We’ve all heard the story of Hansel and Gretel- kids abandoned in the woods encounter a delectable house made of candy, meet an evil witch in the candy house, and get captured and force- fed sweets by the said cannibalistic witch so that she can inevitably gobble up the children. Luckily, the protagonists in this classic fable survive when they push the witch into the very oven she was planning to cook them in, and it’s a happily ever after for everyone (except the witch).
In our case, though, there is no convenient oven to trap the witch in- or rather, the criminal mastermind. Indeed, we have chosen to analyze a scene from the modern- day British television crime drama, Sherlock. A contemporary adaptation of Sir Arthur Conan Doyle’s classic detective stories, the show integrates modern forensic techniques to showcase Sherlock’s incredible skills of deduction in a more realistic light. But what does this have anything to do with a classic German fairy tale?
In a scene from the episode, The Reichenbach Fall, we see Sherlock investigating a case of children gone missing- vanished from their beds. Although seemingly stumped at first, he soon realizes that one of the kidnapped children, having a predilection for spy books, was clever enough to leave behind a trail of linseed oil, which supposedly exhibits fluorescence under the presence of ultraviolet light. Sherlock then takes a scraping of the footprints the kidnapper left by stepping in the spilled oil and analyzes the traces left behind in the oil to determine his precise location. The examination leads him directly to a chocolate factory, where the children had been driven dangerously ill by eating copious amounts of candy and were on the verge of dying in the arson of the factory. Sound familiar?
The kidnapper is the “consulting criminal” Moriarty, a man who acts as a consultant to commit crimes for his clients. He is also Sherlock’s primary rival in the show and the antithesis of Sherlock’s “consulting detective” position. He is clearly establishing the Hansel and Gretel motif here intentionally, being that the kidnapped siblings were German, left behind by their parents over term break and taken to a place where they nearly meet their doom through burning. It’s undoubtedly a relief that the ingenuity of the child paired with Sherlock’s observance saved the victims, but the modernistic juxtaposition of the series makes us consider the unavoidable question: what if this hadn’t been just a plot event in a fictional series? What if it were real? Simply shining a special light over the crime scene to reveal the answer seems a tad too easy to be true. Does linseed oil indeed show itself under UV light? If so, can this behavior be explained chemically?
As it turns out, linseed oil is a triglyceride capable of fluorescence after degradation when exposed to UV light. Derived from the seeds of a flax plant, linseed oil is fatty oil that contains polyunsaturated fatty acids. It is commonly called a drying oil, because when exposed to oxygen in the air, it undergoes a process known as polymerization, or drying. This is partly due to its composition – it contains an unusually high amount of alpha-linolenic acid as well as di- and triunsaturated esters, which makes linseed oil inclined to polymerize when brought into contact with atmospheric oxygen. This autoxidation, the exothermic addition of oxygen to an organic compound, causes subsequent crosslinking and the curing of oils. An atmospheric oxygen molecule in the air immediately pounces on the long hydrocarbon chains of the oil’s fatty acids, inserting itself into carbon- hydrogen bonds adjacent to one of the double bonds within the unsaturated fatty acid. This causes a domino effect in which a number of addition reactions, organic reactions in which two or more molecules combine to form a larger one, occurs in rapid succession. This in turn forms hydroperoxides that are susceptible to crosslinking reactions. A vast polymer network forms as a result of the bonds between neighboring fatty acid chains, and is visible as a skin-like film formation on samples. The polymer network may undergo further change over time; it can transition from a system held together by nonpolar covalent bonds to one governed by ionic forces between the functional groups as well as the metal ions present in the paint pigment. This change is primarily due to the functional groups in the network becoming ionized. Moreover, the eventual polymerization process results in rigid, somewhat elastic films that do not readily flow or deform. The early stages of polymerization can be monitored by changes in weight; linseed oil in particular increases in weight by 17 percent as the reaction occurs. With regard to linseed oil, its structure becomes a huge, chain-like polymer network of molecules, as shown below in Figure 2. As a result of this process, free radicals are produced. Free radicals are substances containing an unpaired electron, which makes it highly reactive. More addition reactions ensue, each step producing additional free radicals. They continue to engage in polymerization until all the free radicals have collided and their unpaired reactions have been combined to form a new bond, at which point the reaction halts. The polymerisation stage occurs over a period of days to weeks, rendering the film dry to the touch.
