What Is an Absorption Spectrophotometer?

An absorption spectrophotometer is an analytical instrument used for determining the quantity of an element present in a sample. It relies on the fact that elements will absorb light at a particular frequency within the light spectrum. When an atom is at its lowest energy — or ground — state, it can absorb a photon of light with a specific frequency and jump to a higher energy state using the energy from the photon. Thus, it is possible to detect the element in a sample, and determine the amount present, by measuring the amount of light of the relevant frequency that is absorbed by the sample. This technique is widely used in chemistry for elemental analysis.

The typical components of an absorption spectrophotometer include a light source; a chamber or cell in which the sample is exposed to the light; a monochromator, which is a device which allows the frequency of interest to be selected and isolated; and a detector that measures the intensity of the light at that frequency. Since the amount of light absorbed at this frequency is proportional to the concentration of the element, the quantity of the element present can be calculated. The data may be displayed on a screen and/or stored electronically.

The light source for an absorption spectrophotometer normally emits light at the specific frequency absorbed by the element for which the test is being conducted. Since the absorption frequency of an element is the same as its emission frequency, the source can use that element to produce light of the required frequency. This can be achieved by an electrical discharge in an inert atmosphere, using the element as the cathode. Normally, different light sources are required to test for different elements, but in some cases, a cathode containing several elements is used, to allow simultaneous testing for these elements.

In the most frequently used type of absorption spectrophotometer, the sample — usually a solution — is converted into an aerosol and introduced into a hot flame, which converts any ions of the element of interest into atoms in their ground state, so that they will absorb light. The type of flame used depends on the element of interest. For many elements, an oxygen-acetylene flame is employed; however, some metals can form oxides if there is sufficient oxygen present in the flame. In these cases, a non-oxidizing flame can be used. Other methods include using a graphite furnace to heat the sample to a high temperature and heating the sample electrically in a thin carbon rod.

There are a wide range of applications for the absorption spectrophotometer. A common application in geology is establishing the content of various metals in a mineral sample. In the field of environmental protection, this instrument can be used to check levels of toxic elements in industrial effluent, river water and ground water or to determine levels of toxic heavy metals in fish. Similarly, it can be used to check levels of various elements in blood samples — for example, essential elements like sodium, calcium and magnesium, as well as toxic elements.