Atomic absorption spectrometry source modulation

In atomic absorption spectrometry (AA) the sample is vaporized and the element of interest atomized at high temperatures. The element concentration is determined based on the attenuation or absorption by the analyte atoms, of a characteristic wavelength emitted from a light source. The light source is typically a hollow cathode lamp containing the element to be measured. Separate lamps are needed for each element. The detector is usually a photomultiplier tube. A monochromator is used to separate the element line and the light source is modulated to reduce the amount of unwanted radiation reaching the detector. 

A remarkable adaptation of technology borrowed from another analytical method was demonstrated for an electrospray ionization (ESI) IMS instrument ion injection to a drift tube was achieved through modulation with a mechanical chopper as found in atomic absorption spectrometry. The chopper was a disk with a small hole that would align with the source and drift tube and would operate as an ion injector. The disk had a second window that was used with optical sensors to synchronize ion injection and drift time, and ion injections were made at pulse rates of 5 to 200 Hz with pulse widths of 200 to 500 ps. 

Figure 3 Principle of construction of atomic absorption spectrometers. (A) Single-beam spectrometer with electrically modulated lamp radiation (B) double-beam spectrometer with reflection and splitting of the primary radiation by a rotating, partially mirrored quartz disk (chopper). 1 - radiation source, 2 -sample cell (atomizer), 3 - monochromator, 4 - detector, 5 -electronics and readout (by permission of Wiley-VCH from Welz B and Sperling M (1999) Atomic Absorption Spectrometry, 3rd, completely revised edition. Weinheim Wiley-VCH).

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