Ultra-sensitive tunable laser absorption spectroscopy: Applications and performance assessment
Roller, Chad B.
Tittel, Frank K.
Doctor of Philosophy
Methodologies using tunable laser absorption spectroscopy (TLAS) for quantifying carbon dioxide stable isotopes (13/12CO 2), measuring atmospheric formaldehyde (CH2O), and analyzing exhaled carbonyl sulfide (COS) in human breath are covered. Fundamental spectroscopic aspects that limit achievable precisions of 13/12CO2 isotopic ratio TLAS measurements have been investigated and include affects of temperature and pressure instabilities, differences in relative humidity between reference standards and sample gases, and the effect of gas heating from optical absorption. Experimental studies were designed and carried out to determine the best possible system configuration and analytical routines for ratiometric measurements. Direct absorption was found to be advantageous over 2nd harmonic detection in regards to precision measurements. Airborne spectral measurements of CH2O using a TLAS based instrument equipped with a lead-salt diode laser onboard a NASA DC-8 aircraft taken during the INTEX-NA 2004 mission was analyzed to assess instrument performance and reliability using standard and newly proposed statistical measures. An improvement in system performance after stabilization of optical components has been unambiguously shown. A retrospective analysis of stored spectral data has shown that methanol (CH3OH) concentrations can also be deduced along with CH2O concentrations using a least-squares singular-value decomposition regression routine. A TLAS instrument equipped with a thermo-electrically cooled pulsed quantum cascade (QC) laser was used to perform absorption measurements of carbonyl sulfide (COS) with detection limits near ∼3-ppbv. This instrument was fully characterized prior to conducting a small clinical study to perform offline measurements of COS in the exhaled breath of lung transplant patients to determine the utility of such measurements in diagnosing acute rejection (AR) of lung allografts. COS at ∼8-ppb was observed in one patient suffering from bronchiolitis.
Biomedical engineering; Electronics; Electrical engineering; Atmospheric sciences