Cyanoacrylate Fuming, Alternate Light and Fluorescent Stains

Ethyl cyanoacrylate (ECA), the compound in most consumer Super Glue formulations and forensic CA products, is a liquid monomer with a strong electron-withdrawing nitrile and ester group on the same carbon. These groups make the adjacent double bond highly reactive to nucleophilic attack. Any nucleophile, including water, an amine, or a carboxylate group, initiates anionic polymerisation of the monomer chain. At room temperature in dry air, ECA is reasonably stable. On a surface presenting moisture and amine groups, such as friction-ridge sweat residue, polymerisation initiates within seconds.

The selectivity of CA fuming for print residue over the surrounding surface relies on two factors:

1. The concentration of initiating groups is higher over the print residue than over the clean substrate.
2. Under humidity-controlled conditions (50 to 80 per cent relative humidity), the moisture film on the clean surface is insufficient to initiate rapid polymerisation, while the moisture and amino-acid content of the print residue provides an effective initiating environment.

If humidity is too low (below about 40 per cent), polymerisation is slow and the white polymer deposit is thin and poorly resolved. If humidity is too high (above about 80 per cent), polymerisation initiates across the whole surface, producing background haze that obscures ridge detail.

Temperature affects the rate of vaporisation of the CA monomer. At room temperature (~20°C) with a moderate amount of CA in the fuming chamber, passive fuming from the liquid surface provides sufficient vapour concentration. Accelerated chemical fuming wands are handheld devices that generate a controlled burst of CA vapour by passing the liquid over a chemically activated surface or a small heating element, allowing the examiner to process large non-removable objects at the crime scene in seconds to minutes rather than transporting the object to a fixed laboratory chamber.

Commercial forensic-grade CA products (Sirchie Cyanoacrylate Fingerprint Developer, Foster + Freeman PolyCyano UV cyanoacrylate, Bvda CA fuming fluid) use ethyl or methyl cyanoacrylate and are formulated to be free of the antioxidants and plasticisers added to consumer adhesives, which can inhibit polymerisation or leave background residue. The UK Home Office CAST and the Australian Federal Police Forensic Services both specify forensic-grade CA in their SOPs.

Fixed fuming chambers are the standard for laboratory processing of removable evidence. Modern chambers from Foster + Freeman (the VMD-ECA system), Sirchie (the SuperFume chamber series), and Bvda (the CA Chamber range) control humidity through a water tray inside the chamber and an optional heated base, with a digital humidity sensor and display. Typical operating parameters are: 10 to 20 ml of forensic CA liquid, 50 to 80 per cent relative humidity, 20 to 30 minutes exposure time, followed by visual inspection and repeat fuming if insufficient polymer deposition is observed. The chamber volume matters for calculating the CA loading. Most commercial chambers between 20 and 200 litres have manufacturer-recommended CA volumes that produce optimal results for their internal surface area.

Where evidence cannot be removed from the scene, accelerated fuming wands are the method of choice. The Mini-Fuming Wand (Sirchie) and the PolyWand (Foster + Freeman) combine a small CA liquid reservoir with an aluminium oxide catalytic surface or a low-temperature heating element; drawing air through the device vaporises the CA and directs vapour onto the surface through a nozzle. A polythene tent, a plastic bag sealed around a car interior, or sheeting over a flat surface creates a temporary enclosure in which vapour concentrates. Field kits for CA fuming at scene are standard equipment in UK SOCO (Scene of Crime Officer) bags, FBI Evidence Response Team kits, and CFSL field investigation kits.

The evidence item must not have been pre-treated with powder before CA fuming if possible. Aluminium powder residue can interfere with CA polymerisation by providing non-specific nucleation sites, increasing background white haze. If powder has already been applied at scene before the decision to fume was made, CA fuming can still proceed. The examiner should note the prior powder treatment in the exhibit record and expect some background artefact.

Post-fuming fluorescent dye stains bind to the white poly(ethyl cyanoacrylate) polymer and are examined under an ALS at the dye's excitation wavelength. The dye introduces spectrally selective fluorescence into the polymer while the background substrate typically has lower fluorescence at the same wavelength, increasing ridge-to-background contrast on coloured, patterned, or light-coloured surfaces.

