Applications of purge-and-trap gas chromatography - Mass spectrometry for the determination of volatile organic compounds in water
Ng, Shu Jun
Date of Issue2018-02-12
School of Physical and Mathematical Sciences
Volatile organic compounds (VOCs) enter water systems through rainfall, through intentional or unintentional industrial or domestic releases as well as through leaching of (from) contaminated sources. The adverse effects of VOCs on health, the environment, and scientific experiments has led to monitoring of VOCs in various aqueous media. Purge-and-trap gas chromatography-mass spectrometry (PAT GC-MS) was applied to three applications of monitoring VOCs in water; (i) to the yearlong monitoring of VOCs present in rainwater collected in the western region of Singapore, (ii) in the qualification and quantification of leachable VOCs from syringe filters, and (iii) in the assessment of the efficiency of water purification with natural materials (lecithin, olive oil, and vitamin E) for the removal VOCs. A PAT GC-MS method for the detection and quantification of VOCs in rainwater was developed for 28 VOCs and described in Chapter 2. Method 524.4 from the Environmental Protection Agency (EPA) of the United States was used as a guideline for the method development of the list of potential VOCs to be monitored in rainwater. Some of the VOCs from the potential list had not been validated or tested by the original method but were proven to be quantifiable via the method with good reproducibility and precision found after the optimization. The concentration of VOCs in rainwater were known from the literature to be low in concentration. Therefore, development of Selective Ion Monitoring (SIM) detection mode was performed. Through the study, a suitable collection set-up was tested and shown not to introduce VOC contaminants while the act of filtering was found to potentially introduce contaminants into the samples collected. The contamination of VOCs from syringe filters was further explored in Chapter 3, and the different filtrates were tested to determine if any one condition may trigger the introduction of VOCs contaminants. A list of different syringe filters with different housing material and filter membrane were tested, and it was determined that housing material significantly affects the type and amount of VOCs detected in the filtrate. Several VOCs such as nonanal could be removed from the syringe filter through prefiltration while some VOCs such as styrene and methyl methacrylate were found to be persistently present in the filtrate after substantial volume of water had been passed. Henceforth, filtration was not used as a sample preparation step to the rainwater collection, but the particulate matter in the samples were allowed to settle down before introduction to the PAT system to reduce the contamination issues to the frit in the sparger of the PAT. A yearlong monitoring of VOCs in rainwater was performed in the western region of Singapore. The sources of emission, as well as the seasonal monsoon, were found to affect the concentration of VOCs determined. One of the most severe haze period in Singapore due to the biomass clearing in the neighboring countries occurred during this yearlong monitoring of VOCs in rainwater and the impact of haze was found to be not limited to particulate matter, but also the increase in VOCs detected in rainwater. A Positive Matrix Factorization (PMF) analysis of the yearlong result deconstructed that 8 factors traceable to the sources of emission in the vicinity of the sampling site contributed to the VOCs detected in rainwater. The traditional methods of water treatment often require the use of methods or materials that may be hard to maintain or expensive, moreover, the goal of water treatments are often not focused on the removal of VOCs that can have much effect on human health. Therefore, the concept of water purification with food safe and readily available material for the removal of VOCs was explored with lecithin and other natural materials. The combination of lecithin and olive oil was found to be efficient in removing VOCs from water through a phased exchange enhanced by the presence of a small amount of lecithin. The results obtained were optimistic in providing a foundation of natural materials as an alternative exploration of a water purification treatment method.