Summary
Highlights
The video introduces the first experiment in CNG 292, focusing on the experimental design of fluorescence spectroscopy. The goal is to differentiate between emission and excitation spectra. The basic instrumental setup for fluorospectroscopy involves a sample in a cuvette, an excitation source, and a detector placed at a right angle to the incident beam. This right-angle placement prevents the detector from capturing the incident light, focusing instead on the sample's emission, such as from the fluorescent compound quinine.
When a sample like quinine is excited, its molecules are promoted from the ground state to the first excited state, which contains different vibrational energy levels. After non-radiative vibrational relaxation, the molecule drops to the lowest vibrational energy level of the first excited state, then undergoes radiative relaxation, which is the fluorescence emission detected. To measure the emission spectrum, an arbitrary excitation wavelength is chosen, often based on the UV-Vis absorption maximum (e.g., 350 nm). The detector then scans a range of emission wavelengths (e.g., 300 nm to 600 nm) to find the emission peak (e.g., 450 nm), representing the highest fluorescence intensity corresponding to a specific energy gap.
The main purpose of running an emission spectrum is to find the emission peak of the fluorescent compound by setting an arbitrary, fixed excitation wavelength. In this setup, the excitation wavelength is a fixed parameter, while the emission wavelength is the variable that is scanned and measured.
After determining the emission peak from the emission spectrum, the next step is to find the excitation wavelength that yields the highest emission intensity for that specific peak. This is the purpose of running an excitation spectrum. The instrumental setup remains the same as for the emission spectrum.
For an excitation spectrum, the detector is fixed to only measure the previously found emission peak (e.g., 450 nm). The excitation source, however, is now varied, scanning a range of excitation wavelengths (e.g., 250 nm to 600 nm) to determine which one causes the highest fluorescence intensity at the fixed emission wavelength. The y-axis of an excitation spectrum shows the fluorescence intensity at the fixed emission peak, while the x-axis represents the scanned excitation wavelengths. This helps identify the optimal excitation wavelength that maximizes the emission intensity for the specific emission peak.
In summary, the key difference lies in the experimental parameters: for the emission spectrum, the excitation wavelength is fixed (arbitrarily chosen), and the emission wavelength is varied to find the emission peak. For the excitation spectrum, the emission wavelength (the peak found from the emission spectrum) is fixed, and the excitation wavelength is varied to find the one that yields the highest intensity for that fixed emission.