Application of GT2002 NMR experiment instrument

GT2002 NMR experiment instrument instruction manual

Purpose request

1. Understand the basic principles of nuclear magnetic resonance experiments.

2. Learn how to use NMR to calibrate magnetic fields and measure g-factors.

〖laboratory apparatus〗

Permanent magnet (including sweeping coil), two probes (samples are water and Teflon), digital frequency meter, oscilloscope.

〖Original〗

Nuclear magnetic resonance is an important physical phenomenon. Nuclear magnetic resonance experimental technology has been used in many fields such as physics, chemistry, biology, clinical diagnosis, metrology science and petroleum analysis and exploration. The American scientist Platinum (Purcell) discovered nuclear magnetic resonance phenomenon in 1945. And Bloch won the Nobel Prize in Physics in 1952. The Swiss scientist Ernst, who made an important contribution to improving nuclear magnetic resonance technology, won the Nobel Prize in Chemistry in 1991.

As we all know, the energy of electrons in a hydrogen atom cannot be continuously changed, and only discrete values ​​can be taken. In the microscopic world, the phenomenon that physical quantities can only take discrete values ​​is very common. The spin angular momentum of the atom involved in this experiment cannot be continuously changed, and only discrete Value, where It is called a spin quantum number and can only take 0, 1, 2, 3, ... integer values ​​or 1/2, 3/2, 5/2, ... half integer values. =h/2 And h is Planck's constant. For different nuclei, I have different determined values. The protons of the experiment and the spin quantum number I of the fluorine core 19F are both equal to 1/2. Similarly, the self of the nucleus The component of the angular momentum in a certain direction of space, such as the z direction, cannot be continuously changed, and only the discrete value pz=m can be taken. , wherein the quantum number m can only take I, I-1, ..., -I+1, -I (2I + 1) values.

A nucleus with a non-zero spin angular momentum has a nuclear spin magnetic moment associated with it, referred to as a nuclear magnetic moment, whose size is

(1-1)

Where e is the charge of the proton, M is the mass of the proton, and g is a factor determined by the structure of the nucleus. For different kinds of nuclei, the value of g is different, called the g factor of the nucleus. It is worth noting that g may be a positive number. It may also be a negative number. Therefore, the direction of the nuclear magnetic moment may be the same as the direction of the nuclear spin angular momentum, or vice versa.

Since the nuclear spin angular momentum can only take (2I+1) discrete values ​​in any given z direction, the nuclear magnetic moment can only take (2I+1) discrete values ​​in the z direction;

(1-2)

The magnetic moment of the nucleus is usually used As a unit, Nuclear magnetic particle. As a unit of nuclear magnetic moment, Can be recorded as And the angular momentum itself is Correspondingly, the size of the nuclear magnetic moment itself is g In addition to characterizing the magnetic properties of the core with the g factor, another physical quantity that can be measured experimentally is usually introduced. , Defined as the ratio of the magnetic moment of the nucleus to the spin angular momentum:

(1-3)

Can be written as , correspondingly .

When there is no external magnetic field, the energy of each nucleus is the same, and all nucleuses are at the same energy level. However, when an external magnetic field B is applied, the situation changes. For the sake of convenience, the direction of B is usually defined as the z direction. Because the interaction between the external magnetic field B and the magnetic moment is

E=

(1-4)

Therefore, the quantum number m has different values, and the energy of the nuclear magnetic moment is different, so that the original degenerate same energy level splits into (2I+1) sub-levels. Since the energy of each sub-level in the external magnetic field is related to the quantum number m. Therefore, the quantum number m is also called the magnetic quantum number. Although the energy of these different sub-levels is different, the energy interval between adjacent energy levels But it is the same. Moreover, for protons, I = 1/2, therefore, m can only take m = 1/2 and m = - 1/2 two values, the energy levels before and after the application of the magnetic field are shown in Figure 1 (a) and (b) in 1.

m=-1/2, E-1/2=

(a)

m=+1/2, E+1/2=

(b)

When the external magnetic field B is applied, the distribution of the nucleus at different energy levels obeys the Boltzmann distribution. Obviously, the number of particles at the lower level is higher than that of the upper level, and the difference is The size, the temperature of the system, and the total number of particles in the system. At this time, if a high-frequency electromagnetic field is applied in the direction perpendicular to B, it is usually the RF field, and the frequency of the RF field is satisfied. It will cause the nucleus to jump between the upper and lower energy levels, but since the core at the lower level is more than the upper level, the net effect is that the transition is higher than the transition, thus making the system The total energy increases, which is equivalent to the system absorbing energy from the RF field.

