Practical Applications of Hydrocarbon and Photoionization Detection Units in Arson Investigations

Matthew D. Baldwin
Evidence Technician
Biddeford Police Department
Biddeford, Maine

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The use of investigative field equipment at crime scenes is an invaluable tool to investigators providing that the equipment is reliable and accurate. At an arson scene, investigators are already at a disadvantage given the nature of the crime. In most circumstances, not only may the evidence be destroyed, but the entire scene may be compromised by smoke, fire, pressurized water and untold personnel from responding emergency agencies. If any one specialized crime scene field investigator needed additional equipment to help locate and interpret evidence at a crime scene, one could argue that an Arson Investigator would be that one.

Specialized equipment and portable resources at a fire scene is not a new philosophy. Canines have long been specially trained and used in searching for signs of accelerants and ignitable liquid residues at fire scenes. Their usefulness and effectiveness at locating sources of ignitable liquids cannot be argued. The canines are trained to detect the vapors from minuscule amounts of remaining ignitable liquid residues that are often buried under heaps of burned debris and rubble. Although an experienced Arson Investigator can often determine the subtle differences between signs of an arson motivated incendiary burn and the resulting burn from an accidental fire, even the most seasoned investigators will still sometimes need assistance and reassurance while determining their hypothetic conclusions. A canine, if available, is a suitable resource to assist in the investigation. What resources does an investigator have however if a canine is not available? What if the remaining debris from a fire does not allow for safe conditions suitable for a canine? What if the suspected ignitable liquid residue is in an area in which the canine does not have access? (ceilings, small openings, etc.) If a canine is not available or it's use is otherwise not practical, a Hydrocarbon Detector may offer additional resources to the Arson Investigator.

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In the field of arson and fire investigations, combustible vapors or vapors resulting from accelerants and ignitable liquid residues are often referred to as "Hydrocarbons." These hydrocarbons are a by-product of the original petroleum based material that may have been used as the original ignitable liquid residue used to start the incendiary fire (e.g. gasoline, kerosene.)

Although there are several derivatives of hydrocarbons, the primary categories in arson investigations are the Saturated Hydrocarbon (single atomic bond, saturated with Hydrogen.) These petroleum-based hydrocarbons are more easily recognized as chemical gases such as Methane and Propane, and chemical liquids such as Hexane, a common constituent of gasoline, and Benzene, a common constituent of crude oil and many solvents. As an arson investigator, recognizing hydrocarbons used in accelerants, (e.g. gasoline) is an important element in determining the origin and cause of an incendiary fire.

In addition to the use of canines during an investigation is the use of portable accelerant detectors. Hydrocarbon Detectors (also sometimes referred to as hydrocarbon "Sniffers") and Photoionization Detectors (P.I.D.s) are both additional devices, which can be used during an investigation where arson may be suspected. These devices are not necessarily new to the field of arson investigation, however the advanced, fast developing and changing technology has only increased the practical applications of such portable equipment when needed. There are many types and models of both Hydrocarbon Detectors and Photoionization detectors. There uses vary from a range of employment fields including Hazardous Material teams, Plumbers, Engineers, Pipe Fitters, Miners, and any other occupation that consistently deals with petroleum products or pressurized petroleum gasses. For the sake of this issue, I'll only be discussing the units that are practical to arson investigation.

The hydrocarbon detector "Sniffer" is offered in many makes and models. Its principle is largely based on a unit-incorporated sensor that can detect minute concentrations of combustible and hazardous vapors. The unit uses a small vacuum pump to pull vapors from your sample area into an opening. This opening, usually a narrow hose like nozzle, draws the vapors into a chemical detector that is specifically designed to detect and sometimes separate the hydrocarbon molecules from the accelerant vapors.

The electrochemical sensors contained within the units can measure the electrical currents that are produced when the target gas molecules and applied reagents react with each other. Applied catalytic bead sensors utilize a combustion chamber to burn the combustible vapors along the surface of the catalytic bead. As a result of the combustion, a resistance level of the burnt vapors is recorded along the bead's surface. The result of resistance is then converted into a measured concentration of vapors by the unit's computer.

