Reflection mode measures the specular reflectance from the surface of a sample.
Transflection measures the absorbance of light passing through material placed on a highly-reflecting surface.
The surface of the sample needs to be optically polished (specular reflectance) or deposited on a highly polished, non-infrared absorbing material such as a gold on glass slide (transflectance).
Please discuss with beamline staff if you plan on doing reflectance measurements.
Specular reflectance measurements produce Reflectance spectra which cannot be directly compared to more common Absorbance spectra.
Transflection measurements were commonly used in earlier spectromicroscopy work, particularly for biological samples. If sample preparation allows, samples prepared for Transmission geometry are preferred for this type of sample.
Papers discussing the potential issues with transflection analysis
P. Bassan, J. Lee, A. Sachdeva, J. Pissardini, K.M. Dorling, J.S. Fletcher, A. Henderson, P. Gardner. Analyst, 138, 144 (2013)
K. Malek, B.R. Wood, K.R. Bambery. FTIR Imaging of Tissues: Techniques and Methods of Analysis; Chapter 15. in book M. Baranska (ed.), Optical spectroscopy and computational methods in biology and medicine. Springer Science, 2014.
M. Miljkovic, B. Bird, M. Diem. Analyst, 137, 3954 (2012)