Forensics Q&A: Fingerprinting

By Kristy Lahoda | @KristyLahoda

Disclaimer: The information provided in this post should not be used for malicious intent unless it is in the form of crime writing. The author is an explosives expert, not an expert in fingerprint analysis. 

QUESTION:  In my novel the protagonist is at a crime scene where fingerprints are found. What are some ways the fingerprints would be readied for analysis and what are some technical bits I can use to make it sound more authentic?

ANSWER: Fingerprints are analyzed by physical or chemical methods.

Types of Fingerprint Evidence

There are three types of prints that can be found at a crime scene: patent, impressions, and latent. Patent prints are those visible prints left as a result of contact with items such as dirt, blood, oil, and grease. Impressions are three-dimensional prints left in soft surfaces such as silly putty, tar, and butter. Latent prints require processing in order to be visible and will be our focus.

Latent Print Residue Composition

Our fingertips have skin called friction ridges that secrete sweat through pores. The composition of this sweat actually forms the latent print as well as any variety of materials that are on the friction ridges.

Latent Print Development

There are two types of methods for print development: physical and chemical methods. The methods used are primarily dependent on the surface the print is found on. The primary physical method is powder dusting. In this method, the powder adheres to the residue allowing for print visualization. Afterward, a piece of transparent lifting tape is applied to the powered print and then mounted onto a background that contrasts with the color of the print powder. Speaking from the limited experience I had during my training in fingerprints, dusting and lifting are art forms. Both procedures require patience and a great deal of skill. The prints can easily be damaged and you are out of luck if only one print is found and it is damaged during processing. Another physical method involves magnetic powder. It has magnetic properties and sticks to a wand. Since there are no brush bristles, the risk of damage to the print is reduced. This method is really cool, but again, I found that it requires technique.

There are several chemical methods. The chemical method that we use often in the crime lab is cyanoacrylate fuming, otherwise known as superglue fuming. The chemical name of superglue is alkyl-2-cyanoacrylate ester. The fumes interact with the latent print residue by polymerizing (i.e., the process of many small molecules combining). We place the item containing the print or prints into a chamber. We place a small dish of superglue in the chamber. The chamber is kept at a certain relative humidity and heat is used as a catalyst (i.e., to speed up the process of the reaction). There are several nice things about superglue fuming. It is an easy procedure, but it also allows for further processing if necessary. The print can be treated with a dye stain that induces fluorescence or luminescence when an alternate light source (ALS) or a laser illuminates the print at a specific wavelength. Examples of two dye stains that the crime lab uses are ardrox and Rhodamine-6G. Another common chemical process is called ninhydrin and is useful for porous surfaces. Ninhydrin reacts with components from the ridge lines containing amino acids. It is commonly applied by spraying, painting, or dipping. The process is slow, but like the superglue process, it can be sped up by adding heat in humid atmosphere. The ninhydrin develops bluish-purple prints. An analog of ninhydrin is 1,8-diazafluorenone (DFO). Ninhydrin and DFO react with components in the latent print residue that are water soluble. A follow-up treatment to ninhydrin and DFO is called physical developer (PD). PD is often used as a follow-up method because it reacts with components in the residue that have not already reacted—water-insoluble components such as lipids. It involves a process based on silver deposition onto the latent print residue.

There are many other chemical methods that can be used. I highlight a few of the more common methods here. The important takeaway is that even though it may seem to be an easy job, it takes a skilled crime scene technician or lab analyst to nondestructively lift the prints.

Kristy Lahoda, Ph.D., is an explosives analyst contractor in a crime lab as well as a technical editor for a scientific journals publishing company.  She writes Christian forensic suspense and discusses forensics on her blog called Explosive Faith.  You can follow her on Facebook and Twitter.

If you have a forensics question for Dr. Lahoda that you'd like to see answered on the QueryTracker Blog, send your question via Carolyn Kaufman using the email link under Contact Us in the right-hand column of the main QTB page.

