Abstract: Two new types of cabinets for cyanoacrylate (CA) fuming were constructed and tested to determine whether either would develop better latent prints than a CA fuming cabinet using a heating element. The first type, a humidity cabinet, consisted of a glove box modified to provide precise control of both the CA vaporization temperature and the humidity level in the cabinet. The second cabinet was a vacuum chamber in which the pressure could be controlled over a wide pressure range (from atmospheric pressure down to < 0.1 torr). The operating conditions and fuming methods that gave the best results with various types of substrates were determined for each cabinet. Prints developed with the optimized methods were compared to results obtained using a normal cabinet (i.e., a cabinet with no pressure or humidity control using a single hot plate for CA heating). Thirteen different substrates were tested to determine which cabinet produced the best prints for each type of surface. Particular attention was paid to determining whether either cabinet would lessen the background discoloration often found after performing CA fuming in a normal cabinet. Less background enhances the contrast between the ridges and the substrate, making the ridges easier to analyze and evaluate. It was determined that both the humidity and the vacuum cabinets produced better results (e.g., less background coloring and sharper, clearer ridge detail) on most substrates than prints developed in the normal cabinet.

Introduction

Forensic science is an applied science in a state of constant development, with new techniques being discovered and refined to achieve better, more reliable, and more efficient results. As forensic scientists have looked for better techniques to develop latent fingerprints, cyanoacrylate (CA) fuming has gained notice. CA fuming is one of many ways to develop latent prints and has been in use for many years. Fuming with CA results in the formation of a cyanoacrylate polymer on the fingerprint through the polymerization of cyanoacrylate ester vapors, initiated by anionic components in the fingerprint residue. Once developed, the visibility of the prints can be enhanced using a fluorescent dye stain or fluorescent powder, and a forensic light source [1]. Prints can be developed on most smooth, nonporous (and even some slightly porous) substances, including skin. Nevertheless, newer CA fuming methods are being explored to further improve the development of latent prints. These improvements include expanding the types of substrates that can be successfully fumed and improving the quality of the developed latent prints by reducing the degree of background deposition that is commonly encountered and may obscure the prints. Two new methods being used for CA fuming are a humidity cabinet and a vacuum cabinet.

In a humidity cabinet, the vaporization of CA is carefully controlled by regulating the temperature of the CA heater. This can minimize the chances of overfuming and also prevents the generation of hazardous cyanoacrylate decomposition products produced at high temperatures. These products begin to appear at temperatures above 250 ºF, and when the temperature exceeds 400 ºF, hydrogen cyanide can be produced [2]. The other important feature of this type of cabinet is that CA fuming can be carried out in a controlled humidity environment, because the level of humidity has a large influence on the development of latent prints. Optimization of both the vaporization temperature and the relative humidity of the surrounding atmosphere can therefore produce better print development.

Vacuum cabinets have attracted attention because of reports that fuming at reduced pressure produces prints with very good detail and low levels of background deposition. Moreover, at reduced pressure, the CA vapors diffuse to fill the cabinet much more evenly than is the case with fuming at atmospheric pressure. The vacuum eliminates the need to rely on circulation equipment to ensure that all surfaces are fumed evenly. In fact, at sufficiently low pressure, very little heating is necessary to produce an adequate amount of CA vapor for uniform fuming. All that is required is to provide enough heat to compensate for the cooling produced by the evaporation process initiated by the vacuum.

Both of these techniques have been studied previously with varying results. One consequence has been some reluctance to fully embrace the vacuum CA technique [3]. Because of the reported various benefits associated with these two alternative CA fuming cabinets, the Midwest Forensic Resources Center elected to undertake the construction and testing of both types of cabinets. This experiment served two primary purposes. The first goal was to determine whether either of the new cabinets offered significant benefits over a normal cabinet. The second, more specific goal was to determine which of the three types of cabinets developed the highest quality latent prints for each of thirteen different substrates, ranging from different types of plastic materials to glass and metal.

