Atomic Absorption Spectroscopy: Instrumentation

An atomic absorption spectrophotometer consists of four main components: a radiation source, an atomizer, a monochromator, and a detector. The radiation source provides narrow-wavelength light, the atomizer converts the sample into gas-phase atoms, the monochromator isolates a specific wavelength range, and the detector converts light into amplified electrical signals for measurement.

Atomic absorption spectroscopy uses either narrow-line sources or continuum sources. Narrow-line sources include hollow-cathode lamps (HCL) and electrodeless-discharge lamps (EDL), which emit specific wavelengths. Continuum sources require very high-resolution monochromators to achieve the narrow wavelength range needed for accurate measurements.

In a single-beam spectrophotometer, radiation passes directly through the atomized sample to the detector. A double-beam instrument splits the source beam into two paths: one passes through the sample while the other bypasses it as a reference. The two beams recombine using a half-silvered mirror before reaching the monochromator.

A mechanical chopper is a circular metal disk placed between the radiation source and atomizer to eliminate interference from thermal emissions. It creates an alternating signal from the source radiation, distinguishing it from the continuous signal produced by thermally excited atoms in the flame, allowing accurate measurement of absorption.

The monochromator selects and isolates a narrow wavelength range from the combined beam after it passes through the atomized sample. This ensures that only the specific wavelength corresponding to the target element reaches the detector, improving measurement specificity and reducing interference from other wavelengths.

The detector, typically a photomultiplier tube, converts incoming light radiation into amplified electrical signals. These signals are proportional to the light intensity reaching the detector, allowing the instrument to measure the amount of radiation absorbed by the gas-phase atoms in the sample.

The two primary atomization methods are flame atomization and electrothermal atomization. Flame atomization uses a flame to convert the sample into gas-phase atoms, while electrothermal atomization uses heat to achieve atomization. Both methods generate atoms capable of absorbing radiation at characteristic wavelengths.