Atomic Absorption Spectrometer

Overview

An atomic absorption spectrometer measures how much of one metal is present in a sample, the lead in drinking water, the iron in a steel alloy, the calcium in blood serum. It does this by exploiting a property unique to each element: free atoms absorb light only at their own characteristic wavelengths. Shine light of exactly the right colour through a cloud of the sample's atoms and the amount swallowed is proportional to how many atoms are in the beam.

The instrument is single-element by nature. The light source is tuned to one metal, so a run measures that metal alone, then the operator swaps to the next. That narrowness is also its strength: by using a source that emits the element's own sharp lines, the measurement is highly specific and resistant to interference.

How it works

The light comes from a Hollow-Cathode Lamp, a lamp whose cathode is made of the very element being measured; a glow discharge inside it emits that element's exact resonance lines. Several lamps sit in the Hollow-Cathode Lamp Turret, which rotates the right one onto the optical axis and pre-warms the next.

The sample must be turned into free atoms. In flame mode the Nebulizer aspirates the liquid and shears it into a fine mist that mixes with fuel and burns in the Slot Burner Head, a long slot flame aligned to the beam. For trace work the Graphite Furnace replaces the flame: a microliter is dried, ashed and flashed to vapour inside a heated graphite tube, concentrating the atoms and pushing detection limits into the parts-per-billion range.

Light that survives the atom cloud is folded by mirrors on the Optical Bench into the Czerny-Turner Monochromator, where a diffraction grating and slits isolate the measured line from the lamp's other emission and from flame glow. The transmitted light strikes the Photomultiplier Detector, whose photomultiplier turns it into a current. The difference between the beam with and without atoms present is the absorbance, and the Control Electronics convert it to concentration against a calibration curve. A deuterium lamp in the optics corrects for broadband background so only true atomic absorption is reported.