Cabinet Fabrication

The humidity cabinet constructed for our study is shown in Figure 1. Table 1 describes the parts indicated by numbers in Figure 1b and referred to in the following text. The humidity cabinet is a glove box (10), roughly 36" wide, 24" deep, and 20" high, with a side antechamber (17), to which have been added a custom-built hot plate, a dehumidifier and humidifier combination with humidity sensor, and fans for air circulation. The dehumidifier (7) and humidifier (8) combination maintains the desired relative humidity in the cabinet. The humidifier is connected to the glove box through a loop of 3" PVC tubing. A humidity fan (5) in this loop draws air out of the glove box, where it is blended with the water vapor produced by the humidifier and then returned to the glove box. Relative humidity is measured by a humidity probe (6) located inside the cabinet. This sensor is mounted in a housing that can be closed during fuming to protect the sensor from CA fumes and thus extend its usable lifetime. A front panel humidity controller (10) provides a digital display of the relative humidity level and the setpoint adjustment. Relative humidity can be controlled from 20% to 80%.

Figure 1a
Photograph of the fuming glove box.

Figure 1b
Schematic drawing with part numbers
referring to the bill of materials in Table 1.

The PVC tubing is also connected to the exhaust tube (3) through a motorized gate valve assembly (4), designed and constructed in-house. The system relies on laboratory ventilation for venting the glove box. A switch on the control panel (12) activates the gate valve and simultaneously opens or closes an exhaust check valve (2) that allows room air to be drawn into the glove box during venting but is closed during fuming.

A programmable, autotuning temperature controller (9) with digital readout regulates the temperature of the hot plate (16). The hot plate is built into a platform that is mounted on a rail so that its height can be adjusted to accommodate the specimens being fumed. All controls and indicators for humidity and heating are located on the exterior of the glove box (11).

Small substrates are hung from clips on a rack that is suspended from the top of the box. Also located near the top of the glove box are two fans on adjustable mounts to circulate the vapors inside the glove box. These fans can be controlled independently. The cabinet has a large window that allows observation of the substrates and gloved ports that permit handling of the substrates so that all surfaces can be examined during fuming. This feature helps ensure that the substrates are not overfumed or underfumed. In addition, using the gloves and antechamber (17), substrates can be removed from the cabinet without the need for venting, which significantly increases the substrate throughput. The antechamber has a thick, tightly sealed plate on each side, one that is opened from the outside and the other must be opened from inside the glove box. We found it convenient to use the antechamber for adding cyanoacrylate to the cabinet.

During typical operation, a batch of various substrates are loaded into the cabinet, which is then sealed and the humidity is brought to the desired level. The hot plate is then preheated to 250 ºF and an aluminum dish of cyanoacrylate is added through the antechamber so that no humidity or heat escapes the cabinet. This also allows careful control of the amount of time the substrates are exposed to CA fumes, because no CA is present during the humidity equilibration period. When fuming is complete, the substrates can be removed from the cabinet through the antechamber, or the cabinet can be vented and the substrates can be removed through the larger side door.

The second cabinet constructed was a vacuum chamber, shown in Figure 2, that is long enough to hold a rifle or shotgun and wide enough to easily hold a gallon milk jug. The vacuum cabinet consists of a tube made of cast aluminum with hinged end caps that are sealed with O-rings and clamps. The inside clearance of the chamber is 54 inches long and 15 inches in diameter. In the center of the tube are three ports: one port consists of a QF40 flange for attachment to the vacuum system, and the other two are viewports for enabling the observation of substrates while they fume, which is useful for preventing overfuming or underfuming. One of the viewing ports can be used to shine light into the chamber while the other port is used for observation.

