The spectrometer may operate in transmission or reflection, but also in attenuated total
reflection (ATR) mode, which have been widely used during the last two decades. The
transmission mode is very suitable for quantitative analysis, since the main parameters to
apply on the Beer-Lambert law are known or easily estimated. The reflection mode is used
when polymer is not well dissolved at room temperature, and a film or pellets of the sample
is characterized. Such is the case of polyolefins and some engineering polymers like PET