Atomic Emission Spectrometry Shrinks Down For The Field

Most spectrometers are large, heavy machines that researchers can't lug easily out of the lab for field work. But researchers have now developed a portable, inexpensive instrument that does atomic emission spectroscopy, opening up the potential for metal analyses in the field (Anal. Chem., DOI: 10.1021/ac1027897).

Atomic emission spectrometry identifies and measures metals present in a sample by examining the light emitted when the sample is heated. Each element emits a characteristic wavelength of light whose intensity is proportional to the number of atoms in the element.

Unfortunately, typical benchtop instruments are large and immobile and can cost between $75,000 and $100,000, says spectroscopist Bradley Jones of Wake Forest University.

The machine he and his colleagues rigged together cost only $2,500. It weighed just a couple of pounds and fit on a sturdy ceramic base that measured 6 × 30 × 1 cm. The instrument consisted of three components: a small commercial spectrometer, a lens, and a hollow aluminum rod 7.5 cm in height, in which the researchers placed a tungsten wire from a 250 W light bulb.

To test the spectrometer, the investigators analyzed samples of polluted water and of peach and tomato leaves with known amounts of different metals. They pipetted a drop of each sample onto the filament through a tiny hole in the rod. After hooking the instrument up to a car battery, they then passed a current through the filament to heat it up and dry the sample. When the filament began to glow with white light, the metals in the sample gave off their characteristic emissions. The emissions exited the rod through another tiny hole and the lens focused the light into the spectrometer. Jones's team could then inspect the spectra using a laptop computer.

They looked for signals of 15 metals in the samples, including alkaline and transition metals, and could detect all of them. They could clearly pick out metals like chromium and barium that emit light between 400 and 600 nm, but they struggled to find elements like manganese and cesium that emit between 250 and 450 nm. This performance was comparable to that of benchtop instruments "at wavelengths above 400 nm but poorer at lower wavelengths," Jones says.  The portable instrument only needs 20 µL of sample, he points out, compared to the few milliliters necessary for conventional analyses.

Because Jones's instrument could detect cesium and europium, two metals with radioactive isotopes that could appear in "dirty bombs," Jones envisions the instrument's use in homeland security applications. The new instrument has also garnered interest from academic researchers in the developing world where funding for scientific equipment is limited, Jones says.

Spectroscopist Vassili Karanassios of the University of Waterloo, in Ontario, says the work demonstrates the importance of redesigning laboratory instruments as miniaturized forms. Any comparison with conventional instruments should take into account that they have developed over decades, he says. With more research, Karanassios expects portable instruments' performance to catch up with that of their laboratory counterparts.