5.3: CHEMICALS

controlled environment, the need for water and a basin, or the specialized equipment needed to complete the process; although some equipment can be acquired that are smaller, more portable versions of their laboratory-based kin.

Ninhydrin

Ninhydrin is a chemical reagent used to visualize latent prints on porous items, such as paper, through a reaction with the amino acids present in sweat secretions, which turn the prints a violet color called Ruhemann’s Purple. The technician will either spray the ninhydrin on a surface using a pre-mixed solution, available from any forensic supply company, in a spray bottle or by brushing or dipping the item with the solution in a pan that contains the solution, which is the preferred method by your author, and then hanging the item to dry. The best results can be achieved using a fuming cabinet heated to 175-degree Fahrenheit with an attached humidifier. Using this method, prints will begin to develop in five to ten minutes. Without the chamber, prints will begin to develop in about an hour. The chamber is invaluable to the technician, who is constantly harried by administrators who are anxious to know if a print was developed on a death threat, forged document, or bank robber’s note, as the reaction time is greatly reduced. The technician will allow the item to hang dry for up to 24 hours to allow faint prints to fully develop. It is not recommend using ninhydrin sold in aerosol cans unless the laboratory is equipped with a good breathing apparatus and a well-ventilated fuming chamber. Even with these precautions, the technician will still have some exposure to the invasive fumes. It is imperative that the technician wears several layers of latex gloves and a respirator when working with ninhydrin; and that the laboratory is well ventilated to an area that will not inconvenience people passing by.

Remember that any uncovered part of the technician’s skin will stain purple when it comes in contact with the chemical, and it might remain that way for a few hours regardless of washing. So, if you do not want to have purple streaks on your cheeks and forehead, do not scratch until you have discarded your gloves and washed your hands. Be careful when using pre-mixed ninhydrin that also contains acetone, as this chemical may obliterate some soluble inks. The same can be said for some fixatives. It is always best to experiment before processing the actual item of evidence. It is also advisable to the document before processing and again immediately after. Using a green colored filter will help correct the purple color if you find it difficult to analyze. You may also use the Alternate Light Source at 535 nm without a barrier filter or 485 nm (450 nm for Manila envelopes) with an orange filter to visualize.

With some experience, the technician will also become an expert at determining which prints belong to the writer of the note and which belongs to people who have handled the document with their bare hands. These prints tend to be partial prints on the extreme corners of the document or thumb prints on the margins about one-third of the way up from the bottom of the page, with corresponding index finger impressions opposing. Suspects who wear latex gloves while drafting their machinations will seldom wear them while carrying the note to the mailbox or delivering the note to a bank teller. Therefore, do not forget to treat the envelope that housed the offensive document. It should be noted that the chemical formula for ninhydrin will likely be banned in the near future. Fear not, technology will replace the current form of ninhydrin with pellet-based vacuum chamber ninhydrin that will provide for the safe use of the development medium well into the future.

Special formulations are available for developing prints on thermal paper. Bear in mind that papers that have been coated in wax, like milk cartons, do not lend themselves to ninhydrin processing. For this type of evidence, it is best to use powders, cyano-acrylate fuming or Sudan Black.

Figure \(\PageIndex{1}\): Ninhydrin developed print. Author's collection

Another great thing about ninhydrin is that even if you only develop partials or smudged impressions, the ninhydrin process does not interfere with DNA typing.

