Cylindrical IR-ATR Sensors for Process Analytics.

Armin Lambrecht,Jochen Erb,Gerd Sulz,Hendrik Fuhr,Carsten Bolwien

doi:10.3390/s20102917

Abstract

Infrared attenuated total reflection (ATR) spectroscopy is a common laboratory technique for the analysis of highly absorbing liquids and solids. However, in a process environment, maintaining a sufficient sample exchange and cleaning of the sensitive surface of the element is a crucial issue. An important industrial application is the measurement of isocyanate concentrations. Isocyanates are necessary for the fabrication of polyurethane materials and are among the chemicals with the highest production volume worldwide. For process applications, narrowband photometers or MEMS spectrometers are more appropriate than the use of bulky FTIR instruments frequently encountered in a laboratory environment. Toluene diisocyanate (TDI) and hexamethylene diisocyanate (HDI) concentrations are measured with a planar ATR photometer setup. Using a miniature Fabry–Perot interferometer (FPI), trace concentrations below 100 ppm (m/m) are detected. By employing an ATR element of the cylindrical shape, sensors can be realized with a smooth surface ideally suited for an automatic cleaning system in a process environment. A laboratory setup with sapphire tubes as ATR elements for incorporation in a liquid flow system is described. Reflection and transmission configurations were investigated. Measurements with acetonitrile as a less toxic substitute showed that with cylindrical ATR sensors’ detection limits for isocyanate concentrations below 100 ppm (m/m) are feasible.

Highlights

Infrared attenuated total reflection (ATR) spectroscopy is a common technique for the analysis of highly absorbing liquids and solids
The use of an ATR photometer for isocyanate concentration determination was previously demonstrated in a non-English publication [22]
The sensor consists of a thermal infrared emitter, a sapphire ATR crystal, and a dual-band pyroelectric detector to register radiation in a spectral window around the main absorption band of the analyte and a reference channel in a spectral window with negligible sample absorption

Summary

Introduction

Infrared attenuated total reflection (ATR) spectroscopy is a common technique for the analysis of highly absorbing liquids and solids. ATR attachments are available for most Fourier transform infrared (FTIR) spectrometers, e.g., [13] In this case, samples are often manually placed on top of or, in the case of solid samples, pressed onto a suitable ATR crystal surface. Immersion ATR probes for the spectroscopy of liquids, e.g., in a laboratory reactor, are available [14,15,16]. They consist of infrared light guides or fibers, which are connected to an ATR prism at the tip of the probe

Results

Discussion

Conclusion