Using UV-Vis for Colour and Routine Quantitation

Colour and concentration are two different questions asked of the same platform. Colour asks whether the sample matches a reference; concentration asks how much of a substance is present. Both can be answered by absorbance, and confusing the two is where routine food QC goes wrong.

Colour is not a wavelength

Colour is a perception, and a number that stands for it has to be defined: transmittance at a fixed wavelength, a platinum-cobalt value, or a set of tristimulus values. Each is valid and each answers a different question. A specification that says “colour within limit” without naming the scale is not measurable.

Fixed-wavelength quantitation

For a single known analyte, a fixed wavelength and a calibration curve are enough. The discipline sits in the details: blank measured in the same cell, wavelength verified, curve bracketing the samples, and every dilution recorded with the result.

Two tasks, one platform

Colorimeter or spectrophotometer

A colorimeter is built for one job and does it quickly with a fixed optical geometry — appropriate when the measurement is a colour scale at production frequency. A spectrophotometer covers the same measurement and adds the ability to locate a wavelength, scan a spectrum and run a second method. Choose by the number of methods, not by the number of samples.

Calibration for colour

Colour standards drift and fade. A reference material that has aged quietly shifts every result that depends on it, so the standards need their own check and their own replacement interval. Write both down, and record the standard’s identifier with the result.

Cell handling

In colour work the cell is part of the method. Path length, cleanliness, filling height and orientation all change the reading. Keep one cell per product where possible, check it against a blank at the start of the shift, and handle it by the frosted faces only.

Where colour results go wrong in practice

Four failures recur. A sample read without filtration, so particles add apparent absorbance. A cell used in a different orientation from the one it was filled in. A reference standard that has aged past its own validity. And a result compared with a value produced on a different instrument type, where the optical geometry differs enough to change the number.

All four are cheap to prevent and hard to detect afterwards, because each produces a plausible value rather than an error message.

The brief

Send the colour scale required, the sample form and path length, the number of methods the laboratory runs, the sample frequency and the record format. Those five decide the platform.