Nuclear Magnetic Resonance Spectroscopy (NMR)
Nuclear Magnetic Resonance (NMR) spectroscopy is a chemistry methodology employed to analyze the ingredients and purity of a sample as well as its structure on a molecular level. For specimens containing known compounds, this analytical process is used for quantitative analysis. For mixtures of unknown makeup NMR can reveal details of the basic molecular structure and use the data to identify the compound by cross-referencing libraries of spectral data. A variety of NMR techniques exist and are useful in determining specific molecular properties of a sample such as solubility, changes of phase, and molecular conformation.
NMR is a powerful tool mainly due to the fact that it is non-disruptive and able to provide complete analysis of an entire spectrum. This technology allows researchers to identify compounds and their molecular structure. This enables scientists to employ a variety of services including:
- Product Failure Analysis
- Material Testing & Identification
- R&D Product Development
- Method Development & Validation
- Litigation Support
The NMR spectrometer functions primarily on the technology of magnetic fields and radio frequencies. It is composed of the following major components:
- A stable magnetic field
- A probe which allows the sample to be close to the charged coils
- A high-power RF transmitter
- A receiver to correctly amplify the NMR signals
- A converter to digitize the NMR signals
- A computer for data analysis and operational control
NMR testing uses nuclear energy transfer principles to identify molecular structures. This is accomplished by applying a magnetic field to a sample and measuring the wavelength emitted by the energy transfer of the charged nuclei. This process relies on the spin of the element’s nucleus. A nucleus has spin if there are either an odd number of protons, neutrons, or both. The spin of the nucleus is what determines the resonance frequency.
The NMR spectrometer uses superconducting magnets which are very stable and range from 500-900 megahertz. These magnets are cooled with liquid helium which is in turn held in a nitrogen bath in order to keep the helium from evaporating too quickly. The probe is simply a tube which passes through the magnet and provides a controlled room temperature region so that the samples are not affected by the extreme temperature of the liquid nitrogen. This probe includes a small coil used to excite the molecules and detect the signal output.
When a sample travels through the NMR’s magnetic field the nuclei experience torque which affects their common spin and causes them to wobble. This wobble, which is induced in the probe, is affected by radio frequency. If the frequency emitted matches the frequency of the nuclei they will reverse their spin in what is referred to as magnetic resonance. The RF transmitter is used to pulse frequencies which disrupt the nuclei’s wobble. During these pulses the nuclei change energy states and then emit energy to lower themselves back to their previous state. The emitted energy equates to a particular resonance frequency which directly relates to the magnetic field’s strength that is being applied to the compound.
This resonance is different for every nucleus. The orientation of atoms within a molecule will affect the resonance of identical atoms which makes it possible to identify the location of similar atoms within differing functional groups.
The NMR signal sent from the probe is quite weak and requires amplification before it can be digitized. The amplifier is placed as close to the probe as possible and boosts the signal as it travels down a cable to the console. The probe, as mentioned earlier, also performs the part of the receiver. In order to prevent the transmittance and reception of frequencies from interacting there is an electronic piece called the diplexer which switches the operations on and off, regulating the timing and preventing interference.
The frequency signals picked up by the receiver are transcribed into binary code by an analogue to digital converter (ADC) and sent to the computer. The ADC sends the results at regular time phases so the data is received as data points. These data points, when reviewed by a knowledgeable analyst, can reveal the identity of the sample compound.
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