The vacuum system consists of three parts: a pump, a trap, and a pressure gauge with controller, all purchased from Kurt J. Lesker Company. The pump is a Vacuum Research (VR600-21) vacuum pump with a mist eliminator (PFEFG24QF4). This is a 3-phase, 2-stage rotary pump with a pumping speed of 17 cubic feet per minute, which allows the pressure in the chamber to be controlled over a wide range (from atmospheric pressure down to <0.1 torr). Between the pump and the vacuum chamber is a micromaze trap with bakeout heater (MMA-151-1QF) and thermocouple gauge (KJL-6000) with controller (KJL-10TC) for measuring the internal pressure of the chamber. A small valve in the vacuum line is used to release the vacuum. All vacuum components, except the thermocouple gauge, are connected using QF40 flanges. The thermocouple gauge has a 1/8" NPT fitting and screws into a female fitting welded into the vacuum line.

Methods and Materials

Substrates, listed in Table 2, were fumed in each of the three cabinets (normal, humidity, and vacuum) using cyanoacrylate. The fumed prints were dye stained, as described below, and illuminated with 530 nm light from a forensic light source, the Omnichrome Spectrum 9000 (Melles-Griot). The stained prints were then photographed through an orange filter using a Polaroid MP4 camera and Polaroid Polapan 665 black and white, positive and negative film.

Figure 2a
Photograph of the vacuum cabinet.

Figure 2b
Schematic drawing.

Table 2
Substrates examined in these experiments.

Prior to testing the two new cabinets, a set of standards were run in the Iowa Division of Criminal Investigation Laboratory’s standard CA fuming cabinet. Ten runs were performed with ten substrates except one run, which was missing duct tape. These standards served as a reference for determining which technique worked the best for each substrate in terms of the amount of ridge detail (quantity) and the clarity (quality) of the developed latent prints, the amount of background color, and the time of development.

The next step was to begin running sets of substrates using the vacuum cabinet. It was quickly determined that some heating of the CA was necessary to achieve satisfactory fuming. Because no provision had been made for installing a heater in the chamber, it was determined to put the CA in an empty glass fingerprint powder jar that had been preheated to approximately 120 °F (50 ºC) with hot water from the tap before placing it in the vacuum chamber. Preliminary runs were made using only glass and plastics to determine the optimum times for running the vacuum and fuming the substrates. A variety of prints were tried including eccrine and sebaceous prints, which were either rolled or pressed on each surface. Initially, the vacuum was run for 15 minutes and then the substrates sat for 15 minutes to fume. However, when a RAM dye stain (a mixture of rhodamine 6G, Ardrox P-133D, and MBD) was used with a petroleum ether carrier to rinse the substrates, the developed latent prints tended to be washed off. Therefore, rhodamine 6G (R6G), MBD, and Ardrox were mixed using methanol as the carrier. These dye stains were made following the directions in the FBI manual [4] except for the substitution of methanol for petroleum ether as the diluent. After further testing, it was determined that MBD and R6G gave better results than Ardrox.

Having achieved satisfactory staining, it was then determined that the best prints resulted when the minimum amount of time was used to reach a pressure of 0.5 torr in the chamber. This took approximately 12 to 13 minutes, leaving 17 to 18 minutes for fuming time. After determining the optimal time, all the substrates were run for a general comparison. The first five runs containing all ten substrates were stained with R6G and the last five runs were stained with MBD. The results were then compared and photographed.

Next, trial runs using the humidity cabinet were begun. The first set of runs was used to determine the optimal humidity. Three runs were made at each of three levels, 80%, 60%, and 40% relative humidity, and one run at 30% relative humidity. These trials were performed as follows: The substrates were hung inside the cabinet and the relative humidity was raised or lowered to reach the test level. The hot plate was then preheated to 250 ºF. The glue was then added through the side chamber and the items were fumed between three and four minutes. The first run was stained using a RAM combination dye stain. However, even though the prints were not washed off, the background was still spotty, so the rest of the runs were stained using the R6G with a methanol carrier. After comparing these runs, it was decided that 60% relative humidity gave the least amount of background color, dotting of the ridges, and filling-in between ridges. Therefore, the last seven runs were performed at 60% relative humidity for four minutes each, to make ten runs at 60% relative humidity. The results for each substrate were then photographed. Finally, one side-by-side comparison of the vacuum cabinet and the humidity cabinet was done using two sets of substrates printed by the same person. The runs were carried out using the optimum conditions for each cabinet. All resulting latent prints were dye stained using R6G and were compared.

