If we want to get more precise and more interference-free measurements, we can use a spectrophotometer. This is very similar to a colorimeter, except that instead of using a filter to select the color of light to pass through the sample, we instead break the white light up into a rainbow (spectrum) of colors using a prism or a diffraction grating. The light is passed through a narrow opening (slit) before reaching the sample.

By rotating the prism or grating, the color {"wavelength") of light can be selected more precisely and we can better match the color with that absorbed by the sample. The principle is shown in the diagram below.Needless to say, spectrophotometers cost more than colorimeters, and are likely to be more delicate and less portable, as well. While many tests are done using visible light, some analyses also make use of the invisible ultraviolet or infrared portions of the spectrum. Scanning spectro photometers can also be used to identify some types of analytes by the wavelengths or colors of the light they absorb.

There is a variation of this type of testing, usually referred to as atomic spectroscopy, which is used mostly for trace metal analysis. The sample is converted to a gas by one of several methods-- usually involving heating. Then the light from a lamp containing the same metal is passed though the gas and the absorbance measured just as with a liquid sample (atomic absorption spectrophotometry). Alternatively, the intensity of the light emitted from the heated atoms of the metal in the gas can be used as a way of measuring the concentration (atomic emission spectrophotometry). A very popular atomic emission method in use today is called inductively coupled plasma spectrometry. The sample is carried in a stream of argon gas surrounded by coils which emit radio frequency energy that converts some of the gas into a very hot, ionized (electrically charged) form. An advantage of this method is that many elements can be measured simultaneously, or in rapid succession.