrumental tests would be carried out. There are many presumptive tests available to the forensic toxicologist such as:

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  • Mandelin test
  • Marquis test
  • Scott test
  • Erlich test
  • Liebermann test
  • Dille-Koppani test
  • Duquenois-Levine test
  • Microcrystalline tests

 Various separation techniques could be used, such as high performance liquid chromatography & gas chromatography. Also analytical techniques such as mass spectrometry, infra red spectrometry & 13C NMR spectroscopy could be used before or after separation has been achieved.


Materials:                The apparatus used in this experiment are listed below.


  • Filter Paper
  • Aspirin, Paracetamol & Caffeine samples
  • Unknown Samples A,B,C &D
  • Small TLC Chamber
  • Solvent 1= Ethyl acetate, Chloroform, 85% Formic acid (3:2:1)
  • Solvent 1= Ethyl ...

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