Results and Discussion

Many changes and additions were made during this experiment to achieve the optimal results for each cabinet. One change involved switching from the RAM combination dye stain to primarily R6G. As explained previously, the RAM dye stain with a petroleum ether carrier sometimes washed off the prints or left splotches on the substrate. Therefore, three dyes (MBD, R6G, and Ardrox) were made using a methanol carrier, which gave better overall results, with MBD and R6G giving better results than Ardrox.

Another important finding was that preheating the glass vessel that held the CA resulted in better prints when fuming in the vacuum cabinet. The reason for this appears to be that supplying a source of heat is necessary to compensate for the cooling that occurs during the evaporation of the CA in the vacuum chamber while it is being evacuated. This cooling lowers the vapor pressure of the CA and hence slows the rate of CA vaporization in the cabinet. The significance of this cooling in the vacuum cabinet was demonstrated by placing a small container (an aluminum weigh boat) of water in the chamber and applying a vacuum. The water froze, showing that for the CA to properly fume at low pressure, some heat was necessary. The amount of heat need not be large. A preheated glass jar or any object with a large enough mass and good thermal contact with the glue dish is sufficient.

Table 3 summarizes the results of the ten substrates that were run in all three cabinets. The results for the standard runs in the normal CA fuming cabinet ranged from very good prints on aluminum foil and duct tape to having no prints appear at all on most runs with substrates such as styrofoam, black plastic bags, and plastic page protectors. In contrast, aluminum foil, black plastic bags, GLAD bags, and duct tape gave the poorest results when the vacuum cabinet was used. The best results in the vacuum cabinet were obtained with clear plastic bags and cups, plastic page protectors, sandwich bags, glass slides, and styrofoam cups. For the humidity cabinet, plastic page protectors, glass slides, duct tape, styrofoam, and aluminum foil gave the weakest ridge detail, and sandwich bags and GLAD bags gave good results.

It is seen in Table 3 that even under the optimum conditions, four substrates still gave poor results using the vacuum cabinet. The most common problem observed, as seen on the black plastic bags, was an extreme faintness or translucent quality. This may have been due to the absence of water vapor in the vacuum cabinet, because high relative humidity tends to make prints whiter and easier to see [3]. In Figure 3, good, clear ridges on a black plastic bag can be seen; however, there are places where it looks to be so faint that the camera could not pick up the ridge detail. Although this made the ridges difficult to see, the ridges were still of good quality. With duct tape and aluminum foil, on the other hand, there was also a lot of spotting of the ridges, making them more difficult to analyze and evaluate. This is shown in Figure 4, which contains a print on aluminum foil that gave very spotty results.

Table 3
CA fuming results.

Figure 3
A print on a black plastic bag, obtained in the vacuum chamber,
demonstrating how the transparent quality of prints developed in
this cabinet sometimes made the prints difficult to see.

Figure 4
A print on aluminum foil, obtained in the vacuum cabinet,showing
the dotting that was observed on this substrate and on duct tape.

One of the biggest advantages of the vacuum cabinet was the lack of background coloring. The glass slides gave the best example of the difference. As can be seen from Figure 5A, the print is very clear and has relatively very little fill-in as well as almost no background coloring, whereas in Figure 5B we see that the glass slide from the humidity cabinet has much more fill-in and background coloring. Although it required a fair amount of time to use the vacuum cabinet, it provided significantly better clarity of the developed prints than a normal cabinet on many substrates. For example, when tested in the normal cabinet, the plastic page protector almost never showed any visible prints and in the humidity cabinet the prints were often dotted and hard to see, but in the vacuum cabinet this substrate gave clear prints almost every time, as can be seen in Figure 6. In our testing, clear plastic bags, sandwich bags, plastic page protectors, clear plastic cups, glass slides, and styrofoam all gave excellent results in the vacuum cabinet.

