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Luminol produces blue chemiluminescence and fluorescein produces yellow-green fluorescence when activated by trace haemoglobin, enabling investigators to locate bloodstains that are invisible to the naked eye, including stains that have been cleaned or diluted.
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A cleaned floor is not a blank slate. Scrubbing with detergent, mopping with bleach, even pressure-washing: none of these reliably eliminates haemoglobin from a porous surface. The haem molecule is tenacious, binding to fibres and mineral surfaces even when the visible red-brown colour is long gone. Luminol exploits this tenacity. Applied to a darkened room, it reacts with residual haemoglobin and glows blue, mapping the original distribution of blood with a precision that no UV lamp or oblique light can match.
Fluorescein works differently in mechanism but serves an overlapping purpose. Instead of generating its own light, it absorbs blue-green excitation light and re-emits it as yellow-green fluorescence. It requires a UV or blue-green light source and an orange barrier filter, which makes field use more equipment-intensive than luminol. In return it is generally regarded as less damaging to downstream DNA, which matters when a scene examination is also a potential DNA recovery exercise.
This topic covers both reagents together because scene examiners often choose between them based on the same set of practical factors: the type of substrate, what cleaning agents have been used, whether DNA recovery is a priority, and how large an area needs to be searched. Understanding the chemistry of both, their detection limits, their false-positive profiles, and their photographic documentation requirements is essential for anyone working on scenes where blood has been deliberately or incidentally obscured.
The chemistry that turns iron into light, in the dark, at a dilution of one in ten million.
Luminol (5-amino-2,3-dihydrophthalazine-1,4-dione) is an organic compound that reacts with haemoglobin in an alkaline solution containing hydrogen peroxide. The mechanism proceeds through several steps. In the alkaline spray solution, luminol is oxidised to a dianion. This reacts with molecular oxygen generated by the haem-catalysed decomposition of hydrogen peroxide, forming a high-energy peroxide intermediate that decomposes to the 3-aminophthalate dianion in an electronically excited state.
The excited 3-aminophthalate dianion drops to its ground state by emitting a photon at approximately 425-435 nm, in the blue region of the visible spectrum. This emission is bright enough to see in a darkened room with the naked eye and to photograph with a camera on a long exposure. Because the light is generated by the reaction itself, no excitation lamp is needed, which simplifies fieldwork in large spaces such as halls, stairwells, and outdoor scenes.
The detection sensitivity is exceptional. Published values range from blood diluted to 1:10,000,000 or greater under laboratory conditions. In practice, on scene substrates, achievable sensitivity is typically 1:100,000 to 1:1,000,000, still far in excess of any colour test. This is why luminol finds blood that has been repeatedly cleaned: residual haemoglobin at concentrations far below visual threshold is enough to produce a detectable glow.
Same targets, different physics: fluorescein needs light to give light back.
Fluorescein works through a completely different physical process. The molecule is itself a fluorophore: it absorbs photons at around 490-510 nm (blue-green light) and emits them at around 520-530 nm (yellow-green light). The Stokes shift between excitation and emission means that with the correct barrier filter, the background illumination (excitation wavelength) is blocked and only the fluorescent emission (from haemoglobin-activated fluorescein) reaches the eye or camera sensor.
The detection chemistry is similar to luminol's: fluorescein is reduced to a colourless leuco form before use and applied in an alkaline hydrogen-peroxide spray. Haemoglobin catalyses the oxidation of leuco-fluorescein back to the fluorescent form. The critical difference is that fluorescein itself does not emit light spontaneously once activated. It only fluoresces while the excitation source is on, which means a dedicated light source and barrier filter are needed but also means the reaction can be re-examined repeatedly without time pressure.
The real power of both reagents is what they find that nothing else does.
Both reagents are delivered as dilute spray solutions. The practical workflow for a luminol or fluorescein search of a room or vehicle begins with preparation: the space must be darkened (luminol) or equipped with a blue-green light source and orange barrier filters (fluorescein). Photography equipment is set up before spraying because re-spraying degrades the substrate and may destroy DNA.
The reagent that finds blood must not destroy the profile inside it.
The DNA damage question is one of the most practically important in the use of both reagents. Early studies reported significant degradation of nuclear DNA by luminol, reducing the probability of obtaining a full short tandem repeat (STR) profile. Later work was more nuanced: dilute luminol formulations, short contact times, and collection of material from treated stains within a short window still yield usable profiles in many cases.
| Factor | Luminol | Fluorescein |
|---|---|---|
| DNA degradation (published evidence) | Moderate; concentration and time-dependent | Lower; generally more compatible with STR profiling |
| Profile success rate after treatment | Variable; dilute formulas improve outcome | Generally higher |
| Re-examination possible | No; glow fades and re-spraying increases damage | Yes; fluorescence resumes when excitation source is reactivated |
| Mitochondrial DNA | Less affected than nuclear DNA in both cases | Less affected than nuclear DNA in both cases |
| Recommended practice | Single careful spray; collect promptly | Single spray; re-examine as needed |
The operational guidance in most accredited forensic laboratories is to use the minimum effective concentration and volume, to collect DNA samples from luminol-treated areas within a short time after treatment, and to prioritise fluorescein on scenes where multiple small stains each need to yield a DNA profile. A luminol search followed by DNA collection is a valid and frequently used strategy; the decision depends on whether the scene examination goal is pattern detection (luminol is better), profile recovery (fluorescein is safer), or both.
Not everything that glows is blood, and not everything that glows from bleach is a false alarm.
Both luminol and fluorescein respond to any substance that catalyses the decomposition of hydrogen peroxide in their alkaline spray solution. This is a broader set of substances than for the Kastle-Meyer and LMG colour tests, partly because the detection is so sensitive that trace contaminants become relevant.
Neither reagent alone is confirmatory. A positive luminol or fluorescein area is followed up with direct collection and either a colour presumptive test (Kastle-Meyer, LMG) or an immunological species-specific test on the collected material. The luminescence or fluorescence maps the distribution; the follow-up tests identify the substance.
If it is not photographed before the glow fades, it did not happen.
Photographic documentation of luminol results is non-optional and requires preparation before any reagent is applied. The glow from a luminol-positive stain is faint and short-lived, typically 30-60 seconds for a strong result, less for a dilute stain. A camera left on auto-exposure in a darkened room will not capture it reliably.
What physical phenomenon produces the glow in a luminol test?
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