Separating water molecules according to the spin on their hydrogen atoms is now possible thanks to researchers in Israel, who used a magnetic field to focus a beam of ortho water. The research could help boost the sensitivity of nuclear magnetic resonance (NMR) spectroscopy [Alexandrowicz et al. Science (2011) 331, 319 doi:10.1126/science.1200433].
Water molecules in which the nuclear spins on the hydrogen atoms are aligned are known as "ortho", water with parallel spins are called "para". Spin isomers may have different physical properties, but separating them for research is difficult. Now, Gil Alexandrowicz and colleagues at the Israel Institute of Technology Haifa, Israel, have turned to the twentieth-century physics of cold molecular beams and magnetic fields to deflect isomers based on spin state. A hexapole magnetic field focuses the beam into a mass spectrometer, with only ortho water being deflected and other spin isomers following a divergent path as if the magnetic field were not present.
The hyperpolarized water molecules produced by this technique might lay the foundations for ultrasensitive NMR experiments in the future. In conventional NMR only a fraction of nuclear spins are aligned, but pre-aligned spins in a sample might boost signal intensity significantly. The ability to separate ortho and para water molecules and study their interaction with surfaces might also have important implications in the field of astrophysics where ortho/para water ratios are derived from measurements of interstellar radiation. These ratios provide a unique method to remotely measure interstellar temperatures; however, reliable interpretation of these measurements requires progress in our understanding of the lifetime of the two spin isomers.
The Alexandrowicz work uses an entirely different approach to that taken by Vladimir Tikhonov and Alexander Volkov of the Russian Academy of Science. In 2002, the Russian team reported [Science (2002) 296, 2363, doi:10.1126/science.1069513] the controversial separation of enriched ortho and para water droplets that were sustained in either state for almost half an hour. Independent researchers are yet to corroborate the Russian results.
"The lifetime of ortho and para-water which was stored in liquid and solid phases and subsequently released to gas phase again, is indeed a controversial and interesting topic, as current estimates for the storage lifetime vary by many orders of magnitude," Alexandrowicz told Materials Today. "Separation methods of the type we presented might be used in the future to provide some definitive answers."
David Bradley