There are various models available of the hydrocarbon detector, many of which have a practical use for Arson investigations. One model available is the SEARCH CGD100. This unit is a portable and battery operated electronic combustible hydrocarbon detector. A LED light bar graph displays the strength of the suspected vapors. An additional and optional audio switch allows for an alarm to sound when the vapors are detected. The alarm, a clicking sound, increases in frequency as the combustible gas vapor signal becomes stronger. Constant adjustment of the sensitivity control knob is not always necessary if the unit is used appropriately.

If the LED signal displays a vapor presence or the audible alarm sounds before the suspected contaminating source can be found, it is likely that the air is already contaminated with a high concentration of suspect hydrocarbons. In these instances, it may be necessary to desensitize this particular unit by turning the sensitivity control knob counterclockwise to a lower sensitivity level.

In searching a general area for contamination, the clicking will speed up in volume and intensity as the suspected vapor source is approached. Just a small amount of combustible gas vapor entering the sensor at the end of the probe will cause this reaction. Some of the detectable contaminants are listed as follows:

  • Gasoline, Acetone and Benzene Vapors
  • Hydrogen
  • Alcohol (various)
  • Ethane, Isobutane, Methane, Hexane & Propane

Some precautions to note while using this unit are that the CGD100 should not be used in a "Class 1" or highly volatile, explosive atmosphere. When arriving at an arson scene and before the unit is put into service, it should always be turned on and adjusted in an atmosphere that is not yet contaminated. The contaminated scene should then be approached with the unit already in the 'ON' position. A properly functioning rechargeable battery will allow for about 3 hours of constant use.

This tool has proven effective for locating even the smallest concentrations of hydrocarbons and the presence of other hazardous vapors at a suspected arson scene. As imagined however, the Unit's sensitivity level allows for the potential of false positives.

Other models designed specifically for arson scenes are the Grace products model 850 and 851. The Model 850 "Investigator's Aid" is described as "A rugged, non-calibrated detector, which is excellent for detecting hydrocarbons/accelerants/gases." This particular unit is designed to detect the presence of hydrocarbons that may be present at a fire scene. It incorporates "Advanced signal processing techniques that make it one of the most effective and reliable survey tools available."

The Model 851 Investigator's Aid is a "Calibrated detector" which is also excellent for detecting hydrocarbons and accelerant gases at arson scenes. This unit features two separate modes of operation: a high sensitivity search mode for locating very small gas leaks, and an additional calibrated mode for providing percentage indications of the lower explosive limits of volatile gases. Methane is used as the test indicator for this option.

To operate these units, the user powers it on and waits approximately 3 minutes so that the sensors can reach temperature stabilization. The Model 851 has a sensitivity rating of approximately 50 parts per million (ppm). When the sensor is exposed to a hydrocarbon gas or petroleum volatile, the meter indicator will react, followed by a series of beep tones. Higher concentrations of hydrocarbons will create a more rapid series of beeping. The unit can be adjusted, as necessary, when scanning levels with high concentrations of hydrocarbon vapors. Proper adjustment can avoid constant alarms and false positives.

When approaching an area suspected of containing accelerants, the investigator should scan the area of concern at a 45 degree angle while holding the sensor approximately 1/2-1 inch from the ground. The suspected area should be examined very slowly to allow sufficient time for the hydrocarbons to be dispersed into the detector. The user needs to keep in mind that the hydrocarbon saturation level can be affected by several factors including temperature, wind, and humidity. These units also allow for them to be "Purged" when their use is completed so that they can be used again without cross contamination.

These are just a few of the dozens of makes and models of various hydrocarbon detectors available to arson investigators. It's important for the investigator to remember however that the units are not without their faults. The catalytic bead present in most models is used as a universal combustible vapor detector and as a result, it is often susceptible to false positive alerts on pyrolysis items that are already saturated with a hydrocarbon base. When used in combination with accelerant detection canines however, the Hydrocarbon Detector can be an efficient and inexpensive tool to improve the quality of your evidence samples collected from the scene.

A Photoionization Detector (PID) is another portable tool practical to Arson Investigations. Much like the Hydrocarbon "Sniffers", it's a tool that recognizes the presence of hydrocarbon vapors as a fire scene. The reason to use more than one kind of detector for hydrocarbon detection is to achieve a more selective and more highly sensitive detection of the specific compounds that are encountered in particular gas chromatographic analyses. The selective determination of "Aromatic" hydrocarbons is the primary function of the photoionization detector. The "PID" is an ion detector that uses high-energy photons in the ultraviolet (UV) nanometer spectrum. This process is able to break molecules into positively charged ions by ionizing the analytes exiting from the GC (Gas Chromatograph) column contained within the unit. As compounds and chemicals emerge from the G.C.'s column they are bombarded by high-energy photons and are ionized when the molecules absorb the high energy UV light. UV light excites the molecules, resulting in temporary loss of electrons in the molecules. The ions produced by this process are then collected by electrodes. The current generated by this process provides a measure of the analyte concentration of hydrocarbon gases and provides the necessary readings.

