# Cryogenic System for FTIR Analysis of Hydrocarbon Fuels at Low Temperature and Atmospheric Pressure
## 2 Materials and Methods  
### Spectrometer Configuration: Experimental Design and Modification  
To achieve reliable results in low-temperature spectroscopic studies of hydrocarbon substances under atmospheric pressure, we carry out experiments using a cryogenic capillary setup equipped with a low-temperature measurement cell. For precise spectroscopic data on hydrocarbons, we utilize a high-accuracy mid-infrared Fourier spectrometer FSM 2203 manufactured by \"Infraspec,\" (Saint Petersburg, Russia) offering a spectral resolution of 0.125 cm\({}^{-1}\) within the 370-7800 cm\({}^{-1}\) range with a DTGS detector and supplied with a diffuse reflection accessory. Each spectrum was averaged over 64 shots. The spectrometer chamber was being continuously blown with gaseous nitrogen to maintain an IR-inactive environment. To perform low-temperature diffuse reflection spectroscopy under atmospheric pressure, it was necessary to design a setup capable of studying hydrocarbon materials in these conditions. A key aspect of this system is the ability to regulate the sample's temperature within the diffuse reflection accessory (Figure 1), where the sample is placed in the cuvette chamber of the spectrometer. A critical feature is the capacity to cool the sample down to 80 K, enabled by the structural modifications we have made. The diffuse reflection technique requires minimal sample preparation and is suitable for analyzing uneven surfaces and a variety of coatings, such as polymers. The attachment can also be used to measure solid samples that have been previously pestled or ground. Additionally, this approach enables spectral analysis over a wide range, providing a notable benefit compared to conventional transmission methods, especially in cases where it is impossible to study with transmission. To enable cooling of the test sample within the cell of the retractable holder in this accessory, a custom modification was developed based on the standard sample holder (Figure 2). The improved holder is constructed from two main materials: copper and polylactide. These materials were selected for specific parts of the holder due to their distinct thermal conductivity characteristics. Polylactide, which has a thermal conductivity between approximately 0.11 and 0.19 W/m*K, is used to minimize the influence of ambient temperature on both the copper rod and the samples placed inside the measuring cells. The rod containing cylindrical recesses (cells) for placing samples (Figure 2, item 5) is made of copper, a material known for its excellent thermal conductivity, to ensure quick and uniform temperature distribution supplied by the cryogenic capillary system. The cryogenic capillary system (Figure 2, item 3) is likewise made of copper for the same purpose. The cooling of test samples to 80 K is achieved by a continuous flow of liquid and gaseous nitrogen through the copper tubing of the cryogenic capillary system. The efficient thermal contact between the copper tube, the copper bar, and the sample cells--combined  
Figure 1: Schematic representation of the diffuse reflection attachment: 1—micrometer screw; 2—double-sided flat mirror; 3—elliptical mirror; 4—retractable sample holder; 5—directive; 6—spherical mirror. Figure 2: Three-dimensional model of modernized specialized retractable sample holder, disassembled. 1—holder handle, 2—heat insulating cover of the holder, 3—cryogenic capillary system, 4—temperature sensor (thermocouple), 5—container. with copper's high thermal conductivity--ensures rapid cooling of the material under investigation (such as hydrocarbons) to the target temperature. To drive nitrogen through the copper tubing, high pressure is generated by heating it in a dedicated Dewar vessel. This heating forces the nitrogen out of the vessel and into the capillary system, thereby cooling the samples. The temperature of the copper bar is tracked using a thermocouple fixed to its surface, which is monitored by a LakeShore 325 PID controller (Figure 2, item 4).