Nuclear Magnetic Resonance (NMR) spectroscopy is a widely used analytical technique in chemistry for exploring chemical composition. NMR spectroscopy provides details about the kinetics and structure of substances, allowing the study of unstable reaction species in situ. Recent advancements in high-power fiber-coupled light-emitting diodes (LEDs) have enabled researchers to integrate these fiber-coupled LEDs with NMR spectroscopy for various applications, especially in the photochemistry field.

A typical LED for an NMR illumination system comprises FC-LED connected to a long (~4m) optical fiber. At the distal end, the fiber is inserted into the NMR tube and submerged into the sample. The fiber may be inserted into a coaxial insert tube to provide additional protection for the fiber.

Optical Fibers for NMR-LED Spectroscopy

To achieve good illumination of the whole sample the distal end of the fiber is stripped from the opaque Jacket (see below for Polymer Optical Fiber Anatomyy) and the Cladding is sanded to create a rough surface to cause the light to escape the fiber along this surface and not only at the tip (See Figure 2).

There are many various types of optical fibers. For the application of LEDs with NMR the best fibers are large core diameter (~1000um) high NA (>0.39) fibers. These fibers accept and transmit a significant part of the light emitted by the LED. Particularly Polymer Optcal-Fibers (POF) with a core diameter of 1000 µm and NA=0.5 were identified as suitable for LED-NMR applications.

These fibers demonstrate efficient coupling with LEDs, good optical transmission, and high flexibility for easy installation. Moreover, they exhibit high durability and are resistant to breakage. The Polymer fibers are compatible with sanding and are cost-effective. The POF fibers have a relatively very thin Cladding that enables effective sanding with good homogeneity of light distribution along the sanded part of the fiber.

An alternative fiber choice suitable for LED-NMR applications is the silica core polymer-clad fibers. With a core diameter of 1000um and high numerical apertures (NA=0.5), these fibers provide versatility, although they may be less flexible than POF and could occasionally experience cracking. Despite these drawbacks, silica core polymer-clad fibers offer transmission at short UV wavelengths (<350nm) and at Near IR (>750nm), providing a wavelength range not supported by POF fibers.

Polymer Optical Fiber Anatomy: the transparent Core is the PMMA media where light is transmitted along the fiber. The Cladding is a very thin (a few micrometers) layer surrounding the core. Rays imping the core-cladding interface at an angle greater than the critical angle are reflected to the core. The Jacket layer is an opaque, thick polymer layer protecting the core and the cladding from mechanical damage. Light entering the core at the proximal end (e.g. at FC-LED) will go out mainly at the distal end of the fiber. The cone of light emitted from the fiber has a specific angle α. This is an important fiber parameter as it also defines the acceptance angle of the light that can be coupled to the fiber. Commonly it is specified as Numerical Aperture, NA=sin(α), rather than actual angle Alpha (n denotes the refraction coefficient of the core material). As a rule of thumb, higher NA enables more LED light to be coupled to the fiber.

Prizmatix offers polymer fibers for NMR applications:

The most popular NMR fiber is:

P/N	Description
NMR-Fiber-1000	High NA Polymer Optical Fiber (POF) Core: 1000um NA: 0.5. SMA Connector. End tip specially processed for NMR tubes: 7cm stripped, 5cm sanded for radial illumination. Standard Length 6m.

Light sources for LED-NMR applications

The Fiber-Coupled LED is designed to maximize the coupling efficiency of the LED light into the fiber. The LED light can be synchronized with NMR spectrometer via TTL input. The level of light power can be controlled manually or from the computer via Analog Input or USB.

Light sources for LED-NMR applications

Prizmatix has a vast offering of Fiber-Coupled LEDs that can be useful for NMR applications.

	The FC-LED product line includes single-channel or multi-channel (up to 10 various LEDs in a single enclosure) fiber-coupled high-power LED products.
	The UHP-LED-FB product line incorporates the highest brightness and power LED chips suitable for more demanding power requirements.
	The CombiLED product line addresses the requirements of multi-wavelength illumination or fast switching between several wavelengths.
	The UHP-T-LED product line facilitates the incorporation of a bandpass filter between the LED and the fiber. This becomes crucial when a specific activation wavelength bandpass is required. Moreover, UHP-T-LEDs are valuable for assembling dual-wavelength or triple-wavelength light sources.

Questions and Answers

Q.: Why LEDs are better than lamps and lasers for NMR applications?

A.: LEDs surpass lamps in NMR applications due to their relatively small emitters and superior directiveness of the emitted light, ensuring enhanced compatibility with fiber coupling. Moreover, LEDs boast a lifespan that is typically ten times longer than lamps and exhibit greater energy efficiency. In contrast to lamps, LEDs offer specific wavelengths, eliminating the need for additional filters in typical NMR applications.

LEDs are preferred in NMR applications over lasers for their cost-effectiveness, and versatile wavelength options enabling precise targeting of specific processes. Furthermore, LEDs sources do not require stringent laser safety protocols and generally require simpler setups compared to lasers. This user-friendly aspect makes LEDs a preferred choice for researchers in NMR applications.

References:

Hanming Yang, Heike Hofstetter, Silvia Cavagnero; Fast-pulsing LED-enhanced NMR: A convenient and inexpensive approach to increase NMR sensitivity. J. Chem. Phys. 28 December 2019; 151 (24): 245102. https://doi.org/10.1063/1.5131452

James P. Yesinowski, Joel B. Miller, Christopher A. Klug, Holly L. Ricks-Laskoski, Optorelaxers: Achieving real-time control of NMR relaxation, Solid State Nuclear Magnetic Resonance, Volume 96, 2018, Pages 1-9, ISSN 0926-2040, https://doi.org/10.1016/j.ssnmr.2018.09.002

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