Detection and analysis of volatile compounds in exhaled breath represents an attractive tool for monitoring the metabolic status of a patient and disease diagnosis, since it is non-invasive and fast. Numerous studies have already demonstrated the benefit of breath analysis in clinical settings/applications and encouraged multidisciplinary research to reveal new insights regarding the origins, pathways, and pathophysiological roles of breath components. Many breath analysis methods are currently available to help explore these directions, ranging from mass spectrometry to laser-based spectroscopy and sensor arrays. This review presents an update of the current status of optical methods, using near and mid-infrared sources, for clinical breath gas analysis over the last decade and describes recent technological developments and their applications. The review includes: tunable diode laser absorption spectroscopy, cavity ring-down spectroscopy, integrated cavity output spectroscopy, cavity-enhanced absorption spectroscopy, photoacoustic spectroscopy, quartz-enhanced photoacoustic spectroscopy, and optical frequency comb spectroscopy. A SWOT analysis (strengths, weaknesses, opportunities, and threats) is presented that describes the laser-based techniques within the clinical framework of breath research and their appealing features for clinical use.

Ammonia concentrations in exhaled breath (eNH3) and skin gas of 20 healthy subjects were measured on-line with a commercial cavity ring-down spectrometer and compared to saliva pH and plasma ammonium ion (NH(+)4), urea and creatinine concentrations. Special attention was given to mouth, nose and skin sampling procedures and the accurate quantification of ammonia in humid gas samples. The obtained median concentrations were 688 parts per billion by volume (ppbv) for mouth-eNH3, 34 ppbv for nose-eNH3, and 21 ppbv for both mouth- and nose-eNH3 after an acidic mouth wash (MW). The median ammonia emission rate from the lower forearm was 0.3 ng cm(-2) min(-1). Statistically significant (p < 0.05) correlations between the breath, skin and plasma ammonia/ammonium concentrations were not found. However, mouth-eNH3 strongly (p < 0.001) correlated with saliva pH. This dependence was also observed in detailed measurements of the diurnal variation and the response of eNH3 to the acidic MW. It is concluded that eNH3 as such does not reflect plasma but saliva and airway mucus NH(+)4 concentrations and is affected by saliva and airway mucus pH. After normalization with saliva pH using the Henderson-Hasselbalch equation, mouth-eNH3 correlated with plasma NH(+)4, which points to saliva and plasma NH(+)4 being linked via hydrolysis of salivary urea.

We present a combined experimental and theoretical study on the radiative lifetime of CO in the aΠ1,23, v=0 state. CO molecules in a beam are prepared in selected rotational levels of this metastable state, Stark-decelerated, and electrostatically trapped. From the phosphorescence decay in the trap, the radiative lifetime is measured to be 2.63±0.03ms for the aΠ13, v=0, J=1 level. From the spin-orbit coupling between the aΠ3 and the AΠ1 states a 20% longer radiative lifetime of 3.16ms is calculated for this level. It is concluded that coupling to other Π1 states contributes to the observed phosphorescence rate of metastable CO.

We report on the Stark deceleration and electrostatic trapping of ${}^{14}\text{N}\text{H}$ $\left(a{}^1\Delta \right)$ radicals. In the trap, the molecules are excited on the spin-forbidden $A{}^3\Pi \leftarrow a{}^1\Delta$ transition and detected via their subsequent fluorescence to the $X{{}^3\Sigma }^{-}$ ground state. The $1∕e$ trapping time is $1.4\pm 0.1\text{s}$, from which a lower limit of 2.7 s for the radiative lifetime of the $a{}^1\Delta$, $v=0$, $J=2$ state is deduced. The spectral profile of the molecules in the trapping field is measured to probe their spatial distribution. Electrostatic trapping of metastable NH followed by optical pumping of the trapped molecules to the electronic ground state is an important step toward accumulation of these radicals in a magnetic trap.

Highly sensitive cantilever-enhanced photoacoustic detection of hydrogen cyanide and methane in the mid-infrared region is demonstrated. A mid-infrared continuous-wave frequency tunable optical parametric oscillator was used as a light source in the experimental setup. Noise equivalent detection limits of 190 ppt (1 s) and 65 ppt (30 s) were achieved for HCN and CH₄, respectively. The normalized noise equivalent absorption coefficient is 1.8 × 10⁻⁹ W cm⁻¹ Hz^−1/2.