At the time, the above-mentioned transition is called a resonance transition, which is simply called resonance. Therefore, the frequency of the RF field is required to satisfy the resonance condition:

(1-5)

If angular frequency Said that the resonance condition can be written as

(1-6)

If the unit of frequency uses HZ, the unit of magnetic field uses T (Tesla), for exposed protons, after a large number of measurements 42.577469MHZ/T, but for protons with different groups in atoms or molecules, due to the different chemical environments of different protons, the shielding by surrounding electrons is different. The values ​​will be slightly different. This difference is called the chemical shift. For protons in water samples in a spherical container at a temperature of 25 ° C, MHZ/T, this experiment can use this value as a good approximation. By measuring the resonance frequency of protons in magnetic field B Calibration of the magnetic field can be achieved, ie

(1-7)

Conversely, if B has been calibrated, measure the resonant frequency of the unknown nucleus You can find the nucleus Value (usually used Value representation) or g factor:

(1-8)

g=

(19)

among them 7.6225914MHZ/T.

Through the above discussion, the resonance must be satisfied In order to observe the resonance phenomenon, there are usually two methods: one is to fix B, and the frequency of the RF field is continuously changed. This method is called the frequency sweep method; the other method, which is the method used in this experiment, is fixed RF. The frequency of the field, continuously changing the size of the magnetic field, this method is called the sweep method. If the magnetic field changes not too fast, but slowly pass and frequency In the corresponding magnetic field, the system can detect the absorption signal of the RF field by a certain method, as shown in Figure 1-2(a), which is called the absorption curve. This curve has the characteristics of the Lorentz type curve. However, if The sweep changes too quickly, and the resulting decaying oscillation curve with a wake is shown in Figure 1-2(b). However, the speed of the sweep is relative to the specific sample. For example, this experiment uses The sweeping field is an alternating magnetic field with a frequency of 50 Hz and an amplitude of 10-5 to 10-3 T. For a solid polytetrafluoroethylene sample, the magnetic field changes very slowly, and the absorption signal will be as shown in Figure 1-2(a). As shown, for a liquid water sample, it is a magnetic field that changes too fast. The absorption signal will be as shown in Figure 1-2(b), and the more uniform the magnetic field, the more the oscillations in the wake.

1-2(a)

1-2(b)

〖Experimental equipment and utensils〗

The block diagram of the experimental device is shown in Figure 1-3. It consists of permanent magnet, sweeping coil, GT2002 nuclear magnetic resonance instrument (including probe), GT2002 nuclear magnetic resonance instrument power supply, digital frequency meter, and oscilloscope.

Permanent magnets: The requirements for permanent magnets are strong magnetic field, large enough uniform area and good uniformity. The center magnetic field B0 of the magnet used in this experiment is about 0.48T, and the uniformity is excellent in the range of the magnetic field center (5mm)3. At 10-5.

Sweeping coil: used to generate a tunable alternating magnetic field with an amplitude of 10-5~10-3 T for observing the resonance signal. The current of the sweeping coil is isolated by the transformer and the output voltage is 6V. The magnitude of the amplitude can be adjusted by adjusting the sweep current potentiometer on the NMR power panel.

Probe: This experiment provides two probes, one of which is water (doped with copper sulfate) and the other is solid polytetrafluoroethylene.

The tester consists of a probe and a marginal oscillator. The liquid 1H sample is placed in a glass tube. The solid 19F sample is made into a stick. The coil is wound around a glass tube or a stick-shaped solid sample. This coil is an inductor L. The coil is inserted into the magnetic field, and the orientation of the coil is perpendicular to B0. The leads at both ends of the coil are connected in parallel with the varactor diode (acting as a variable capacitor) in the tester in reverse connection to form an LC circuit and oscillate with a nonlinear component such as a transistor.

When the circuit oscillates, the RF field is generated in the coil and acts on the sample. Change the reverse polarity of the diode

Figure 1-3

The magnitude of the voltage changes the capacitance C between the two diodes, thereby achieving the purpose of adjusting the frequency. This coil also serves as a coil for detecting the resonance signal. The detection principle is as follows:

The oscillator in the tester does not work in a state where the amplitude is stable, but works in the edge state of the just-starting (the edge oscillator is named), and any change in the circuit parameters will cause a change in the operation. When resonance occurs, the sample absorbs the energy of the RF field, so that the quality factor Q of the oscillating coil decreases. The decrease of the Q value will cause a change in the working state, which is reflected by the change of the envelope of the oscillating waveform. This change is the resonance signal. After detection and amplification, the NMR output is connected to the oscilloscope to observe the resonance signal from the oscilloscope. The undetected high-frequency signal of the oscillator is directly output to the digital frequency meter via the “frequency output” terminal, so that it can be directly Read the frequency of the RF field.

