Evanescent wave spectrometry

Attenuated total reflection (ATR) has grown into the most widely practiced technique in infrared spectrometry. The reasons for this are fairly straightforward the technique requires little or no sample preparation, and consistent results can be obtained with relatively little care or expertise. The technique is not foolproof, but it can be very forgiving. ATR spectrometry is known by a number of alternative names, for example, multiple internal reflection (MIR), which is not to be confused with mid-infrared, frustrated multiple internal reflection (FMIR), evanescent wave spectrometry (EWS), frustrated total internal reflection (FTIR), which is not the same as Fourier transform infrared (FT-IR) spectrometry, and internal reflection spectrometry (IRS), but IRS is better known, at least in the United States, as the Internal Revenue Service. 

ECD = electron capture detector EPA = Environmental Protection Agency FEWS = fiber evanescent wave spectroscopy FID = flame ionization detection FT-IR = Fourier transform infrared GC = gas chromatography HECD = Hall electrolytic conductivity detector HRGC = high resolution gas chromatography HSD = halide-sensitive detector H2SO4 = sulfuric acid LDPE = low-density polyethylene MS = mass spectrometry NIOSH = National Institute for Occupational Safety and Health NR = not reported PI = photoionization UV = ultraviolet detection... 

When the phenomenon of total reflection is examined in detail, it becomes clear that the evanescent radiation plays a central role in ATR spectrometry [3,4]. The electric field of this evanescent wave penetrates the sample and decays exponentially with increasing depth of penetration. If no absorption of the incident radiation occurs, the radiation is totally reflected, but if the energy of radiation is transferred to the sample at a wavenumber at which an absorption by the sample occurs, the reflectance at this wavenumber is reduced by the amount of the absorbed energy. Accordingly, if the spectrum of the total reflection is measured, a spectrum similar to a transmission spectrum is obtained. 

---