Rationale
UV-Vis spectrophotometer is an instrument that analyzes the selective absorption characteristics of ultraviolet and visible electromagnetic radiation based on matter. It works according to Lambert-Beale's law, which describes the quantitative relationship between absorbance and solution concentration and path length. When a beam of parallel monochromatic light passes through a homogeneous, non-scattering solution of absorbing substances, the absorbance of the solution is directly proportional to the concentration of the solution and the thickness of the liquid layer. Its mathematical expression is:
A = εbc
Among them, A is absorbance, ε is the molar absorbance coefficient (L·mol⁻¹·cm⁻¹), b is the path length (cm), and c is the concentration of light-absorbing substances in solution (mol· L⁻¹）。 This law is the basis for quantitative analysis, which calculates the concentration of the target component by measuring the absorbance of the solution to be measured at a specific wavelength.
Most heavy metal ions themselves are not significantly absorbed in the UV-visible region and cannot be directly measured. Therefore, before UV-Vis spectrophotometry is used, the target heavy metal ions need to be chemically converted into colored complexes with strong absorption at specific wavelengths. This process is called the chromogenic reaction. Commonly used chromogens can form a stable complex with a high molar absorbance coefficient with specific metal ions, thereby amplifying trace amounts of metal ion concentration signals into accurately measurable optical signals. The selectivity, sensitivity and stability of the complex are the key to determining the performance of the method.
Common heavy metal detection methods
For different heavy metal ions, a variety of mature and standardized spectrophotometric detection methods have been established. These methods typically result in the appropriate pretreatment of the sample (e.g., digestion, masking of interfering ions), the addition of a specific developer for a reaction, and then the absorbance is measured at the maximum absorption wavelength of the complex. The following are some key points of detection methods for typical heavy metals.
| Target heavy metals | Commonly used color rendering methods and characteristic wavelengths |
| Hexavalent chromium | Diphenyl carbonyl dihydrazine is chromogenic, with a wavelength of around 540nm |
| General Railway | O-phenylline or 1,10-phenylline chromogenic, wavelength around 510 nm |
| Total copper | Sodium diethyl dithiocarbamate develops color, wavelength around 440nm |
| Total zinc | Disulfiohydrazine is chromatic, with a wavelength of around 535nm |
| Total lead | Disulfide is chromatic, with a wavelength of around 520nm |
Notes:
The standard process for the detection of heavy metals in water using UV-Vis spectrophotometers typically includes: sample collection and preservation, sample pretreatment (filtration, digestion), chromogenic reaction condition control (pH, reagent dosage, reaction time and temperature), absorbance measurement, blank and calibration curve measurement, and result calculation. Throughout the process, factors that can affect the accuracy of the assay are strictly controlled, such as the cleanliness of the vessel, the purity of the reagent, the masking of interfering ions, the stability of the chromogenic system, and the baseline drift and stray light levels of the instrument itself. Following standard operating procedures and implementing effective quality control are prerequisites for reliable data.
Scope of application
UV-Vis spectrophotometry has the characteristics of high equipment penetration, relatively simple operation and low operating cost in water quality heavy metal detection. Its sensitivity can meet the routine monitoring requirements of some heavy metals in most environmental water quality standards (such as surface water, drinking water, wastewater). However, this method also has certain limitations, such as: usually only one or a few elements can be determined at a time; For complex matrix samples, the pretreatment and masking disturbance steps may be cumbersome. Its lower detection limit is higher than that of atomic absorption spectrometry or inductively coupled plasma mass spectrometry. Therefore, it is more suitable for batch analysis of samples with known targets, moderate concentration ranges, and manageable interference.
With the advancement of technology, the performance of UV-Vis spectrophotometers is also improving, such as the use of dual-beam design for stability and the use of array detectors for fast spectral scanning. In terms of methodology, the automation, analysis speed and anti-interference ability of the method can be further improved by studying new high-selectivity and high-sensitivity chromophagints, as well as combining with automated injection techniques such as flow injection and sequential injection. These developments have allowed the classic method to continue its application value in environmental monitoring, industrial process control and other fields.