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In the spectrophotometric analysis(uv-vis-NIR) of ITO Nanoparticles in tetrachloroethane using a double beam instrument baseline was measured with 2 identical cuvette with solvent only. This gives me only negative values for absorbance. Does this mean there is significant difference in transmission between the blank and the reference even if they are the same compound?

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    $\begingroup$ With zero measurement noise(s) and an ideal double beam spectrophotometer, the baseline would be zero and perfectly flat. Real instruments are not ideal, measurement noise is not zero and baselines have some noise, which can give small negative values. Remember that what the instrument does is measure two transmitted light intensities, one for each optical path, then compute a ratio and negative logarithm. $\endgroup$ – Ed V Jan 17 at 15:29
  • $\begingroup$ That is why you run a baseline correction just prior of a serie of analogues measurements. And periodically check that all is OK. $\endgroup$ – Alchimista Jan 17 at 15:35
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Negative absorbance in a double beam instrument has no physical meaning because absorbance, by definition, cannot be negative. It ranges from $0$ to $\infty$.

Instrumentally, all that means is that somehow the sample cuvet is transmitting more light than the reference cuvet. In other words, the measured absorbance of the sample is less than than measured absorbance of the reference cell. Usually this can happen when the cuvets are not optically matched (i.e., they may look identical physically but their optical transmission characteristics are very slightly different).

Try swapping the reference cuvet with the sample cuvet and see if the readings remain negative again. If you are getting large negative absorbance values with the same solvent, try another cuvet set and if the problem persists, the spectrophotometer must be checked. However, if the negative values are quite small such -0.001 or -0.0002, this is tolerable and you can subtract the baseline from the measured signal in order to correct this systematic error.

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    $\begingroup$ As EdV has pointed out the noise issue, check the magnitudes of the negative absorbance. If they are random, small, and you also have positive values, it is not a problem. Make sure there is no drift in the baseline in the scan. $\endgroup$ – M. Farooq Jan 17 at 17:53

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