What Is Present - Field Sampling: Principles and Practices in Environmental Analysis - page 234

ethylbenzene, and the xylenes. This group of compounds is often referred to as BTEX,
meaning some combination of all of these compounds. Any benzene ring with an alkyl
group may also be referred to as an alkylbenzene. Compounds of this type are shown at
the top of Figure 9.3. Hydrogen atoms are at each corner of the hexagon rings, as shown
for benzene C 6 H 6 , but they are omitted in the other diagrams for simplicity.
A second group of compounds is made up of benzene rings bonded to each other by a
single bond or where two or more rings share carbons. An example of the first type of
compound is biphenyl, which has two benzene rings joined by a single bond. Compounds
containing only the benzene rings carbon hydrogen and halogens can be either toxic or
carcinogenic. One important group of these compounds is the polychlorinated biphenyls
(PCBs). An example of a polychlorinated biphenyl is shown in Figure 9.3. There are 10
carbons that can have a chlorine substituted for hydrogen on either or both of the two
rings. There are thus a large number of possible PCBs.
Examples of the second type of compound are situations in which the benzene rings
share two carbons (e.g., naphthalene or anthracene). These condensed ring compounds
may have the rings joined in many different ways, as shown in Figure 9.3, producing
different compounds. The