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?

Why can different instruments give different results for the same sample?

First align sample type, concentration units, dilution factor, blank, wavelength and path-length normalization. Different settings can convert identical absorbance into different concentration results without an instrument fault.

Measure fresh aliquots of the same homogeneous sample independently under consistent conditions. Compare full spectra and absorbance, not only concentration; control elapsed time, temperature and evaporation.

For persistent systematic differences, use suitable independent reference materials and assess wavelength, bandwidth, path length and photometric performance. A higher reading does not establish accuracy. For particulate samples, optical geometry also affects collection of scattered light.