Glows in the Dark

Think back to the late sixties when kids had blacklights in their rooms to illuminate posters that were treated to "glow in the dark". They were taking advantage of the fact that paper treated with certain chemicals would fluoresce under ultraviolet (UV) light. With much more intense ultraviolet lights, this same principle can be used by an analyst today in a variety of applications. The technique can be used as the first step in problem solving by detecting fluorescent substances to be subsequently sampled for identification by chemical analyses. Alternatively, in many cases, an ultraviolet light inspection alone will be sufficient to examine and document physical evidence. Chemists also take advantage of ultraviolet light when performing sophisticated fluorescence analyses which are beyond the scope of this article.

Let's look first at situations where UV light has been used to isolate a fluorescent substance for the purpose of obtaining a sample. This technique can be applied to compounds which are either naturally fluorescent or contain a fluorescent additive. A good example is the detection of antifreeze splashed in the interior of a vehicle. The major component of antifreeze, glycol, is not fluorescent but additives in the antifreeze produce a "green glow" when exposed to UV light. This is especially useful to isolate antifreeze splashes on dark colored vehicle carpet, a task that is difficult if not impossible under ordinary lighting conditions. It then becomes a relatively easy task to obtain samples for confirmation of the chemical composition, map the location, and/or to photograph the antifreeze distribution in the vehicle. This information is useful in automobile accident reconstruction.

UV light inspection truly shines (pun intended) when the substance being sought after is a clear liquid, small particle, or thin layer that would otherwise be difficult to observe. The figure above is an example of Loctite residue on a metal fitting which could not be seen under ordinary lighting conditions (left side), partly due to rust and dirt that had accumulated on the surface. By comparison, the same fitting under UV light (right side) shows Loctite glowing brilliantly in the center threads. It is then an easy task to sample the fluorescent area for chemical analysis to confirm the identity. We have successfully isolated thin layers of residues and contaminants by taking advantage of the fluorescent properties of a chemical. One example was the detection of silicone which interfered with adhesion between a polymer and the adjoining metal surface. Another involved locating residues that contaminated computer circuit board surfaces, degrading performance of the component. Liquids can also be examined using UV light. A water sample that appeared to be cloudy was found to actually contain fine paper fibers whose brighteners were easily observed using UV.

Being a chemist/surface analyst, my natural inclination is to use UV light for the detection of fluorescent materials which could be sampled for subsequent identification. However, my mechanical engineering colleagues would use this technique in other ways. One of the most recognized is ultraviolet detection of a fluorescent material through the use of a chemical dye marker to penetrate small cracks in metal. A dye is sprayed on the metal surface and allowed to seep into cracks which may not otherwise be noticeable to the naked eye. The metal surface is wiped clean and a UV light is used to observe the dye caught in the crack crevice. This technique is commonly referred to as dye penetrant non-destructive testing (NDT).

In the above example, a fluorescent dye is intentionally placed on the surface to aid in the inspection process. We have also found UV inspections to be useful for detecting a substance whose presence is not only unintentional but also unexpected. Grease and oils are commonly found substances that fluoresce. A steel cable surface that appeared to be clean glowed brightly revealing grease trapped between individual strands. This could permit a clamp to slip along the surface and not hold properly. Observation under UV light could prove useful to document the area traveled by a screw which had been greased sometime in the past. It may be possible to observe subtle wear patterns on lubricated fittings, especially those which are relatively new and do not have any alteration to the metal surface. If the technique is reversed so that the areas of interest are those which do not exhibit fluorescence, one can employ ultraviolet light to look for such things as defects, inclusions, etc. in polymeric material which should be uniformly fluorescent.

To conclude this article, I should mention that while the use of an ultraviolet light is certainly a valuable tool during an inspection, there are some drawbacks. While checking various household chemicals for fluorescent detection, I happened to shine the light on my stovetop that appeared to be clean. I turned the light off.