Fig. 5.

Laser power dependence of photo-CIDNP intensities in the presence of a cryogenic probe. Photo-CIDNP data at variable laser power were collected on the Trp amino acid in 90% H₂O at variable Trp concentration, namely, (A) 50 μΜ, (B) 5 μM, and (C) 500 nM. At each Trp concentration, the S/N was normalized relative to the highest S/N within the series. (D) Block diagram illustrating the fractional S/N achieved at low laser power (0.1 W, 0.2 s irradiation time) at various Trp concentrations, relative to high-power irradiation (1.5 W, 0.2 s irradiation). This plot shows that low power irradiation is particularly effective at low (ca. submicromolar) sample concentration.