Gas Detection Tools

Gas Detection Tools

AEL believes in using the right tools when approaching an environmental site investigation. For sites with contaminants in soil or groundwater that readily off-gas, such as Petroleum Hydrocarbons (PHCs) which include diesel, gasoline, kerosine, etc., or Volatile Organic Compounds (VOCs) which include dry cleaning fluids, degreasing agents, and other industrial chemicals, the right toolset often involves the use of a gas detector for on-site environmental testing.

Depending on the target compound, the type of gas detector can vary. The main types of gas detectors are Photoionization Detectors (PID), Flame Ionization Detectors (FID), and Halogen Specific Detectors (XSD).

Photoionization Detectors (PID)

PID’s operate by drawing in vapours from an off-gassing sample of soil or water and shining ultraviolet (UV) light on them. Any compounds in the vapour stream that have an ionization potential less than the UV light (10.6 eV) are ionized. The ions are drawn into a collector anode, which produces a current that is proportional to the amount of ions present. This signal is then amplified and displayed to the operator in real time. Compounds that will ionize (i.e., have an ionization potential less that 10.6 eV) are hydrocarbons that tend to be aromatic in structure, for example benzene, or have double bonds between some or all of the carbon atoms, for example trichloroethylene. Compounds that will not ionize tend to be aliphatic in nature with single bonds between carbons. However, it should be noted that these are tendencies, and there is overlap. At the end of the day, it is best practice to look up the ionization energy for the target contaminant(s) to see if the PID will be able to detect them.

Flame Ionization Detectors (FID)

FIDs operate similarly to the PID in that they draw in vapours from off-gassing samples but diverge in the measurement method. Whereas PIDs are non-destructive, FIDs use a flame to combust the vapours, creating positive and negative ions. An electrode senses the ions, generating a current, which is proportional to the amount of ions. An FID will measure all volatiles with a combustion point below its flame temperature. This essentially encapsulates all hydrocarbon vapours. As such, the FID is often used to get a sense for the total concentration of volatiles in the measured vapours. The amount of volatiles with an ionization point above 10.6 eV can be determined by subtracting the PID reading from the FID reading. This can be a useful and targeted approach if contaminants on site are known to have a higher ionization point.

Halogen Specific Detectors (XSD)

XSDs also function by drawing in vapours but have a significantly different system which allows them to strictly measure halogenated solvents (i.e., any volatile hydrocarbon bonded with one or more halogen atoms such as fluorine, chlorine, etc.). The XSD uses pyrolysis to convert halogenated compounds into oxidation products and free halogen atoms. The halogen atoms are absorbed into and react with a cathodic surface, which pulls electrons from the halogen atoms. The electrons are measured as a current which directly reflects the amount of halogenated atoms in the vapours. XSDs are very useful to identify chlorinated solvents, which are a frequently encountered form of VOC, and are widespread throughout both industrial and residential areas, as they were used for industrial processes such as degreasing and in residential areas for processes such as dry cleaning. The XSD can be combined with the PID to determine what fraction of the PID reading is due to halogenated solvents versus other volatile contaminants.

Regardless of the detector used, if the sample is not prepared in the correct way, the data generated will not be useful. Samples must be collected within an enclosed space such as a ziplock bag, the mass of the soil and the soil grain size must be taken into account, and the soil needs to be homogenized and left to equilibrate with the headspace of the enclosed container it resides in. Repeatable sample preparation and measurement technique by an experienced technician is imperative to generate useful data to support decision making. This is even more the case if secondary screening instruments for volatiles are used, such as AEL’s on-site gas chromatography/mass spectrometer (GC/MS) are deployed.

AEL is proud to rely on field technicians with an abundance of practical knowledge and uses the triad method to ensure that field technicians and project managers are in constant communication during a project, allowing for an iterative optimization of workflow as the project moves forward, thereby reducing project costs, timeline, and providing clients with data they can be confident in.

If you have a project that requires precise environmental testing or if you would like to learn more about our services, contact us to discuss your project requirements and discover how AEL can assist you.