Gloria Torres

Published Nov. 4, 1999

Did you ever imagine being a Sherlock Holmes? Did you dream of solving the perfect murder? If you like chemistry, you can be a Holmes of the 21st Century by using your brain and some chemical reactions, writes Gloria Torres.

Who has never read a book about the detective from Baker Street? Sherlock Holmes could solve the most difficult crime just by observing the crime scene and using his brain. Unfortunately, clues are not always obvious to our senses and what your senses donít catch your brain cannot process. It is then when science comes to the rescue. And crime-solving techniques have improved a lot since the good old times So detectives of the 21st century are getting much closer to science. And chemistry has a lot to say here.

Imagine that you want to know if a man has shot a gun. Can chemistry help you? Of course it can! When someone uses a gun some waste remains on his hand. It could be gunpowder or remains of the fulminant used to start the shot, but be sure something will remain. Common gunpowder is a mixture of potassium nitrate, carbon and sulfur, and some of the fulminants used nowadays contain antimonium, lead or barium. It is possible to determine the presence of these ions on a hand by rubbing it with a gauze wet with dilute hydrochloric acid. The gauze is then cut into small pieces, and some drops of sodium rhodizonate and buffer solution are added to each piece. A brown-pink color means that there was barium on the hand, if the color is scarlet red then there was lead, and a mixture of colors tells you both were present. These are the colors developed when a salt between rhodizonate and lead or barium is formed. Figure 1 shows how these reactions work. Those characteristic color changes are positive proof. On the other hand, the absence of color does not mean the hand is innocent. The killer could have already washed hands. Here, as usually, innocence is so difficult to prove... Thatís why the hand is innocent unless proven otherwise.

Figure 1

Did you know that iron is one of the components of ink? Neither did the murderer who left proof on a burnt paper. In his mind, throwing the note to the fireplace was enough to destroy the message. Certainly nobody else could ever read again the revealing words that had been written there. But flames don't destroy the iron in ink, and it is possible to read the text again by following a simple procedure. First of all, keep the burnt paper in a chamber at 20ºC and 60% of humidity for 48 hours, in order to let it recover part of the humidity and flexibility lost during the combustion. Then, wet a paper with a solution of potassium ferrocyanide and put it in contact with the back of the burnt paper. On the other side of it you have to put a white paper and a filter paper wet with hydrochloric acid. After several minutes of pressing the papers together, the words appear again on the white paper. How does it work? There are iron residues where there was ink before the paper was burnt. These are dissolved by hydrochloric acid and then react with potassium ferrocyanide to form Prussian Blue (a blue dye), showing the words again.

Fingerprints have been used for years to identify criminals because they are unique for every person. But in order to compare a fingerprint found at the scene of the crime with the ones classified at the police station it is necessary to obtain a good picture of the fingerprint. There are some chemicals that react with the inorganic salts of sweat and the grease always present on our skin, and change them into colored substances, so that the fingerprint is perfectly visible and can be identified. Silver nitrate solutions have been used for this purpose since long ago. The fingerprint support was sprayed with the solution and dried in a dark room. In the darkness, silver nitrate reacts with the chlorides of sweat to form silver chloride. In turn, this is photochemically reduced by light to form colloidal particles of metallic silver. In this way an image very much like a photographic negative is obtained.

Nevertheless, nowadays an organic molecule called ninhydrin (triketo hydrindene) is used instead of silver salts. This reacts with the amino group of the aminoacids segregated with sweat, giving a blue color due to the formation of the blue-colored molecule shown in figure 2.

Figure 2

These are only a few examples of how chemistry can help police to solve a crime, but in fact hundreds of chemical reactions are used by scientific police. They use chemical reactions to identify different drugs and distinguish them, to determine if an industry is polluting a river, or even to find out if a document has been forged.

Yet, the most important breakthrough in this area has been the use of DNA evidence. Every person in the world has a different DNA (except twins), and this is the same in all the cells of his body. So, if you have a sample that contains human cells (that is, blood, semen, skin, saliva or hair follicles) it is possible to identify the person who left it, with an estimated error rate of less than one in a trillion. It is incredible, isn't it? The first murderer who got caught just by pulling the string of his DNA was Colin Pitchfork, in 1987. Two teenage girls were raped and killed, and English police used the semen recovered from their bodies to compare its DNA with the DNA of more than 5000 men between 13 and 30 who lived in three villages near the scene of the crime. Pitchfork's DNA matched the DNA of the semen, and he was given a life sentence. This technique has also allowed scientists to solve some historical doubts: DNA evidence has proved that Thomas Jefferson, president of the United States of America from 1801 to 1809, had a child with his slave Sally Hemings, and has helped to identify the remains of the last Russian tsar and his family.

Sometimes we feel that science is very far away from our lives and that chemistry is too difficult for us to understand, but this is not true. Every day science is closer and closer, improving our life and helping us to solve some of our problems. And very frequently, chemical reactions that can be found in elementary chemistry textbooks can provide the clue to catch a criminal.
It's really getting more and more difficult to carry out the perfect crime.

F.Antón Barberá, J.V. de Luis y Turégano, "Manual de Técnica Policial", Tirant lo Blanch Ed., Valencia, 1991
A.Streitwieser, C.H.Heathcock, "Química Orgánica", Interamericana Ed., Madrid, 1986
J.A.Lorente Acosta, M.Lorente Acosta, "El ADN y la identificación en la investigación criminal y en la paternidad biológica", Comares Ed., Peligros (Granada), 1995
J.Adler, J.McCormick, "The DNA detectives" in "Newsweek", November 23, 1998
F.Burriel Martí, F.Lucena Conde, S.Arribas Jimeno, J.Hernández Méndez, "Química Analítica Cualitativa", Paraninfo Ed., Madrid, 1989.

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