Introduction
The use of fluorescence intensity for quality control of fluorescent whitening agent products has the advantages of convenience, speed and accuracy, but there are many factors that affect fluorescence intensity, among which the most important The most important ones are the solvent, solution pH, solution concentration, and the influence of light.
1. Influence of solvents
Absorption of fluorescent whitening agents The spectral characteristics are closely related to the solvent used. The position and intensity of the fluorescence spectrum of the same fluorescent whitening agent in different solvents may be significantly different. Therefore, when measuring the fluorescence intensity of fluorescent whitening agents, it is necessary to select an appropriate solvent when converting the sample to be measured into a solution. Improper selection of the solvent will cause the solution to not comply with Lambert-Beer’s law, and accurate measurement results will not be obtained.
Generally, the following principles should be followed when selecting solvents:
(1) The selected solvent will not chemically react with the sample to be tested.
(2) The sample to be tested has a certain solubility in the solvent.
(3) Within the measured wavelength range, the solvent itself has no absorption.
(4) Solvents should be selected that are less volatile, less flammable, less toxic and less expensive.
Among these principles, maintaining good solubility of the sample to be tested in the solvent is crucial for accurate measurement of absorbance. This can be learned from the fluorescent whitening agent It is well illustrated in the determination of CXT fluorescence intensity.
2. The influence of solution pH
The pH value of solution It has a greater impact on the absorbance of anionic and cationic fluorescent whitening agents. The absorbance of cationic fluorescent whitening agents drops significantly when pH ≥ 9; the absorbance of anionic fluorescent whitening agents drops sharply under acidic conditions. Different products have different degrees of reduction. Not the same. The effect of solution pH on the absorbance of fluorescent whitening agents is bound to have a certain effect on fluorescence intensity. The optical brightener VBL is a good example of the effect of solution pH on extinction value and fluorescence intensity.
3. The influence of light
Some fluorescent whitening agents The solution will not undergo significant changes under sunlight (or excitation light containing sufficient ultraviolet light) for a long time. However, some fluorescent whitening agents, such as dilute solutions of stilbene fluorescent whitening agents, will change under direct sunlight. It is very easy for cis-to-trans isomerization changes to occur. This cis-to-trans isomerization change causes changes in the extinction value, thereby affecting the stability of the fluorescence intensity. Therefore, when dissolving and measuring the extinction value of the fluorescent whitening agent, pay attention to the rapid and avoid light.
4. Influence of solution concentration
Application of light absorption law There are certain conditions, that is, the concentration of the solution should be controlled within an appropriate range. Within this concentration range, the absorbance A and the concentration have a linear relationship. For fluorescent whitening agents, the linear relationship between absorbance and solution concentration is limited to relatively dilute solutions. For concentrated solutions, the absorbance not only does not increase with the increase in solution concentration, but also often decreases with the increase in concentration, which deviates from the If the law of light absorption is violated, the measured fluorescence intensity is no longer reliable. This is mainly due to the quenching of fluorescence and the internal filtering effect.
In addition to the several influencing factors introduced above, temperature, coating effect, surfactant, etc. also have an impact on the fluorescence intensity of optical brighteners. Pay attention to this during the measurement process.
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