Definition
A laboratory spectrophotometer is an optical analytical instrument used to quantitatively or qualitatively analyze the concentration of specific components in a sample based on the selective absorption properties of a substance. It is widely used in laboratory routine testing in various fields such as environmental monitoring, food analysis, materials science, biochemistry, and industrial production quality control by measuring the absorption of light by measuring the absorption of light by a specific wavelength or wavelength range.
Principle
The core working principle of a spectrophotometer is Lambert-Beale's law, which describes the quantitative relationship between light absorption and the concentration and path of light-absorbing substances. When a beam of parallel monochromatic light passes through a homogeneous, non-scattering solution, the absorbance of the solution is directly proportional to the concentration of the absorbing substance and the thickness of the liquid layer. Its mathematical expression is: A = εbc, where A represents absorbance, ε is the molar absorbance coefficient, b is the length of the optical path, and c is the concentration of the solution. The instrument generates a continuous spectrum through the light source, and after the monochromator splits, a specific wavelength of monochromatic light is obtained, which passes through the sample cell and is received by the detector, and finally converts the light signal into an electrical signal and calculates the absorbance value.
Measurement method
Routine measurements typically include the following steps: first pre-warm the instrument with baseline calibration, and adjust the absorbance zero point using a reference solution. A series of concentration standard solutions are then configured, absorbance is measured at the selected wavelength, and a standard operating curve is drawn. The absorbance of the sample to be measured is then measured under the same conditions, and its concentration is calculated according to the operating curve. For unknown samples, absorption spectra can be obtained by full-wavelength scanning to aid in qualitative analysis. Some instruments support multi-wavelength measurement, kinetic mode, and quantitative analysis software to meet different analysis needs.
Influencing factors
The accuracy of the measurement results is influenced by several factors. In terms of optical parameters, the bandwidth and wavelength accuracy of monochromatic light directly affect the selectivity. Stray light levels can cause absorbance deviations. Sample handling factors include solution uniformity, scattering caused by bubbles or suspended solids, cuvette cleanliness and matching. Chemical factors involve whether the concentration range of the DUT conforms to the law, linear intervals, pH of the solution, or chemical equilibrium shifts due to ionic strength. Environmental conditions such as temperature fluctuations can alter the absorbance coefficient or cause instrument drift. During operation, it is necessary to ensure data reliability through regular calibration, standardized sample preparation and control environment.
Application:
In the field of environmental monitoring, it is used to determine heavy metals, nitrates, chemical oxygen demand and other indicators in water bodies. In the food industry, nutrients, additive content, or contaminant residues can be analyzed. In materials science, it is used to characterize dye concentrations, film light transmittance, or chemical reaction process monitoring. Biochemistry experiments are often used for quantitative analysis of proteins and nucleic acids and enzyme kinetics. In addition, it can be used for raw material purity inspection and product quality control in the chemical production process. Different industries usually refer to the corresponding international standards, national standards or industry standard methods for operation.
Selection
When selecting an instrument, it is necessary to comprehensively consider the analytical needs and technical parameters. The wavelength range should cover the characteristic absorption band of the substance to be measured, with the visible light model suitable for routine analysis and the UV-Vis model expanding the range of applications. The spectral bandwidth affects the resolution, and the narrower bandwidth helps distinguish between overlapping absorption peaks. Photometric accuracy and stray light indicators affect the accuracy of low-concentration measurements. The sample adapter needs to match common cuvettes or support trace sample detection. The operation interface and data processing function should facilitate method development and result analysis. In addition, the long-term stability, ease of maintenance, and compliance of the instrument are also aspects to consider. It is recommended to evaluate based on the actual sample type, throughput, and budget range.