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Transcript of NMR in a low field of a permanent...
Univerza v Ljubljani
Fakulteta za matematiko in fiziko
Oddelek za fiziko
Seminar Ia – 2.letnik, II.stopnja
NMRinalowfieldofapermanentmagnet
Author: Janez Lužnik
Advisor: prof. dr. Janez Dolinšek
Ljubljana, March 2014
Abstract
In this seminar we first look at the experimental setup of nuclear magnetic resonance spectroscopy
in a magnetic field of a permanent magnet at room temperature. Then we look at two possible
practical applications of such a system. In conclusion we also discus some of the advantages and
disadvantages of both applications.
2
Tableofcontents
1 Introduction ..................................................................................................................................... 2
2 Nuclear magnetic resonance ........................................................................................................... 3
2.1. Spin‐lattice relaxation .................................................................................................................. 4
2.2. Spin‐spin relaxation ...................................................................................................................... 5
3 Experimental setup ......................................................................................................................... 5
4 Measuring the pore size distribution of cement pastes and mortar .............................................. 6
4.1 Theoretical background ........................................................................................................... 6
4.2 Results ..................................................................................................................................... 7
5 NMR based liquid explosive detector ........................................................................................... 10
5.1 Measurements of a series of samples ................................................................................... 10
5.2 Simulation of different alcoholic beverages and shielding effect ......................................... 11
6 Conclusions .................................................................................................................................... 12
7 References ..................................................................................................................................... 13
1 Introduction
Nuclear magnetic resonance (NMR) is a powerful non‐invasive technique used to measure various
physical and chemical properties of molecules or materials. It usually requires strong magnetic fields
generated by large superconducting magnets, which have to be cooled by liquid nitrogen and helium.
The use of this technique is therefore limited to laboratories with suitable equipment.
However systems that enable NMR in permanent fields at room temperature do exist. Some like the
one that will be discussed in this paper are also small and portable. They broaden the use of NMR as
an investigative technique as they allow on‐site testing and give fast results. Two practical
applications of such system will also be presented.
3
2 Nuclearmagneticresonance
Nuclear magnetic resonance is a phenomenon in which nuclei in a magnetic field absorb and re‐
emit electromagnetic radiation. The nuclei with spin have a dipole magnetic moment [9,10]:
ħ Eq. 2.1.
is the magnetic moment, ħ is Planck’s constant divided by 2π and is the gyromagnetic ratio of the
nucleus (the ration between the charge and mass of the nucleus). Gyromagnetic ratio is different for
each isotope. For protons it is 42,6 / .
Nuclei with magnetic moment can only have discrete energy states in the external magnetic field. In
the tha case of protons only two states are possible. Either in the direction of the filed +½ or in the
opposite direction of the filed ‐½. The external magnetic field 0,0, determines the energy
difference between these two states.
∆ ħ ħ Eq. 2.2.
Graph 3: Two possible energy states of protons in the external magnetic field B
Graph 1: When the external magnetic
field is not present, the nuclei are
randomly ordered and the sum of their
magnetic dipole moments is zero [11].
Graph 2: When the external magnetic field B ispresent the nuclei allign wiht the filed and the sumof their magnetic dipole moments is now differentfrom zero [11].
4
The occupation of both energy states is defined by Boltzmann’s distribution:
∝ħ
. Eq. 2.3.
At our conditions (frequency of 2MHz and temperature of 300K) the ratio ħ
is about 10‐7. Therefore
we can only take into account the linear part of the extrapolation of the exponential function. We
can then define the difference between the occupations of both energy states as:
∆ħ
. Eq. 2.4.
There is an initial magnetization in the sample in the thermal equilibrium, which is aligned with the
direction of the field:
∆ħ
. Eq. 2.5.
The net magnetization behaves in the external filed just as a single magnetic moment would. We
can flip the magnetization out of its initial direction for an angle , using a short
radiofrequency pulse (perpendicular to the initial magnetization) with the frequency . The flipped
magnetization starts to precess around the external magnetic field with the frequency and
induces a signal in our detection coil.
2.1.Spin‐latticerelaxationAfter the RF pulse is turned off, the magnetization slowly returns towards the equilibrium state.
