Communication Relaxometry of insensitive nuclei: Optimizing dissolution dynamic nuclear polarization Pascal Miéville a , Sami Jannin a,⇑ , Geoffrey Bodenhausen a,b,c,d a Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, EPFL, Batochime, 1015 Lausanne, Switzerland b Département de Chimie, Ecole Normale Supérieure, 24 Rue Lhomond, 75231, Paris Cedex 05, France c Université de Pierre-et-Marie Curie, Place Jussieu, 75005 Paris, France d CNRS UMR 7203 Paris, France article info Article history: Received 23 December 2010 Revised 3 February 2011 Available online 9 March 2011 Keywords: Dynamic nuclear polarization Hyperpolarization Relaxometry Scavenging abstract We report measurements of spin-lattice relaxation of carbon-13 as a function of the magnetic field (‘relaxometry’) in view of optimizing dissolution-DNP. The sample is temporarily lifted into the stray field above a high-resolution magnet using a simple and inexpensive ‘shuttle’. The signals of arbitrary mole- cules can be observed at high field with high-resolution and sensitivity. During the dissolution process and subsequent ‘voyage’ from the polarizer to the NMR magnet, relaxation is accelerated by paramag- netic polarizing agents, but it can be quenched by using scavengers. Ó 2011 Elsevier Inc. All rights reserved. Dynamic nuclear polarization (DNP) [1] using the so-called dissolution process [2] has become a method of choice to enhance the sensitivity of 13 C in urea [3], pyruvic acid [4], bicarbonate [5], sodium acetate and glycine [6], of protons in alanine–glycine [7,8], of 15 N in acetylcholine and choline chloride [9], of 89 Y in yt- trium chloride and its complexes with DOTAM and similar ligands [10–12], of 6 Li in lithium chloride [13], and many other nuclei. DNP can enhance nuclear spin polarization by about four orders of magnitude. This can be achieved through microwave saturation of the EPR transitions [14–16] of stable radicals such as trityl, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), or nitroxide biradicals such as TOTAPOL, mixed with the analyte in a solvent that forms glassy ‘beads’ at low temperatures (on the order of 1.2 K) in a polarizing magnet (3.35 or 5 T in our labora- tory). In the so-called dissolution method [2], the sample is rapidly heated and transferred from the polarizer to an NMR or MRI magnet. As illustrated in Fig. 1, during the ‘voyage’ from one magnet to the other, the sample is exposed to low fields (on the order of 0.5 mT), except if the polarizing and NMR magnets are housed close together in the same cryostat [17], or if the transfer tube is enclosed in permanent or electro-magnets. The orientation of the static field may vary during the voyage, but the spins follow the field adiabatically. The relaxation rates of hyperpolarized nuclei depend on the field, the chemical shift anisotropy, the nature and concentration of the radicals, the molecular mass, the translational diffusion coef- ficient D and the rotational correlation time s c . Relaxation induced by paramagnetic polarizing agents such as nitroxides is particu- larly efficient at low fields [18]. This is expected to be of critical importance for macromolecules [19]. To design the best strategy for the transfer between the two magnets, e.g., to sustain the mag- netic field above a threshold during the ‘voyage’, to select the best scavenging agents to eliminate the radicals after dissolution [20], and to predict which molecules are good candidates for dissolu- tion-DNP, it is essential to determine the longitudinal relaxation times T 1 of the hyperpolarized nuclei as a function of the static magnetic field. Conventional relaxometers using variations of the current that drives the electro-magnet [18,21,22] are usually designed for 1 H, and do not offer reasonable sensitivity for less sensitive low-c spins such as 13 C. More elaborate relaxometers using superconducting magnets and involving shuttling of the sample into a second mag- net [23], or moving the entire probe assembly in the stray field [24], provide ways to conveniently address low-c spins [25–27]. We have developed a simple and inexpensive mechanical ‘shuttle’ (Fig. 2a) to measure slow longitudinal relaxation (T 1 > 1s) typical of nuclei such as carbon-13 as a function of the static magnetic field. The nuclear spin polarization is allowed to reach Boltzmann equi- librium at high field during a delay d 1 , and the sample is then lifted in less than a second to a variable height in the stray field above the magnet, where spin–lattice relaxation is allowed to occur during a variable delay d 2 (see Fig. 2c). If desired, the sample can be moved to an area, where the field is shielded to 0.1 mT by a l-metal sheath. The sample is then shuttled back to high field 1090-7807/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2011.02.006 ⇑ Corresponding author. Fax: +41 21 693 94 35. E-mail address: sami.jannin@epfl.ch (S. Jannin). Journal of Magnetic Resonance 210 (2011) 137–140 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr