UV visible spectrophotometer is a scientific instrument based on the absorption characteristics of substances towards UV visible light to analyze the composition and concentration of substances. Its core principle and operating logic can be broken down into the following key steps:
1、 The interaction between light and matter: Generation of absorption spectra
When a substance is irradiated with light of a specific wavelength, electrons in the molecule absorb photon energy and transition from the ground state to the excited state. Due to differences in molecular structure, different substances have different absorption wavelengths (absorption peaks) and intensities (absorbance) for light. For example, conjugated double bond structures (such as vitamin A) may exhibit strong absorption in the 280-320nm wavelength range, while metal ion complexes (such as Fe ³ ⁺) may exhibit characteristic peaks in the 500-600nm range. The instrument decomposes composite light into monochromatic light through a monochromator, scans the wavelength range of 200-800nm, records the absorption degree of each wavelength of light by the substance, generates a unique absorption spectrum curve, and becomes the "molecular fingerprint" of the substance.
2、 The cornerstone of quantitative analysis: Lambert Beer law
The quantitative relationship between substance concentration and absorbance is described by Lambert Beer's law (A=ε bc): absorbance (A) is proportional to solution concentration (c), optical path length (b, typically a 1cm cuvette), and molar absorptivity (ε, an intrinsic property of the substance). For example, if the ε value of a substance at 510nm is 1.2 × 10 ⁴ L/(mol · cm), and the optical path length is 1cm and the solution concentration is 1 × 10 ⁻⁵ mol/L, the absorbance A=1.2 × 10 ⁴× 1 × 1 × 10 ⁻⁵=0.12. By measuring the absorbance of a standard solution and drawing a standard curve, the concentration of an unknown sample can be inferred based on its absorbance.
3、 From Spectrum to Essence: Applications of Qualitative and Quantitative Analysis
Qualitative analysis: By comparing the spectra of unknown samples and known substances, identify the position and shape of characteristic absorption peaks, and determine the type of substance. For example, when detecting phenol in water, its absorption peak at 270nm can be used as a qualitative basis.
Quantitative analysis: In environmental monitoring, the concentration of pollutants is calculated by measuring the absorbance of water samples at specific wavelengths and combining it with standard curves; In the field of biomedicine, measuring the absorbance of proteins at 280nm (due to the presence of aromatic amino acids such as tyrosine and tryptophan) can quickly estimate their content.
UV visible spectrophotometer captures the "absorption signal" of substances towards light, and correlates the microscopic molecular structure with macroscopic concentration, becoming an analytical tool in fields such as chemistry, environment, and biology.