During this process the spin system exchanges the energy with its surrounding and the longitudinal
component of the magnetization changes. The process is called spin‐lattice relaxation and is
characterized by the spin‐lattice relaxation time T1. The relaxation is described by the following
equations [11]:
Eq. 2.6.
0 Eq. 2.7.
Typical relaxation times for protons are around 2 to 3 seconds.
Graph 4: The induction of NMR signal in the detection coil
5
2.2.Spin‐spinrelaxationWe can flip the initial magnetization (oriented along the z axis) around the x axis using the π/2 pulse.
The magnetization is now perpendicular to the external magnetic filed nad we refer to it as
transversal magnetization. After the pulse that was used to flip the magnetization is turned of, the
spins preces around the external magnetic filed B0 with the Larmor frequency in the shape of a
cone spiral. Initiall all spins precess in phase. However as time passes the spins begin to dephase and
the transverzal magnetization decays towards its equlibrium value of zero. The process is described
by the following equations:
Eq. 2.8.
Eq. 2.9.
The spin‐spin relaxation is characterized by the spin‐spin relaxation time T2. It is usually faster than
spin‐lattice relaxation.
3 Experimentalsetup
In the two experiments we used a commercial system Magritek 2 MHz Rock CoreAnalyzer [1]. It is a
cylindrical shaped magnet with the proton Larmor frequency of 2MHz. The system uses a heater to
keep the magnet temperature at 30 °C needed for field stability. The samples of maximal dimensions
l = 62 mm and φ = 39 mm sit in a sample chamber, which is isolated form the magnet. All mea
surements were done at room temperature.
Grpah 5: The complete Magritek 2 MHz Rock CoreAnalyzer system is shown on the left. On the right is a picture of the probe and sample chamber [2].
6
4 Measuringtheporesizedistributionofcementpastesandmortar
Made by mixing water, cement and sand (mortar) or only water and cement (cement pastes),
mortars and cement pastes are widely used construction materials. As they harden they become
porous, which affects different properties of these materials such as strength and durability.
Measuring the pore size distribution gives us information about the types of porosity present in the
material and helps us determine the quality of the material [3,4].
During our experiment we analysed four different groups of samples, each group containing six
samples. Types of samples are shown in the table below.
Type of sample Number of sample (x) Composition
cp5 (cement pastes) 1 to 6 1 kg cement paste +500 ml water
cp6 (cement pastes) 1 to 6 1 kg cement paste +650 ml water
mr5 (mortars) 1 to 6 1 kg cement paste +3 kg sand+500 ml water
mr6 (mortars) 1 to 6 1 kg cement paste +3 kg sand+650 ml water
4.1 Theoreticalbackground
In porous cement materials, the relaxation can be attributed to three main processes [5]:
1
2
1
2
1
2
1
2 Eq. 4.1.
Our experiment was done in a homogeneous magnetic field so that the diffusion contribution
( ) is negligible. Bulk relaxation ( ) is a slow process compared to the relaxation at the pore
surface ( ), which happens much faster due to paramagnetic ions in the concrete, such as
iron. Therefore we only measured the surface relaxation that is directly proportional to the pore size
ratio [6,7]:
Eq. 4.2.
V is pore volume, S is the pore surface and ρ2 is the relaxivity (μm/s), defined as the ability of
magnetic compounds to increase the relaxation rates of the surrounding water proton spins. The
spin‐lattice relaxation is also proportional to pore size distribution. Relaxivity is different but the
relation is the same as for spin‐spin relaxation:
Eq. 4.3.
We fitted the T2 and T1 data to a broad distribution of relaxation times, using a NNLS (non‐negative
least squares) algorithm. The stability and accuracy of results are better for T2 data due to the fact
that 100 points per T2 experiment were used. For T1 measurements only 20 points per experiment
were used.
Table 1: Sample types
7
4.2 ResultsWe measured the samples after they were immersed in the water for five days, so all the pores
accessible to external water were saturated. The CPMG sequence was used in spin‐spin relaxation
measurements. It consists of a π/2 pulse followed, after a specific time τ, by a series of π pulses.
These pulses are 2τ apart from one another.
The observed NMR magnetization curve depends upon the T2 of the broad distribution of all pores.
