1. The Physics: Absorbance Cannot Really Go Negative

Absorbance is defined as log₁₀(I₀/I). If the sample transmits more light than the reference — I > I₀ — the log goes negative. That usually means the reference/blank is absorbing more than the sample, or the baseline shifted mid-run.

2. Common Causes & Fixes

Cause	Symptom	Fix
Baseline drift	Slightly negative across whole scan	Re-zero with both cells empty; check lamp warm-up (15–30 min)
Blank absorbing more than sample	Negative at specific wavelengths	Blank contaminated or wrong solvent — prepare fresh blank
Dirty or mismatched reference cell	Consistent negative offset	Clean both cells; use matched pair for reference
Scratched/etched windows	Negative or noisy at UV end	Replace the cell; verify with clean reference
Wavelength or bandwidth error	Negative near steep peaks	Validate wavelength calibration
Solvent absorbance	Negative where solvent absorbs	Use spectroscopic-grade solvent; correct baseline

3. Cell-Related Checklist

• Matched pair? — sample and reference cells should be paired (transmission-matched). Use the same path length and quartz grade.
• Windows clean? — fingerprints and films absorb and scatter. Follow the cleaning protocol.
• Orientation? — always insert the cell the same way; the polished windows face the beam.
• Volume? — sub-micro cells must be filled above the beam height (check the Z dimension guide).

4. Quick Fix Sequence

• 1. Re-zero with both cells empty.
• 2. Replace the blank with a fresh preparation.
• 3. Clean both cells, re-zero again.
• 4. Swap sample and reference positions — if the sign flips, it is a cell mismatch.
• 5. If none of the above works, check lamp age and wavelength calibration.

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