Optical density for biomass: When measuring cell mass by light scattering (turbidity), readings are most commonly taken at which wavelength range?

Introduction / Context:
Turbidimetry is a rapid method to estimate microbial biomass in liquid culture by measuring light scattering. The optical density at 600 nm (OD600) is the long-standing convention in microbiology labs for bacterial growth curves and kinetics.

Given Data / Assumptions:

• Spectrophotometer or colorimeter with visible-range filters is used.
• Standard bacterial cultures (e.g., E. coli) in clear media are measured.

Concept / Approach:
At ~600 nm, absorbance by most media components is minimal while scattering by bacterial cells is sufficient for a linear correlation with biomass over a useful range. Shorter wavelengths (UV/blue) increase interference from nucleic acids and media; longer near-IR wavelengths may reduce sensitivity for small cells or require specialized optics. Hence, 600–700 nm is the common operational window, with OD600 being the canonical single-wavelength measure.

Step-by-Step Solution:
Blank the instrument with uninoculated medium. Measure culture turbidity at ~600 nm at intervals. Plot OD versus time to obtain growth phases (lag, exponential, stationary).

Verification / Alternative check:
Correlate OD600 with dry weight or cell counts to validate linear ranges for a given organism and path length; dilute samples above the linear range.

Why Other Options Are Wrong:

• 300–500 nm: Higher background absorbance and potential photodamage; less standard for routine biomass.
• 500–600 nm: Usable but less canonical than OD600–OD660 range.
• 700–800 nm: Sometimes used for dense cultures or algae, but not the most common standard for bacteria.

Common Pitfalls:
Forgetting to mix cultures before reading; bubbles and clumps cause scattering artifacts. Always use the same cuvette path length.

Final Answer:
The conventional range is 600–700 nm, notably OD600.