RU2333475C1 - Method of search and registration of nuclear quadrupole resonance spectra - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: present invention can be used for search and registration of nuclear quadrupole resonance spectra. Invention implies that substance analysed is irradiated with multipulse sequence, each pulse having phase shift by 180° relative to previous one. NQR signals are accumulated and processed. Multipulse sequence consists of two components, the first one, of duration 0.7T₁, including preparatory 90° pulse and following sequence of 180° pulses, and the second component of sequence including pairs of 90° pulses.

EFFECT: reduced detection time and increased sensitivity at analysis of substance by quadrupole resonance method.

1 dwg

Description

The invention relates to radio spectroscopy and can be used both in pulsed nuclear quadrupole resonance (NQR) spectrometers intended for basic scientific research, and in equipment for the remote detection of explosives and explosives.

Almost all explosives and drugs have nitrogen atoms in their composition, the nuclei of which have a quadrupole moment, and, therefore, it is possible to observe NQR spectra in these compounds. However, their intensity is extremely low, since the NQR frequencies of nitrogen N ¹⁴ in these substances are low (0.7-6 MHz), which requires multiple accumulation of NQR signals. To reduce it, coherent multipulse sequences are used [1-3].

There is a method for searching and recording NQR spectra, which is also used to detect explosives and drugs, including irradiating a test substance with a multi-pulse sequence, each pulse in which has a phase shift of 180 ° relative to the previous one, when the frequency of the radio frequency field coincides with the frequency of the NQR of the substance transient signals, the accumulation of these signals and their subsequent processing [see A.S. No. 958935, IPC G01N 24/00).

There is also a method of detecting explosives, including irradiation of the test substance, an object with two multipulse sequences followed by accumulation and signal processing (see AS No. 1824559, IPC G01N 24/00).

The disadvantages of these methods are low sensitivity at frequencies NQR N ^{14 of} most drugs and explosives 0.7-6 MHz and, therefore, a small signal-to-noise ratio leading to an incorrect result, and also requiring long-term accumulation and, consequently, increasing the detection time of the test substances.

The technical task of this invention is to reduce the detection time and increase sensitivity in the study of substances by nuclear quadrupole resonance.

The problem is solved due to the fact that in the method for searching and recording spectra of nuclear quadrupole resonance using a multi-pulse sequence, including irradiating the test substance with a multi-pulse sequence, where each pulse has a phase shift of 180 ° relative to the previous one, the accumulation of NQR signals and their subsequent processing, the multi-pulse sequence consists of two parts, the first of which, with a duration of 0.7T _1, includes a preparatory 90 ° pulse and the next the duration of 180 ° pulses, and the second part of the sequence includes pairs of 90 ° pulses.

The method is as follows.

The test substance, placed in the coil of the oscillatory circuit of the NQR spectrometer, is irradiated with a multi-pulse sequence with alternating phases, consisting of two parts, the first of which is 0.7T _{1 in} duration and consists of a preparatory 90 ° pulse and pairs of 180 ° pulses following it, the radio frequency filling phases differ by 180 °, the second part of the sequence includes 90 ° pulses with the same phases.

Theoretical and experimental study of the two most commonly used sequences in NQR (multi-pulse spin-locking (MPLS) φ _y - (τ-φ _x -τ) _n and multi-pulse sequences with alternating phases (MPAF) φ _x - (τ-φ _{- х} - 2τ-φ _x -τ) _n ) was performed in [4]. Here, φ is the angle of rotation of the nuclear magnetization vector by the preparatory pulse, φ is the angle of rotation of the sequence by the pulse, x and y are the axes in the spin space along which the radio frequency field vector is directed, n is the number of cycles in the sequence, and τ (2 τ) is the interval between pulses in the loop.

The experimental results are shown in the drawing, which shows the envelopes of the echo signals in the MPAF at the transition frequency f ₊₀ = 4.93125 MHz at a temperature of 77K for the cases: φ = φ = 90 ° -1 and 2φ = φ = 180 ° -2; τ = 0.5 ms. It follows from them that the maximum of transitional signals takes place after the first pulse of the 180 ° MPAF with the 90 ° preparatory pulse and is equal to the intensity of the echo signal. Then their intensity drops to zero in a time of the order of the spin-lattice relaxation time T ₁ . The intensity of the transition signals in the 90 ° sequence is 1/2 of the maximum intensity and does not depend on the number of cycles or on the presence or absence of the 90 ° preparatory pulse.

Therefore, in the case when the signal detection time should be no more than the time corresponding to a twofold decrease in the signal intensity in the sequence (of the order of 0.7 T ₁ ), or when the increase in sensitivity during this time is sufficient for reliable signal detection, it is advisable to use the 180 ° multipulse sequence. The 90 ° sequence can give a significantly larger increase in sensitivity if the search time is several tens of T ₁ . Typically, 10–20 seconds are given in the search for explosives and drugs (T ₁ in compounds of interest at room temperature not more than one to two seconds).

From the above analysis it follows that to obtain the maximum gain in sensitivity, it is most advisable to use a combined sequence. Its initial section of duration 0.7T ₁ should consist of a 180 ° sequence, and the next one of a 90 ° sequence.

Similar studies of MPSL were published in [5]. These articles also provide theoretical studies that are fully consistent with the experiment.

Information sources

1. R. A. Marino, S. A. Klainer, J. Chem. Phys., 67, 3388 (1977).

2. D.Ya. Osokin. Copyright certificate 958935, IPC G01N 24/00. “A method for searching and recording spectra of nuclear quadrupole resonance”, 09.25.82.

3. V.G. Kuznetsov, S.D. Koblev. Copyright certificate 1824559 A1, IPC G01N 24/00. "A method for detecting explosives using nuclear quadrupole resonance", 05/14/91.

4. D.Ya. Osokin. Molec. Phys., 48, No.2283 (1983).

5. D.Ya. Osokin. Journ.Molec.Struct., 83243 (1983).

Claims (1)

A method for searching and recording nuclear quadrupole resonance spectra using a multi-pulse sequence, including irradiating the test substance with a multi-pulse sequence, each pulse having a phase shift of 180 ° relative to the previous one, accumulation of NQR signals and their subsequent processing, characterized in that the multi-pulse sequence consists of two parts, the first of which, the duration 0,7T ₁ includes a preparatory 90 ° -th pulse and the following sequence of 180 ° -x momentum s, and the second portion of the sequence includes a pair of 90 ° -x pulses.

Images