Areas where the distribution is different form 0 represent different sized pores. We can estimate the
pore size value of each area from the total T2 (Eq. 4.2.) calculated by integrating the same area. Area
1, which is the relaxation of crystal bound water, and Area 2, relaxation of capillary bound water are
present in all samples. They are also comparable in size due to the logarithmic scale. Area 3
(relaxation of protons in larger pores) and Area 4 (relaxation of protons in cracks and very large
pores) appear only in a few samples. The comparison of relaxations from different groups is
presented in the graph below. The relaxation curves are shifted vertically for a better view.
Graph 6: CPMG pulse
1E-3 0,01 0,1 1 10 100 1000 100000
2
4
6
8
10
12
14
16
18
Area 3 Area 4
Am
plitu
de (
a.u
.)
log T2 (ms)
Area 1
Area 2
Graph 7: Integration areas of T2 measurements shown on a typical cp group representative (cp53)
8
We used the inversion recovery sequence (π pulse followed by π/2 pulse after specific time τ) for T1
measurements.
We also integrated areas from T1 measurements, but the borders changed as shown on the graph
below.
Graph 8: T2 comparison
Graph 9: Inversion recovery sequence
Comparison of typical T2 from different sample groups
9
1E-3 0,01 0,1 1 10 100 10000,0
0,1
0,2
0,3
0,4
Area 4
Area 3
Area 2
Am
plitu
de
(a.u
.)
log T (ms)
Area 1
The comparison of the relaxation curves from different groups are shown on »Graph 6». Again we
shifted them vertically for a better view.
In »Table 2« the average values of integrated areas for each sample group are presented.
Areas calculated from T2 measurements
Areas calculated from T1 measurements T2 T1
Name Area 1 Area 2 Area 3 Area 4 Area 1 Area 2
Area 3 Area 4
A1/(A1+A2) A2/(A2+A3)
Average of cp5 group
0.287 0.484 0.240 65.487 3.365E‐04 0.004 0.017 0.019 0.357 0.188
Average of cp6 group
0.220 1.075 0.267 113.210 4.638E‐04 0.008 0.027 0.023 0.176 0.231
Average of mr5 group
0.334 0.371 0.163 87.746 2.334E‐04 0.009 0.022 0.031 0.440 0.476
Average of mr6 group
0.219 0.642 0.620 9.006E‐05 0.004 0.022 0.041 0.307 0.246
Graph 10: Integration areas of T1 measurements shown on a typical cp group
Graph 11: T1 comparison
Table 2: Integration results
Comparison of typical T1 from different sample groups
10
As expected because of the different relaxivities, areas calculated from T1 and T2 do not match.
Instead we compared the area ratios (last two columns in »Table 2«), which matched quite well. They
are important, because they offer us direct comparison between the water located in the pores (Area
2) and crystal bound water (Area 1).
5 NMRbasedliquidexplosivedetector
In the past there were some attempts by terrorists to detonate liquid explosives on commercial
planes. As a result of that the amount of liquid that a person can bring on‐board in hand luggage has
been limited to several containers with a volume of 100ml each. A detector that could identify
potential threats could increase security checks and possibly allow some lighter restrictions. In this
experiment we tested the Magritek 2 MHz Rock CoreAnalyzer as a possible detector, which could
discriminate between various liquids on the basis of spin‐lattice and spin‐spin relaxation times.
5.1 MeasurementsofaseriesofsamplesAs in the previous experiment the spin‐lattice relaxation was measured with the inversion recovery
sequence and the spin‐spin relaxation with CPMG echo train. We used a large set of samples to
construct a model database of relaxation values. Both relaxations were measured in a two‐step
procedure. The first step was a faster and less accurate measurement intended to get an estimated
values of T1 and T2. In the second step the parameters were corrected according to the estimated
values. The difference between the estimated and true values was less than 20% for T1
measurements, while in the T2 measurement the estimated and true value mostly matched. T1
measurement took about 20s and T1 measurement was significally shorter taking only a few seconds.
Our results are shown in T2 vs T1 plot on the graph below [8].
