https://doi.org/10.3390/s23010180
“The measurement setup consisted of the FTIR spectrometer combined with a 10 m long-path gas cell (Pike Technologies, Madison, WI, USA), mass flow controllers (Bronkhorst, The Netherlands), and a lid (a). The lid was needed to evacuate the sample compartment of the spectrometer to avoid absorption from atmospheric gases. However, the long-path gas cell was higher than the sample compartment, so the sample compartment could not be evacuated with the standard lid of the spectrometer. A lid with corresponding dimensions, as seen in a, was constructed and designed. An aluminum cylinder (250 mm × 15 mm × 360 mm) with a polycarbonate plate (250 mm × 15 mm) was fixed through screws to a rectangular aluminum plate (282 mm × 293 mm × 10 mm). Four holes for straight bulkhead fittings were inserted in the bottom plate. The gas inlet was connected to one of the bulkhead fittings on the outside of the lid, and a second bulkhead fitting was connected to the gas exhaust (see b). The inlet and the outlet of the long-path gas cell were connected to the respective bulkhead fittings on the inside of the lid. In case the sample compartment was intended to be flushed with nitrogen gas (N2), the third bulkhead fitting was connected to the N2 source and the fourth one was connected to the gas exhaust. For evacuating, these two were closed so that the entire measurement setup remained vacuum tight.
Figure 2. (a) CAD model of the designed lid. An aluminum cylinder (250 mm × 15 mm × 360 mm) with a polycarbonate plate (250 mm × 15 mm) was fixed with screws to a rectangular aluminum plate (282 mm × 293 mm × 10 mm). (b) Measurement setup with the lid fixed to the FTIR spectrometer with the long-path gas cell inside. The gas inlet was connected to one of the bulkhead fittings on the outside of the lid, and a second bulkhead fitting was connected to the gas exhaust. The inlet and the outlet of the long-path gas cell were connected to the respective bulkhead fittings on the inside of the lid.
Before adjusting different concentrations of each gas and recording the resulting spectra with the FTIR spectrometer, the measurement parameters, such as the mirror velocity, resolution, etc., were set. For all measurements, consistent parameters were defined. A mercury–cadmium–telluride (MCT) photo detector cooled with liquid N2 was used to achieve high sensitivity. summarizes all parameters.
Table 1. Parameters used for the transmission measurements with the FTIR spectrometer (Vertex 80v, Bruker).
Table 1. Parameters used for the transmission measurements with the FTIR spectrometer (Vertex 80v, Bruker).
| Detector Type |
LN-MCT Photoconductor |
| Resolution |
0.08 cm−1 |
| Mirror velocity |
80 kHz |
| Acquisition mode |
Single-sided, forward–backward |
| Phase correction method |
Mertz |
| Apodization function |
Three-term Blackman–Harris window |
The transmission spectra were recorded after setting different gas concentrations, starting from the gas cylinder concentration down to 5–15 ppm. Certified gas bottles were obtained from Westfalen, Germany (1000 ppm SO2F2 in N2), and TEGA, Germany (refrigerants), and used as received. At high gas concentrations, the strong absorption bands were already saturated, while weak absorption bands were not saturated and were clearly measured. On the other hand, at low gas concentrations, the strong absorption bands were clearly measured, while the weak ones were not. The decadic absorption coefficients were calculated from the transmission spectrum at each concentration, and the calculated spectra were merged. The resulting spectrum then contained both the weak and strong absorption bands with the desired resolution ().
Figure 3. (
a) Decadic absorption coefficient spectrum of SO
2F
2 between 2500 and 15,000 nm and the decadic adsorption coefficient spectra of the possible substituents R227ea (
b); R134a (
c) [
32]; R125 (
d) and propene (
e) in standard conditions.
“
