## 5 Recent Applications Using Selective Enrichment
### Environmental Analysis
Coating the surface of the internal reflection waveguide (ZnSe crystal) with the ethylene/propylene copolymer facilitated direct detection of the compounds, which exhibited well separated absorption features in the MIR wavelength range. Limits of detection varying between 20 (xylene)-80 (toluene) ppb were reported [96]. The development of molecularly imprinted polymers (MIPs) that selectively extract specific molecules in the presence of others facilitates improved sensitivity of enrichment-based MIR methods for water quality monitoring. One of the first reported papers in this area came from [PERSON] _et al._ in 1997, who developed MIPs selective for the herbicide 2,4-dichlorophenoxyayacetic acid (2,4-D) [97]. The MIP film was generated on a ZnSe ATR crystal, mounted into a flow cell through which analyte solutions were pumped. Saturation of the film was achieved after 15 min and the sorption process could be completely reversed using a buffer solution. Limits of detection ranging from 3 to 210 umoles were reported, depending on the wavenumber range employed in the analysis. More recently, [PERSON] _et al._ developed an MIR sensing methodology in which the properties of the sensing phase can be modified [98]. This is achieved using the sol-gel process in which variations of sol-gel precursors and processing conditions facilitate tailoring of the polymer properties, such as porosity, functionality and polarity. A novel phenyl-trimethoxysilane diphenyldimethoxysilane medium for detection of \(p\)-nitrochlorobenzene was designed. The medium was coated on a ZnSe ATR crystal which was placed in a temperature controlled flow cell. Analyte solutions were pumped over the polymer surface. The residence time required for this method was reduced by increasing the temperature of the system, and limits of detection of 0.7 ppm were reported. Subsequent developments in the use of silver halide fibres for water quality monitoring have been reported. [PERSON] _et al._ employed tapered silver halide FOs to improve the sensitivity of FEWS measurement of chlorinated hydrocarbons in water. Decreasing the fibre diameter increases evanescent water absorbtion and thus the sensitivity of the system. The tapered fibres were coated in polyisobutylene (PIB) and TeCE was used as a test analyte in this study. A minimum limit of detection of 50 ppb was achieved using the tapered fibres, while that for non-tapered fibres was in the range 100-300 ppb. The time taken for regeneration of the sensor in this case was around 10 min [99]. Current methods for pesticide detection require sample enrichment followed by GC/GC-MS/LC/enzyme immunoassay and are unsuitable for field testing, continuous monitoring or screening. As a consequence, there has been much interest in the development of MIR sensors for pesticide detection in water. [PERSON] _et al._ developed a sensor that could detect pesticides in water employing a polymer film coated on either an ATR crystal or silver halide FO coated in a polymer film. Atrazine and alachlor were used as test analytes in this study; the polymer film (PVC with a chloroparaffin pasticiser) enriched their concentration by up to a factor of 30. With the proposed system, detection limits of 2 ppm were achievable [88]. [PERSON] in 1999 reported the development of an MIR-FEWS sensing system for pollution monitoring in the subsea environment [100]. This consisted of an FTIR spectrometer coupled to a silver halide optical fiber sensor coated with E/PCo. A conventional lab-based ATR spectrometer with an E/PCo layer was used to demonstrate the feasibility of the proposed system and it was shown that it could detect chlorinated hydrocarbons in artificial seawater at low ppb (100-115 ppb) concentrations. The development of a sensor head for this system, consisting of U-shaped silver halide fibres coated with polymer membranes, has also been reported [101]. Simultaneous detection of tetrachloroethylene (TeCE), 1,2-dichlorobenzene and the three xylene isomers at concentrations ranging from 100 to 5 ppm were reported.