The equipment for bullet-DNP is proven in Fig. 1.
Bullet-DNP gear. a Sketch of the general setup, comprising a polarizer magnet with cryostat and DNP insert, a switch tube (6) with a solenoid alongside its whole size, and an injection machine inside a liquid-state NMR magnet. The polarity of the superconducting magnets is indicated by the purple arrows. The course of the utilized pulsed magnetic discipline alongside the switch modifications on the valve (5), the place two orthogonal pairs of Helmholtz coils (four) are used to rotate the sector. b Rendering of the decrease a part of the DNP insert displaying the drive fuel tube (three) on the left, the principle pattern tube (middle, 2) and the microwave tube (1). The central brass piece (11) hosts small circuitboards for tuning and matching. SMA connectors are soldered to the highest brass piece (9) that additionally pushes the coil help (10) onto the underside brass piece (12) to seal the switch path. c Rendering of the decrease a part of the injection machine with a mounted NMR pattern tube. The underside of the solvent reservoir (7) could be seen on the prime of the picture, adopted by the pinch valve (eight) and a 3D printed nylon piece (13) with an inside channel that connects a three/16” metal tube to the NMR pattern tube to facilitate evacuating the latter. Two additional nylon items (14) are screwed onto the NMR tube (15) utilizing a compression becoming, and facilitate a quick alternate of NMR tubes in between experiments
Whereas the polarizer shares many components with beforehand reported systems2,23, the 2 distinguishing options are a provision to quickly eject samples utilizing pressurized fuel and a rigorously engineered magnetic tunnel alongside the whole switch path. A rendering of the decrease a part of the DNP insert is proven in panel (b) of Fig. 1. The DNP insert contains three metal tubes for helium drive fuel, pattern insertion and ejection, and microwave irradiation. The drive and pattern tubes are 1/four” metal tubes with four.2 mm ID, whereas the tube carrying the microwave is a three/eight” tube. The underside of the DNP insert contains three brass items. The underside brass piece connects the drive tube with the pattern tube through an inside channel that’s linked to the skin helium tub throughout regular operation however closes throughout ejection of the pattern utilizing an extra stream valve. Thus throughout regular operation liquid helium flows into the pattern tube for environment friendly pattern cooling. The middle brass half carries a gold-plated mirror for the microwave irradiation and helps a printed circuit board for tuning and matching the radio-frequency (RF) saddle coil. The RF coil is wound onto a PEEK coil help that hosts the bullets. The bullets are miniature cylindrical buckets constructed from PTFE. The bullet is 12 mm tall, with an outer diameter of three.9 mm and a 10 mm deep, three.5 mm diameter concentrical bore to accommodate the pattern. Every bullet can accommodate pattern volumes as much as 80 μL. The PEEK coil help is clear for the microwave and RF fields required to polarize the pattern and monitor the polarization degree, however confines the helium fuel contained in the switch path throughout pattern ejection. Brass screws push the middle brass piece onto the PEEK coil help that’s thereby pushed onto the underside brass piece. The pattern tube suits into the highest of the coil help that compresses on cooling, affording seal of the switch path.
A 1/four” inch T-union (Swagelok, UK) on prime of the plug valve (SS-4P4T, Swagelok) on the prime of the pattern tube is used to use a small stream of helium fuel when loading the bullet into the chilly pattern house, thereby stopping contamination of the system with air. After loading the pattern, the versatile switch tunnel is linked on the prime of the Swagelok T-union.
