Author: Howard Altman

Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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The main goals of the project is to develop a method of maintaining the required temperature of the sample for the duration of the experiment; and to improve upon the connection between the transfer line and the probe dewar.

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Senior Design Group Two is assisting the NMR (Nuclear Magnetic Resonance) Facility at the NHMFL (National High Magnetic Field Laboratory) with the improvement of an already developed probe for the 900 MHz UWB magnet. This probe allows for magic angle spinning which allows for experiments to take place at very low temperatures. Having a spin angle of 54.74° and a spin frequency ranging from 1 – 25 kHz allows for the study of temperature-dependent behavior to be studied in a variety of materials and other forms.

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Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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As an introduction to the group, we are all seniors at the FAMU-FSU College of Engineering studying mechanical engineering. All of us have had very different life experiences. These experiences give the group diversity in thought, ideas, and procedure. Overall, this makes the team stronger and more productive when we work hard and incorporate these ideals.

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Rebecca Altman

Rebecca is graduating with a B.S in Mechanical Engineering in May 2009. During her free time she likes to play the piano, read about current events, travel, cooking, and listening to a variety of music artists. She is an officer in the local chapter of SWE and a member of ASME.

Jason is a returning student with a B.A in Anthropology from Florida State University, who is now earning a B.S in Mechanical Engineering for completion in the spring of 2009. He works as an engineering assistant at the National High Magnetic Field Laboratory. Current work interests include control systems and fluid mechanics. Jason and his wife Amy have a two year daughter named Emily; they are eagerly expecting the arrival of another daughter in the spring of 2009. Home interests include church, bicycling, cooking, and lots of playtime with his daughter.

Zachary Stevenson

Zac is a senior in Mechanical Engineering and will be graduating in the spring of 2009. He is currently working in the kitchen at the Mellow Mushroom to help get through school. He is a huge Tampa Bay Buccaneers fan and during his free time likes to exercise, play poker, and listen to music.

E-mail: zgs03@fsu.edu

Jessica Vanterpool

Jessica is Tallahassee native. She is a senior in Mechanical Engineering and will be graduating with her B.S in May 2009. She enjoys listening to music and is currently building up her collection. Some of her favorite artists include Israel and New breed, Lauryn Hill, Jazmine Sullivan, and more.

E-mail: jdv04c@fsu.edu

E-mail: rla04c@fsu.edu

Jason Kitchen

E-mail: ja.kitchen@yahoo.com

Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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The system for cooling samples in NMR radio frequency (RF) probes used in the 900 MHz magnet to cryogenic temperatures needs to be improved in several aspects. The client has a closed-cycle refrigerator capable of delivering gases to temperatures reaching -70 °C. A heat exchanger in a liquid nitrogen bath is currently used in series with this refrigerator to achieve outlet temperatures reaching -188 °C. However, this system does not account for the automatic and indefinite refill of liquid nitrogen surrounding the heat exchanger. In addition to engineering an automatic refill system, it is necessary to ensure that that the system works cohesively with the high magnetic field.