Graph 12: T2 vs T1 plot of a series of samples
11
We divided the samples in several groups. The first group (laboratory chemicals such as ethanol,
methanol, toluene, etc.) and the second group (various non‐alcoholic and alcoholic drinks) overlap
slightly with only a few samples form the second group such as milk having shorter relaxation times.
95 and 100 octane petrol and diesel represent the next group. Here the T1 for diesel (0,7s) was much
shorter than T1 for 95 (2,35s) and 100 (2,64s) octane petrol. Last group were viscous edible samples
such as jam, honey, fruits, butter and several vegetable oils. These samples mostly had much shorter
T1 and T2 values than the rest.
5.2 SimulationofdifferentalcoholicbeveragesandshieldingeffectWe also did two more experiments. In the first one we simulated different types of alcoholic drinks
by mixing together distilled water and ethanol to get the T1 and T2 values as a function of ethanol
concentration.
We can observe a minimum at 60% ethanol concentration. The T1 and T2 values for real drinks are
usually smaller than those of our model system due to the presence of paramagnetic nuclei and
some other molecule that affect relaxation dynamics in real drinks.
The metal containers often used for liquids can present problems for NMR measurements because of
the skin effect. That is the presence of magnetic screening fields generated by RF induced eddy
currents in the metal. In our frequency range (2MHz) the skin depth of aluminium is about 1mm,
which is close to the thickness of aluminium can. We wanted to test those effects at our system. To
Graph 13: T1 (a) and T2 (b) as a function of ethanol concentration. The fitted curve and some data from real alcoholic samples are added
12
do that we used a 40ml sample of water with added CuSO4. First we measured the unshielded bottle.
Then the bottle was wrapped with aluminium foil 30μm thick. It was still possible to measure both T1
and T2 after wrapping the bottle in though the signal was two times smaller than without the shield.
The problem was that we could not tune the resonant circle after adding two layers of foil due to the
limited range of capacitors.
6 Conclusions
Measuring the pore size distribution of cement pastes and mortar
It is essential to know the properties of mortars and concretes to guarantee their stability and
durability. NMR spectroscopy is an ideal tool with which we can inspect such materials without
influencing their structure. The pore‐size distribution ratios were not very specific. We can
distinguish among different groups but not among individual samples. Another useful measurement
would be effective porosity, which could be done by modifying our system with gradient coils
allowing us to measure diffusion constant of water saturated samples.
NMR based liquid explosive detector
As a liquid explosive detector our system offers simple T1 and T2 measurements of large sample
volumes. Though the measurements are quite fast for an efficient use they would still need to be
faster. Other disadvantages are the lack of spatial resolution and that the proton density cannot be
determined. Again as was the case in the first experiment we could benefit from diffusion
measurements which are not possible without the gradient coils.
Graph 14: T1 (left) and T2(right) relaxation curves with and without aluminium shielding
13
7 References
[1] http://magritek.com/
[2] http://www.act‐aachen.com/halbach_magnets.html
[3] McCain & Dewoolkar : Porous Concrete Pavements: Mechanical and Hydraulic
Properties. http://www.uvm.edu/~transctr/publications/TRB_2010/10‐2228.pdf
[4] Hildegard Westphal, Iris Surholt, Christian Kiesl, Holger F. Thern, Thomas Kruspe; Pure
appl. geophys. 162 (2005) p.549
[5] NMR Petrophysics. http://www.slideshare.net/sstromberg/NMRCOURSE
[6] J.H. Strange, J. Mitchell, J.B.W. Webber; Magnetic Resonance Imaging 21 (2003) p.221
[7] R M E Valckenborg, L Pel and K Kopinga; J. Phys. D: Appl. Phys. 35 (2002) p.249
[8] A. Gradišek, J.Luzar, J.Lužnik, T. Apih; Magnetic Resonance Detection of Explosives and Illicit Materials, NATO Science for Peace and Security Series B: Physics and Biophysics 2014, pp. 123
[9] M. H. Levitt: Spin dynamics: Basics of Nuclear Magnetic Resonance; Wiley, Chichester,
UK (2008)
[10] C. P. Slichter: Principles of Magnetic Resonance; Springer Verlag, Berlin (1996)
[11] J. Dolinšek (2012); Spektroskopija trdne snovi, p.33