Fig. 1.

FEL irradiation method. a The FEL beam can be generated using an electronic gun. The electron beam is accelerated to 1.5 MeV through the linear accelerator. The periodic length of the undulator is tunable and can give various wavelengths of FELs for users. b The FEL beam output is transported through the vacuum tube and directed by means of the gold-coated mirror onto the sample. The insulin sample was placed within the circle of the beam spot on the glass slide and irradiated with the FEL. The sample was air dried or redissolved in water and subsequently analyzed by various methods. c Power monitoring during irradiation. The FEL beam was tuned to the amide I band and was directed onto the sample as described earlier. The beam power was measured at 10-min intervals for 60 min. The triangles, squares, and circles represent the first, second, and third experiments, respectively