A negative optical density (OD) value usually indicates a measurement error, often due to improper calibration or issues with the blank reference. Such results should not be considered valid and require troubleshooting of the spectrophotometer and measurement setup.

Thank you sir for your reply.

It is likely because your baseline absorption exceeds the measured one. This can happen for example if you measure in a mixed solvent and use water as a reference or if you used cuvettes with different dimensions for baseline and samples. So result is not valid, almost certainly. In some odd cases such as with the highly scattering samples this may occur even with proper measurement procedures. If the reference (blank) has low scattering and the sample has high scattering, the sample can appear to transmit more light than expected. This mismatch creates the illusion of negative OD values. There is always a possibility that something is wrong with the instrument. This can be checked by measuring reference sample like holmium oxide cuvette. If you don't have this in your lab you may buy or even prepare standardized potassium dichromate solution in diluted sulfuric acid.

Yes, it make sence and thank you for this detailed and instructive explanation.

You are welcome and I am glad if it was helpful. Best of luck!