The front panel of the tester consists of a ten-turn potentiometer as a frequency adjustment knob. In addition, there is an amplitude adjustment knob (operating current adjustment). Adjusting this knob appropriately can maximize the signal absorbed by the resonance, but it will change due to the adjustment of the amplitude knob. The interelectrode capacitance of the oscillating tube has a certain influence on the frequency. The "frequency output" is connected to the digital frequency meter, and the "NMR output" is connected to the oscilloscope. The "voltage input" is connected to the "power output" on the power supply.

The front panel of the NMR power supply consists of “sweeping power switch”, “sweep adjustment”, “X-axis deflection adjustment” and “power switch”. The “sweep power output” is connected to the sweep scene input on the permanent magnetic base. The “power output” is connected to the “voltage input” on the tester. In order to synchronize the horizontal scan of the oscilloscope with the magnetic field sweep, the “X-axis deflection output” of the sweep signal is added to the X-axis (external) of the oscilloscope. To ensure that a stable resonance signal is observed on the oscilloscope.

〖Experimental content and experimental methods〗

1. Calibrate the magnetic field B0 of the permanent magnet center

Insert the probe with water (copper sulfate) into the center of the magnet, and make the probe at the front end of the tester in the same horizontal direction as the magnetic field. Move the tester left and right to make it roughly in the middle of the magnetic field. The “Frequency Output” and “NMR Output” on the panel are connected to the frequency meter and the oscilloscope respectively. Place the oscilloscope's scan speed knob at 1ms/div and the vertical zoom knob at 0.5V/div or 1V/div. “X The axis deflection output is connected to the X axis (external connection) of the oscilloscope to the oscilloscope. The power switch of the frequency meter, oscilloscope and NMR power supply and the power switch of the oscilloscope are turned on. At this time, the frequency meter should have a reading. Connect the "sweep" Field power output" and "sweep power input" on the magnetic field base turn on the power switch and adjust the output to a large value. Slowly adjust the tester frequency knob to change the oscillation frequency (small to large or large to small) while monitoring Oscilloscope, search for resonance signals.

Under what circumstances will the resonance signal appear? What is the resonance signal?

Nowadays, the magnetic field is the result of superposition of the magnetic field B0 of the permanent magnet and a 50 Hz alternating magnetic field. The total magnetic field is

(1-10)

among them Is the amplitude of the alternating magnetic field, Is the angular frequency of the mains. The total magnetic field is ~ Within the range of Figure 1-4, the sinusoidal curve changes with time. From (1-6), we can see that only Resonance occurs in this range. In order to easily find the resonance signal, it is necessary to increase (ie, adjust the output of the sweep to a larger value) to increase the range of the magnetic field that may resonate; on the other hand, adjust the frequency of the RF field so that Fall in this range. Once Falling in this range, the total magnetic field at some point in the change of the magnetic field At these moments, the resonance signal can be observed. As shown in Figure 1-4, resonance occurs at The horizontal dashed line corresponds to the point of intersection of the sinusoid representing the change of the total magnetic field. As mentioned above, the resonance signal of water will be as shown in Figure 1-2(b), and the more uniform the magnetic field, the more the number of oscillations in the wake, Therefore, once the resonance signal is observed, the left and right positions of the tester should be further carefully adjusted to maximize the number of oscillations in the wake, even if the probe is in the most uniform position of the magnetic field in the magnet.

As you can see from Figure 1-4, as long as Resonance signals can be observed falling within the range of (B0+B1) to (B0-B1), but this time It may not be exactly equal to B0, as can be seen from the figure: ≠B0, the time interval of each resonance signal is not equal, and the resonance signal is unevenly arranged on the oscilloscope. Only when When they are evenly arranged, the resonance occurs at the time when the alternating magnetic field crosses zero, and the time interval from the oscilloscope's time scale can be measured as 10ms. Of course, when or Evenly arranged resonance signals can be observed on the oscilloscope, but their time intervals are not 10ms, but 20ms. Therefore, only when the resonance signals are evenly arranged and the interval is 10ms At this time, the reading of the frequency meter is the resonance frequency of the proton corresponding to B0.

