Spectroscopies use electromagnetic radiation to determine changes, such as absorbance or fluorescence, to allow reactions to be studied.
Absorbance spectroscopy is a cheap and easy technique used in many labs, allowing the concentration of the sample to be determined. Coupled assays may be able to use UV-vis spectrophotometry, making it easier to study the reaction of interest.
NMR is an advanced and expensive assay technique, able to identify almost any molecule. Interference in the data can be caused by buffers, water, and other compounds present. However, due to the high cost and complexity of NMR, it is generally avoided unless it is the only option available.
Absorbance spectroscopy and NMR are both better suited to slower reactions – with NMR taking minutes to obtain a recording. Absorbance spectroscopy is faster, but still takes time to load the sample into a spectrophotometer. A quench flow could be used with absorbance spectroscopy, as well as continuous flow (through the CCD detector), also allowing fast reactions to be studied.