Cyanoacrylate Ester

Cyanoacrylate ester fuming has replaced powders as the work horse of the fingerprint laboratory. With the advent of easy-to-use fuming kits, the ever-popular and inexpensive cotton-ball method, and fuming wands there is no reason for a laboratory not to be equipped with a fuming chamber or to take cyanoacrylate into the crime scene. The technician will place the item of evidence in the fuming chamber and place either the laboratory grade super glue on a foil tray on top of a heating plate with a humidity source or will introduce a fuming kit, such as Sirchie’s Cyano-Shot™ or Lynn Peavey’s Hot Shot™, by placing the activator canister into the jar containing the activator solution and emptying the glue vial on top of the activator canister. It is best to have a fan in the chamber for the circulation of the fumes evenly onto the items being fumed. Cyanoacrylate esters react with the amino acids, fatty acids, and proteins in the latent fingerprint to create a visible print that is white along the ridge lines and absent on the furrows. Cyanoacrylate also will not degrade DNA, so if you only get partials, you can still submit the item for DNA typing. Fuming chambers can be made from any cabinet, fish tank, or can be commercially purchased. The technician should keep a constant eye on the fuming process to make sure that the items of evidence are not over-fumed. To reduce the chance of over-fuming, Placing a black fingerprint backing card with a nice greasy nose-oil thumb print prominently in the center on a string in the chamber where a keen eye can be kept is a good way to know when to stop the fuming process. The best method for documenting the visualized print is with digital photography using a format that is acceptable to the agency that administers the automated fingerprint database. Do not forget to add a metric scale to the frame. The California Department of Justice recommends either JPEG-Fine or TIFF. The experienced fingerprint comparison analyst usually has no difficulty matching the white ridges on the developed print to the black ridges of the inked exemplar. However, when making exhibits for the jury, it is always best to color-reverse the image, with an explanation of the steps taken, and a reason for the enhancement, so that the jury will not be confused by the color contradiction.

So, what happens if the surface upon which you want to develop the print is of a white color? There's a dye for that. Treat the item as normal and examine it under magnification. If you detect only partials or fragments, submit the item for DNA typing, should the investigation warrant. If you detect good impressions under magnification, you can treat the item with Ardrox, Basic Yellow, Basic Red or Rhodamine 6G. If a laboratory does not have dyes on hand, the technician can try using a fluorescent medium and an Alternate Light Source. Small Particle Reagent can also be used to dye the ridges black; providing the surface is washable.

Figure \(\PageIndex{2}\):

Latent fingerprint developed with cyanoacrylate fuming. Color-reversed image. Exemplar fingerprint. Author's collection

Sticky-Side Tape

The sticky-side (or adhesive-side) tape formula method is another staple of the fingerprint technician in the jail or prison setting owing to the abundance of inmate manufactured weapons with tape-wrapped handles. There are several formulations on the market including Forensics Source Sticky-Side Powder™ kit, and Wet Wop™, and Evident’s Wet Powder™ that are extremely easy to use. The technician merely paints the adhesive side of the tape with the formula using a brush, waits a few minutes, gently rinses the tape using tap water or a Nalgene wash bottle, and allows the tape to dry. The resulting developed prints will appear on the tape. The trickiest part of using sticky-side tape formula is pulling the tape apart from itself and the object. Pulling the tape steadily and slowly is the best method. The other tricky part is holding the tape down while you apply the formula. A pair of forceps and an old ridge pick are handy for taming the wild sticky tape, especially when the technician is wearing latex gloves. Sticky-side tape formula will also recover lip impressions if tape is used to cover a victim’s mouth. Think how great it would be to have the suspect’s fingerprints and the victim’s lip print on the same item of evidence.

Figure \(\PageIndex{3}\): A latent fingerprint developed with the Sticky-Side tape method. Color-reversed court exhibits. Exemplar print. Author's collection

Skin Print Black

Doje's Skin Print Black™ is a product that is kept in the laboratory for those “just in case” events. It is a magnetic powder that is formulated to have less ink than normal magnetic powders. The technician inserts a small dowel into a roll of clean cotton cash register tape (do not use thermal paper) and rolls it, with some pressure, on the skin of a victim as a means to transfer the impressions to the paper. The technician then dusts the paper with the powder. This product is great if the technician can get to the victim before they wash or rub the area that was grasped. Thus, if a person is strangled or grabbed hard upon the arm, this method may well recover either fingerprint impressions or other sweat transfers like those produced by the assailant’s inner arm. Even if the technician develops an impression on the paper that does not yield significant ridge detail for identification purposes, the technician will still be able to see where the victim was grabbed or strangled and therefore, they can vigorously swab that particular area for a DNA specimen. This is the least obtrusive examination of a decedent person, with the least amount of contamination, a technician can make while awaiting personnel from the coroner’s office to arrive.