Within the experiment introduced right here, the pattern is shot by way of a three.2 m lengthy switch tunnel that’s linked to the highest of the DNP insert and extends 50 cm into the injection machine within the secondary magnet. A minimal bend radius of zero.5 m is chosen to restrict pattern deceleration within the tunnel bends. A solenoid wound alongside the whole switch path is energized with a present of 60 A previous to pattern ejection, utilizing a 15 kW energy provide (Keysight Applied sciences, US). A solenoid can also be wound across the pattern tube contained in the polarizer, guaranteeing that the sector skilled by the pattern by no means drops beneath 75 mT. The solenoids are made by tightly winding 1 mm diameter insulated copper wire (RS) onto the switch tube, resulting in an estimated discipline of 75 mT for a present of 60 A. The polarizer and the secondary magnet in our lab each have the identical polarity. Due to this fact, a reversal of the sector course alongside the switch path is required. Any change in discipline course ought to happen adiabatically, that means that the Larmor frequency ωL ought to all the time be considerably bigger than the frequency of the rotation of the sector as seen from a body fastened with respect to the shifting bullet24. In our setup, that is achieved utilizing two pairs of Helmholtz coils across the plug valve on the prime of the pattern tube. The person coils have a diameter of ~6 cm. The primary pair of Helmholtz coils is used to use a transverse discipline, the second pair is a longitudinal anti-Helmholtz pair. Contained in the polarizer the magnetic discipline is utilized alongside the course of journey. Following passage by way of the Helmholtz coils the magnetic discipline is utilized reverse to the course of journey. The sector rotation is carried out over a distance of ~6 cm (see Supplementary Fig. 1 for a schematic drawing of the Helmholtz coils). At a bullet pace of 100 m/s this corresponds to a frequency of ~1 kHz, nicely beneath the Larmor frequency ωL/(2π) = 800 kHz of 13C at 75 mT. Notice that the utilized discipline of 75 mT additionally ensures that the Zeeman interplay dominates different spin interactions—the spins are quantized alongside the utilized discipline always. Particularly, the sector can also be sufficiently robust to considerably suppress thermal mixing of the proton and carbon spins within the pattern20. That is essential, since in singly labeled 13C pyruvic acid the 1H spin tub has a ~50 instances bigger warmth capability than the 13C tub20. Thermal contact would subsequently quickly quench the 13C hyperpolarization.
The passage of the bullet by way of the switch tube is monitored utilizing optical limitations19 on the entrance and exit of the switch tunnel. For the experiment described right here, the drive fuel stress was set to 10 bar, resulting in a switch time of ~70 ms. Preliminary experiments present that the switch time could also be lowered additional by rising the drive fuel stress. The pattern is simply ejected after the switch coils are totally energized. The time dependence of the present within the coils, and the indicators from the optical detectors are proven in Fig. 2a, b, respectively. The optical sensors are positioned ~40 cm above the sector facilities of the 2 major magnets.
Timing of the pattern ejection, relative to the beginning of the NMR knowledge acquisition at t = zero. At t = −1200 ms the coil alongside the switch path is energized. The voltage throughout the coil is ramped as much as 200 V in ~150 ms. The present (panel a) takes ~200 ms to achieve its most, and subsequently decreases barely because the coils resistance will increase because of resistive heating. At t = −900 ms the pattern ejection routine is executed. The actuator chain contains a mechanical relay, a pilot valve and an actuated plug valve, resulting in a delay of one other 100 ms earlier than the bullet passes the primary optical detector (panel b, prime curve) close to its begin place, at t = −800 ms. ~70 ms later the bullet passes the second optical detector contained in the second magnet (panel b, backside curve), the place the pattern is dissolved in pre-heated solvent. At t = −500 ms the pinch valve within the injector is opened, and the pattern is sucked into an evacuated NMR tube. To show the loading of the NMR tube, a bullet containing 60 μL of a 1:1 combination of purple dye and pyruvic acid was shot from the polarizer into the injection machine that was preloaded with 700 μL of degassed H2O, heated to 50 °C. Chosen frames from a video taken as the answer flows into the NMR tube are proven in panel c. The time origin is identical as in panels a, b. The NMR acquisition is triggered at t = zero
Two totally different injection units have been constructed. A easy machine has been used for observations of quantum-rotor-induced polarization25,26. It consists of a 3D printed receiver that connects to the NMR tube on the backside, and to the switch tube on the prime. Contained in the receiver a recession in diameter retains the bullet – the pattern itself carries enough momentum to slip out of the bullet and into the NMR tube that’s preloaded with solvent. An analogous machine has been described by Hirsch et al.21. Venting slots on the prime of the receiver present an escape path for the pressurized helium fuel. This machine is quick and works nicely for natural solvents, however the affect of the pattern on aqueous options usually causes bubbles and results in inhomogeneous options, specifically when utilizing 5 mm NMR tubes.