Rhodamine 6G is a xanthene dye with an excitation peak at approximately 525 nm and an emission peak at approximately 555 nm. It is dissolved in an acetone or methanol carrier at concentrations of 0.025 to 0.1 per cent and applied by dipping, spraying, or swabbing the fumed surface. A rinse in clean methanol or acetone removes excess dye from the background and leaves the dye concentrated in the polymer. Under ALS illumination at 530 nm (green) with a matching orange barrier filter, Rhodamine 6G-stained fumed prints produce bright orange fluorescence. Rhodamine 6G is effective on surfaces of all colours but produces highest contrast on dark substrates where the white CA polymer alone is poorly visible. It is the most commonly used post-CA dye stain in UK, Australian, and North American latent print laboratories.

RAM (Rhodamine, Ardox, and MBD) is a multi-dye mixture formulation developed to improve performance across a wider range of substrate types than any single dye. The three components cover overlapping excitation windows, giving the combined stain excitation response from 400 nm to beyond 550 nm. RAM is available from Sirchie as a ready-to-use solution and is used in FBI Latent Print Operations Unit SOPs as a post-CA dye option.

Basic Yellow 40 (BY40) is a fluorescent dye with excitation near 450 to 490 nm (blue) and emission at approximately 530 to 540 nm. Its main advantage is that it can be used immediately after CA fuming by dipping the fumed item into the BY40 solution followed by a water rinse, making it a rapid workflow for items processed in the laboratory. The Netherlands Forensic Institute and several northern European national laboratories use BY40 extensively.

Ardrox (a commercial formulation from Chemetall, originally developed for aircraft-skin crack detection under UV) contains a mixture of fluorescent optical brightener compounds that fluoresce broadly under UV (365 nm) excitation and emit in the blue-to-green region. Applied by dipping or spraying after CA fuming, Ardrox-stained prints are visualised under a UV source. The advantage of Ardrox is that many substrate backgrounds do not fluoresce strongly under UV, giving good contrast even on light-coloured surfaces. It is widely used in the UK and Australia but less common in the US, where Rhodamine 6G is the dominant post-CA dye.

Lumicyano (Foster + Freeman) is a proprietary formulation that combines a fluorescent cyanoacrylate (modified CA with a fluorophore group incorporated into the monomer) with a standard CA developer, allowing simultaneous development and fluorescent marking in a single fuming step. Lumicyano-fumed prints emit fluorescence without a separate dye-staining step, simplifying the workflow for high-throughput environments. Validated for operational use at UK Home Office CAST, Swiss Federal Criminal Police, and Australian Federal Police Forensic Services.

An alternate light source (ALS) in forensic fingerprint work is any instrument that delivers high-intensity, narrow-bandwidth illumination at a controlled wavelength. The illumination may excite fluorescence in the print residue (inherent fluorescence), in a dye stain applied to the fumed print, or in a fluorescent powder applied during development. The matched barrier (or rejection) filter, worn by the examiner as goggles and placed over the camera lens, blocks the excitation wavelength and passes only the emitted fluorescence, producing the dark-background contrast that makes fluorescent examination effective.

Three wavelength regions dominate forensic fingerprint ALS practice:

Commercial ALS instruments used across global forensic laboratories include:

• Foster + Freeman Crime-lite series (Crime-lite 82S, Crime-lite MINI): 450, 490, 530, and 590 nm standard bands with a UV option at 365 nm.
• Rofin Polilight (Omnichrome/Rofin, now Foster + Freeman): continuously tunable wavelength from 400 to 700 nm.
• Sirchie Mini-CrimeScope (MCS-400): broad-spectrum source used widely in North American law enforcement.

UV sources at 365 nm (commonly called Wood's light or long-wave UV) are useful for detecting Ardrox-stained prints, for scanning documents for invisible security features, and for detecting fluorescent trace evidence including bodily fluids and optical brighteners. At 365 nm, many common organic residues show characteristic fluorescence that allows rapid scene screening. Standard UV-blocking eye protection is mandatory for all 365 nm work. Direct ocular exposure to 365 nm UV at close range risks corneal and retinal damage.

Inherent fluorescence refers to the luminescent signal produced by latent print residue itself under ALS illumination, without any development agent applied. The eccrine and sebaceous composition of that residue determines the strength and spectral range of inherent fluorescence. The fluorescent components in sebaceous print residue include squalene (excitation around 330 nm, emission around 420 nm), certain fatty acids, and terpene-derived compounds that vary with diet, cosmetics, and body chemistry. Eccrine residue contributes little inherent fluorescence, so inherent fluorescence is typically stronger in prints from individuals with higher sebaceous output and in prints on surfaces that have been in contact with the palm or fingertips repeatedly.

