Gold

Gold (Au) is a precious metal, meaning that even in ancient times people recognized that it has special properties and is extremely rare. The symbol Au comes from the Latin word aurum. Aside from its beautiful appearance, gold does not tarnish. Chemically this means that gold does not react with oxygen (O2) in the air, so it retains its lustrous appearance over time. Gold also does not react with water or many acids or bases, so it is extremely durable. For this reason gold is frequently used as a catalyst or for wiring in specialized instruments and devices in addition to its use in jewelry and currency. It is not known when people began to use gold as a sign of wealth, but it was mentioned in ancient Egyptian texts dating back nearly 3,000 years. Gold is soft and extremely malleable, which means that it can be hammered into very thin sheets. For making jewelry or coins, especially using ancient tools and techniques, this is an important property. Dentists also utilize gold for making fillings and other dental appliances.

The role of gold in the story of forensic chemistry relates to how people tested for it. Once gold was used to represent wealth, people needed to know how pure a particular nugget or coin was to assign a proper value. At first this was nearly impossible because chemists did not know how to separate gold from other materials. The first methods that worked involved fire, in which the metal was heated to drive off impurities. It was not until the Middle Ages that acids became available and chemists learned how to dissolve solid samples. Once they were dissolved, chemists could devise tests to detect the presence and quantity of gold in the sample. The need to separate gold from other materials was an important factor in the development of analytical chemistry.

A coin weighing a gram contains gold, but how much? Chemists in the 1500s could use an acid to dissolve the coin. They could then add a reagent that would form an insoluble solid with the gold, such as gold chloride (AuCI), which chemists knew is 85 percent gold by weight. They next dried the solid and weighed it. Since this solid is 85 percent gold, the coin contained 85 percent of the total weight of the solid, 0.59 gram in this case, of gold, or about half a gram. The coin is 50 percent pure.

How much gold is in the coin? Coin weighs 1.0g

How much gold is in the coin? Coin weighs 1.0g

Dry and weigh It

0.59g AuCI 85% by weight gold

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Sample containing three components A, B, and C

Filter paper (top view)

Filter paper (side view)

Filter paper (top view)

Filter paper (side view)

Sample containing three components A, B, and C

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Filter paper (top view)

Filter paper (side view)

Filter paper (top view)

Filter paper (side view)

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Spot tests. A drop of the dissolved sample is applied to a piece of filter paper and immediately diffuses outward. In this example there are three components in the mixture, and each moves at a different speed. When the analysis is complete, the three components are separated by distance traveled.

18 drugs, poisons, and chemistry to perform separations based on dissolution using wet chemistry, advances quickly followed in both qualitative and quantitative analytical chemistry.

An important test for modern forensic chemistry, spot testing, appeared in the 1800s. To perform a spot test the chemist dissolves a sample that might contain several compounds in a water solution. A tiny drop is applied to the center of a piece of paper so that the solution is concentrated at this one small point. The sample diffuses outward with the individual components in the sample moving at different speeds. As a result the components also travel different distances. The distance depends on the compound and how much it interacted with the paper matrix; the more it interacts, the slower it moves. A spot test can be modified by impregnating the filter paper with chemical reagents that would react selectively with different components in the sample. This approach of separation followed by detection is still used in modern separations. Spot testing reached a peak in the early 1900s when several books were published on the subject.

As instrument methods (discussed later in the chapter) were developed, the role of simple spot tests changed. In modern forensic chemistry labs the analysts initially use simple qualitative tests to narrow down the list of substances that might be present in evidence. Many of these qualitative tests are based on the formation of colored compounds and are often referred to as color tests. Many chemists still call simple qualitative tests "spot tests" even though the methods differ from earlier versions. Genuine spot tests survive in the modern lab in the form of chromatography, particularly a technique called thin-layer chromatog-raphy, which is discussed in a later section.

chemistry of color

One way chemists know that a chemical reaction has occurred is by seeing a change in color. Forensic chemists use color change tests (color tests) to determine what an unknown sample may contain. A color test is usually performed by placing a small amount of the sample into a well on a plate or into a test tube. Special chemical solutions are added, and any change in color is noted. A change in color indicates that a change has occurred in the molecules that make up the sample.

The organic molecule carotene, which imparts an orange color to carrots. The color arises from the way light interacts with the alternating double bonds between carbon atoms.

Broadly speaking, there are two types of chemicals that produce color. Organic compounds that contain carbon and hydrogen molecules can be colored if they also have a pattern of alternating double bonds between carbons. Most dyes are based on this type of structure. When a color test reagent is added to a sample containing drug molecules, chemical changes can result in the formation of compounds that contain alternating double bonds. If this happens, a color develops or an existing color changes.

The other way a color can be produced is by a unique chemical structure called a complex. Complexes form between ions of metals such as cobalt (Co) and compounds that have nitrogen and oxygen atoms. Many drugs have nitrogens in their structure and can form colored complexes with metal ions. The color test used for cocaine and related drugs is based on cobalt, which forms a deep blue complex with cocaine. Color change reactions like this are used in many aspects of forensic chemistry.

If blood and death belonged to the English and the Americans, forensic chemistry belonged to the Germans and the Austrians. Much of this heritage is unappreciated outside Europe, given language barriers and the lack of English translations of many pioneering papers. Once the Stas-Otto method had provided the means to extract poisons from tissues, the issue of which poison was present remained. This led to color-based tests that were used to tease out likely identities. The color produced in most tests is the result of dye formation. Many of the

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© Infobase Publishing earliest tests developed to screen for certain drugs and toxins carry the name of the people who developed them, including Marquis (a student of the German chemist Kobert), Dragendorff, Mecke, Ehrlich, Frohde, Liebermann, and Zwikker. Most of these tests were described in publications from 1870 to 1905. Forensic chemists and law-enforcement officers still use these tests today. In the 1960s and the 1970s German chemists published articles describing how the color tests worked on a molecular level.

