  Absolute quantity imaging of biomolecules on a single cell level is critical
for measurement assurance in biosciences and bioindustries. While infrared (IR)
transmission microscopy is a powerful label-free imaging modality capable of
chemical quantification, its applicability to hydrated biological samples
remains challenging due to the strong water absorption. We overcome this
challenge by applying a solvent absorption compensation (SAC) technique to a
home-built quantum cascade laser IR microscope. SAC-IR microscopy improves the
chemical sensitivity considerably by adjusting the incident light intensity to
pre-compensate the IR absorption by water while retaining the full dynamic
range. We demonstrate the label-free chemical imaging of key biomolecules of a
cell, such as protein, fatty acid, and nucleic acid, with sub-cellular spatial
resolution. By imaging live fibroblast cells over twelve hours, we monitor the
mass change of the three molecular species of single cells at various phases,
including cell division. While the current live-cell imaging demonstration
involved three wavenumbers, more wavenumber images could measure more
biomolecules in live cells with higher accuracy. As a label-free method to
measure absolute quantities of various molecules in a cell, SAC-IR microscopy
can potentially become a standard chemical characterization tool for live cells
in biology, medicine, and biotechnology.
