Ducted or Ductless? Why Forensic Labs Need to Review their Choice of Flow Hoods

Forensic scientist at work

Last week saw the much anticipated, and subsequently critiqued, launch of Apple’s latest generation of i-products. From the new and improved Apple TV to the updated Apple Watch with integrated SIM card, fresh innovation seemed to be the theme of the keynote held at the newly unveiled Steve Jobs Theater in Cupertino, CA. But it was the next generation of iPhones that caused the most controversy, specifically the iPhoneX. Why? Well, it’s mainly about what the phone has lost – the ‘homekey,’ that small round, erstwhile pressable button at the center of the base. The homekey acted as way to activate the device and, via finger print recognition, to authenticate the user. And it’s safe to say that its incorporation was a game-changing novelty.

So why do away with this technology? According to Apple exec Phil Schiller, although the odds of a malicious user accessing an iPhone by thwarting the fingerprint recognition are around 1 in 50,000, that’s still too high a risk. So with iPhoneX, Apple developed FaceID, a new biometric that will allow the device to unlock only when it ‘sees’ its owner and when its owner is paying it direct attention. Naturally, controversy is swirling around this move as commentators ponder the implications of personal privacy and of law enforcement, but what most agree upon is that this technology, with its one in one million chance of being hacked, leaves finger print recognition in the dirt.(1)

To date, fingerprints have been the cornerstone of biometric analysis, and boasts a longer history that you might imagine.

Although the tools we are now familiar with from TV shows like CSI and NCIS offer 21st century tech, the recognition that our fingerprints are unique identifiers goes back some four hundred years to Marcello Malpighi, professor at the University of Bologna, Italy. In 1686, Malpighi, for whom a layer of skin is named, was the first to note the loops, ridges, and spirals on the fingertips using a newly invented microscope. Malpighi went on to become the first to identify capillaries in animals and to observe red blood cells under a microscope. Following an illustrious career in medicine, anatomy, and botany, Malpighi became the papal physician to Pope Innocent XII in 1691 whose fingerprints he seemingly did not catalog.

During the 1870s, Henry Faulds, British Surgeon-General in Tokyo, Japan, built on Malpighi’s work by creating a classification system for the prints, a system he subsequently forwarded to that über-classifier of things, Charles Darwin. Meanwhile, here at home, Gilbert Thompson was the first to authenticate a document using his own thumbprint in 1882. By 1897, the Calcutta Anthropometric Bureau in India created the world’s first database of fingerprints, developing the Henry System of classification. Within five years, at the turn of the twentieth century, the New York State prison system deployed the new technology, with the U.S. military quickly following suit. And until the advent of DNA analysis, fingerprint identification technology was the mainstay for every law enforcement and intelligence agency worldwide – the same technology now pulling double duty to secure both personal electronic devices and legal convictions.

Fans of fictional crime lab shows are very familiar with investigators dusting for prints and preserving them as evidence. But how does the process actually work? We’re glad you asked. In forensics’ terms, there are three major types of prints: plastic prints, which are three dimensional and left on soft surfaces like soap, fresh caulk, or wet paint; patent prints which are visible and left on hard surfaces such as paper, wood, or glass; and latent prints which are invisibly deposited when the body’s sweat or oils come into contact with a surface. While plastic and patent prints can be photographed, the preservation of latent prints requires the use of fingerprint powders in collection.

Having isolated the prints by means of an alternative light source (ALS) such as an LED or a blue light, forensic technicians will apply their choice of several powders to reveal the print. Black powder is composed of rosin, lampblack, and black ferric oxide, for example, while other options contain inorganic elements such as bismuth, copper, titanium, mercury, silicon, and lead. According to Evidence Technology Magazine, a trade publication with a focus on evidence collection, storage, and preservation, ‘Black is by far the most commonly used latent print powder color [and is] the workhorse of latent print processing.’(2) But into the mix comes a powder composed of nano-engineered silicon balls termed Supranano particles. Larger in size than true nano particles, suprananos do not permeate the skin but do have the ability to isolate far more than the loops, ridges, and spirals of the past. In fact, supranano powders in conventional, magnetic, and fluorescent powder formulations can identify important demographic characteristics such as race and ethnicity, along with evidence of drug use (both illicit and prescribed), and can establish the presence of ‘contact residuals,’ such as explosives.

Prints preserved using supranano powders are analyzed using MALDI-TOF lasers – Matrix Assisted Laser Desorption/Ionization – Time of Flight Mass Spectrometry.(3) That’s quite a mouthful but in essence what this laser does is to allow a sample on a matrix to absorb ultra-violet light and convert it to heat energy. As it heats, a 100nm thickness of analyte – the material under analysis – is burned off, becoming vapor. The velocity of the ionized materials traveling across the ‘drift space’ between the sample and the detector, as determined by the law of conservation of energy, is analyzed to identify the smaller or more highly charged ions, which hit the detector most rapidly – the time of flight value.(4)

At the time of collection, many factors will affect the integrity of the lifted print.

