Proposition of Hyper‐Chemical Exchange Saturation Transfer Subtraction Spectroscopy to Detect Very Weak and Broad Signals Hidden Under Baseline and Widen Range of Materials Accessed by Hyperpolarized ¹²⁹ Xe NMR

A recirculating hyperpolarization system is used for the hyper-Chemical Exchange Saturation Transfer (CEST) experiment after a stopped-flow scheme and a subtraction mode are added. The resulting hyper-CEST subtraction spectroscopy (HCSS) is effective in providing evidence for weak or broad signals. The chemical shift of these broad and weak signals has been successfully determined thanks to a thorough examination of the saturation frequency dependency. An application to polyimide reveals a very broad signal at 206.5 ppm with a line width of 96 ppm (11 kHz). The tiny dimension of the void space in polyimide, where the embedded Xe atoms are in a severely constrained state in dynamics, is supported by this broadness combined with the low field chemical shift. The weak signal that is not easy to be made evident in the standard hyperpolarized ¹²⁹Xe spectrum is successfully observed with additional application to gelatin capsules. As the storage year progresses on, the chemical shift determined for this sample reveals a low field shift.