TEL-PS-15

Introduction

In 1922, Stern and Gerlach provided experimental confirmation that nuclei had magnetic moments, leading Wolfgang Pauli to postulate that certain nuclei posses a spin angular momentum. In 1939, Rabi first demonstrated resonance absorption of an oscillating electromagnetic field by molecules placed in a constant magnetic field. Both Purcell and Bloch observed NMR in both solids and liquids for the first time in 1946, for which they recieved the Nobel Prize in 1952.

Hahn implemented an ingenious idea of replacing continuous wave excitation of polarized nuclei by pulse excitation. In 1951 Arnold went beyond the limits of magnet homogeneity and obtained the first high-resolution spectra discovering 1H chemical shifts. Pulse spectroscopy matured in the 1960’s with the wider availability of computers. Anderson and Ernst began to use fast Fourier transforms to change from the time domain to the frequency domain. This work eventually culminated in a Nobel Prize in Chemistry for Earnst.

The age of medical applications started in the early 1970’s after Lautertbur demonstrated the feasibility of using NMR for imaging. Liberated from the obsession of perfect magnetic field homogeneity he deliberately applied gradients to encode the spatial information into an NMR spectrum. This and Damadian’s discovery in 1971 about tissue contrast available through variation of nuclear relaxation times opened Pandora’s box for medical application.

Since its discovery, NMR has proved to be a versatile technique in basic research (Physics, Chemistry, Biochemistry). It found application in Geology (oil and ferrous compounds search), Agriculture and Food Industries (moisture contents and purity measurements), and in Archeology (tracing changes of the Earth’s magnetic field through the ages). Finally materializing under the MRI acronym Magnetic Resonance Imaging; (for “political” reasons the word “nuclear” was removed) Lauterbur’s idea on “Image Formation by Induced Local Interactions” proved to be a perfect modality for clinical noninvasive anatomical and functional imaging. Not surprisingly the 2003 Nobel prize in Medicine was awarded to Chemist Lauterbur and Mansfield, a Physicist who invented modern NMR Imaging.

Spectrometer Description

TEL-Atomic, Inc. presents a desktop pulse NMR system, the PS-15, that combines all of the sophisticated features that mainframe spectrometers have including:

o        Electromagnet with three stage magnetic field stabilization (current, flux, and NMR lock on 19F)

o       Broadband transmitter

o        Multiphase RF pulses

o        Low-noise quadrature receiver

o Temperature control of the sample in range 5-60C

o        Intuitive software for experiment preparation, experimental data acquisition, storage and processing

o     Collection of modern NMR pulse sequences dedicated for:

o    NMR spectroscopy

o    nuclear relaxation times measurements (T1, T2, T1σ)

The PS-15 hardware and software provides a convenient means for NMR spectroscopy as well as for relaxation experiments on 1H nuclei (protons) at a magnetic field of 350 mT and at frequency of 15 MHz.

The spectrometer can operate with or without NMR magnetic field stabilization. The NMR stabilizer provides excellent long-term stability of the electromagnet by compensating for thermal drift. This stability is necessary for experiments that require long times (for example multiple signal accumulation, samples with very long relaxation times).

With the NMR flux stabilizer turned off diagnostics like confirming the magnet’s homogeneity (shimming) and adjustment of initial current (I0) of the basic magnet current stabilizer can be performed.

Attenuators

The PS-15 includes 2 attenuators for changing the pulse power. The main attenuator has a range of 0-31.5 dB in 0.5 dB steps. This attenuator changes the power of pulses simultaneously in all channels. This means very accurate adjustment of the exciting pulse. Samples with short relaxation times (solids) need short high-power pulses. For samples with long relaxation times (liquids) long, low-power is more adequate. The Y channel attenuator changes the power of pulses in Y channel only. It is active only during rotating frame experiments. It is used for calibration and selecting of the locking B1 field for T1ρ measurements.

Goiniometer

The PS-15 also comes with a 360 degree dial attached permanently to the magnet and a rotating sample holder. This can be used for measurements of spectra shape angle dependence in monocrystal samples. 1H NMR spectra of solid samples can give structural information that X-ray crystallography cannot deliver due to poor X-ray scattering on the hydrogen single electron. A gypsum monocrystal is used as an illustration of line split due to proton interaction with the local magnetic dipolar field.

Software Description

The spectrometer operation is controlled by a package of dedicated software for user-friendly assistance during the preparation of an experiment, the actual acquisition of data and later data processing. The software provides a convenient means for the acquisition of any form of nuclear signal related to NMR spectroscopy or relaxometry and its subsequent processing. The software graphic interface consists of three windows: setup, acquisition, and processing.

Experimental setup window

This window is used for the preparation of an experiment . Data obtained during any experiment that is run on the setup window are shown continuously in real time. Any change in setup takes effect immediately on the displayed data. The user can create, save and load his own setups to preserve and retrieve specific settings.

Data acquisition window

The final experiment and data storage is performed in the acquisition window. This task requires the definition of the name of the destination binary file, the accumulation number, the variable delay file name if relaxation measurements take place, and any comments the user wants to make about the experiment. During relaxation measurements, amplitudes of consecutive FIDs corresponding to executed interpulse delays are displayed to show the magnetization recovery.

Data processing window

This window is used for data viewing and processing. It allows the user to perform the following operations:

o     loading binary data files and viewing them on the computer monitor,

o     manipulating spectroscopy data

o    exponential multiplication

o    left and right points shifts

o    Fast Fourier Transform (FFT)

o     processing spectra after FFT

o    spectra integration

o    calculation of line width at half maximum

o     extracting data points from relaxation binary data files

o     fitting extracted relaxometry data points to calculate

o    T1 by Inversion Recovery, Saturation and T1ρ

o    T2 by CP and CPMG methods,

o    converting binary files and storing them as text files for processing with independent software (Excel, Origin, Matlab, Mathematica, etc)

Documentation

An experimental manual, on CD, gives the student full flexibility with respect to the level of involvement and the applications of magnetic resonance spectroscopy one wishes to explore. Twenty plus basic and advanced, as well as instrumentation experiments are included in the experimental manual. Basic experiments include, NMR Specta, and Relaxation experiments. Advanced experiments such as multiple pulse sequences and rotating frames of reference, as well as, angle dependence on spectra shape lets the student explore more deeply into the uses of NMR. Instrumentation experiments are designed to teach the student about NMR instrumentation. A number of samples for use with the experiments in the experimental manual have been included for convenience and ease of use. The student however is not limited to these samples. The number of potential samples (chemical compounds, commercial substances or products) and measurements are practically unlimited.

(Computer and shelf not included in the system.)