A compact sensor system for in-line gas analysis based on heart-shaped substrate-integrated hollow waveguides and a micro-NIR spectrometer

In recent years, chemical sciences have witnessed a rapid and crescent development in analytical techniques and instrumentation, due to technological improvements in areas such as new materials, optics and electronics. Hence, the construction of portable analytical devices as robust as their bench counterparts has been facilitated. There is a wide variety of portable instruments based on UV-Vis, Raman, X-ray fluorescence, and mid- and near-infrared spectroscopy, which have been increasingly employed in process analytics, biomedical applications, and in situ monitoring.
Recently, we have described the first portable sensor system based on substrate-integrated hollow waveguides (iHWGs) coupled to a micro-NIR spectrometer (iHWG-µNIR) for sensing natural gases (Analyst 2014, 139, 3772). Although this system can be used in field measurements, it has a major remaining limitation, as it requires an external radiation source. Therefore, in the present study we describe a new heart-shaped substrate-integrated hollow waveguide (hiHWG), which directly takes advantage of the light source integrated within the µNIR, and guiding the NIR radiation through the gas sample towards the detector array, which is again part of the µNIR. The hiHWG was evaluated using two different µNIR spectrometers covering the spectral ranges 915-1650 nm, and 1150-2150 nm for the determination of mixtures of methane, ethane, propane and butane, which are the main components of natural gas. A third NIR spectrometer constructed in the laboratory based on an acousto-optic tunable filter (AOTF) covering the spectral range of 1500-2750 nm was also tested for comparing the performance to the of hiHWG-µNIR systems. In addition, multivariate models were developed with synthetic mixtures of methane (30 – 95 %), ethane (10 – 50 %), propane (3 – 30 %) and butane (0.8 – 20 %), prepared via calibrated flowmeters. A total of 70 samples were analyzed, whose compositions was validated by GC-MS analysis. Samples were divided into two sets, using the Kennard-Stone algorithm, i.e., 50 samples for calibration and 20 for external validation. All spectral data were submitted to a mean-centering and first derivative pre-treatment using a Savitzky–Golay algorithm with 7-point window and 2nd-order polynomial. PLS models using the NIPALS algorithm and one-leave-out cross validation were obtained for the target gases.
The RMSEP values obtained for methane and ethane with both hiHWG-µNIR systems were similar to those provided using the AOTF spectrophotometer, thereby indicating that the spectral region used for calibration is of minor importance, if the gas constituents are present at high concentrations. However, RMSEP values are much better for propane and butane using the AOTF spectrophotometer. However, regardless of the spectrometer it was clearly demonstrated that the hiHWG in combination with any kind of NIR spectrometer has significant potential for the development of compact, in-line, and molecularly selective gas sensing systems facilitating monitoring of (petro)chemically relevant processes or other significant analytical scenarios that require particularly short response times.