Development of lab-on-a-chip technology for the analysis of ions in natural waters

Fallatah, Ahmed M.

Abstract

There is a demand for portable in-situ measurement systems for measuring ions in environmental water samples. Commercially available flow injection analysis systems are not easy to install, have high power requirements, and use large volumes of reagents. Miniaturising the measurements is promising, and quite successful microfluidic automated systems have been developed. These systems, however, tend to only measure one species at a time, and what is really required is a portable system that simultaneously measures several anions and cations. The aim of this thesis was, therefore, to develop a novel analytical device that has the ability to measure several nutrient levels in water samples in-situ, at high-frequency measurements, by miniaturising the sample preparation and measurement. The system involved: ion extraction, ion separation, and detection with a C4D contactless conductivity detector. This analytical system to be developed needs to be able to replace the current, typical system being used in the field.

To avoid the effects of the sample matrix on the separation process, the extraction of ions from the sample before separation, was investigated. Different ion extraction approaches were explored, including ion extraction through a potassium silicate monolith and electrodialysis. The potassium silicate monolith was used to extract ions from water samples; this worked as a barrier to prevent unwanted materials present in the river water from entering the measurement system. The presence of these materials would influence the separation process.

Extraction techniques using the potassium silicate monolith were investigated, using both glass chips and glass capillaries. To prevent the electroosmotic flow (EOF) caused by the negative charge on the surface of the glass and the monolith, the surface was silanised. Three different silanisation methods were investigated: trichloro perfluorooctyl silane (FDTS), a commercial coating, and an end-capping procedure. The extraction of the ions was achieved with all three of the silanisation methods but greater reproducibility was needed.

Electrodialysis (ED) was examined to see if it could provide a more reproducible sample introduction method. The advantage of this method was that it could be effectively combined with electrophoresis (CE) for rapid pre-concentration and the subsequent determination of inorganic ions in the river water. Different ED techniques were investigated for extracting the inorganic ions. This involved exploring different membranes including polytetrafluoroethylene (PTFE), Parafilm, cellulose acetate, and ion-exchange membranes. Different ED systems were evaluated, including a commercial flow design and microfluidic chips manufactured from cyclic olefin copolymers (COC), with both single and multiple membranes.

The cellulose acetate membrane provided good results for cation extraction from a real river water sample. Sodium, calcium, potassium and magnesium were extracted at 85%, 45%, 23%, and 10%, respectively. The anion-exchange (AEX) membrane in the ED system was successful and demonstrated good results. Cation extraction with a single cation-exchange (CEX) membrane also provided good results.

Several ED microfluidic chips were designed to improve the extraction, using the ion-exchange membranes. In this system, a gold electrode was used and positive results were obtained for both anions and cations, and the anion extraction was demonstrated using a real river water sample. Five anions were extracted: respectively comprised sulphate 65%, phosphate with 31%, 82% for chloride, nitrite with 10% and nitrate with 6%.

Initial work on the separation of the inorganic ions concentrated on a monolithic column, as reported in the literature. For the separation of anions, two different anion exchangers were investigated, lysine and DDAB. The monoliths were prepared in-house and were then coated with the anion exchanger. A commercial C18 monolith was used to compare the results obtained from the homemade monolithic column. Problems were encountered with the coated monoliths, especially in terms of high back pressures, and it was decided that capillary electrophoresis (CE) would provide a better separation solution and would more easily integrate with the ED ion extraction.

A proposed design of an integrated system was presented including the pumps that are required, the reagents, and the energy use. The proposed integrated system only required one buffer for all processes in the system, which reduces the environmental impact of the chemical reagents. The suitability of the buffer (MES/His) was extensively investigated, and the amount needed for a month was also calculated to be less than 3 L, which is ideal for the system portability. It was also calculated that the required power could be provided by a battery, although the inclusion of solar panels would be advantageous.

The proposed integrated system meets most of the requirements of the project, and promising results were obtained. Further optimisation of the design will focus on increasing the robustness of field applications.

Citation

Fallatah, A. M. (2015). Development of lab-on-a-chip technology for the analysis of ions in natural waters. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4220270

• Thesis Type: Thesis
• Deposit Date: Feb 14, 2018
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4220270
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Jan 1, 2015