The main UV-Vis signal for DoP is likely in the 220-250 nm region you suggested, or possibly at slightly higher wavelengths, up to around 280 nm? This range typically corresponds to n → π* electronic transitions of the aromatic phthalate group, although the exact detection may vary depending on the solvent and concentration.

I would suggest the following approach. First, try preparing a saturated solution by adding an excess of solid DoP to water and treating it with ultrasound if necessary. Then, perform serial dilutions of this saturated solution. Measure the spectrum of the saturated solution as your reference, and you may even use it as a baseline spectrum. Next, measure the spectra of the diluted solutions and subtract them from the reference spectrum. If you used the saturated solution spectrum as the baseline, the subtraction is done automatically by the software; otherwise, you can do it manually using the exported data. This method is convenient because minor spectral differences are easier to observe in difference spectra. It is likely best to start with highest concentrations (saturated solution), as spectral differences are most noticeable when the concentration difference is large. After identifying the relevant spectral region where changes occur, you can proceed to lower concentrations (e.g., 100 ppm to 0.01 ppm).

If measuring higher concentrations is necessary and pure water is not a strict requirement, you might consider dissolving DoP in ethanol or another water-miscible solvent. You could then add small amounts of these solutions to water for measurements, ensuring you add the same amount of solvent to the blank to adjust the baseline.

Dear Channa Naimatullah

I think you are not likely to get meaningful results by UV-Vis spectroscopy.

First, bear in mind that solubility of dioctyl phthalate in water is