Emma Crooke

Using Plasticizers to Control the Hydrocarbon Selectivity of a Poly(Methyl Methacrylate)-Coated Quartz Crystal Microbalance Sensor

Chemical sensors based on a polymer coated quartz crystal microbalance (QCM) generally present poor molecular selectivity for compounds that contain similar functional groups and possess the same chemical properties. This paper shows for the first time that the selectivity and sensitivity of a poly(methyl methacrylate) (PMMA) based QCM sensor can be significantly enhanced for aromatic hydrocarbons by incorporating a plasticizer into the polymer film. The sensor was fabricated by spin coating PMMA onto a quartz crystal, and the influence of plasticizer type and amount on the response was evaluated. It was shown that the hydrocarbon sensitivity of plasticizer-free PMMA is negligible, while the sensitivity of plasticized PMMA was similar to or in some cases greater relative to highly responsive rubbery polymers such as polyisobutylene (PIB). Detection limits of 4.0, 1.5, 0.4, 0.6, and 0.1 ppm were obtained on a PMMA film containing 25% w/w di(2-ethylhexyl) phthalate for benzene, toluene, ethylbenzene, p-xylene, and naphthalene, respectively. We found that at low plasticizer levels (∼10% w/w) the PMMA film was more sensitive toward ethylbenzene and p-xylene over naphthalene when compared to a PIB film under similar measurement conditions. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements were performed to understand the sensing mechanism, and these studies confirmed a higher hydrocarbon uptake by PMMA in the presence of plasticizer. Positron annihilation lifetime spectroscopy (PALS) studies detected variations in the free volume properties of the polymer films as a function of plasticizer content. The accessible free volume as measured by PALS was significantly less in the PMMA films compared to the PIB, and this result correlates favorably with differences in the QCM response pattern. The QCM results have been rationalized in terms of free volume theory which is responsible for the higher hydrocarbon diffusion/sorption with increased plasticizer content.

The impact of water and hydrocarbon concentration on the sensitivity of a polymer-based quartz crystal microbalance sensor for organic compounds

Long-term environmental monitoring of organic compounds in natural waters requires sensors that respond reproducibly and linearly over a wide concentration range, and do not degrade with time. Although polymer coated piezoelectric based sensors have been widely used to detect hydrocarbons in aqueous solution, very little information exists regarding their stability and suitability over extended periods in water. In this investigation, the influence of water aging on the response of various polymer membranes [polybutadiene (PB), polyisobutylene (PIB), polystyrene (PS), polystyrene-co-butadiene (PSB)] was studied using the quartz crystal microbalance (QCM). QCM measurements revealed a modest increase in sensitivity towards toluene for PB and PIB membranes at concentrations above 90 ppm after aging in water for 4 days. In contrast, the sensitivity of PS and PSB coated QCM sensors depended significantly on the toluene concentration and increased considerably at concentrations above 90 ppm after aging in water for 4 days. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed that there is a change in the sorption mechanism at higher toluene levels for PS and PSB. Positron annihilation lifetime spectroscopy (PALS) studies were performed to investigate the free volume properties of all polymers and to monitor any changes in the free volume size and distribution due to water and toluene exposure. The PALS did not detect any considerable variation in the free volume properties of the polymer films as a function of solution composition and soaking time, implying that viscoelastic and/or interfacial processes (i.e. surface area changes) are probably responsible for variations in the QCM sensitivity at high hydrocarbon concentrations. The results suggest that polymer membrane conditioning in water is an issue that needs to be considered when performing QCM measurements in the aqueous phase. In addition, the study shows that the hydrocarbon response is concentration dependant for polymers with a high glass transition temperature, and this feature is often neglected when comparing sensor sensitivity in the literature.

Mid-Infrared Spectroscopic Method for the Identification and Quantification of Dissolved Oil Components in Marine Environments

The use of mid-infrared sensors based on conventional spectroscopic equipment for oil spill monitoring and fingerprinting in aqueous systems has to date been mainly confined to laboratory environments. This paper presents a portable-based mid-infrared attenuated total reflectance (MIR-ATR) sensor system that was used to quantify a number of environmentally relevant hydrocarbon contaminants in marine water. The sensor comprises a polymer-coated diamond waveguide in combination with a room-temperature operated pyroelectric detector, and the analytical performance was optimized by evaluating the influence of polymer composition, polymer film thickness, and solution flow rate on the sensor response. Uncertainties regarding the analytical performance and instrument specifications for dissolved oil detection were investigated using real-world seawater matrices. The reliability of the sensor was tested by exposition to known volumes of different oils; crude oil and diesel samples were equilibrated with seawater and then analyzed using the developed MIR-ATR sensor system. For validation, gas chromatographic measurements were performed revealing that the MIR-ATR sensor is a promising on-site monitoring tool for determining the concentration of a range of dissolved oil components in seawater at ppb to ppm levels.

Development of an arrayed hydrocarbon sensing system for the detection of seeped hydrocarbons

Hydrocarbon seeps in the marine environment are used as proxies for subsurface accumulations. Typically marine surveys to delineate these subsea features us a verity of techniques, which generally do not allow timely chemical identification of hydrocarbons. This paper outlines current research on the development of an integrated underway arrayed hydrocarbon sensing system that can be used on a variety of vessels to detect seeped hydrocarbons. The aim of the research is to increase the number of successful surveys through the deployment of this equipment. Field trials of the system have demonstrated its successful operation in the marine environment and testing of the hydrocarbon sensors have shown that the majority of the sensors used can detect dissolved hydrocarbons at ppb levels. Laboratory experiments using 23 oil-in-synthetic sea water extracts has shown that the sensor array has the potential to differentiate between oils (refined and unrefined) from different sources, based upon their seawater extracts.

Improvements to ATR-FTIR based chemical sensors for the detection of organic contaminants dissolved in water

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) based chemical sensors have been widely used for the detection of a variety of hydrocarbon compounds. The selection of an appropriate chemoselective membrane material to coat the zinc selenide (ZnSe) internal reflectance element is a crucial step in achieving acceptable sensor performance. Although there are many materials available, membrane degradation and biofouling is still an issue that has limited long-term sensor deployment in environmental waters. It is proposed that superhydrophobic membranes can suppress the diffusion of water and biomolecules to the sensing surface making the sensor less susceptible to fouling problems. This paper explores whether single-walled carbon nanotube/polymer dispersions can be used to improve sensor performance for BTEX (benzene, toluene, ethylbenzene, xylenes) compounds dissolved in water.

Real-time data processing and visualization of a hydrocarbon sensor network for hydrocarbon environmental monitoring

The development of a graphical user interface for a hydrocarbon sensor network system is presented in this paper. The software introduces two mathematical algorithms for realtime data processing and visualization: The Kalman filter is employed to remove spikes from acquired data; PCA (Principal Component Analysis) is used to reduce the dimension of the multiple sensor dataset and project the dominating differences among measured water samples on a 2D scores plot. The software also offers easy interface for simultaneous monitoring of system health status and logging activities and observations. This design greatly facilitates fast identification of anomalies, detection of sensor failure and timely collection of representative samples. Performance of the software interface is evaluated by applying real-world data collected during the Gulf of Mexico survey.