Typical photoionization detectors measure hydrocarbons in concentrations from sub parts per billion to 10 000 parts per million (ppm). The photoionization detector is arguably the most efficient and one of the most inexpensive types of hydrocarbon detectors. As an arson investigation tool, it can be another valuable asset.

Like the Catalytic based Hydrocarbon detectors, there are also several makes and models of the Photoionization Detectors. The IST-AIM 450 Photoionization Detector has been described as a compact, easy to use, and portable monitor which incorporates a photoionization detector for detection of hydrocarbons such as benzene, chlorinated hydrocarbons, ethyl benzene, toluene, and xylene. The AIM 450 (PID) features a large LED display audible and visual alarm indications. A selector switch allows you to choose between two available ranges and allows the flexibility to monitor a variety of different applications at fire scenes. The unit includes a built in sampling pump and is easy to decontaminate after use. It operates on 4 'C' size alkaline batteries or on an AC adaptor/charger.

The MiniRAE 2000, manufactured by RAE systems, is another PID that allows for quick and easy use at an arson scene. This model incorporates a "Survey Mode" that when used properly, will indicate the presence of hydrocarbon concentrations. The Geiger Counter feature will send both audible and visual alarms as concentrations of hydrocarbons increase to assist in narrowing down suspected areas.

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A third unit, the Drager Multi-PID 2, is described as "The next generation of reliable photoionization detection." This unit additionally detects volatile organic compounds (VOCs) such as hydrocarbons. The Drager model can be used for various applications like soil, water or jar headspace screening, leak detection and confined space measurements. It comes equipped with a standard 10.6 eV UV lamp that can cover a measuring range from 0 to 2,000 ppm. If the optional dilution probe is used it can extend the measuring range up to 20,000 ppm. This Drager model is said to have an "Ergonomic design" which makes it easy to operate, even when wearing heavy duty gloves and safety equipment. It incorporates a three button menu navigation that makes the use of this instrument very user friendly.

All of the various models of detection units mentioned above were found to range in price, effectiveness, and use of operation. Depending on the experience of the operator, the condition of the chosen equipment, and the circumstance of the fire scene, the detection unit used may provide varying degrees in reliability and effectiveness.

In summary, it's clear then when used appropriately and under the right circumstances, Hydrocarbon detection equipment can be an invaluable extra investigation tool in the field. In today's environment, the new, inexpensive and portable equipment is more easily obtained and constantly being updated as technology advances. It's important to keep in mind that these detection units are not without their flaws. The catalytic beads in the vapor vacuum models will often need to be replaced or cleaned as pyrolysis can occur. Some models also need to be recalibrated on a regular basis. The Photoionization units have UV lights that need to be replaced occasionally and the GC tubes need to be checked often to ensure that there is no cross contamination. There's no argument that these portable detection units can be substituted fully for a qualified Canine at a suspicious fire scene and a seasoned Arson Investigator still needs to know how to interpret whatever evidence and samples are collected from the scene. On that occasion however when a Canine is not available or when investigating an area in which a canine does not have access, these tools can assist in locating those hard to find samples that may have otherwise gone unfound. Once samples are collected and moved to a stable environment, the Canine can be used to check the samples again to verify or dismiss the suspicious sample collected.


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NFPA 921 Guide for Fire and Explosion Investigations 2008 Edition National Fire Protection Association, Copyright 2008

"Technical Information SEARCH Hydrocarbon Detector Catalog No.GCD100." SIRCHIE Fingerprint Laboratories, March 5th, 2009.

"DRAGER Photoionization Detectors." Dräger Safety AG & Co. KGaA, 2008, March 17th, 2009

"PID's as an Arson Investigation Tool." RAE Systems 2009, March 5th, 2009

"Gas Detection Units." Grace Industries 2009, March 15th, 2009

Article submitted by the Author
Article posted February 25, 2015