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|Posted: Fri Dec 15, 2006 3:15 pm Post subject: (Chem) Spectroscopy: NQR Spectroscopy and Illegal Substances
NQR spectroscopy: Detecting illegal substances
By Sharon Lin, Georgetown University
Ever since the terrorist attacks of 9/11 on the United States, there has been a large demand for inexpensive, quick, and highly accurate methods for detecting illegal substances, mainly explosives and drugs. Conventionally, the U.S. has used X-ray technology to scan carry-on and luggage for dangerous explosives and trained dogs to detect drugs, but these methods have proved to be not as effective compared to the more sophisticated methods of detection available today.
One of these more sophisticated technologies is called nuclear quadrupole resonance (NQR) spectroscopy. This spectroscopic method is capable of imaging solid substances and therefore is capable of detecting illegal substances, such as explosives and drugs. This method uses the fact that quadrupole (four charges) nuclei, which exist in a wide variety of chemical compounds, can be used to explore intra (within the molecule) and intermolecular (between molecules) structures. NQR also has an ability to differentiate between various chemical compounds, due to the fact that it can precisely determine the electric charge distributions in molecules. An important advantage that NQR spectroscopy has over other conventional methods, such as NMR, is that it does not require the use of a magnet (to induce a magnetic field) or exposure to high energy radiation, therefore preventing any permanent changes in the molecule being studied.
Some nuclei have a non-zero electric quadrupole moment, which results from the fact their electric charge distribution is not shaped like a sphere, but rather like an oval. NQR requires that these nuclei must possess a nuclear spin (I) greater than ½, so that when the nucleus is exposed to an electric field gradient (EFG), distinct energy states arise. In general, for nuclei with a nuclear spin equal to 1 (e.g. 14N), three transition frequencies between the quadrupole energy levels arise: . For nuclei with a nuclear spin equal to 3/2 (e.g. 35Cl), only one transition frequency arises: . These transition frequencies usually fall in the range of 0-6 MHz for 14N, and 28-43 MHz for molecules with C-Cl bonds. Transition frequencies are the frequencies that arise from moving from one energy level to another. This concept might be easier to understand if the distinct energy levels are compared to the distinct levels of a house. The lowest level of the house has the lowest energy, the second level has a higher energy, the third level has one higher than that, and so on. Walking up the stairs from the first level to the second requires energy (it is hard to go uphill), which corresponds to a specific frequency since energy is related to frequency. Similarly, walking down the stairs from the second floor to the first releases energy (it is easy to go downhill), which also corresponds to a specific frequency. This is the transition frequency.
The electric field gradient provides insight into the arrangement of electronic charge distributions about the nucleus being studied. This depends on the nature of nucleus as well as the chemical environment, and therefore the EFG is different for different materials. As a result, each material will have a characteristic transition frequency and hence NQR can accurately differentiate between various materials.
Nuclear quadrupole resonance imaging (NQRI) is the technique used to image and characterize solid state substances containing quadrupole nuclei. There are many different NQRI techniques, the most advantageous one being based on the rotating frame method. This technique is usually called -NMR. In -NMR, there is no static magnetic field applied to the sample so that no Zeeman splitting effects have to be considered. In the absence of the static magnetic field (no magnet), the complete spectroscopic information about the nuclei of the sample is completely accessible. Radio frequency pulses are applied to the object to begin imaging. The end result of imaging is a spectrum that gives the values of the transition frequencies that were detected within the object of interest.
NQR spectroscopy is particularly useful in detecting explosives and drugs because many of them contain nitrogen or chlorine atoms. 14N and 35Cl have a pretty large electric quadrupole moment, which can be easily detected by NQR. Due to the fact that NQR can precisely differentiate between various materials according to the frequencies at which the NQR signal is obtained, it is possible to distinguish between the illegal substances and regular, every day substances. The NQR technique for searching for explosives and drugs offers many advantages over X-ray technology, trained dogs, and other techniques. NQR can detect explosives and drugs even when they are sealed tightly, something that sniffing dogs and vapor detectors have difficulty doing because these methods rely on environmental factors for detection. Also, the procedure for detecting illegal substances through NQR is relatively simple. The object of interest (e.g. luggage) is moved through a probe coil. Then the sequence of RF pulses near the correct NQR frequency is applied to the object. The probe coil then detects a weak signal at the NQR frequency corresponding to 14N and 35Cl, if those isotopes are indeed present within the object.
A major setback in this method is that it requires that the sample contains grams or even tens of grams of the compound of interest. It would be extremely beneficial to explore and develop ways to improve NQR sensitivity.
NQR spectroscopy is a unique and extremely advantageous method for detecting explosive substances and drugs. This method proves to be particularly useful in the security field because it is relatively inexpensive, harmless, non-destructive, it does not require for samples to be prepared beforehand, and it does not require the use of an external magnetic field. Future improvements on the NQR technique call for more sensitivity, and revealing the location of the illegal substance within the object under suspicion.
