UV/VIS in the Spot Light
UV/VIS spectroscopy refers to absorption/reflection measurements performed in the ultraviolet and visible light spectrum.
The Beer-Lambert law, which relates the attenuation of light to properties of the material the light is passing through, states that the absorbance of a sample is directly proportional to the concentration of the absorbing analyte. You can frequently see this axiom at work in analytical chemistry to quantify analytes, monitor processes and reactions, and detect certain organic compounds.
Atmospheric chemistry plays a central role in our understanding of the mechanisms of local climate conditions and factors into global radiative balance as well. Researchers study a class of compounds called Secondary Organic Aerosols (SOAs) which are the reactive products of gas-phase photooxidation of both naturally-occurring and man-made volatile organic compounds (VOCs). There have been many studies on the reaction mechanisms in the propagation of SOAs, Dr. Kun Li and a team of fellow researchers from the Institute of Chemistry at the Chinese Academy of Sciences and the Beijing National Laboratory for Molecular Sciences looked more closely at the optical properties of these aerosols under varying reactive conditions .
The scattering and absorptive properties, the direct components of the refractive index are more dependent on the composition of the aerosols than of the concentration or particle size. Understanding the link between the chemical composition of aerosol pollutants and their optical properties allows for a much more accurate estimate of the global radiative effects of localized reactive conditions.
Dr. Li’s team’s work tested the optical properties of SOA particles generated in a Teflon smog chamber in the lab from several different precursor compounds and under varying NOx levels. Using the AvaSpec-2048L spectrometer, the particles were shown to be nonabsorbent at wavelength 532 nm. Retrieving Refractive Indices (RI) for each sample at that wavelength yielded values ranging from 1.38-1.59 depending on which precursor compound generated the SOA and under what concentration of NOx, but independent of the concentration of SOA or of particle size. Ultimately their work suggests that many environmental models may overestimate the Refractive Index, and in turn the global radiative effects.
Colloids and Nanoparticle Reactions
One area of study in Chemistry that has received a great deal of attention in recent years is Nanoparticles. These nano-scale particles are between 1 and 100 nanometers in size and are surrounded by an interfacial layer of ions, organic and inorganic compounds which can react with other substances. Nanoparticles can exist as a powder or in a solid matrix but are often found in colloidal form, dispersed in an aqueous solution or gel.
Nanoparticles are formed by either breaking down larger particles or by a controlled chemical reaction assembly process. These microscopic particles can be used for molecular tagging, DNA probes, gene therapy, and even cancer treatment. They can be found in consumer products such as sunscreens, anti-glare/non-scratch eyeglasses, anti-microbial or heat resistant coatings, and even in our food supply. The processes of chemical assembly of nanoparticles can be temperamental and the stability and reproducibility of a reaction is dependent on many factors including solution temperature and humidity, and the purity and concentration of the reagents. Any study of nanoparticles necessarily begins with their production and one of the most widely accepted means of reaction monitoring is UV/VIS spectroscopy.
A group of researchers Kaunas University of Technology, Department of Physics, Kaunas Lithuania analyzed colloidal silver nanoparticles created via silver salt reduction. The chemical reduction of the silver salt solution was monitored throughout the reaction process using the AvaSpec-2048L UV/VIS spectrometer in the wavelength range 300-700 nm. Colloidal silver exhibits a wide absorption band from 350-550 nm with an absorption peak at 445 nm. As nanoparticles begin to form, absorption increases, then as the particles grow in size, the absorption peak shifts toward the red wavelengths. Stabilization of the absorption peak indicates that new nanoparticles have ceased forming. The data collected during the reaction process also allowed the researchers to calculate average particle size .
Another group of researchers from the School of Chemistry and Chemical Engineering at the Beijing Institute of Technology studying Nano-Hydrogel Colloidal (NHC) array photonic crystals for the detection of humidity, relay on UV/Vis spectroscopy during the precipitation polymerization synthesis process of developing their NHCs. The novel design of this colloidal gel humidity sensor allows the water absorbing properties of this hydrogel to swell and change volume in response to environmental stimulus. As the particles swell, the change in the size of the particle causes a shift of the absorption bands toward the red. This displays as a change in color visible to the naked eye and tunable, covering the full visible wavelength range from 400-760 nm corresponding to a 20-99.9% range in humidity. These experiments, supported and verified using the AvaSpec-ULS2048-TEC spectrometer, may lead to new sensor technologies which can be applied to a variety of organic and inorganic molecular detection sensors .
Complementary Measurement Methods
While UV/VIS spectroscopy can be a powerful tool on its own, this technique is frequently used in combination with other measurement techniques. The use of complementary techniques can provide researchers with richer sample data than either technique can achieve on its own. Alternatively, these methods can serve as a check of accuracy, with data deviations between methods pinpointing sampling discrepancies.