Figure 1- 900 MHz Magnet

Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR) spectroscopy allows scientists to assess chemical and structural properties of a sample in non-destructive manner. The technique was first applied to liquid and solid samples in 1946 by Felix Bloch and Edward Mills Purcell. NMR spectroscopy of liquid samples has since matured as a scientific discipline and for the last several decades following this discovery has been an important tool in the pharmaceutical and chemical industries. Application of NMR to solid-state samples followed as techniques such as magic-angle spinning were developed to increase their
inherently poor signal resolution.
NMR takes advantage of a quantum
property of all atomic particles called
spin. Electrons, neutrons, and protons
each exhibit this property, with
individual spin values of 1/2. NMR
spectroscopy is measured from the
interactions of applied radio frequency
(RF) electromagnetic radiation to the
spin polarity of neutrons and protons
– hence the name nuclear magnetic
resonance. Only isotopes which have
unpaired neutron and or proton spins
are directly observable using NMR.
Isotope examples include 1H, 2H, 15N,
31P, 14N, 13C, and 19F. When a sample
containing NMR-sensitive isotopes is
placed in an external magnetic field, 900 MHz Magnet
called the B0 field, some spins align
themselves in a low energy state opposite to B0 magnetic polarity, while the remainder align in a high energy state with the same B0 magnetic polarity. Photons of frequency v cause a spin energy level jump, where the frequency is proportional to the B0 field multiplied by a proportionality constant specific to the isotope, called the gyromagnetic ratio, γ.
On a macroscopic scale, the average of all the individual spin polarities yields the net magnetization, M0. As RF energy is applied at the resonant frequency of the isotopes being studied, the net magnetization vector along the Z axis can be brought to zero. Additionally, if enough energy is applied, the net magnetization vector can be brought into the XY plane, where it will rotate around the Z axis at the v frequency of the photons which cause the energy level jumps. This is called the frequency of precession or the Larmor frequency. After the net magnetization is brought into precession in the XY
plane, scientists measure the time it takes for it to return to zero along the Z axis. Also, the time for the vector to return to equilibrium along the Z axis at M0 is measured. Both of these measurements are respectively called the T2 and T1 relaxation times. Through Fourier transforms, these relaxation response times are converted to the frequency domain. In this way the frequency peaks in an unknown sample can be compared to known frequencies of the elements, and the atomic ordering of a molecule can be
determined. In addition, other more complex NMR techniques allow the inter-atomic distances and angles to be determined within a molecule, so that the 3-D structure of the molecule can be known.
For modern NMR experiments, a solid or liquid sample is placed inside a probe which locates the sample in the center of the B0 magnetic field created by a superconducting electromagnet. The weak signal to noise ratio of NMR spectroscopy requires signal averaging over time, so a homogeneous magnetic field is needed across the sample so that field variations do not overly skew the frequency peaks. In order to increase field homogeneity, non-superconducting shim coils are placed around the probe bore and
currents are applied for various terms, such as Z, Z2, X, Y, XY, X2, and so on. In the NMR probe, the sample is place in a solenoid coil oriented in the X direction. When RF pulses are applied to this coil, a B1 field is created which acts to bring the net magnetization vector into precession in the XY plane. The simple NMR experiment consists of pulses to bring the net spin vector into precession, followed by an acquisition phase, where the frequency of the rotating magnetization vector induces current in the solenoid which is amplified and recorded by the spectrometer console.
Magic Angle Spinning
For non-crystalline chemical samples in the solid-state, dipolar and quadripolar interactions between atoms and also chemical shift anisotropy cause the spectral lines to become broad. Line broadening serves to mask individual frequency peaks, and so information about the atomic structure of
a molecule is lost. By spinning the sample at 54.74° to the B0 vector, the
frequency peaks can be narrowed, raising the resolution. This technique is
referred to as magic angle spinning (MAS), where spin rates of 1 to 70 kHz
are applied to the sample. The solid powder samples in MAS NMR probes
are placed in rotors which vary in diameter from 2 to 7 mm, with smaller
diameter rotors capable of higher spinning speeds for better averaging,
while compromising signal due to the smaller sample size. These ceramic
rotors are placed in a precision machined stator, which use gas bearings and gas driven impellers to achieve the high spin rates. In order to study material properties of solid samples at cryogenic temperatures, such as magnetic phase transitions, MAS probes have been developed which are capable of variable temperature (VT) experiments.
MAS Stator in 900 Probe