As a quantitative measurement, in addition to the value to be measured, we also care about how to reduce the measurement error and try to make a quantitative estimate of the error to determine the effective number of the measurement. As can be seen from Figure 1-4, once observed For the resonance signal, the error of B0 does not exceed the amplitude B1 of the sweep. Therefore, in order to reduce the estimation error, the amplitude B1 of the sweep should be gradually reduced after the resonance signal is found, and the frequency of the RF field is adjusted accordingly to make the resonance signal Maintain a uniform arrangement with an interval of 10ms. Under the premise that the resonance signal can be observed and resolved, try to reduce B1 to a minimum, and note that B1 is at a minimum and the resonance signal is kept at intervals of 10ms.

Figure 1-4

Frequency Using protons in water The value and formula (1-7) find the B0 value of the area to be tested in the magnetic field. By the way, when B1 is small, since the range of the sweep is small, the number of oscillations in the wake is small, which is normal, not The magnetic field becomes uneven.

In order to quantitatively estimate the measurement error â–³B0 of B0, the size of B1 must first be measured. The following steps can be taken: keep the amplitude of the sweep field unchanged, adjust the frequency of the RF field, so that the resonance occurs successively (B0+B1) and (B0-B1), at this time in Figure 1-4 with The corresponding horizontal dashed lines will be tangent to the peaks and valleys of the sine wave respectively, that is, the resonance occurs near the peaks and valleys of the sine wave respectively. At this time, the resonance signals seen from the oscilloscope are evenly arranged, but the time interval is 20ms, The following two resonance frequencies with Using formula

(1-11)

Can find the magnitude of the sweep.

In fact, the estimation error of B0 is smaller than that of B1. This is because the judgment error of the uniformity of the resonance signal is usually less than 10% with the help of the grid on the oscilloscope. Since the sweep size is a sine function of time, it is easy to calculate the corresponding The estimation error of B0 is about 80% of the sweeping range B1. Considering that the measurement of B1 itself also has an error, 1/10 of B1 can be taken as the estimation error of B0, that is,

(1-12)

Equation (1-12) shows that 1/20 of the difference between the peak top and the valley resonance frequency is utilized. The value can be used to find the estimated error △B0 of B0. In this experiment, △B0 only needs to retain one significant digit, and then the effective number of B0 can be determined, and the complete expression of the measurement result is required, namely: B0=measurement±estimation error

Phenomenon observation; increase B1 appropriately, observe as many wake oscillations as possible, and then gradually move the left and right positions of the tester in the magnetic field to the left (or to the right), so that the sample probe at the front end gradually moves from the center of the magnet to the edge. At the same time, observe the change of the resonance signal waveform during the movement and explain it.

Optional experiment: Using the probe with water as the sample, move the tester to the leftmost (or rightmost) of the magnetic field and measure the magnetic field at the edge of the magnetic field.

2. Measuring the g factor of 19F

Replace the sample probe with water as a sample Teflon probe and place the tester in the same position. Adjust the oscilloscope's vertical magnification knob to 50mV/div or 20mV/div, using the same method and procedure as the calibration magnetic field. Resonance frequency of 19F and B0 in PTFE And the resonant frequency near the peak and valley and ,use And formula (1-9) find the g factor of 19F. According to formula (1-9), the relative error of g factor is

(1-13)

Where B0 and △B0 are the results obtained by calibrating the magnetic field, which is similar to the above method of estimating ΔB0, which is preferable As Estimation error.

Find The calculated g factor can then be used to find the absolute error. , Only one significant digit is reserved and the complete expression of the g-factor measurement is determined by it.

When the resonance signal of fluorine in polytetrafluoroethylene is observed, the difference of the resonance signal waveform of protons in the water sample mixed with copper sulfate is compared.

PD Toys plastic Co., Ltd is OEM & ODM manufacturer of inflatable products in the mainland of China with more 17 years of manufacturing experience. products ranges are Inflatable Toys, inflatable pools, Inflatable Pool floats, towable tubes, Air Furniture and Promotional Items etc. total have more than 1500 employees (4 factories) related to PVC inflatable products.

Operated under ISO 9001:2015 management system, We had passed factory Audit by Walmart, Taret, Disney ect, also passed all necessary certificates and testing such as ICTI, BSCI, SEMTA,Target FA, NBC Universal, FCCA, SGS, CVS Security, GSV, Disney FAMA ect. We have our own PVC raw materials manufacturing company, all the PVC we produced are compliance with European EN71, American ASTM standard and NON PHTHALATE (6P) standard.

Armband & Lifevest

Arm Band,Inflatable Arm Float,Inflatable Life Vest,Kids Life Vest

P&D Plastic Manufacture Co., Ltd , https://www.pdinflatable.com