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  • Quantum Cascade Laser Improves Smoke Analysis

Photonics Spectra
Dec 2003
Sally B. Patterson

Understanding the exact chemistry of smoke could guide the development of tobacco products. Mid-infrared laser spectrosco-py is a well-established analytical method that can provide accurate measurements of gaseous constituents in cigarette smoke. However, traditional lead-salt tunable diode laser light sources require cryogenic cooling, which, besides being unwieldy, can result in unstable mode structure and nonreproducible frequency output when the laser is cycled from liquid nitrogen temperature to room temperature.

Quantum Cascade Laser Improves Smoke Analysis<
Puffs of smoke, created by an innovative sampling method using bypass flow to regulate air pressure in a cigarette, are analyzed via a pulsed quantum cascade infrared laser spectroscopic system. Courtesy of Aerodyne Research Inc.

Philip Morris USA has commissioned Aerodyne Research Inc. to develop a research instrument sensitive enough to analyze gases in both mainstream and sidestream smoke -- the portion drawn through the cigarette during a puff and that released from the smoldering tip.

In the prototype spectrometer, the researchers used a distributed feedback quantum cascade laser. These lasers are inherently stable in mode structure, are generally single-mode and have minimal wavelength drift. When operated in pulsed mode, they can be used near room temperature and can be stabilized thermoelectrically.

The Aerodyne researchers coupled DFB-QCL-2 series quantum cascade lasers from Alpes Lasers of Neuchâtel, Switzerland, to a proprietary multiple-pass gas cell that increases absorption by passing the light between two mirrors 182 times. The cell's small volume (0.3 liters) allows a flow response time of less than 0.1 seconds, which is important for temporal resolution during a puff. They analyzed concentrations of gases in two types of standard reference cigarettes, measuring ammonia and ethylene with one laser and nitric oxide with another. The data rate of 20 Hz provided sufficient resolution to obtain profiles from a 2-second-long puff and to analyze sidestream smoke before, during and after the mainstream puff.

Scientists found that in pulsed mode the laser linewidth can be excessively broad, which would reduce the spectral resolution unless very short electrical pulses close to the lasing threshold were used. They determined the optimal pulse width for the laser measuring NH3 to be about 15 ns, resulting in a laser linewidth of 0.006 cm–1. The laser was operated at 3.8 °C, yielding a line frequency close to 967 cm–1. The laser measuring NO worked best with 18-ns pulses and a linewidth of 0.010 cm–1. Operated at <–6 °C, it produced a frequency near 1900 cm–1.

Lead researcher Mark S. Zahniser said that, although quantum cascade lasers have been around for several years, their enormous potential has not yet been tapped. "A light source in the mid-IR that doesn't require cryogenic cooling has been a holy grail for a long time," he added. His team chose Alpes' lasers because the company was one of the first to make quantum cascade lasers commercially available, it provides a wide variety of wavelengths and, to encourage application of the devices, it supplies pulse electronics and the Peltier cooling units.

The scientists adapted proprietary spectral analysis software -- previously developed at Aerodyne for continuous-wave lead-salt tunable diode lasers -- to work with pulsed-mode quantum cascade lasers to calculate the absolute concentrations of the gases in both mainstream and sidestream smoke. To reduce sampling variations in the sidestream analysis, they used CO2 measurements made by a nondispersive infrared analyzer as a tracer against which the other emissions could be compared. They also created a streamlined sampling method that eliminated the valves and traps commonly used in smoking machines and, instead, controlled the stream of air through the cigarette by changing the flow rate of the carrier gas in the sampling line.

The findings, published in the Oct. 1 issue of Analytical Chemistry, demonstrate the feasibility of a cryogen-free, real-time measurement of smoke constituents. Aerodyne researcher Quan Shi said the next steps are to develop an instrument using multiple lasers that can quantify up to eight gases simultaneously and to investigate the method to measure denser molecules, such as acrolein and 1,3-Butadiene. He added that the pulsed quantum cascade laser technology is applicable in many other areas, including measuring air pollutants, greenhouse gases and automobile exhaust. A commercial version of the Aerodyne instrument has been continuously measuring ammonia emissions from swine and cattle farms in Iowa for the past few months.

laser spectroscopy
That part of the science involved in the study of the theory and interpretation of spectra that uses the unique characteristics of the laser as an integral part in the development of information for analysis. Raman spectroscopy and emission spectroscopy are two areas where lasers are used.
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