Fluorescence is generally characterized by the presence of aromatic compounds and their delocalized electrons. In organic chemistry, aromaticity is a property of conjugated cycloalkenes whose p-orbital electrons are delocalized and provide a framework for a stable and planar molecule. Typically, fluorescent compounds will be highly conjugated. In addition, it is well established that these fluoresce when exposed to ultraviolet light as shown in Sherlock. When a vegetable oil, such as linseed oil, is heated, its fluorescence is initially diminished and indiscernible due to the decomposition products and nitromethane, but as the oil progressively becomes polymerized, its fluorescence increases in intensity. In the episode The Reichenbach Fall, Sherlock did not heat the linseed oil, but instead the oil simply revealed itself under the presence of UV light.
Figure 3. When electrons of a substance absorb energy gained from UV light, they are excited to higher energy levels and then fall back to their ground states, emitting energy in the form of light. Fluorescence is light emitted that is in the visible spectrum.
In order to be fluorescent, a molecule must be highly absorbing, and possess the appropriate structure andelectronic energy levels. Fluorescence occurs when electrons absorb energy from the UV light to become excited to a higher energy level. When these electrons come back to the ground state, energy is emitted in a form of a light. Because the light that is emitted usually has a longer wavelength than the energy absorbed, we are able to see the light emitted as fluorescence. This is due to something known as the Stokes’ Shift in which a molecule that absorbed light is now in the excited state releases a bit of the initial energy first before releasing the rest of the energy in the form of a weaker proton to get back to the ground state. Linseed oil is able to exhibit fluorescence because of both the chemical and physical properties that it has. As stated before, linseed oil contains many fatty acids. Some of these fatty acids contain pi bonds which are very good at absorbing and emitting light. This is due to the fact that the energy separation for an isolated pi bond is very large, and so the ultraviolet light with its large energy and short wavelength can excite the electrons in the pi bond. However, linseed oil does not fluoresce brightly because linseed oil does not contain conjugated pi systems (a system in which double bonds alternate with single bonds).
Figure 4. The Stokes’ Shift occurs when an excited molecule releases some of the initial energy before releasing the remainder of the energy in form of a weaker proton to return to the ground state. This allows us to view the light emitted as fluorescence.
We thought this investigation seemed a tad too easy to be 100% chemically accurate, and we were correct- partially, at least. As discussed above, linseed oil does in fact dry into a somewhat elastic film through a process termed polymerization- however, the entire affair can range time- wise from several days to weeks, indicating that by the time Sherlock and Watson had arrived at the scene, the state of the oil most likely would not have sufficed for UV testing. Moreover, linseed oil’s fluorescent properties are significantly less notable than that of conjugated pi system- containing compounds, the light emitted being of longer wavelength and weaker energy than that absorbed. And now for the final kicker- dried linseed oil barely exhibits any fluorescence under UV light than it does as a viscous liquid. The non conjugated system in the oil compound has its double bonds broken when dried, thus notably lessening the already minimal degree to which the substance fluoresces; no question that it certainly would not have glowed with the same quality of brilliant luminescence as it did in the scene. Nonetheless, we applaud the Sherlock forensics team, whoever they are, on their ingenuity on thinking to use a drying oil as a means of tracing a kidnapping to parallel the trail found in Hansel and Gretel. Though it might not have worked in reality, we also know that bending the truth is sometimes necessary on the screen- and this definitely receives full marks for originality!