You may be sure that, at least in the mentioned paper, the absorbance ratio (e.g., A1320/A1290) is the ratio of peak heights measured from a baseline drawn by hand.

Cornel Radu Thanks for the answer. That's what I figured, but can you elaborate on how you know this? If it's simply your experience then I can likely trust that this is the same case in the other papers. Also, do you have any advice as far as measuring at the reported peak wavenumbers or the measured maximum for each peak?

I agree with Prof. Cornel

The Beer's law is valid for absorbances measured at a defined wavelength/wavenumber. Thus absorbance ratio always means peak heights ratio. Any other practice must be clearly declared and explained.

They explictly state they draw a baseline between 850 and 1500 cm-1. No where is it stated that they used curve fitting procedures to isolate the peaks to determine the area of eah peak. In addition, in 1974 the calculational tools were pretty crude and extracting the peak areas would have been a major effort, probably requiring signficant coding to achieve and would have been discussed. Peak positions can shift a bit depending on the preparation of the KBr salt matrix. As long as your spectra are essentially the same the shift isn't going to be a problem in using your data for similar analysis. So I agree with Cornel Radu above as well.

Remember that Beer's law is just empiric, but it actually can be derived from the Lorentz-Lorenz relation under certain approximations. The first approximation is to neglect the polarization of matter by light. Under this approximation it is the band area rather than the peak height that is proportional to the molar concentration. The next approximation linearizes the relationship and leads to Beer's law, which means that both area and peak height are equivalent. Read the whole story in Article The Bouguer-Beer-Lambert Law: Shining Light on the Obscure