So in their collection and analysis, fingerprints are subjected to physical assault – dusting – and the later rigors of mass spectrometry which begs the question: How vulnerable are they to damage? At the time of collection, many factors will affect the integrity of the lifted print. Extreme heat, for instance, will cause the oil in prints to be less viscous which can lead to smudging, and those collected in extreme cold must be thawed carefully before processing. Curiously, as Dick Warrington points out in his article for Forensic Magazine, Fingerprinting in Adverse Conditions, since oil and water are not miscible, latent fingerprints collected in wet weather conditions are likely to be well preserved, assuming that best practices for collection are followed.(5)

For obvious reasons, great care is taken by forensic technicians to ensure that fingerprints from actual or potential crime scenes are preserved and protected.

But the need for protective measures goes both ways with the use of laboratory chemicals necessitating safety protocols for those who analyze the materials. Processing latent prints typically involves the use of a variety of equipment, from a fume hood to a cyanoacrylate fuming chamber to a fingerprint development chamber, all of which are subject to specific cleaning and calibration protocols. A cyanoacrylate fuming chamber (CAE) is a microprocessor-controlled unit that heats cyanoacrylate – also known as superglue – and circulates the fumes via a fan mounted either to the soffit or beneath the workstation. When the fumes meet the moisture of the print’s ridges they stick to them and are converted into the white polymer polycyanoacrylate. Once the relative humidity and fume cycle time is set by the operator, the chamber’s door auto locks to prevent exposure to the fumes which can represent an allergen and a source of irritation. In fact, although the Health and Safety Executive in the United Kingdom concurs with the U.S. National Toxicology Program that the use of cyanoacrylate is safe, 5% of the U.S. population is sensitive to CA fumes, suffering flu-like symptoms upon exposure. Given this sensitivity, the American Conference of Governmental Industrial Hygienists (ACGIH) has recommended a threshold exposure limit value of no more 200 parts per billion. To protect technicians who may come in repeat contact with the chemical, the air within a CAE unit is purged through filters and circulated multiple times through a specially blended carbon filtration bed to trap cyanoacrylate particles. The units range from desktop sized to chambers large enough for bicycles, so whether the technician is dusting a small weapon such as a knife or gun or larger evidentiary items the CAE has it covered.

…as a result of the powder mess that is created inside the hood, the chemical fume hood becomes used exclusively as a “fingerprinting hood.”

In addition to the CAE, forensics labs also require the use of fume hoods or Ductless Downflow Workstation (DWS), the latter being a more expensive proposition than its ducted counterpart. A typical DWS has a filter pack within the fume hood that actively filters the air before returning it directly into the cleanroom and although this seems like a small difference there are significant implications. For one, not all chemicals can be used within a ductless workstation which means that, before purchasing, technicians must supply a list of potential compounds to the supplier for vetting. And this might be galling given that the upfront cost of a 5-foot long ductless hood is approximately $28,000 as compared with its $6,000 alternative, although the associated costs of infrastructure modification to accommodate a ducted hood (exhaust fans, ductwork, et cetera) do raise that price tag a little. And from a contamination control perspective, there is a problem. When fingerprint powder is used within a fume hood it generates what Kelly Williams writing for Labconco Corporation, a laboratory equipment manufacturer based in Missouri and Kansas, calls ‘a big mess.’ ‘In most cases,’ she notes, ‘as a result of the powder mess that is created inside the hood, the chemical fume hood becomes used exclusively as a “fingerprinting hood.” Other types of procedures once performed in the hood are no longer done in the “fingerprinting hood” because it is impossible not to contaminate other samples with the latent print powders that are in it.’(6) So it’s worth researching whether a lab really needs that costly Ductless Downflow Workstation for this purpose…

…whether using a ductless or a ducted hood, the controlled environment of the cleanroom must serve to protect forensic technicians from potentially dangerous exposure to reagent grade chemicals.

But the advantages of the DWS are myriad: they are cleaner and more efficient than ducted units, and they significantly lower the operating costs of exhausting and replacing tempered air. Downflow units combine catch trays with prefilters and HEPA filtration systems to trap powder particulates and return the air to the cleanroom, and without the need for ductwork, they’re also mobile and versatile. They are the tool of choice for many institutions, assuming that SOPs allow for it and the operating budget will stretch to the initial investment outlay. But one thing is certain: whether using a ductless or a ducted hood, the controlled environment of the cleanroom must serve to protect forensic technicians from potentially dangerous exposure to reagent grade chemicals. Given that these include methanol, petroleum ether, acetone, and glacial acetic acid, and weighing the importance to law enforcement of the tasks performed, it is critical that the health and safety of all laboratory technicians is valued as a number one priority. So, until the kind of facial recognition embraced by Apple in the iPhoneX is more broadly adopted and finally edges physical fingerprint analysis out of the lab, keeping our forensic personnel safe and healthy is one of the most significant weapons we have in the fight against crime.

Do you work with ductless or ducted flow hoods? Do you have a preference? We’d love to hear from you!


  1. http://money.cnn.com/2017/09/12/technology/gadgets/apple-iphone-event/index.html
  2. http://www.evidencemagazine.com/index.php?option=com_content&task=view&id=1387
  3. ibid
  4. http://www.shimadzu.com/an/lifescience/maldi/princpl1.html
  5. https://www.forensicmag.com/article/2014/11/fingerprinting-adverse-conditions
  6. http://www.labconco.com/news/contain-the-cost-mess-of-latent-print-powder-fin



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