Practical inherent fluorescence examination is part of the standard examination sequence before any treatment. The examiner places the item in a darkened examination room or uses a light-shield hood, illuminates it with the ALS at multiple wavelengths (typically starting at 450 nm and stepping through to 530 nm), and photographs any fluorescent prints observed. On substrates that themselves fluoresce strongly (some types of white paper, certain polymer surfaces, fluorescent-dyed textiles), the background fluorescence may equal or exceed the print fluorescence, making inherent detection impractical. On glass, polished metal, and dark polymers, inherent fluorescence detection is often effective.

The no-treatment workflow serves two functions: it avoids destroying the inherent fluorescent signal before it is documented, and it avoids unnecessary chemical or physical treatment of items that may be needed for other forensic examinations such as DNA sampling from sebaceous residue or ink chemistry from a document substrate. Any item targeted for inherent fluorescence detection must be examined before any treatment that could alter, contaminate, or consume the surface residue.

In the UK, the sequential processing guide from CAST recommends ALS examination at multiple wavelengths as step one in the examination sequence, before powder application, before CA fuming, and before any chemical processing. Australian ANZPAA NIFS and FBI Latent Print Technical Reference protocols both include inherent fluorescence screening as the opening step.

Photographing fluorescent latent prints under ALS requires a camera configuration distinct from conventional oblique-light macro photography. The camera must capture emitted fluorescence while rejecting the reflected excitation light, which requires placing the same matched barrier filter used as examiner goggles in front of the camera lens.

The photography protocol follows a consistent structure across jurisdictions:

• Camera: a full-frame or APS-C DSLR or mirrorless camera. Canon EOS 5D series, Nikon D850, and Sony Alpha series are common in UK, US, Australian, and Indian forensic laboratories.
• Lens: a true macro lens capable of 1:1 reproduction at minimum focus distance. Canon EF 100mm f/2.8L Macro and Nikon AF-S Micro-Nikkor 105mm f/2.8G are standard references; equivalent Sigma and Tamron macro lenses are also used.
• Aperture: f/8 to f/16 for sufficient depth of field across the print plane. Fluorescence intensity diminishes at very small apertures because exposure time must increase.
• Shutter speed: variable, often 1/15 to 2 seconds for fluorescence photography in a darkened room.
• ISO: 400 to 1600 depending on fluorescence intensity.

The barrier filter over the lens must be secured and verified before the exposure sequence; partial displacement allows excitation light into the frame and degrades the image. For Rhodamine 6G staining, an orange barrier filter (Wratten 21 or equivalent, or a Schott OG570 gelatin filter) is standard. For Ardrox under UV, a yellow barrier filter (Wratten 2B or equivalent) is used.

The scale marker in the frame must be present in all latent print photographs submitted as evidence. The ABFO No. 2 scale (with circular reference spots and millimetre markings at right angles) is the most widely used in US laboratories. The UK fingerprint photography standard (College of Policing APPro, Crime Scene Photography) specifies a metric ruler with minimum 1 cm divisions and a case reference card in the field of view of every fingerprint photograph.

1. Inherent fluorescence screen
   In a darkened room, illuminate the item with ALS at 450 nm, 530 nm, and 365 nm sequentially. Wear matched barrier goggles. Photograph any fluorescent marks with the matched barrier filter on the camera lens. Document the result before any treatment.
2. White light examination and oblique lighting
   Examine the item under white oblique light to detect any prints visible without development. Photograph with scale marker at 1:1. These photographs are the baseline before any development agent is applied.
3. CA fuming in humidity-controlled chamber
   Load the item into the CA chamber. Set humidity to 50-80%. Add forensic-grade CA (ethyl or methyl cyanoacrylate; 10-20 ml per 100-litre chamber volume). Seal and fume for 20-30 minutes. Remove and inspect under oblique white light.
4. Photograph white CA polymer development
   Photograph all visible fumed prints under oblique white light at 1:1 before any dye staining. This is the primary evidence record of the fumed print quality.
5. Apply fluorescent dye stain if required
   If background colour or pattern obscures the white polymer: dip or spray the item with the appropriate dye stain (Rhodamine 6G, BY40, Ardrox, or RAM). Rinse with clean solvent to remove excess. Allow to dry.
6. ALS examination and photography of dye-stained prints
   Illuminate with ALS at the excitation wavelength of the dye. Wear matched barrier goggles. Place matched barrier filter on camera lens. Photograph each developed print at 1:1 with scale marker and case reference card in frame. Record ALS wavelength, barrier filter type, aperture, ISO, and shutter speed in the photographic log.