Principle
UV-Vis spectrophotometers perform quantitative analysis based on Lambert-Beale's law. This law describes the linear relationship between the absorption of light by a solution at a specific wavelength and the concentration and path of the absorbing substance. Its mathematical expression is:
A = ε * c * l
where A represents absorbance (unitless), ε is the molar absorbance coefficient (L·mol⁻¹·cm⁻¹), c is the concentration of the substance to be measured (mol/L), and l is the optical path length of light through the sample solution (cm). For ink pigments, they are usually dissolved or evenly dispersed in a suitable solvent to form a test solution. The concentration of pigments in the sample is calculated by measuring the absorbance of the test solution at the characteristic absorption wavelength of the pigment and comparing it to a standard curve of known concentrations.
Sample preparation
Accurate measurements begin with canonical sample preparation. First, the representative part needs to be quantitatively weighed from the ink sample. Subsequently, a solvent that is compatible with the ink system and transparent to UV-visible light, such as a specific organic solvent or water-based solution, depending on the pigment nature, is diluted or dissolved. The goal is to obtain a test solution that is uniform, stable, and has absorbance values that fall within the instrument's optimal detection range (typically between 0.2 and 0.8 A-values). Filtration or centrifugation is necessary to eliminate interference from light scattering caused by suspended particles. At the same time, a series of standard pigment solutions at known concentrations were prepared for the establishment of calibration curves.
Instrument operation
During operation, the instrument needs to be turned on first and preheated and self-tested. Key parameter settings include: selecting the wavelength corresponding to the maximum absorption of the target pigment for fixed-point measurement, or scanning within a certain wavelength range to confirm the characteristic peak; Set the appropriate spectral bandwidth to balance resolution and signal-to-noise ratio; Select the correct measurement mode (usually absorbance mode). The measurement procedure is usually as follows: baseline correction is performed using a pure solvent as a reference, and then the standard solution and the sample solution to be measured are measured sequentially. It is recommended to average the measurement multiple times per sample for accuracy.
Concentration calculation
After obtaining the absorbance data, the calibration curve was drawn using the concentration of the standard solution as the abscissa and the measured average absorbance as the ordinate. The curve should have good linearity, and the linear correlation coefficient should be close to 1. The absorbance value of the sample to be tested is then substituted into the linear regression equation of the calibration curve to calculate its concentration. If the sample is diluted, it is multiplied by the corresponding dilution factor to obtain the actual pigment concentration in the original sample. The calculation formula can be expressed as:
cSamples = (ASamples - b) / m * D
where m is the slope of the calibration curve, b is the intercept, and D is the dilution multiple.
Influencing factors
In practical applications, the following factors need to be paid attention to and controlled to ensure reliable results:
| Chemical factors | The stability of pigments in solvents, whether aggregation or chemical reactions occur. |
| Optical factors | Light scattering caused by sample turbidity, absorption background of the solvent itself, cuvette cleanliness and matching. |
| Instrumental factors | Light source stability, wavelength accuracy, stray light level, detector sensitivity. |
| Operational factors | Weighing and dilution errors, cuvette placement, temperature fluctuations. |
By optimizing sample preparation, rigorous instrument calibration, and standardizing operating procedures, the effects of these factors can be effectively controlled.
Scope of application:
This method is suitable for the determination of ink pigment concentrations that can be dissolved or stably dispersed in transparent solvents and have characteristic absorption in the ultraviolet or visible region. It has the advantages of high sensitivity, relatively easy operation, and fast analysis speed. However, its limitations are as follows: for mixed pigment systems, if the absorption spectra overlap severely, chemometrics are required to be used to solve the spectrum. For insoluble or insoluble pigment particles, the measurement error may increase; The method relies on standards and stable calibration curves. In actual industrial testing and quality control, this method is often combined with physical tests (such as fineness and chromatic aberration) to comprehensively evaluate ink performance.