Atomic Absorption Spectrometers

Atomic absorption spectrometers (AA) use light absorption to measure the concentration of gas-phase atoms. An analyte, usually a solid or liquid, is vaporized in a flame or in a graphite furnace with a temperature of 1000° - l 200° K. Atoms absorb ultraviolet or visible light and achieve higher energy levels. Absorption amounts determine the analyte concentration.

Atomic absorption spectrometers can measure analyte concentrations over a spectral range or a dynamic range. The spectral, or wavelength, range captures the dispersion of the grating across the linear array. This amount is also expressed as the “size” of the spectra on the array. The dynamic range, also known as the linear dynamic range or the linear range, is the range over which a response is a well-defined (usually linear) function of the analyte concentration. To vary the dynamic range, operators adjust instrumental parameters to, for example, decrease the absorption path length and sample volume.

Flame atomic absorption and graphite furnace atomic absorption (GFAA) are vaporization methods used with atomic absorption spectrometers. Flame atomic absorption, a common technique for detecting metals and metalloids in environmental samples, is based on how ground state metals absorb light at specific wavelengths. Applying a flame converts metal ions in a solution to an atomic state. Light of the appropriate wavelength is supplied and the amount of light absorbed is measured against a standard curve. Graphite furnace atomic absorption (GFAA) is a highly sensitive spectroscopic technique that measures each element sequentially. GFAA is useful when samples are very small, when very low levels of detection are required, and when matrices are dilute or volatile. Furnace parameter adjustments and matrix modifiers allow operators to optimize the analytical method for each element.

To convert radiant energy from a light source to electricity, atomic absorption spectrometers include a detector. Typically, the detector is a photomultiplier tube; however, some instruments use a solid-state detector. Photomultiplier tubes (PMTs) are photo-detectors with adjustable voltages that translate optical signals into electrical current. Increasing the PMT voltage increases the output signal for a given amount of light. Photodiodes are semiconductor devices used to detect light and generate an electrical current. Typically, photodiodes are used in forward scatter (FSC) detection.

Atomic absorption spectrometers allow operators to adjust sample temperatures and program fluorometers. Some spectrometers are self-calibrating, position lights automatically, or compensate for stray light emissions and various spectral interferences that bias analytical results. Intrinsically safe (IS) instruments do not release sufficient electrical or thermal energy to ignite hazardous atmospheric mixtures.

Atomic absorption spectrometers provide different adjustment and display options. Some spectrometers have an analog panel with dials and switches; typically, an analog meter or simple visual indicator displays data. Other instruments have a digital front panel with keypads and menus, and provide digital readouts or video displays. Some atomic absorption spectrometers also include application software or serial, parallel, or other computer interfaces.

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