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Dual Laser Pulses Enable Portable Lead Testing

Photonics Spectra
May 2008
Hank Hogan

To diminish the negative effect that the metal lead has on public health, the US Environmental Protection Agency (EPA) has set a limit of 15 parts per billion (ppb) by weight of the metal in drinking water. If samples from a water supply exceed that limit too many times, steps must be taken to remedy the exposure.

One challenge lies in testing the water. Laboratory-based techniques are suitable but not on-site. Conversely, portable methods can test water at the source but lack the needed sensitivity.

AA_lead_schem.jpg

A schematic shows the setup of a combined laser ablation and laser-induced fluorescence experiment designed to provide high-sensitivity detection of lead in water. A frequency-quadrupled Nd:YAG laser ablates the water jet, with pulse energy controlled by a λ/2 plate and a Glan-Taylor prism. The pulse for fluorescence excitation of the ablated sample is a frequency-doubled dye laser pulse pumped by a XeCl excimer laser. Printed with permission of the American Chemical Society.


Now a group from the University of Alberta in Edmonton, Canada, has shown that laser-induced breakdown spectroscopy (LIBS) combined with laser-induced fluorescence could provide the necessary sensitivity and portability. The demonstration was performed using large research-grade lasers, but technological advances promise to shrink the setup considerably.

“With the emergence of nanosecond-pulse fiber and microchip lasers with 100-mJ energies and the ability to tune fiber lasers with fiber Bragg gratings, it is now becoming feasible to think of compact and portable commercial systems using this technology,” said Robert Fedosejevs of the university’s electrical and computer engineering department.

Current testing involves taking tap water samples to a laboratory and determining the lead content using techniques such as mass spectrometry or atomic absorption spectrometry that can pinpoint the amount of lead with a sensitivity of <0.1 ppb, far below the EPA’s action limit of 15 ppb. However, the equipment is too cumbersome and the procedures too complex for field work.

Laser-induced breakdown spectroscopy, in contrast, is portable, and micro-LIBS systems have been shown capable of microanalysis of surfaces. With optimization, the micro version performs as well as the macro technique. The problem is that the sensitivity is 12.5 parts per million, orders of magnitude too high.

This relatively poor performance is a consequence of the laser pulse that ablates the sample. In the full-scale version, a pulse typically in the tens to hundreds of millijoules creates a plume in which the atoms of interest emit light. However, the plume also emits a lot of light from other sources, and this additional light masks the atomic emission of interest. To overcome this, detection is performed after the plasma has cooled for up to a few microseconds. By then, the atomic emission stands out but has faded.

The researchers therefore decided to combine micro-LIBS, in which the pulse is in the microjoule range, with laser-induced fluorescence. They did this by producing a second pulse, one tuned to resonantly excite the lead atoms inside the plasma. They describe their work in the March 15 issue of Analytical Chemistry.

In their setup, they used a frequency-quadrupled Nd:YAG laser from Big Sky (now Quantel USA) of Bozeman, Mont., to produce 100 μJ pulses at 266 nm with a width of 10 ns. They focused these pulses onto a 10-μm-diameter spot on the surface of a jet of water with a known concentration of lead. They used a frequency-doubled XeCl-pumped dye laser from GSI Lumonics of Novi, Mich., to create the second pulse, which was tuned to 283 nm, to excite the lead atoms in the plasma that was created by the first pulse. The generated emission was picked up by an intensified CCD camera from Andor Technology of Belfast, UK, sitting behind an imaging spectrometer from Newport Oriel of Stratford, Conn.

The investigators adjusted various parameters, such as the separation between ablation- and fluorescence-inducing pulses, the detector gate width and the number of shots. They achieved a limit of detection of about 35 ppb, an improvement of several orders of magnitude but still about twice that needed for testing to EPA standards.

The performance gains resulted from more efficient excitation and detection of atoms, and Fedosejevs believes that there is room for further progress. “We expect to do better with improvements in the technique, including larger-aperture light-collection systems and switching to filtered photomultiplier or avalanche photodiode detectors.”

The goal is to improve the limit of detection to about 5 ppb, he added. Other plans call for integrating the technique into a lab-on-a-chip platform. The group already has a collaborator, MPB Technologies Inc. of Montreal, that might be interested in commercialization once significant development work on lasers, compact detector systems and systems integration at a reasonable cost is finished.

Contact: Robert Fedosejevs, University of Alberta; e-mail: rfed@ece.ualberta.ca.


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