Forensics Q&A: Chain of Custody

By Kristy Lahoda | @KristyLahoda

Disclaimer: The information provided in this post should not be used for malicious intent unless it is in the form of crime writing. The author is an explosives expert, not a crime scene expert. While every attempt was made to ensure the accuracy of this information, for security purposes, some details may have been withheld.

QUESTION: In my crime novel, there are scenes involving courtroom testimony about the evidence that was collected at the crime scene. What happens to evidence at a crime scene once it’s been collected?

ANSWER: An investigator from the agency that had jurisdiction over the crime scene will take the evidence to the proper lab. Depending on the state, it could be the state crime lab, state highway patrol lab, state fire marshal’s lab, a city or regional crime lab, or even governmental labs such as the Federal Bureau of Investigation, the Drug Enforcement Agency, or the Bureau of Alcohol, Tobacco, Firearms and Explosives. It really just depends on the jurisdiction. 

So what is chain of custody?

Chain of custody provides a record of time and possession/location of the evidence. This is documentation that later becomes important if the case goes to trial. If the evidence cannot be accounted for from cradle to grave—from detection at the crime scene, to delivery to the crime lab, to analysis, and to either destruction or its return to the submitting agency—it is impossible to prove that the evidence has not been tampered with. This is often a tactic of the defense attorney and rightfully so. The evidence needs to stay secure at all times. 

How does a crime lab keep track of chain of custody?

Once the evidence is collected and packaged, the investigator secures it with evidence tape, initial and date it, and bring the packaged evidence to the lab from the crime scene. Paperwork is submitted with the evidence that provides a description of the evidence in each container along with the type of evidentiary tests that the investigator wants run on the sample. For example, if the investigator suspects that an item of evidence was used to commit arson and this item potentially has latent prints on it (such as a gas can), then there will be an indication such as checked boxes that will let the lab analysts know to analyze the evidence for arson and latent prints.

In many crime labs, each piece of evidence is received and logged into the Laboratory Information Management System (LIMS) by the evidence-receiving technician. The investigator’s name and submitting agency will be on the evidence receiving form and once the evidence is logged into the LIMS system, it will be transferred from the investigator’s possession to the evidence storage area. Once the lab analyst wants to analyze the evidence, he or she will first transfer it into his or her possession, then cut through the evidence tape, breaking the seal. After analysis, the lab analyst will place the evidence back in the original container, add another piece of evidence tape to seal the package once again and write his or her initials and possibly the date. The initials are written half on the tape and half on the package. The evidence is then transferred back into the evidence storage area. Each transfer is logged into the LIMS indicating the time and date of transfer.

What happens if the chain of custody is broken?

If proper chain of custody is not documented and maintained, there will most likely be serious repercussions for the case in court. This could lead to the perpetrator being found not guilty even if s/he was actually at fault for the crime. 

Kristy Lahoda, Ph.D., is an explosives analyst contractor in a crime lab as well as a science content editor for a major educational publishing company.  She writes Christian forensic suspense and discusses forensics on her blog called Explosive Faith.  You can follow her on Facebook and Twitter.

If you have a forensics question for Dr. Lahoda that you'd like to see answered on the QueryTracker Blog, send your question via Carolyn Kaufman using the email link under Contact Us in the right-hand column of the main QTB page.

Forensics Q&A: Not All Explosives Are Created Equal

By Kristy Lahoda | @KristyLahoda

Disclaimer: The information provided in this post should not be used for malicious intent unless it is in the form of crime writing. The author is an explosives expert, not a crime scene expert. While every attempt was made to ensure the accuracy of this information, for security purposes, some details may have been withheld.

QUESTION: I am writing a thriller and my antagonist is planning to make a pipe bomb.  Does it matter what type of explosives she uses?  For example, could something like TNT be used as an explosive in the pipe?

ANSWER: It does matter as not all explosives are created equal.  TNT is not an explosive typically used in a pipe bomb—it does not need to be contained to cause mass destruction.  Explosives are generally grouped into two categories: low explosives (LEs) and high explosives (HEs).  These are differentiated based on the speed of explosion.  LEs deflagrate, HEs detonate.

