# Electrodegradation of Selected Water Contaminants: Efficacy and Transformation Products
## 3 Results and Discussion
### Development of Methods for Parent Compounds and Their Transformation Products
#### 3.1.3 Method Development for Triton X-100, Alkylphenols, and Their TPs  
Triton X-100 (t-OPPE) usually contains 9-10 ethoxylate units; other isomers in addition to 4-_tert_-octylphenol may also be present in the commercial product and even in the standard. In different advanced oxidation processes, similar typical TPs are formed, which are alkylphenol ethoxylates with a shorter chain (e.g., monoethoxylate and dietoxylate) and 4-_tert_-octylphenol (t-OP) or its isomers. While t-OPPE is non-volatile and thus best suited for analysis with LC, its TPs exhibit sufficient volatility to be amenable to GC analysis. However, their moderate polarity may cause distorted peak shapes in GC and derivatization is recommended to enhance volatility and improve peak shapes.  
Therefore, an HPLC-DAD method was developed to monitor the degradation of t-OPPE. The available standard contained several congeners with the polyethoxylate chain of 7-12 units, but mostly 9-10 units, as shown in Figure 2. The aim of HPLC method optimization was to elute all of the possible congeners and isomers in one narrow chromatographic peak to enable better quantification after EO degradation. Such a focused elution was achieved by using a reverse-phase LC column, an acidic mobile phase, and a steep gradient of acetonitrile vs. the aqueous phase (80% increase in 3.5 min). The same conditions were used for LC-MS, albeit with a lower flow rate. The linear range with DAD detection was 0.05-15 mg/L and \(R^{2}\) was 0.9998.  
The GC-MS method for alkylphenols and their mono- and diethoxylates was developed on the basis of a previous method [41]. One of the challenges was the choice of a suitable internal standard to enhance the repeatability of the method due to the derivatization step. After several trials, triiodophenol (TIP) was chosen because of its similar polarity to alkylphenols, phenolic group amenable to derivatization, and very unlikely presence in the real samples. A GC-MS/MS method in SRM mode was used for the determination of derivatized t-OP, n-OP, n-NP, and TIP, with a linear range of 0.0001-5.0 mg/L and \(R^{2}\) > 0.9999. Mono- and diethoxylates, however, were even in a derivatized form much less volatile and required higher temperatures for elution. Therefore, a separate GC-MS method in SIM mode was developed with a linear range for t-OPME of 0.0001-0.1 mg/L and \(R^{2}\) > 0.97. The diethoxylate standard, however, was a mixture of different isomers which eluted as several peaks, enabling only a semi-quantitative evaluation.  
For the isolation of the expected TPs from the aqueous samples, SPE was needed because water is an unsuitable solvent for GC and also for derivatization. The SPE method optimization was based on an older method [41]. Recoveries were improved by using ethyl acetate as an elution solvent. The recoveries were 86.3% for t-OP, 46.2% for n-OP, and 29.7% for n-NP, but in spite of lower recoveries for the linear alkylphenols, the extraction method was deemed suitable because of the sufficiently low LODs reached by the final analytical method, 0.02-0.4 ug/L, while RSDs were 6.7-7.6%. The same SPE method was also suitable to extract mono- and diethoxylates, with recoveries around 50%, and even t-OPPE, with a high recovery above 80%.