Nonlinearity of Beer's law in gas-phase FT-IR spectroscopy

The nonlinear deviation from Beer's law due to a finite instrumental resolution has been studied by taking the shape of the spectrum into account. The phenomenon has been studied as a function of resolution theoretically and by simulations. The results show that the nonlinearity is constant in the part of interferogram between the vibrational structure and rotational structure since there is very small signal in that region. The study suggests that the optimal resolution has to be decided by the width of the vibrational bands rather than by the width of the rotational lines.