Current Cryogenic Delivery System
The 900 MHz ultra-widebore NMR magnet at the NHMFL allows scientists to perform spectroscopy at higher sensitivities using large diameter probes, which can house more instrumentation than for similar narrow-bore 900
MHz magnets found in other labs globally. The RF Program of the NHMFL, led by Dr. William Brey and Mr. Peter Gorkov, have developed specialized NMR probes for this and other magnets at NHMFL which feature novel coil geometries, improving signal to noise ratio and reducing RF-induced sample heating in sensitive biological protein samples. The RF Program built a transmission line circuit MAS probe for the 900 MHz magnet, utilizing a Chemagnetics style MAS stator with a 3.2 mm diameter rotor. The probe was intended for use in VT experiments, and features a vacuum insulated glass dewar and internal probe heater for accurate delivery of cooled or heated gas to the sample chamber inside the stator.
FTS refrigerator
For MAS experiments to temperatures reaching -70 °C, an Airjet XRII851 closed cycle refrigeration unit from FTS Systems is utilized to cool a room temperature nitrogen gas stream. This unit is capable maintaining a cold gas stream for volumetric flow of up to 1 standard cubic foot per minute (SCFM). When colder sample temperatures are required, the pre-cooled output of the FTS refrigerator is fed into a heat exchanger which is submerged in a liquid nitrogen bath inside a 50 L dewar manufactured by MVE Biological Systems. The heat exchanger was supplied by Doty Scientific, a manufacturer of commercial NMR and MRI probes and related accessories. It consists of counterflow pre-cooling coils, where incoming gas is cooled by the vaporized nitrogen at -195.8 °C which is exiting the dewar as it vents to atmospheric pressure. The gas then flows through the heat exchanger coils which are submerged in liquid nitrogen (LN2) and exits the unit through a
stainless steel transfer line to the NMR probe. The transfer line is manufactured by Cryofab, Inc. and is vacuum insulated for higher thermal efficiency.
Doty Hx
A design challenge with the current probe is where the metal transfer line adapts to the glass VT dewar at the base of the probe. Currently a Torlon adapter is used to mate with the brass quick-disconnect fitting at the exit of the transfer line, and then a small length of PTFE tubing is band clamped
between the Torlon adapter and over the stem of the vacuum insulated glass dewar. This series of joints and adapters has proven to be an area where both gas leaks and higher thermal losses are evident from the formation of
ice during cold bench tests of the system. Furthermore, since the transfer line enters the glass dewar at the horizontal but the length of the dewar is vertical, the Torlon adapter clamp must be secure enough to support the weight of the stainless steel transfer line, while at the same time allowing for thermal contraction in the vertical direction of the brittle glass dewar over the approximate 6 ft length of the NMR probe. Our client has explained that this is a desired area for improvement in our team’s final system design.
Ice Build-up
Also inserted into the base of the glass probe dewar is a resistance heater with internal thermocouple, used in conjunction with a PID temperature controller at the spectrometer console to provide final accurate setting of the sample temperature. The heater is manufactured by Bruker Biospin and is commonly used in their commercial NMR probes. The internal thermocouple provides a safety shutdown in the event of the heater temperature reaching an unacceptably high temperature either from electric fault or by removal of VT gas flow. There is a T-type thermocouple closer to the MAS stator which provides the actual feedback to the PID temperature controller. In a standard length NMR probe for smaller magnets, the distance between the heater and this VT thermocouple is approximately 2 to 3 feet, so there is not a significant lag between temperature detections and heater control. Since this probe is closer to 6 ft in length, there twice as much lag time in this feedback loop. It
was expressed by our client that if our final system design requires the heater to be moved further upstream, such as somewhere in the transfer line, that sample temperature variations could possibly become wider and fall out of desirable limits.
The cooled gas stream travels up the probe through the glass dewar, which is actually a series of three interconnected glass dewars of shorter lengths to facilitate manufacture by the glass blower. The gas is routed through Tygon tubing in order to transition through a 180° U-turn since the VT entry port is located on the top of the MAS stator. Here, the gas blows on the sample rotor and surrounding solenoid, and exits the stator through an outlet port and out the top of the probe. It was determined during cold bench tests of the current system, that gas flows of 1 to 1.3 SCFM are required in order to cool the VT
thermocouple to -188 °C. NMR experiments using temperature sensitive lead nitrate samples are planned for the near future to determine actual sample temperature under these conditions, since heating is introduced by the spinning of the rotor and from electric fields of the RF coil.

Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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The group is being sponsored by the National High Magnetic Field Laboratory (NHMFL) Nuclear Magnetic Resonance (NMR) Facility. The liaison engineers that will be assisting and advising for the duration of the project are Dr. William Brey and Peter Gor’kov. The NMR Facility researches and fabricates many different types of hardware for NHMFL research. Current projects consist of: Development of Probes for 900 MHz Ultra-Wide Bore (UWB), HTS Probe for Natural Products, Low-E Probes for Biological Solid-State NMR, 29 mm Family of Narrow Probes for High Field 830 MHz and Resistive Magnets, and NMR in Resistive and Hybrid Magnets.
For more information, please visit there website at:
http://nmr.magnet.fsu.edu/rf/index.html

Dr. William Brey

Sponsor

He came to the NHMFL in 1999. Prior to joining the NMR group, Brey had primary responsibility for RF coil design for the Conductus/Varian team that developed the superconductive NMR spectroscopy probe. His graduate work with Thomas Mareci and Raymond Andrew at the University of Florida included the development of new designs for gradient coils. Before his graduate study, Brey worked on the research staff at the University of Texas Health Sciences Center, Houston, where he developed pulse sequences, gradient, and RF hardware for MRI and MR spectroscopy. He also worked as a consulting engineer for Tecmag, Inc., which produces retrofit hardware upgrades for NMR spectrometers. At the NHMFL Brey has continued his work with superconductive probes, guided the commissioning of the 900 MHz ultra-wide bore magnet, and is now developing probes and related technology for NMR on the Series Connected Hybrid magnet that is under development.

Peter Gor’kov

Sponsor

He came to NHMFL in 1999 from the group of Paul Lauterbur at the University of Illinois in Urbana-Champaign where he designed MRI probes for small animal MRI. At the NHMFL, Gor'kov has designed and built over 25 user probes and accessories for solid state NMR in superconducting and resistive magnets, including RF probes for the 900 MHz UWB magnet. While other labs have acquired high field NMR magnets, the strength of the Magnet Lab's NMR program rests in large part on providing users with unique RF probes that are not available elsewhere. Peter's recent interests focus on developing instrumentation for NMR of biological solids to aid with solving structure of insoluble proteins in a native environment. Gor'kov has come up with several user RF probes that use specially designed low-E resonators instead of conventional solenoids to minimize the high-frequency electric fields that heat protein samples. This allows users to perform sensitive biological NMR experiments in the high field 900 MHz magnet. Several such probes were built for use at other facilities, including Pacific Northwest National Laboratory and Karlsuhe Institute of Technology.

Dr. Steven Van Sciver

Faculty Advisor

He is a Distinguished Research Professor in Mechanical Engineering and Director of Magnet Science and Technology at the National High Magnetic Field Laboratory (NHMFL). He joined the FAMU-FSU College of Engineering and the NHMFL in 1991, initiating and teaching in the graduate program in Magnet and Materials Engineering and in cryogenic thermal sciences and heat transfer. Dr. Van Sciver was previously the Director of Magnetic Science and Technology at the NHMFL. In this capacity he manages the group of about 70 professional staff and students that develop high field magnet systems for the NHMFL. Dr. Van Sciver is a Fellow of the ASME and the American editor for the journal Cryogenics.

Prior to joining Florida State University, Dr. Van Sciver was Research Scientist and then Professor of Nuclear Engineering, Engineering Physics and Mechanical Engineering at the University of Wisconsin-Madison from 1979 to 1991. He was also Associate Director of the Applied Superconductivity Center. Dr. Van Sciver received his PhD in Low Temperature Physics from the University of Washington-Seattle in 1976.

Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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To view the documents below, please click on report or presentation. Reports open with Microsoft Word or Adobe PDF and the presentations with Microsoft Powerpoint.

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Report

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Spring Proposal

Report

Progress Report

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Mid-point Review

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Operations Manual

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Final Project Review (Open House)

Report Presentation Poster Pamphlet

Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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Update System
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Senior Design Open House

Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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Heat Exchanger Coil Picture

Schematic

Additional Links:

The Basics of NMR
CRYOFAB

Senior Design 2008—2009, Group 2

National High Magnetic Field Laboratory (NHMFL) 900 MHz NMR Probe

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NHMFL NMR Department

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Dr. William Brey (Sponsor)
Peter Gor’kov (Sponsor)
Dr. Fu Andy Powell (Shared Magnet Time)

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FAMU-FSU Mechanical Engineering Department

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Dr. Chiang Shih (Department Chair)
Dr. Daudi Waryoba (Senior Design Instructor)
Dr. Steven Van Sciver (Faculty Advisor)

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