High Explosives (HEs)

Detonation occurs when the reaction front propagates (i.e the reaction proceeds) through the explosive at a speed greater than the speed of sound,  Detonation occurs at velocities above 3,300 ft/sec (2,250 mph).  For detonation to occur, the assistance of a primary explosive is required.  To provide sufficient energy for the high explosive to begin its energetic decomposition (i.e. the breakdown into chemical components as a result of the energy of the reaction), only small amounts of the primary explosive are needed because they are reactive to shock, friction, or heat.  Detonation results in a rapid release of energy and an accompanying shock wave.

Types of HEs

High explosives are categorized as primary or secondary based on ease of initiation.  When comparing the two, primary explosives are more sensitive to heat, friction, and shock and have less energy, and therefore less power, than secondary explosives.  Secondary explosives are less sensitive to heat, friction, and shock and are more powerful.  Primary explosives are used in detonators and initiation systems.  They can be used to ignite secondary explosives.  Secondary explosives are used in large quantities relative to the primary explosive and are typically used as the main charge.

Primary Explosives

Two common initiating explosives include lead azide and lead styphnate.  Lead azide is very sensitive to initiation by friction, heat, or shock.  The velocity of detonation for lead azide is around 17,500 feet per second.  That’s just over 11,900 miles per hour!  Lead styphnate is sensitive to static electricity and fire, but is less sensitive than lead azide to friction and shock.  Its velocity of detonation is similar to that of lead azide.

Secondary Explosives

Trinitrotoluene (TNT) is one of the most universally known HEs.  Cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN) are two favorites of terrorists.  In fact, PETN was the explosive that Richard Reid, aka The Shoe Bomber, had concealed in his shoe in an attempt to blow up American Airlines Flight 63.  Another example of a high explosive is ammonium nitrate-fuel oil (ANFO, used by Timothy McVeigh and Terry Nichols in the bombing of the Alfred P. Murrah federal building in Oklahoma City in 1995.

Low Explosives (LEs)

Low explosives are propellants that burn in open air, but deflagrate (burn rapidly) when initiated in confinement.  LEs might not sound that dangerous since they burn, but they can be extremely perilous when used improperly.  In fact, they can even “act” as high explosives upon deflagration given the proper conditions when initiated, such as containment.  Simply stated, deflagration = explosion when confined.  When the LE deflagrates, the burn is faster than in open air.  However, it does not burn as rapidly as detonation occurs.  The reaction front propagates through the low explosive at a velocity less than the speed of sound.  As a result, low explosive deflagration reactions occur at a lower reaction front pressure, velocity, and temperature than HEs.  

Types of LEs

There has been a strong emphasis in counterterrorism literature on high explosives analysis, but the majority of incidents in the US have been due to LEs such as black powder, smokeless powder, improvised explosives, and fireworks. These are the types of explosives that are used in pipe bombs.

In summary, low explosives require containment to deflagrate.  High explosives require primary explosives to initiate detonation of secondary explosives, but are destructive without containment.  After an explosion, there are a number of law enforcement activities that are launched at the crime scene and crime lab including an investigation, a crime scene search, evidence collection, sample preparation, and forensic analysis on the decomposition products of the explosives—called post-blast residue analysis.  The wealth of forensic information obtained post-blast is amazing and should give pause to anyone considering bomb construction.  

For more information on these topics, stay tuned for the installments in the upcoming months of Forensics Q&A.

Kristy Lahoda, Ph.D., is an explosives analyst contractor in a crime lab as well as a science content editor for a major educational publishing company.  She writes Christian forensic suspense and discusses forensics on her blog called Explosive Faith.  You can follow her on Facebook and Twitter.

If you have a forensics question for Dr. Lahoda that you'd like to see answered on the QueryTracker Blog, send your question via Carolyn Kaufman using the email link under Contact Us in the right-hand column of the main QTB page.