Application of UV Spectrophotometer in the Determination of Lignin in Pulp

Overview

As an analytical instrument based on the absorption characteristics of substances to ultraviolet light, UV spectrophotometer plays an important role in the lignin determination of the pulp and paper industry. As one of the main components in plant fiber raw materials, the content and structure of lignin directly affect the yield, bleaching performance and final paper quality of pulp. The determination of lignin by ultraviolet spectrophotometry has the advantages of easy operation, fast analysis and low sample consumption, and has become one of the routine detection methods in the laboratory. This method mainly based on the characteristic absorption of aromatic groups in the molecular structure of lignin at specific ultraviolet wavelengths (e.g., around 280 nanometers), and establishes the quantitative relationship between absorbance and lignin concentration to achieve accurate determination of their content.

Principle of determination

Lignin is a complex natural polymer composed of phenylpropane units connected by ether bonds and carbon-carbon bonds, and its aromatic ring structure has characteristic absorption in the ultraviolet region. According to Lambert-Beale's law, when a beam of parallel monochromatic light passes through a homogeneous solution, its absorbance is directly proportional to the concentration of the solution and the length of the optical path. The relationship can be expressed by the following formula:

A = ε × b × c

Among them, A represents absorbance (dimensionless), ε is the molar absorbance coefficient (unit: L·mol⁻¹·cm⁻¹), b is the length of the optical path (unit: cm), and c is the concentration of lignin solution (unit: mol· L⁻¹）。 In practical assays, lignin is usually extracted from pulp using a specific solvent (such as acetic acid aqueous or alkaline solution) and its absorbance is measured at the selected wavelength, and the lignin content is calculated using a calibration curve.

Assay method

A typical UV spectrophotometry process for determining lignin content in pulp includes sample preparation, solution preparation, instrument calibration, measurement and calculation. First, the pulp sample is dried and crushed for extraction using an appropriate solvent to remove the interfering substances. Subsequently, the treated sample is dissolved in a specific solvent to prepare a solution to be tested at the appropriate concentration. Before measurement, the UV spectrophotometer is subjected to baseline calibration and wavelength calibration, and a calibration curve is drawn using a standard lignin sample. When measuring, the solution to be measured is placed in a quartz cuvette, the absorbance value is read at a set wavelength, and finally the lignin content is obtained by calibrating the curve or calculating the formula.

Notes:

To ensure the accuracy and repeatability of the measurement results, it is necessary to pay attention to multiple experimental links. The sample preparation, solvent selection, solution uniformity, measurement wavelength accuracy, and the stability of the instrument itself can all affect the results. For example, the absorption peak position of lignin from different sources may be slightly offset, and the optimal wavelength needs to be determined by scanning. In addition, suspended particles or bubbles that may be present in the solution can cause light to scatter and interfere with absorbance readings, so the solution needs to be filtered or centrifuged. The operating environment should avoid direct light and cuvettes should be kept clean and free of scratches.

Relevant standards

The determination of pulp lignin by ultraviolet spectrophotometry has the characteristics of high sensitivity, fast analysis speed and relatively low cost. A number of domestic and foreign standards organizations have published relevant test methods, which standardize sample preparation, reagent specifications, operation steps and result calculation. These standards ensure that the results of assays are comparable between different laboratories. The following table lists some of the core information of the relevant criteria:

Application examples

In actual production and R&D, this method is often used to evaluate the removal efficiency of lignin in pulping, monitor the bleaching performance of pulp, and study the lignin properties of different raw materials. For example, process parameters can be optimized by tracking changes in lignin content in the slurry before and after cooking or bleaching. Advancements in instrument technology, such as UV spectrophotometers equipped with autosamplers and data processing software, further improve analytical throughput and result reliability. In the future, this method is expected to be combined with other analytical techniques, such as chromatography, to more comprehensively characterize the structure and properties of lignin.