HOW TO QUANTIFY THE ACTIVE PRINCIPLE BY THE SPECTROPHOTOMETRIC METHOD?
The spectrophotometric method is commonly used in the pharmacopoeia and generally in the pharmaceutic industry. A lot of active principles found in drugs have chemical groups in their structure that absorb UV-light and that can be thus quantified.
The quantic theory defines the total energy of a molecule as the sum of three energies:
- Electronic energy: Ee
- Vibration energy: Ev Et = Ee + Ev + Er
- Rotation energy: Er
UV absorption depends essentially on Ee. The concerned area of the electromagnetic spectrum goes from 10 to 380 nanometers. In reality, the classic apparatus explore the area between 200 and 380 nm, because below 180 nm, the radiation is absorbed by the air and the measure needs to be done under vacuum.
• Qualitative: useful for identification (comparing to a standard); however, UV/Visible absorption is not sufficient to identify a substance alone, but can sometimes usefully complete infra-red, mass spectrometry, NMR and other spectral methods.
• Quantitative: useful for dosage; it is a rapid and precise method, based on the exploitation of the Beer-Lambert law that links absorption to the concentration of the molecules in solution, given a wavelength λ.