51Why does the Bradford assay respond differently to different proteins?52What is the basic principle of the Lowry assay?53Why do colorimetric protein assays require a standard curve?54What might cause a flattened spectral peak?55Why can absorbance be negative?56What should I check for a jagged spectrum or large differences between repeats?57Why can different instruments give different results for the same sample?58Is OD600 truly absorption?59How can OD600 be converted to cell count?

What should I check for a jagged spectrum or large differences between repeats?

First distinguish random fluctuations from a sustained drift. Weak signals, bubbles or unstable liquid columns may cause fluctuations; evaporation, settling, residue or reference changes may contribute to drift. Retain spectra and measurement order.

Check the pipette, sampling and mixing procedure, surface cleanliness and liquid column. Mix appropriately for the sample and reload fresh aliquots. Repeated readings of one droplet do not replace independent reloads.

Check a matched blank and suitable reference sample. If only the test sample is unstable, investigate its condition first; if both are unstable, investigate shared measurement conditions.