In the humidity cabinet, aluminum foil and duct tape again gave the poorest results, as well as plastic page protectors, styrofoam, and glass slides. With duct tape and aluminum foil, the ridges were often very dotted, just as they were in the vacuum cabinet. This can be seen in Figure 7, which shows the spotty ridges that were often obtained using these substrates in the humidity cabinet. However, for some substrates, like styrofoam, even though the results may not have been the best, they were often much better than those achieved using the normal cabinet. Often no prints were observed on styrofoam using the normal cabinet; however, in the humidity cabinet, prints could often be seen, although with poor contrast, as shown in Figure 8. Furthermore, although the glass slide had more background when fumed in the humidity cabinet than when fumed in the vacuum cabinet, the ridges were still often very clearly visible and had less background coloration than the normal cabinet.

A few additional substrates were tried in only a couple of runs. Aluminum cans, a solid white plastic cup, and opaque plastic cups were run in both the vacuum and humidity cabinets. In general, the aluminum cans and white plastic cups gave good results in both cabinets. The opaque plastic cups had more contrast between the cup and the print when developed in the humidity cabinet. The cups developed in the vacuum cabinet were observed to have better ridge detail. However, both cabinets gave good results on this substrate as well.

Figure 5
Comparison of background coloring on glass slides run in (A)
the humidity cabinet and (B) the vacuum cabinet.

Figure 6
A print on a plastic page protector, obtained using the vacuum cabinet and dye stain.

Figure 7
Examples of spotty ridge detail obtained on some substrates using the humidity and
vacuum cabinets: (A) prints without dye stain on duct tape,
and (B) prints with dye stain on duct tape.

Figure 8
A print on styrofoam developed using the humidity cabinet and dye stain.

Conclusions

Compared to the normal cabinet used for CA fuming, both the humidity and vacuum cabinets gave better results on most substrates. Both cabinets gave good results on clear plastic bags, sandwich bags, and clear plastic cups. However, for some substrates, one cabinet gave better results than the other did. The vacuum cabinet tended to have better results with the glass slides, plastic page protectors, and styrofoam, whereas the humidity cabinet often gave better results with the GLAD bags, black plastic bags, and, in general, any plastic substance. Duct tape and aluminum foil, however, often gave only fair results in both cabinets. Both substrates, in general, gave good prints that were of value for comparison, but they always had more dotting and faintness than other substrates. In conclusion, both cabinets were found to give better results on certain substrates and definitely contained less background coloring and better ridge detail than the prints developed in a normal cabinet.

In the course of this work it was also observed that a dye stain made with methanol as a carrier instead of petroleum ether gave better, clearer results, especially with R6G as the dye. In addition, allowing the prints to set for some amount of time after fuming often gave better results. It was also found that a small amount of heating of the CA is highly recommended for fuming in vacuum cabinet.

Acknowledgments

This project was initiated as part of the founding of the Midwest Forensic Resource Center (MFRC). This Center is a cooperative undertaking of Ames Laboratory-U.S. Department of Energy, Iowa State University, and forensic laboratories around the Midwest. Funding for this project was provided by the Iowa State University Institute for Physical Research and Technology.

For further information, please contact:


Carl W. Bessman, Criminalist
Iowa Division of Criminal Investigation
Criminalistics Laboratory
Des Moines, Iowa 50319
(515) 281-3666
Bessman@dps.state.ia.us

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3. Watkin, J. E.; Wilkinson, D. A.; Misner, A. H.; Yamashita, A. B. Cyanoacrylate Fuming of Latent Prints: Vacuum Versus Heat/Humidity. J. For. Ident. 1994, 44 (5), 545-57.
   
   
4. Chemical Formulas and Processing Guide for Developing Latent Prints; US Department of Justice, Federal Bureau of Investigation, Laboratory Division, Latent Fingerprint Section: Washington, DC, 1994.