Osán Tristán M., Cerioni, Lucas M.C., Forguez, Jóse, Ollé, Juan M., and Pusiol, Daniel J. “NQR: From Imaging to explosives and drugs detection”. Physica B: Condensed Matter, Sept 2006. ScienceDirect. Georgetown University Library. 11 December 2006.
Questions to further explore the topic:
What is an electric quadrupole moment?
What is a magnetic field?
What is nuclear spin?
What is an electric field gradient?
What is transition frequency?
What is an electric charge distribution?
What is resonance frequency?
What are wavelength and frequency?
How are Energy and frequency related?
What is MRI?
What is radio frequency?
What is a magnetic field gradient pulse sequence?
What is line broadening?
What is the Zeeman Effect?
What is free induction decay (FID)?
What is the Fourier transformation?
What is spectroscopy?
What is light?
How does light interact with matter?
How does light affect matter?
What is X-Ray spectroscopy?
What is NMR spectroscopy?
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|Posted: Sat Jun 30, 2007 9:11 am Post subject: Scientists seek marijuana's isotopic fingerprint
|University of Alaska Fairbanks
21 June 2007
Scientists seek marijuana's isotopic fingerprint
Scientists at the Alaska Stable Isotope Facility can tell whether marijuana confiscated in a traffic stop in Fairbanks likely came from Mexico or the Matanuska Valley.
They're also working on a way to determine whether it was grown indoors or out.
A few more years and enough samples and they hope to have something even more precise: an elemental fingerprint that could tell police where and under what conditions a sample of marijuana was grown.
"There are scientists already doing this for drugs like heroin and cocaine," said Matthew Wooller, Alaska Stable Isotope Facility director. "The potential is there for being able to do this for marijuana as well."
The key lies at the atomic level. Of particular interest to Wooller and his colleagues are the stable isotopes of four elements: carbon, oxygen, nitrogen and hydrogen.
Isotopes are atoms of elements that have the same number of protons and electrons but different numbers of neutrons. A stable isotope is one that doesn't decay over time. Those additional or missing neutrons in an isotope slightly alter the mass of the atom, allowing scientists to use a stable isotope ratio mass spectrometer to separate the light isotopes from the heavy ones and form a ratio for each sample. That ratio can tell scientists about the sample and its origins.
"The marijuana holds a signature of the environment that it used to be grown in," Wooller said. "It is laid down in time and preserved in the materials that make up a plant."
For example, oxygen and hydrogen ratios can reveal information about the water a plant used while growing and, as a result, where it was grown. Water in Alaska and other high latitudes generally has a larger proportion of light oxygen and hydrogen stable isotopes than water from locations at lower latitudes. Carbon tells another story, he said. It can offer information on whether a plant was grown outdoors or inside. Nitrogen could provide even more information.
The testing at the UAF facility is novel because, for each sample, scientists are taking the isotopic signatures of four elements, rather than for just a single one, Wooller said. "We have the potential to create a precise chemical fingerprint."
The marijuana research began approximately two years ago and was initially supported by a grant from the University of Alaska President's Special Projects Fund. The UAF Police Department provided the lab samples of marijuana confiscated locally. "We started off running samples of unknown origin," Wooller said, noting that even those samples yielded some surprising results.
Scientists initially assumed that most of the samples would show that they had been grown in Alaska rather than being imported from the low latitudes. "In fact, what we saw is there are samples that are almost certainly grown in high latitude," he said. "Then you had marijuana that was clearly grown at lower latitudes."
Since then, the project has expanded beyond samples of unknown origin. The federal Drug Enforcement Administration and the Alaska Bureau of Alcohol and Drug Enforcement have started providing samples from grows in Alaska.
Wooller hopes that, with enough of those samples, he can create a marijuana isotope map for Alaska and beyond, which could eventually allow scientists to match unknown samples with known growing locations.
The project has potential to help police on multiple levels, according to Investigator Stephen Goetz at the UAF Police Department.
From an evidentiary standpoint, it could tie a growing operation to marijuana seized on the street, he said, and offer evidence of both the production of marijuana and its distribution.
"The common denominator that people use as their defense is that (they) are growing it for their personal use only," Goetz said. If marijuana seized from a dealer, for example, matched that growing operation, it could counter such a defense, he said.
It could also help the state's drug enforcement officials track the trafficking patterns of marijuana by comparing where the marijuana was grown to where it is seized, Goetz said. "It could, theoretically, focus law enforcement's efforts on where to look for (growing operations.)"
In order to get the method to that level, though, Wooller said he needs time, money and many more samples of marijuana, either from known locations or that are grown in a laboratory, such as the state crime lab, under controlled conditions.
"We need more data," Wooller said. "We need more analyses of marijuana samples from known locations so we can create these base marijuana isotope maps."
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