Polyaromatic hydrocarbons (PAHs) are a large group of organic compounds consisting of two or more fused rings. They are also one of the most widespread groups of organic pollutants in the environment. Also referred to as polynuclear aromatic hydrocarbons or polycyclic aromatic hydrocarbons, PAHs are formed during the incomplete combustion of coal, oil, wood and other organic material. Thus, they occur naturally and from man- made activities and are suspected carcinogens, even at very low levels. There is some evidence that chimney sweeps and tar workers developed skin cancer due to dermal exposure to PAHs in the workplace.
Human exposure to PAHs occurs in several ways. Most of us are exposed when we breathe smoke (cigarettes), auto emissions, or industrial exhausts. Charbroiled meats are also a source of PAHs, and it may also be in the groundwater near certain industrial sites. Roofing tar and driveway sealers are also known sources of PAHs, thus workers in those industries are at a higher risk for exposure. The toxicity and widespread occurrence of these compounds in our environment has resulted in regulation by the EPA. The EPA has set maximum allowable levels in drinking water and also requires the reporting of releases into the environment. Thus, there is the need for reliable, sensitive and very robust analytical methods and instrumentation for the determination of PAHs.
How are PAHs determined?
There are two common methods for the determination of PAHs in environmental samples. The first is high performance liquid chromatography (HPLC) using EPA methods 610 or 8310. In this method, a combination of UV and fluorescence detection is used to determine 16 common PAHs. This method can be somewhat problematic due to matrix interferences. In addition, there is no positive confirmation - identification is determined by retention time only.
The second common environmental method for the determination of PAHs is EPA method 8270, which uses gas chromatography/mass spectrometry (GC/MS) in the full scan mode. This is a fairly routine method and provides absolute identification because of the availability of spectral libraries for compound confirmation. However, this method is not capable of achieving low enough detection limits for some applications.
To overcome this disadvantage, the GC/MS can be operated in the Selected Ion Monitoring (SIM) mode. The instrument is programmed to collect data at masses of interest instead of stepping the mass filter over a wide range of masses. Because the mass spectrometer collects data at only the masses of interest, it responds only to those compounds that possess the selected ions. Because only a few masses are monitored, much more time is spent looking at these masses, thus increasing sensitivity.
PAH determination by GC/MS has additional limitations. Some of the higher molecular weight PAHs tend to be absorbed on any active or cold site in a GC system. Therefore, typical PAH analysis on a GC/MS system can show decreasing response and sensitivity with higher molecular weight compounds.
Recent developments in technology have addressed some of the issues concerning PAH determination by GC/MS. One of these was the introduction of the Agilent Technologies 7890A GC, coupled with the 5975C Series Mass Spectrometer. The 5975C MS has an inert ion source that allows for higher operating temperatures, which increases the response for active and late-eluting PAH compounds. The inert ion source eliminates surface activity reactions and requires less frequent cleaning, which means less down time and increased productivity. Using this system in the SIM mode allows for low-level detection of these compounds with high sample throughput.
Microbac Laboratories, Chicagoland Division is currently running low level PAHs using Agilent Technologies equipment and achieving very low regulatory limits as required by Indiana RISC (Risk Integrated System of Closure) and Illinois TACO (Tiered Approach to Corrective Action Objectives).
Authored by: Brian Mills, Organics Chemist, Microbac Laboratories, Chicagoland Division