Near-Infrared Study of the Light Liquid Alkanes

A cryostat cell has been designed and constructed to enable the in situ liquification of methane, ethane, and natural gas from the mains. This cell, which was operated with pathlengths of 3 mm and 62 mm, was used to study the near-infrared spectra of methane, ethane, propane, n-butane, n-pentane, n-hexane, n-heptane, liquified mains gas, and bulk storage liquid natural gas (LNG) over the range 3000 to 15,000 cm⁻¹ with a resolution of 8 cm⁻¹. Band positions and their extinction coefficients corresponding to overtone and combination bands have been obtained for the first time under standard conditions. It was shown that significant differences are seen between the spectra of methane and ethane, and hence their relative concentrations can be estimated in the different samples of LNG, while the concentrations of the longer chain alkanes are so small and their spectra so similar that it would be impossible to determine their individual concentrations in the LNG samples without the aid of chemometric manipulation.