Small Particle Reagent

Small Particle Reagent, or SPR, is a chemical reagent that is extremely useful in developing fingerprint impressions on difficult surfaces like vehicles that are wet with dew, cold soda cans, surfaces that are covered in oils or dirt, and glass. SPR is easy to use and inexpensive. The technician can either use a pre-mixed solution, available from all of the forensic supply companies, or can purchase the individual vials (containing Molybdenum disulfide) and mix them with water, shaking vigorously. The solution is then sprayed on the surface with a gentle misting spray. Wait a few minutes and then spray the area again with clean water from another spray bottle. The prints will appear as the excess SPR washes away. The process can be repeated for faint impressions. Once the surface has dried completely, the impressions can be photographed and lifted using common lifting methods. One thing to remember is to be sure that the technician has black and white reagent on hand as there is nothing worse than being called out to a vehicle crime scene with a dark colored car and all that the technician has brought is black SPR. A quick look at some of the forensic supply sources reflect a fluorescent SPR is now also available.

Figure \(\PageIndex{4}\): Fingerprint developed on a soda can fresh from the vending machine using SPR. Author's collection

Enhancing ridge print impressions in blood

On occasion, the technician will come across a faint impression deposited on a surface in blood, whether ridge print or footwear impression, and the hope is that the impression can be enhanced in a manner in which it can be visualized and documented using photography. Coomassie Blue is an enhancement chemical used to dye blood-contaminated fingerprints on porous or non-porous surfaces, which produce bright blue visible prints. Amido black is commonly used for the same purpose and will produce a dark blue stained impression. Amido black is sprayed onto the area of interest and then rinsed with a destaining product. Acid fuchsin, also known as Hungarian red, is an aqueous solution used to enhance fingerprint deposits in blood but must be used with an Alternate Light Source with a wavelength between 520nm-560nm. Your author prefers Leuco-Crystal Violet because of its ease of use, safety, versatility, and ability to find impressions in large scenes that were previously unseen. Leuco-Crystal Violet (LCV), also known as Aqueous LCV, is a reduced form of gentian violet; it is a highly sensitive formula to visually enhance blood prints and the reaction to blood will produce a vibrant violet result.

Figure \(\PageIndex{1}\): Blood print enhanced with LCV. Author's collection

Of course, all the above listed latent print development techniques are all for naught if the items being processed were not collected in a manner that preserved the fragile deposited evidence. The technician should tirelessly impress upon first responders, crime scene investigators, and civilian employees that fingerprints are delicate and easily obliterated. The technician should instruct all personnel that wearing latex gloves while handling evidence is essential; however, they are not sufficient to prevent the destruction of latent fingerprints, which are composed, depending on the source, from 80% to 90% water. Items of evidence must be handled in a way to preserve the latent print impressions. In other words, you do not pick up a knife by the handle and expect the prints on the handle to be intact. You must pick up the item in a manner that is not instinctive, such as by the end, hilt, or the very edges of the blade near the handle. Once the item is in an evidence envelope or paper bag, hold the bag or envelope so that you do not press the inside against the object. In other words, hold it by the top or the bottom, but not with your hand grasping the item as the impression may be absorbed into the paper or smudged beyond individualization. Be sure to get that evidence into the lab as soon as possible. The less handled and the fresher the evidence, the better the chance of recovering prints.

Vacuum Metal Deposition

Vacuum Metal Deposition or VMD is an advanced latent fingerprint development technology that works by depositing very thin layers of atomized metal on to the surface of an item of evidence inside a vacuum chamber to visualize latent fingerprints. The benefits of VMD is that it can develop latent fingerprints on aged items of evidence that have been exposed to adverse environments and conditions. The process does not interfere with subsequent DNA analysis. It can develop fingerprints on fabrics, fired cartridge casings, thermal receipt paper, positive and negative sides of adhesive tape, and unfired and fired cartridge casings. VMD is also non-destructive to electronic devices and electronic digital storage media. It can also develop latent transfer impressions of body surfaces like faces, ears, lips, hands, feet, and arms, which provide target areas for DNA collection. It works on porous, semi-porous, and non-porous surfaces. The vacuum removes air, moisture, and solvents from the evidence and chamber environment to allow the metal atoms to travel through the chamber without interference to the evidence item surface and to condense onto the surface thus providing a smooth, even coating. The metals used during the VMD process are gold, silver, sterling silver, zinc, and copper. Your author believes that VMD will be the go-to latent fingerprint development method in the future.

Figure \(\PageIndex{6}\): West Technology Model VMD360. Author's collection

Figure \(\PageIndex{7}\):Fired rifle cartridge casings developed with VMD. Author's collection