Since a number of articles have been published in this area, it seems timely to review the development of IR-based sensors in addressing the issues and challenges facing environmental monitoring of hydrocarbon contaminants. The most significant advances in relation to MIR-ATR sensing of organic compounds in aqueous environments will be reviewed. The article will pay particular attention to sensor design and type of materials used to prepare the sensing surface.2.?Principles of Attenuated Total Reflectance (ATR)Since the development of the Fourier transform infrared (FTIR) spectrometer in the early 1960s there has been a significant rise in the application of infrared spectroscopy to investigate and understand a wide range of problems [11].
The majority of the infrared studies performed in the past involved collecting IR spectra in the direct transmission mode. However, in recent years a number of accessories and refection-based methods have been developed, and these have extended the capability of FTIR to measure a wide range of complex samples in the laboratory. In particular, attenuated total reflectance infrared spectroscopy is one of only a few techniques that allows the interfacial phenomena of many important chemical systems and materials to be investigated in situ [12]. Although, ATR has been around for several decades, the technique has only recently been exploited as a tool for chemical sensing.
ATR, also known as internal reflection spectroscopy (IRS) or evanescent field spectroscopy (EFS), is a versatile and non-destructive technique.
To reduce confusion, the term ATR will be used exclusively throughout this entire manuscript. When light strikes an interface between two materials of different refractive indices some of the light will be reflected and some will be transmitted. A standing wave normal to the reflecting surface is established in the denser medium and an evanescent non-propagating field in the rarer medium. An optically transparent material of high refractive index, known as the internal reflection element (IRE), is used to ensure that the IR beam propagates through a series of internal reflections at the sample/IRE interface. Although complete internal reflection occurs at the interface, some of the radiation (i.
e., evanescent wave) does penetrate into the sample, noting that the sample is in direct contact with the IRE. Only molecules in Drug_discovery the region of the evanescent wave will undergo interaction with the IR radiation, since the evanescent wave decays exponentially Entinostat in amplitude with distance from the IRE surface into the adjacent sample.