Article written by Laura Hayes, Chemist, Analytical Products Group, Inc.
Complete article from the Edition 23 APG eNewsletter
Introduction
Aromatic hydrocarbons are important components of petroleum and its refined products. The extensive occurrence of aromatic hydrocarbons through accidental spills and from leakage of underground storage tanks has caused an enormous amount of contamination to surface and groundwater environments.
The aromatic hydrocarbons of low molecular weight, such as the BTEX compounds (benzene, toluene, ethylbenzene, and three isomers of xylene) are key targets of importance with respect to environmental monitoring. These compounds have become the focus of many environmental studies because of their toxic and carcinogenic potential.
Importance of Aromatic Hydrocarbon Analysis
Aromatic hydrocarbons are among the most important materials produced in the chemical industry. They are commonly used as solvents, fuel additives and chemical intermediates.
Because all aromatic hydrocarbons are derivatives of benzene, its properties are very important. In turn, the chemical structure of benzene plays a key role in determining its properties.
Figure 1: Structural representation of benzene
As you can see in Figure 1, the structure of benzene is a six carbon ring which includes three double bonds. Each of the carbons, represented by a corner, is also bonded to one other atom. In benzene itself, these atoms are hydrogens. Benzene and its derivatives comprise the naturally occurring hydrocarbons and are sometimes characterized by an ‘aromatic’ smell (hence the name “aromatic hydrocarbons”).
Among the most important aromatic hydrocarbons are toluene (also referred to as methylbenzene), ethylbenzene, xylenes (m, p, and o-xylene) and dimethylbenzenes (1,2; 1,3 and 1,4 dimethylbenzene). The organic chemicals of the BTEX group make up a significant portion (approximately 18%) of petroleum products. Of the different components that make up gasoline, the BTEX group is the largest group that has been directly correlated to any negative health effects. Because these compounds can and are known to be associated with adverse health consequences, it is essential to keep their status in the environment in check.
Analysis
There are a number of methods that have been developed by the American Society of Testing Materials (ASTM) and the United States Environmental Protection Agency (EPA) to measure the purity of aromatic hydrocarbons. The EPA recommends the EPA 8015B method for the analysis of aromatic hydrocarbons. At APG, we use the HP 6890 series gas chromatograph (GC) to simplify the analysis of aromatic hydrocarbons and improve lab efficiency. Gas chromatography is the most widely referenced analytical technique used to establish the purity of specific aromatic hydrocarbons.
GC analysis is a common determinative test of aromatic hydrocarbons. GC analysis separates all of the components in a sample. The technician injects the sample into the injection port of the GC device. The GC instrument next vaporizes the sample and then separates and analyzes the various components. The amount of time that a compound is retained in the GC column is called the “retention time.” The retention time can help to differentiate between some of the compounds in the sample(s). The size of the peaks in the chromatogram is proportional to the quantity of the corresponding substances in the sample that was analyzed. The peak is measured from the baseline to the tip of the peak.
The GC instrument uses a detector to measure the different compounds as they emerge from the column. Because of its relatively high sensitivity to most organic compounds, the flame ionization detector (FID) is a very powerful tool for GC in detecting aromatic hydrocarbons. The FID uses a flame as its ionization source. When a sample enters the detector it is passed through the flame and flammable components are ionized and analyzed. This detector is selective to the flammable components, which is a benefit when aromatic hydrocarbons are being analyzed. The FID detector responds to any molecule with a carbon-hydrogen bond, but not at all, or poorly to compounds such as H2S, CCl4, or NH3.
Properly set gas flow rates are important to achieving maximum sensitivity with an FID, in addition to preventing the flame from being extinguished. Typically, the total gas flow to the FID ranges from around 300-500ml/min, of which around 30ml/min. is the hydrogen flow plus the carrier gas flow. The make-up gas used (the gas which balances the flow) is generally nitrogen.
One difficulty that may be encountered when doing aromatic hydrocarbon/BTEX analysis is a decrease in VOC (volatile organic compound) concentration at a rather rapid rate. In an open container, volatilization can decrease the VOC concentration by 80% over a 30-minute period. To help prevent this from happening, it is best to keep the samples you will be using in a refrigerated, airtight container until you are ready to perform analysis.
Other difficulties that may arise during your analysis may be due to problems with the FID. Contamination and a clogged jet (used to optimize the shape of the flame) are common problems that can be associated with using an FID in the analysis of volatile compounds. These compounds can cause deposits to build up on the detector and they must be removed to prevent a decrease in sensitivity or to prevent spiking.
Figure 2: Jet used for FID in Gas Chromatography
 Typically, small-bore jets provide the greatest sensitivity, but they are more prone to contamination than larger jets. Generally, a compromise between the smaller and larger jets may be necessary when deciding which is best for you to use for your analysis.
References:
Brown, William H. "Introduction to Organic Chemistry, 2nd Ed." Saunders.
Restek. “EPA Office of Underground Storage Tanks (OUST): Recommended Methods.” www.restekcorp.com.
Scientific Instrument Services. Flame Jets. www.sisweb.com.
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