UV/VIS Spectroscopy with Attenuated Total Reflection (ATR)
Another spectroscopic technique that is frequently paired with UV/Vis spectroscopy is attenuated total reflectance (ATR). In analytical chemistry, when with strongly light absorbing, or optically dense material, ATR techniques can yield data about a sample by exploiting the electromagnetic evanescent field. During ATR, a beam of IR light is passed through a crystal with a high refractive index. Once the sample is in contact with the crystal, the incidence beam reflects off the interior surface of the crystal in contact with the sample. An electromagnetic field generated by the vibrations of this reflection penetrate into the sample revealing a band of absorption where the evanescent wave is attenuated.
Dr. Thomas Bürgi at the University of Geneva, Switzerland, Department of Physical Chemistry pairs ATR with UV/VIS absorbance measurements to study oxidation reactions at the point of catalytic interfaces at an in-depth molecular level. During experiments, Dr. Bürgi collected simultaneous and synchronized ATR and UV/VIS spectra. Changing reaction conditions yielded complementary information from the combination of measurement techniques. Attenuated total reflection (ATR) was used with the AvaSpec-ULS2048 spectrometer to identify dissolved reaction products, while UV/VIS spectroscopy was very sensitive to changes of the catalyst .
UV/Vis Spectroscopy with Attenuated Total Reflection (ATR), and Raman
Studying the synthesis of mixed oxide catalyst precursors, Dr. Ursula Bentrup and a team of scientists at the Leibniz Institute of Catalysis at the University of Rostock used multiple methodologies during their experiments for a deeper understanding of how varying synthesis parameters affect the structure and crystallinity of the precursors and the performance of the resulting catalysts.
The in-situ UV/VIS measurements were used in the monitoring of the chemical reactions to track the formation an extinction of reactant species. Paired with Raman, researchers could track changes at an atomic scale revealing a gradual crystallinity change of Co2+ from octahedral to tetrahedral. Using the AvaSpec-ULS2048 spectrometer with a specialized ATR probe, researchers observed absorption bands for molybdenum at 838, 879, 930, 1442, and 1632 nm, as well as a nitrate band around 1338 nm, become broader and more intense as precipitate formation increased. This combined method of investigation has a high potential for real-time monitoring of complex reactions in liquid phase, including the precipitate of solids .
UV/VIS Spectroscopy with Nuclear Magnetic Resonance (NMR)
One technique that is often combined with UV/VIS Spectroscopy is Nuclear Magnetic Resonance. German researchers publishing in the journal Angewandte Chemie studied the efficacy of combined UVNMR methodology in reaction monitoring to gain insight into the acid-based chemistry of strongly hydrogen-bonded complexes in aprotic solutions [N*]. These slow reactions are very sensitive to concentration, temperature, and the solvent used. Using the AvaSpec-ULS2048 spectrometer for UV/VIS spectroscopy allowed the researchers to monitor reaction states through changes in the spectra surrounding the phenol group absorption bands found at 315 nm in low pH solutions and shifting to 400 nm in high pH solutions. Meanwhile, simultaneous use of NMR methods yields insight into the hydrogen geometry of the reaction product .
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Webinar: Raman spectroscopy; the solution for product identification in chemistry and pharmaceutical industry
- Li, Kun, et al. "Optical properties of secondary organic aerosols generated by photooxidation of aromatic hydrocarbons." Scientific reports 4 (2014): 4922.
- Šileikaitė, Asta, et al. "Analysis of silver nanoparticles produced by chemical reduction of silver salt solution." Mater. Sci 12.4 (2006): 1392-1320.
- Wang, Zhe, et al. "Self-assembly of a nano hydrogel colloidal array for the sensing of humidity." RSC Advances 8.18 (2018): 9963-9969.
- Bürgi, Thomas. "Combined in situ attenuated total reflection infrared and UV–vis spectroscopic study of alcohol oxidation over Pd/Al2O3." Journal of Catalysis 229.1 (2005): 55-63.
- Bentrup, Ursula, et al. "Linking simultaneous in situ WAXS/SAXS/Raman with Raman/ATR/UV–vis spectroscopy: comprehensive insight into the synthesis of molybdate catalyst precursors." Topics in Catalysis 52.10 (2009): 1350-1359.
- Tolstoy, Peter M., et al. "Combined NMR and UV/Vis spectroscopy in the solution state: study of the geometries of strong OHO hydrogen bonds of phenols with carboxylic acids." Angewandte Chemie International Edition 48.31 (2009): 5745-5747.