Anal. Chem. 2003, 75, 5180-5190
Quantum Cascade Infrared Laser Spectroscopy for Real-Time Cigarette Smoke Analysis Quan Shi,* David D. Nelson, J. Barry McManus, and Mark S. Zahniser
Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821 Milton E. Parrish, Randall E. Baren, and Kenneth H. Shafer
Research Center, Philip Morris USA Inc., P.O. Box 26583, Richmond, Virginia 23234 Charles N. Harward
Nottoway Scientific Consulting Corporation, P.O. Box 125, Nottoway, Virginia 23955
A compact, fast-response, infrared spectrometer using pulsed quantum cascade (QC) lasers has been applied to the analysis of gases in mainstream and sidestream cigarette smoke. QC lasers have many advantages over the traditional lead salt tunable diode lasers, including near-room-temperature operation with thermoelectric cooling and single mode operation with improved long-term stability. The instrument uses a 36-m, 0.3-L multiple pass absorption gas cell to obtain a time response of 0.1 s for the mainstream system and 0.4 s for the sidestream system. A precision of 10-4 absorbance units in 1 s is achieved using pulse normalization to reduce pulse-topulse intensity variations. Laser line widths (hwhm) of 0.006 cm-1 at 967 and 0.010 cm-1 at 1900 cm-1 are obtained in pulsed operation. With this instrument, we have measured the concentrations of ammonia and ethylene using a QC laser at 967 cm-1 and nitric oxide using a laser at 1900 cm-1 in both mainstream and sidestream smoke from two types of cigarettes. A data rate of 20 Hz provides sufficient temporal resolution to reveal the concentration profiles for these gas components during each 2-s puff in the mainstream smoke. In addition, the concentration profiles before, during, and after the puffs are observed in the sidestream smoke. Simultaneous measurements of CO2 are obtained using a nondispersive infrared analyzer, and the emission ratios of the analyte gases relative to the amount of CO2 produced during combustion are calculated using two types of reference cigarettes. Tunable infrared laser differential absorption spectroscopy (TILDAS) has been used for many years for detecting gaseous components from cigarette smoke.1-7 TILDAS with conventional (1) Forrest, G. T. Appl. Opt. 1980, 19, 13, 2094-2096. (2) Vilcins, G.; Harward, C. N.; Parrish, M.; Forrest, G. T. SPIE Proc. 1983, 438, 48-54. (3) Parrish, M.; Harward, C. N.; Vilcins, G. Beitr. Zur Tabakforsch. Int. 1986, 13, 169-181. (4) Parrish, M.; Harward, C. Appl. Spectrosc. 2000, 54, 11, 1665-1677. (5) Plunkett, S.; Parrish, M.; Shafer, H.; Nelson, D.; McManus, J. B.; Jimenez, J. L.; Zahniser, M. SPIE Proc. 1999, 3758, 212-221.
5180 Analytical Chemistry, Vol. 75, No. 19, October 1, 2003
lead salt tunable diode lasers (TDLs) provides high sensitivity and selectivity for detecting combustion product molecules with resolvable ro-vibrational structure in the midinfrared spectral region.8 However, the requirement of cryogenic cooling for lead salt lasers makes them inconvenient to use in many analytical chemistry applications. Quantum cascade (QC) lasers are semiconductor devices that have several advantages over lead salt lasers in terms of their spectral characteristics. The key virtues of QC lasers are higher output power, near-room-temperature operation, pure single mode operation, and minimal wavelength drift.9-11 When QC lasers are operated in the pulsed mode, thermoelectric (TE) cooling can be used to control the laser temperature in the range from -30 to +30 °C. Although the laser line width is considerably wider (0.003-0.03 cm-1) in the pulsed mode than in continuous wave mode, the spectroscopic detection of trace gases is still quite feasible.12-16 This paper describes a compact, fast-response QC laser spectrometer which couples a pulsed, TE-cooled QC laser to a 36-meter, 0.3-L astigmatic multiple pass gas cell.17 The small volume of the gas cell allows the flow response to be
