Biophotometer

A biophotometer is a laboratory instrument used to measure the absorbance of biological samples in the ultraviolet and visible light range. It is essentially a spectrophotometer optimized for quantifying nucleic acids and proteins, and for assessing sample purity.

The device directs light from a calibrated source through a sample in a cuvette or microvolume attachment, selects a wavelength with a monochromator or filter, and detects transmitted light with a photodetector. Absorbance values are reported and, with the appropriate path length, concentrations can be calculated using Beer-Lambert law.

Common measurements include A260 for nucleic acids and A280 for proteins; ratio metrics such as A260/A280 and A260/A230 are used to judge sample purity. Typical usable wavelengths cover roughly 190 to 750 nm in benchtop models; some instruments measure beyond into the near-UV and visible.

Applications include rapid quantification of DNA, RNA, and purified proteins, preliminary quality assessment of samples, and routine checks before downstream experiments such as PCR or sequencing.

Design variants range from benchtop single-beam or double-beam units to microvolume or nanovolume models that require only a few microliters of sample. Most current devices provide onboard software to enter extinction coefficients and compute concentrations.

Calibration involves blanking against an appropriate buffer and periodic validation with standard solutions. Limitations include interference from contaminants that absorb at similar wavelengths, the need for known path length, and assumptions about sample composition that may affect accuracy.