Problems with UV-Vis spectrophotometers
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When measuring the absorbance of a substance, as the concentration of that substance increases, the wavelength corresponding to the maximum absorption peak also increases. Why is this, everyone? Thank you first!
A spectrophotometer operates on the principle of light complementarity; the wavelength should be determined by the color of the solution and does not need to be changed. Did you adjust the wavelength during the sample preparation process? If so, this can explain it – the same instrument emits different intensities of light at various wavelengths, and the sensitivity of the sensing element to light of different wavelengths also varies. The change in concentration you mentioned, along with the shift in the maximum absorption peak, is likely due to this reason. Try conducting experiments to verify this.
Without changing the wavelength, it remains unchanged. I didn’t seem to encounter such an issue when I was working on it； Ugh~! Is the wavelength at which a color-developing agent forms a complex within a specific range?
The wavelength remains the same during the measurement of the same sample. . Unchanged. .
It is possible that the substance measured by LZ underwent aggregation; is what LZ represents a polymer? Under normal circumstances, J-type aggregates cause a redshift in the ultraviolet absorption spectrum, that is, the phenomenon described by lz of an increase in the absorption wavelength ； And H-type aggregates cause a blue shift in the UV absorption spectrum.
This is not absolute; based on the LZ’s description, the absorption wavelength increases as the concentration rises. For certain substances, an increase in concentration leads to the packing of molecular pi bonds (i.e., the aggregation mentioned earlier), which in turn increases the effective conjugation length and results in a red shift of the ultraviolet absorption spectrum.
1# Yan Qing: This kind of phenomenon is quite rare. It could be due to a problem with the spectrophotometer itself, resulting in drift, or it might be because the color development time is insufficient and the solution is not stable. Another possibility is that the wrong wavelength was selected
First, thank you all. Now, to elaborate on the issue: For this particular substance, as the concentration decreases, the absorbance also decreases, and the wavelength of the maximum absorption peak changes as well. The data is as follows: Concentration (mol/L), Peak/Valley, Wavelength (nm), Absorbance. For a concentration of 0.038 mol/L, there is a peak at 197.80 nm with an absorbance of 1.417; another peak occurs at 200.10 nm with an absorbance of 1.579; and yet another peak is at 202.70 nm with an absorbance of 1.622. The issue is that the wavelength should remain constant, and it’s only the absorbance that should change. And the changes are quite significant. Could that guy tell me what’s going on here? Thank you
Spectrophotometry is a method for the qualitative and quantitative analysis of a substance by measuring its absorption of light at a specific wavelength or within a certain range of wavelengths. Just find the right wavelength
Molecular aggregation leads to a redshift or blueshift of the absorption spectrum, and this generally occurs only when the concentration exceeds a certain critical value. In dilute solutions, molecular aggregation is minimal, so there is no spectral shift. LZ can try diluting the solution step by step and then observe how the absorption wavelength changes as the concentration decreases. If the concentration drops to a certain value at which the absorption wavelength no longer changes, it can be essentially determined that the previous wavelength shift was caused by molecular aggregation. Before conducting experiments, LZ can consult relevant literature; generally, molecules with similar structures exhibit similar behavior, so it’s worth checking whether there are any reports by others on this topic.