Photometric analysis determines the absorption of light at a specific wavelength after the addition of color forming reagent to the sample. The degree of light absorption is proportional to concentration. The absorption wavelength is a function of the chemical reaction between the color reagent and the analyte. The analyst prepares a series of standards in increasing concentration and adds the color reagents. A plot of concentration versus absorbance is created. The concentration of unknown samples, reacted with the same reagent with absorbance measured under equivalent conditions as the standards, is determined by a calibration curve.

A simple filter colorimeter is often considered sufficient for routine environmental analysis because most environmental tests can be done with very few wavelengths (420, 505, 540, 578, 600, 630, 660, 815, and 880 nm), however, a double beam UV-Visible spectrophotometer provides a more accurate means of determining the concentration of an analyte. Double beam spectrophotometers accurately split monochromatic light into two identical beams passing one beam through the sample and a reference beam to a reference detector. 

The signal for the absorbance of the contents of a reference cell is automatically subtracted from the sample cell providing a net signal corresponding to the absorbance of the sample solution. High quality, matched optical glass or quartz cells minimize the variability of results due to imperfections in the cell increasing precision and decreasing detection limits. Many spectrometers have two light sources, a tungsten halogen lamp used for visible light measurements, and a deuterium lamp used for ultraviolet measurements covering a wavelength range of 190 – 1100 nm. A high quality spectrophotometer provides the environmental laboratory with the following benefits:

1) Greater accuracy at lower concentrations

2) Lower Method Detection Limits (MDL)

3) Ability to measure metals, and non-metals, for which there are no other reliable methods

Examples of environmental methods requiring the use of a high quality spectrophotometer are:

1) Standard Methods 3500-Fe B Ferrous Iron by the Phenanthroline Method

2) Standard Methods 3500-Cr B Hexavalent Chromium 

3) Standard Methods 4500-P E Phosphorus by the Ascorbic Acid Method

4) Standard Methods 4500-SiO2 D Silica by the Heteropoly Blue Method

5) Standard Methods 5910 B UV-Absorbing Organic Constituents by the Ultraviolet Absorption Method

6) Standard Methods 10200 H Spectrophotometric Determination of Chlorophyll

A modern spectrometer also enables computerized acquisition of multiple point calibrations and allows various fitting models ensuring accurate calibration. Data is securely stored in USB memory and reports can be printed directly from a PC.

The spectrophotometer is used for determination of metals and non-metals for which there are no reliable alternatives. A high quality spectrophotometer is important in achieving low detection limits and high precision.