Thin-layer chromatography is a tried and true staple of an organic chemistry lab, but it can be of limited use. Compounds with similar properties and polarities often don’t separate, and chemicals that are invisible under ultraviolet light can be difficult to detect. But through a new technique that combines TLC and surface-enhanced Raman spectroscopy (SERS), chemists can identify compounds without full separation. In addition, because the technique scans the entire TLC plate, scientists can find new by-products and determine their primary structures via the Raman spectra (Anal. Chem. 2014, DOI: 10.1021/ac5017387).
SERS is a sensitive spectroscopic technique that can identify small amounts of compounds. Researchers apply a sample to a metal surface, often silver or gold nanoparticles, and shine light on it. The metal surface enhances the characteristic Raman signal produced by the adsorbed molecules.
Linking TLC and SERS is not new. In some previous methods, chemists dropped silver nanoparticles on the spots of developed TLC plates and then collected the SERS spectra from each spot. So this method only worked with spots that were visible on the TLC plate, says Jing-Fu Liu, an environmental analytical chemist at the Chinese Academy of Sciences.
In the new method that Liu and coworkers developed, researchers spray the entire plate with an even layer of gold nanoparticles and then obtain SERS data along the compound’s path on the TLC plate at intervals less than 1 µm. This way, Liu says, the researchers can conduct a continuous analysis of all the chemicals, including ones that are not readily identified on the plate. “Our work has low risk of missing chemicals such as new products or by-products of a reaction,” he says.
As proof of concept, Liu and coworkers followed the Suzuki coupling between phenylboronic acid and 2-bromopyridine to form 2-phenylpyridine. After mixing the reaction solution and activating the TLC plate, the group pipetted 1-μL samples onto the plate and placed it in a developing chamber with solvent. After developing the plate, they sprayed it with a dispersion of gold nanoparticles and placed it on the stage of a commercial Raman spectrometer. They found the Raman signatures of both the reactants and products. In addition to these spots, they also picked up a signal that they attributed to a previously unknown by-product of the reaction. Based on the Raman data, they speculate that the compound is either biphenyl or 2,2-bipyridine.
Since the intensity of a Raman peak increases with a compound’s concentration, Liu says that this method could help organic chemists estimate the yield of a reaction. “Any sample that can be separated and purified by TLC and analyzed by SERS can be handled by our method,” he says.
One of the nice things about this method is its quantitative nature, says Yi-Tao Long, an analytical chemist at East China University of Science & Technology. However, he says that he would have liked to see the researchers follow a set of diverse reactions to demonstrate the method’s capabilities.
Liu’s team is now working on implanting the gold nanoparticles inside TLC plates so scientists don’t have to spray them, which could disturb sample spots.