Which instrument can operate with a true zero blank, and what feature enables this?

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The instrument being described relies on detecting light that is scattered by particles in the sample, not light that passes straight through. When there are no particles in the blank, there is virtually no scattered light reaching the detector, so the baseline is essentially zero. The feature that enables this true zero blank is the optical design that collects scattered light at a specific angle while rejecting transmitted light, making the signal directly reflect particle presence and allowing an almost perfect zero in the absence of scatterers. Fluorometers measure emitted light, which leaves some background and instrument noise to account for, so a true zero is not inherently guaranteed by a blank alone. Spectrophotometers depend on transmitted light and the blank sets a baseline, but stray light and detector dark current can prevent a perfect zero without calibration. Turbidimeters rely on transmission or general scattering, which also doesn’t yield a true zero as cleanly as a nephelometer.