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Spectrometer Made Smaller, Less Expensive

Photonics Spectra
Mar 1999
Michael D. Wheeler

Scientists at the University of Ulm in collaboration with LaserSpec Analytik GmbH have unveiled an atomic absorption spectrometer that uses a laser diode as a light source and a tungsten coil to atomize samples -- advances that could lead to smaller, less expensive commercial systems.
Current atomic absorption spectrometers are highly sensitive instruments that employ light absorption to measure the concentration of gas-phase atoms. They are particularly useful in determining the concentration of metals in solutions or tissues. However, most systems are bulky, require as much as 4 kW of power and cost as much as $70,000.
The semiconductor industry and others have called for smaller, less expensive systems suitable for online and on-site analysis. With these requirements in mind, a team led by the university's Viliam Krivan theorized that developing such a system would require a low-cost atomizer and a compact, inexpensive light source.

Inexpensive components
Instead of the graphite or metal tube atomizers used in conventional systems, the new spectrometer uses tungsten coils. Typically manufactured in large quantities for commercial lamps, the coils are readily available and less expensive. And they require only about 150 W to reach the 3000 °C needed to atomize sample material.
The system uses a laser diode manufactured by LaserSpec Analytic in place of a hollow cathode lamp. Laser diodes have excellent spectroscopic and operational properties, and their radiation has a high spectral purity, small linewidth and high intensity. An added benefit is the ability to modulate the wavelength, which enables background correction. Conventional systems require additional components to reduce noise.

Saturation test
Krivan's team tested the prototype by determining the saturation of chromium and aluminum in human blood serum. After frequency-doubling the diodes, the wavelengths were 396.15 nm at 0.2 µW of power and 427.48 nm at 3 µW, respectively.
The absorption of the laser radiation was detected with an H5783 photomultiplier with built-in high voltage from Hamamatsu Photonics KK in Hamamatsu, Japan. They found the results were in close agreement with those obtained using a conventional atomic absorption spectrometer from Perkin-Elmer Corp. The only limitation with the prototype came from the laser diodes. Because commercially available diodes are limited to 630 to 1600 nm, elements that have absorption lines outside that range cannot be detected.
"That is the drawback of the technique -- that we are restricted in our wavelength selectivity," said Christoph Schnürer-Patschan of LaserSpec, who is involved in the research. LaserSpec, in collaboration with Krivan, is investigating developing a commercial system.

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