Phenolic compounds can be some of the most challenging compounds to extract from the compound lists in EPA Method 8270 and EPA Method 625.1. The recovery of these compounds suffer tremendously compared to some of the other target analytes on the list. So what exactly are phenols and why are they challenging to extract and quantitate?
Phenolic compounds are a group of organic compounds that contain a hydroxyl group bonded to one or more aromatic rings. The base compound of this class is phenol, which is simply a benzene ring mono-substituted with an -OH group. There are many derivatives of this base structure that create the large class of compounds called phenols. These compounds have a high affinity for water and are considered persistent organic pollutants (POPs) by both the U.S. EPA and the European Union (EU). Phenols are introduced into water by both natural occurrences and by human interaction.
Phenols occur in nature from the degradation of polyphenols, which are synthesized by plants and act as antioxidants. Phenols bio-accumulate in our water sources due to the decomposition of plants and animals in water supplies. This decomposition can be catalyzed by certain naturally occurring acids or exposure to UV radiation from the sun. Aside from the intake of polyphenols as antioxidants, phenolic compounds serve many functions to humans.
Most of our industrial phenol usage comes in the form of polymers. Phenolic resin polymers are frequently used in the production of appliances and coatings for wood products. One of the most widely used chemicals worldwide is Bisphenol A which serves many purposes and is discussed in detail in the blog post (Simplified BPA Analysis). Aside from polymer chemistry, phenols are used to produce many commercially available pesticides and insecticides. Whether directly or indirectly, these industrial phenols can pollute our water sources by means of discharge from wastewater, application of pesticides and insecticides, or by the degradation of polymers in landfills.
Given their high affinity for water, and more specifically acidic water, these compounds can be very challenging to extract, recover and quantify. When doing extractions according to EPA Methods 8270 and 625.1, several steps must be performed to successfully elute acid, base and neutral fractions. First, you extract the water sample in an acidic solution, then the pH of the aqueous sample is adjusted to a basic pH and extracted under basic conditions. When performing liquid liquid extraction (LLE), the phenols should be extracted during the acidic portion of the process, however, given their affinity for acidic water, extraction with methylene chloride is very challenging.
A better option for extracting phenols is using ion exchange via solid phase extraction (SPE). The use of a strong cation exchange (SCX) media will capture the phenols and other partially positive compounds electrostatically. Electrostatic interactions are much stronger than the other governing intermolecular forces that we are relying on when performing a liquid liquid extraction. The phenolic compounds and other cations are eluted from the SCX media using a low concentration of base, such as 1% ammonium hydroxide, followed by acetone and methylene chloride. The important thing to remember when performing SPE for these methods is to dry the acidic and basic portions separately and to use new sodium sulfate or a new phase separation membrane for each fraction. This ensures that the phenols do not back extract into the acidic water.
The SCX media is usually paired with an HLB (hydrophobic lipophilic balance) or DVB (divinylbenzene) media to form a mixed-mode disk to extract the full suite of 8270 and 625.1 compounds. If efficient extraction of phenols is an issue for you, maybe it is time to try a mixed-mode SPE extraction for EPA methods 8270 and 625.1.