How low can it go: ATR-FTIR characterization of compounds isolated from ginger at the nanogram level

This proof-of-concept study demonstrated the potential of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy for the structural characterization of natural products when only very small quantities of the target compound are available. Four known compounds (6-gingerol, 6-shogaol, 8-gingerol and 10-gingerol) were isolated from ginger (Zingiber officinale) rhizome using semi-preparative high performance liquid chromatography (HPLC). A portion of each fraction was evaporated on the ATR plate and spectra collected using a standard FTIR instrument. The minimum amount required to detect some spectral features appeared to be around 50 ng for the gingerols, and around 25 ng for 6-shogaol. Various peaks are assigned and interpreted to demonstrate the range of structural information that can be obtained. Evaporated ATR-FTIR spectroscopy could be an inexpensive and rapid method to aid structural elucidation of natural compounds, even when collected from a single semi-preparative HPLC run.