Increase in signal-to-noise ratio of > 10,000 times in liquid-state NMR

Abstract

A method for obtaining strongly polarized nuclear spins in solution has been developed. The method uses low temperature, high magnetic field, and dynamic nuclear polarization (DNP) to strongly polarize nuclear spins in the solid state. The solid sample is subsequently dissolved rapidly in a suitable solvent to create a solution of molecules with hyperpolarized nuclear spins. The polarization is performed in a DNP polarizer, consisting of a super-conducting magnet (3.35 T) and a liquid-helium cooled sample space. The sample is irradiated with microwaves at approximately 94 GHz. Subsequent to polarization, the sample is dissolved by an injection system inside the DNP magnet. The dissolution process effectively preserves the nuclear polarization. The resulting hyperpolarized liquid sample can be transferred to a high-resolution NMR spectrometer, where an enhanced NMR signal can be acquired, or it may be used as an agent for in vivo imaging or spectroscopy. In this article we describe the use of the method on aqueous solutions of [13C]urea. Polarizations of 37% for 13C and 7.8% for 15N, respectively, were obtained after the dissolution. These polarizations correspond to an enhancement of 44,400 for 13C and 23,500 for 15N, respectively, compared with thermal equilibrium at 9.4 T and room temperature. The method can be used generally for signal enhancement and reduction of measurement time in liquid-state NMR and opens up for a variety of in vitro and in vivo applications of DNP-enhanced NMR.

Fig. 1.

Schematic drawing of the DNP polarizer and parts. 1, DNP polarizer; 2, vacuum pump; 3, VTI; 4, microwave source; 5, pressure transducer; 6, sample port; 7, microwave container; 8, sample holder; 9, sample container; 10, dissolution wand.

Fig. 2.

¹³C polarization in the solid state as a function of time with 15 mM radical concentration. The increasing curve shows the polarization build-up with microwaves on (93.952 GHz, 100-mW source output). The decreasing curve shows the polarization decay after shutting off the microwaves. Mono-exponential fittings of the data yield a polarization build-up time constant of 4,900 s and a relaxation time of 28,200 s.

Fig. 3.

The dependence of the ¹³C polarization on the microwave frequency with a radical concentration of 20 mM. The output power is 100 mW, and the temperature is ≈1.1 K.

Fig. 4.

(A) ¹³C spectrum of urea (natural abundance ¹³C) hyperpolarized by the DNP-NMR method. The concentration of urea was 59.6 mM, and the polarization was 20%. (B) Thermal equilibrium spectrum of the same sample at 9.4 T and room temperature. This spectrum is acquired under Ernst-angle conditions (pulse angle of 13.5° and repetition time of 1 s based on a T₁ of 60 s) with full ¹H decoupling. The signal is averaged during 65 h (232,128 transients).

Fig. 5.

¹⁵N (natural abundance) spectrum of ¹³C-labeled urea. (Inset) The normalized partial integrals are shown in the graph. The polarization of ¹³C was 37% (difference between partial integrals), and the polarization of ¹⁵N was 7.8% (determined by comparison with a reference sample of known ¹⁵N concentration). The solid sample contained 0.6% (wt/wt) [¹³C]urea in glycerol and 15 mM radical. After dissolution, the concentration of [¹³C]urea was 8.24 mM.