The Case of Mary Blandy


An important test used in early toxicology was developed in 1836 by chemist James Marsh (1794-1846), nearly 80 years after the Mary Blandy case. The Blandy case marked the beginning of a wider reliance on and demand for trustworthy scientific tests for detection of poisons. Shortly after Blandy's execution Swedish chemist Carl Wilhelm Scheele (1742-86) developed a postmortem test for arsenic. Scheele was an apothecary's assistant and an excellent technician who discovered the element chlorine. His arsenic test involved heating arsenic powder (As2O3) placed in a solution containing metallic zinc and nitric acid. The arsenic formed the arsine gas AsH3 that smelled of garlic, a test that was used in many cases, including that of Blandy. Other chemists improved and enhanced this test and attempted to extend it to use on human tissues and stomach contents. These early methods were difficult, required great skill, and could not detect relatively low levels of arsenic. A famous case revealed this limitation to light and brought English chemist Marsh into the story.

In 1832 John Bodle had been accused of poisoning his grandfather with arsenic placed in coffee. Marsh was asked by the prosecution to check the stomach contents of the victim. He used a hydrogen sulfide method and was able to produce a yellow solid consistent with the presence of arsenic. Unfortunately, the solid degraded between the time it was prepared and when it was presented to a jury. To Marsh's dismay Bodle was acquitted. To add insult to injury Bodle later bragged about his guilt. Marsh took his anger and frustration and disappeared into his lab with one simple goal: develop a reliable and visually convincing method to detect arsenic in messy and complex samples like tissue and stomach contents.

First, he turned to Scheele's procedure in which arsenic was converted to arsine gas. Marsh knew that under the proper conditions compounds containing arsenic, such as arsine, could be manipulated to form arsenic metal. Magnus had demonstrated that conversion centuries earlier. Marsh realized that metallic arsenic is stable, and if he could capture the arsine gas, he could manipulate it so that metallic arsenic would form on a solid surface. This process is sometimes called "plating out." This simple idea took Marsh four years to perfect, and the method became known as the Marsh test. This famous procedure was the first reliable

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The Marsh test for arsenic. The sample, such as stomach contents, is heated in the presence of zinc metal and acid. The arsine gas rises into the glass tube. With more heating arsenic metal forms a metallic mirror on the tube.

analytical test for arsenic. For his efforts Marsh received wide acclaim and a gold medal from the Royal Society of Arts.

The Marsh test starts by placing the sample, be it a powder, stomach contents, or body tissue, in solution by adding hydrochloric acid. Shavings of metallic zinc are included in the mixture, which is then heated. Under these conditions arsenic forms arsine gas (H3As) that rises away from solution. The innovation Marsh added was to trap this gas and direct it through a tube. The gas is heated again, causing the arsine gas to decompose to metallic arsenic, which plates out a gray coating on glass or on a piece of porcelain. Unlike the arsenic compound formed with hydrogen sulfide gas, the metallic "mirror" is stable and makes ideal visual evidence for display before a jury. One critical feature of the Marsh test was that it was fairly reliable when used on biological samples.

The Marsh test came to the public's attention during the case of Marie Lafarge (1840) and was particularly notable because of who used it, a man named M. J. B. Orfila (1787-1853). Owing to his skills in forensic toxicology and his role in this case, Orfila is often referred to as the father of forensic toxicology.

The Lafarge case revolved around the death of Marie's husband, Charles, who died of an illness that was consistent with arsenic poisoning. Initial tests of his stomach showed no arsenic, nor did other organs removed from the body. However, arsenic was detected in food and beverages that Charles had been given. The court asked Orfila, a recognized expert in forensic toxicology, to sort out the analytical results. He and other experts reviewed and reanalyzed the samples, finding small amounts of arsenic in the remains of Charles Lafarge. Orfila, a consummate chemist, also showed that the arsenic could not be attributed to the soil in which the body had been buried or to contaminated reagents. To this day such awareness of possible contamination and the use of control samples are an integral part of forensic toxicology. The Lafarge case was pivotal for forensic chemistry, forensic toxicology, and forensic science. It marked the first time that modern chemical testing was used, accepted, and instrumental in obtaining a conviction in a murder case. From that moment poisoning was no longer easy to mistake for other causes of death.

Orfila's testimony and the acceptance of the Marsh test by the scientific and legal communities did not end poisoning, nor did it help prevent accidental exposures to arsenic. As is often the case, the scientific advance it highlighted provided great assistance to law enforcement but also forced those who would break the law to alter their tactics. The Marsh test made arsenic poisoning a less attractive means of murder, so murderers moved on to other poisons, such as those obtained from plants.

The most common plant-based poisons are chemically classified as alkaloids. These substances were originally called "vegetable bases" as they were extracted from plants and the extracts were found to be basic, or alkaline. A basic substance tastes bitter and when dissolved in water will cause the pH of a solution to rise above 7.0. A pH of 7 is neutral, 0-6 acidic, and above 7, basic. Examples of alkaloids include nicotine,

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  • virpi
    Why is mary blandy case important in forensic science?
    7 years ago
  • Kaija S
    How the case of mary blnady impacted chemistry?
    2 months ago

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