So I did an experiment today using ATR-IR spectroscopy, thus I didn't know what the exact pathlength was (I should have just used traditional FT-IR!). Anyhow, I know that Beer's Law states that: A(Absorbance)= a(wavelength) * b (pathlength) * c (concentration) I know absorbance and wavelength, and am trying to find the concentration of my sample. However, I have no idea what the pathlength was since I used ATR. I looked up a formula online for penetration depth (which may be interchangeable with pathlength, not really sure), and it read: dp= λ / 2(π)(n1) sqrt(sin^2θ - (n2/n1)^2) I'm just confused as how to find the incident angle, ATR crystal refractive index and sample refractive index? Any help would be much appreciated! thanks.