Spectrophotométrie: Introduction

Spectrophotometry is the quantitative method of evaluating radiation interacting with a material by measuring its relative intensity. Electromagnetic radiation of a particular wavelength passes through a sample to a detector. The difference between incident radiation intensity and transmitted radiation intensity reveals how much radiation the material absorbed, reflected, diffracted, or emitted.

An absorption spectrum is a plot of intensity lost to absorbed radiation versus the radiation wavelength. Since molecules have distinct energy gaps for different transition types, the range of wavelengths indicates which transition type occurred. The specific wavelength of absorption reveals information about functional groups present in the molecule.

A spectrophotometer contains three essential components: a source of electromagnetic radiation, a slot for placing the material to be analyzed, and a detector to measure incident and exiting radiation intensities. The detector captures transmitted, reflected, or diffracted radiation. The difference between incident and transmitted intensities is used to calculate absorbed radiation.

The wavelength range of radiation determines which type of transition occurs in a material. UV-visible radiation causes electronic excitation, while infrared radiation excites vibration levels of specific bonds. Different functional groups absorb infrared radiation at different wavelengths, allowing infrared spectra to identify functional groups in molecules.

Spectrophotometry generates multiple spectrum types by plotting different radiation measurements against wavelength. Absorption spectra show absorbed radiation, transmission spectra show transmitted radiation, reflection spectra show reflected radiation, and diffraction spectra show diffracted radiation. Each spectrum type provides distinct information about how the material interacts with radiation at specific wavelengths.

Spectrophotometry is nondestructive because the sample material can be recovered after analysis, unlike destructive methods such as titration. The technique measures radiation interaction without chemically altering or consuming the sample. This allows the same sample to be analyzed multiple times or used for subsequent analyses.

Scanning exposes the sample to radiation of different wavelengths, often one at a time, generating a series of data points with absorption details for each wavelength. Since material energy levels are finite, absorption occurs only at specific wavelengths. This systematic process builds a complete absorption spectrum showing which wavelengths the material absorbs.