This litany of names shows how German chemists excelled in two areas intimately related to forensic science—dyes and drugs. There is a neat symmetry here; the color tests mentioned earlier work, in many cases, by forming a dye; many drugs were made accidentally while the chemists were busy trying to make dyes, and vice versa. Explosives manufacturing closely connected to both. It might seem that drugs and dyes have little in common with each other, but chemically, these families share common roots. Drugs are of obvious forensic importance, but the role of dyes is less so.

Dyes, along with pigments, are colorants that impart color to the substrate to which they are applied. Solubility dictates the distinction between them; dyes dissolve in solution like food coloring in water, while pigments form a suspension in solution and dry as a coating on a surface. The chemical structure alone does not automatically dictate which is which, because a change in solvent can change solubility. The forensic importance of dyes and pigments, particularly from a chemical point of view, is a recent development that traces back to the introduction of materials that use them. Pigments are widely used in paints and inks and as colorants for fabrics and fibers. It is this last application that spurred the chemical developments that would have the greatest impact on forensic science.

The mention of dyes appeared during 3000-2000 b.c.e. The dye compounds came from extracts of plants, leaves, and other colored materials. The ancients derived pigments for paints by grinding up colored minerals and suspending them in water or other slurry materials. The Chinese and the Egyptians made inks and paints using charcoal suspended in oils or animal fats. Along with medicines and metallurgy, interest in colorants was a primary driver of early chemistry. Once beyond simple extracts and slurries, progress in colorant chemistry had to wait for techniques of chemical synthesis to evolve, a process that picked up dramatic speed in the late 1800s.

Many naturally derived drugs share chemical similarities with naturally derived dyes. The alkaloids, for example, a class of drugs that include heroin, morphine, and cocaine as well as thousands of others, are based on a structure called a tertiary amine. Indigo, the dye used to make blue jeans blue, also contains tertiary amine groups. The first synthetic dye created, mauve, was an accidental by-product of an attempt to make quinine to treat malaria. It is not surprising, then, that the chemical histories of drugs and dyes run parallel. Before the advent of synthetic organic chemistry, drugs and dyes could only be obtained from natural sources, such as extracts, teas, and mineral preparations. Chemists lacked that knowledge and tools to create new molecules and, thus, new drugs from precursors.

Drug chemistry diverges from that of dyes in an important and obvious way: People can abuse drugs. Pinning down the definition of abused drugs begs a definition of abuse. This is a social rather than a scientific question, the answer to which depends on the circumstance and the place in history. The Sumerians and Egyptians were excellent brewers, with the former showing a penchant for brewing beer, while the Egyptians were particularly adept at wine making. Hair analysis from ancient samples from Peru revealed the presence of metabolites of cocaine, which supports the notion that chewing coca leaves began in South America around 2000 b.c.e., if not earlier. Marijuana was mentioned in Chinese texts from the third century b.c.e., and because the marijuana (hemp) plant is hardy (i.e., a weed), it grows almost anywhere. It is not surprising that use of its products was widespread in the ancient world. The pursuit of medicines and treatments remained separate from chemistry until Paracelsus and his followers began to bring chemistry into the search for medicines, a search that predictably led to the discovery of drugs of abuse. What was not predictable was that dye chemists would be the ones leading the way.

Of all the colors, the most sought after in ancient times were the blues and the purples. Indigo, one of the earliest blue dyes comes from the Indigofera tinctoria plant. The leaves and their extracts are not blue and require chemical processing for the blue color to emerge. The extract is first soaked in a basic solution and then oxidized by exposure to air. Likely, this was an accidental discovery, but the realization that colors could be coaxed from colorless materials was a chemically important step, even if ancient dye makers did not understand the fundamental chemistry they were practicing.

Chemists exploited indigo for analytical purposes in the 1800s. A reagent consisting of indigo dissolved in sulfuric acid. If the sample contained nitric acid, the indigo was bleached from the characteristic blue to a clear solution. The test was used forensically in the early 1800s until William Brooke O'Shaughnessy (1809-89), published his first paper in the medical journal The Lancet in 1830, describing the shortcomings of the test. This was important because some poisoners used nitric acid. O'Shaughnessy, who was 21 when he wrote the report, pointed out that other compounds besides nitric acid reacted with indigo. In noting this, he was contradicting some of the early English forensic scientists of note, including Robert Christison, who had a medical degree from the University of Edinburgh. It was a courageous action for such a young chemist.

Not satisfied with debunking the existing method, O'Shaughnessy described three tests that were specific for nitric acid: first, nitric acid would turn orange in the presence of morphine (forming another dye); second, it would form a solid when added to urea nitrate; and third, it would facilitate formation of silver fulminate. Detection of the latter was simple, obvious, and hazardous. In publishing the paper, O'Shaughnessy was following Christison's advice to forensic chemists that it was the job of the analyst to provide more than some evidence but to provide the best evidence possible given the limits of scientific knowledge of the time. For a short period, indigo dye and other tests represented those limits.

Another famous ancient dye was royal purple, also called Tyrian purple for the coastal city where its manufacture was centralized. Tyre, once part of the ancient Phoenician Empire, is on the coast of Lebanon, a city build on mounds of mollusk shells used in the production of the dye. The process of making Tyrian purple started by gathering mollusks and extracting their glands. Next, the extracted material oxidized to form

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