We, subsequently, constructed a brand new injection machine, proven in Fig. 1c. On this machine the bullet can also be retained, however the pattern itself is shot right into a solvent reservoir positioned above the NMR tube. The solvent reservoir is machined from titanium and is provided with a heater and a temperature sensor. With an inside diameter of 10 mm it facilitates quick dissolution and mixing10. A 1/eight” OD PTFE tube connects the solvent reservoir to a home-built valve by which a bit of silicon tubing is compressed with pressurized fuel to dam stream. The output of this valve is linked to an evacuated 5 mm NMR tube utilizing an extra piece of PTFE tubing. The valve opens upon releasing the stress, and the answer is sucked from the solvent reservoir into the NMR tube. The evacuating line is closed to forestall boiling of the answer at low stress. The filling of the NMR tube with aqueous resolution is proven in Fig. 2c. The filling of the tube is basically full at t = zero ms, ~860 ms after the bullet passes the optical barrier on the tunnel entrance contained in the polarizer, and NMR acquisition is triggered. Remaining bubbles on the prime of the pattern collapse inside the first second after the set off. After the experiment, the injection machine is faraway from the magnet, the empty bullet is eliminated manually and the NMR tube is exchanged for a brand new one.
The solid-state polarization in our polarizer is estimated as 50 ± 10%, primarily based on a comparability of the hyperpolarized sign to the thermal sign measured at a temperature of four.three Okay. This polarization degree is within the anticipated vary for the polarization of neat pyruvic acid at 6.7 T and 1.four Okay14,27.
A 13C NMR spectrum of 80 μL hyperpolarized 1-13C-labeled pyruvic acid after fast switch and dissolution in 700 μL of degassed D2O is proven in Fig. three. The spectrum displays two strongly enhanced peaks which can be as a result of presence of pyruvic acid in each the oxo and the hydrated type28. As described previously2, substantial polarization ranges result in an asymmetry within the doublet of close by J-coupled spins. The inset of Fig. three exhibits a close-up of the doublet at 93.5 ppm, which is as a result of pure abundance carbonyl carbon of pyruvate hydrate. The asymmetry of the doublet yields an estimate for the 13C polarization of 32 ± 1percent2. A direct comparability of the hyperpolarized sign with the thermal equilibrium sign yields a barely smaller estimate for the 13C polarization of 28%.
Two 13C NMR spectra of hyperpolarized 1-13C-pyruvic acid recorded instantly, and two seconds after fast switch, dissolution in degassed D2O and injection (decrease and higher blue curve, respectively). The spectra had been recorded with a flip angle of ~10°. The spectra present two strongly polarized peaks at 175.7 and 164.2 ppm as a result of presence of pyruvate and pyruvate hydrate, respectively. The inset is a close-up of the sign at ~93.5 ppm which is as a result of pure abundance carbonyl of pyruvate hydrate. This sign is break up right into a doublet by the 67 Hz J-coupling to the (labeled) carboxyl carbon. The asymmetry within the doublet is modeled with two Lorentzians (purple dotted line) and yields an estimate for the 13C polarization of 32 ± 1%. The underside, orange line exhibits a 1000-fold magnification of the thermal equilibrium sign (four transients averaged)
The spectrum additionally exhibits a set of strongly enhanced peaks between the 2 major peaks at round 170 ppm. These indicators are attributed to impurities corresponding to para-pyruvate and zymonic acid28,29,30.
At current the violent affect of the stable pattern onto the solvent results in the incorporation of tremendous helium bubbles that result in line broadening of ~30 Hz 2 s after dissolution. Schemes to quickly take away helium bubbles throughout injection into the NMR tube are at the moment being explored.
Pattern heating throughout switch
To estimate the pattern heating through the switch, a platinum temperature sensor was inserted right into a bullet containing pyruvic acid. The bullet containing the sensor and the pyruvic acid was frozen in liquid nitrogen and lowered right into a helium dewar, the place it equilibrated at ~20 Okay. Then, it was quickly delivered to the highest of the dewar the place it was uncovered to quickly flowing ambient temperature helium fuel. This experiment, detailed in Supplementary Strategies and Supplementary Fig. 2, exhibits that the pattern heats up quickly at low temperatures, with a typical heating price of 60 Okay/s at 30 Okay.