# Cryogenic System for FTIR Analysis of Hydrocarbon Fuels at Low Temperature and Atmospheric Pressure
## 2 Materials and Methods
### Experimental Methodology  
Spectra were recorded every 5 degrees during heating from 80 to 100 K. At each step, the temperature was held constant for 1-2 min to ensure thermal equilibrium. Spectral image acquisition time was approximately 30 s per scan, and triplicates were averaged for each condition. The liquid nitrogen flow rate was \(\sim\)0.5 L/h with active cooling. Warming was performed by heating samples via heater cables and monitoring the temperature increase. There is schematic diagram of the entire experimental installation in Figure 5, offering a clearer overview of the system. Actual photos of the installation have been added to Appendix A.  
Figure 4: Gasifier generating nitrogen for inert IR-inactive environment. “1—Dewar vessel, 2—hose, 3—radiators, 4—pressure gauge, 5—capillary tube, 6—the valve, 7—check valve, 8—the lid”.  
Here, we provide a step-by-step explanation of the experimental procedure. Once all necessary equipment and instrumentation have been properly prepared, the investigation begins into the optical properties of the test materials (hydrocarbon compounds) at low temperature (80 K) and atmospheric pressure. The experiment is carried out by following these steps:  
_Stage 1:_ Begin by filling both the gasifier and the cryogenic capillary system with nitrogen and ensuring they are securely sealed. Connect the components as follows:  
(1) Attach the gasifier to the spectrometer's cuvette compartment inlet using a hose to maintain an inert environment.  
(2) Link the cryogenic capillary system to the temperature PID controller via the E-type thermocouple (chromel-constantan), and connect it to a heater using wires with alligator clips to generate additional pressure inside the flask, thereby improving nitrogen circulation through the capillary system.  
_Stage 2:_ Seal the cuvette compartment using layers of polypropylene and polyethylene film, leaving only openings for the micrometer screw and the retractable holder. This helps preserve the nitrogen-based IR-inactive environment within the cuvette, which enhances the quality of the recorded IR spectra. Since nitrogen and air have relatively similar molecular weights, maintaining this environment is feasible.  
_Stage 3:_ Place a pre-ground potassium bromide sample--known for its tendency to absorb ambient moisture--into the first cell of the container bar on the retractable holder. This sample will serve as a reference spectrum for later dividing the spectra of the test substances. Then, place the hydrocarbon test samples into the second and third cells and insert the holder into the diffuse reflection attachment.  
_Stage 4:_ Adjust the vertical micrometer screw to identify the position of the sample holder that yields the highest IR beam intensity for each cell individually. Once the optimal position is determined, cease further adjustments. Use this setting to record the spectra of the samples with the \"FSpec\" software, and subsequently analyze the data in \"Origin.\"  
_Stage 5:_ Acquire the spectra while gradually cooling the samples from room temperature (290-300 K) down to near boiling point of nitrogen (80 K). Use the temperature controller to monitor the sample temperature in real time, and activate the heater to increase nitrogen flow through the capillary by building pressure inside the flask. At 80 K, record the spectra. Then, the sample heater is turned on, allowing precise control of the  
Figure 5: Schematic diagram of the setup (top view). 1—Fourier spectrometer, 2—cuvette compartment with diffuse reflection attachment, 3—cryogenic cooler with Dewar vessel, 4—capillary system, 5—gasifier, providing nitrogen to the cuvette compartment, 6—computer desk with specific software.  
sample temperature. Continue spectra recording during the heating process at certain temperatures. Finally, compare the spectra obtained during cooling and heating to evaluate any differences.