focal point review SCHOOL

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SCIENCE, TIANJIN UNIVERSITY

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LIZHU ZHANG, GUANG TIAN TECHNOLOGY AND EDUCATION, TIANJIN 300220, CHINA

JINGSONG LI,* BENLI YU KEY LABORATORY OF OPTO-ELECTRONIC INFORMATION ACQUISITION AND MANIPULATION OF MINISTRY OF EDUCATION, ANHUI UNIVERSITY, HEFEI 230601, CHINA

Applications of Absorption Spectroscopy Using Quantum Cascade Lasers Infrared laser absorption spectroscopy (LAS) is a promising modern technique for sensing trace gases with high sensitivity, selectivity, and high time resolution. Mid-infrared quantum cascade lasers, operating in a pulsed or continuous wave mode, have potential as spectroscopic sources because of their narrow linewidths, single mode operation, tunability, high output power, reliability, low power consumption, and compactness. This paper reviews some important developments in modern laser absorption spectroscopy based on the use of quantum cascade laser (QCL) sources. Among the various laser spectroscopic methods, this review is focused on selected absorption spectroscopy applications of QCLs, with particular emphasis on molecular spectroscopy, industrial process control, combustion diagnostics, and medical breath analysis. Index Headings: Laser spectroscopy; Quantum cascade laser; QCL; Spectroscopic applications; Gas analysis.

INTRODUCTION aser absorption spectroscopy (LAS) is an emerging discipline in many fields such as atmospheric environmental monitoring, industrial process control, medical diagnostics, and is promising for applications in which the continuous monitoring of targeted gases with sensitivity, selectivity, and fast response are required. Compared with other measurement techniques, LAS has a rapid response, is nondestructive, and is amenable to remote sensing. Advances in laser sources and spectroscopic techniques have led to an increase in the use of LAS for atmospheric and industrial measurements of trace gases.1–5 Bunsen and Kirchhoff first proposed spectral analysis in the middle of the 19th century.6 Many important gases exhibit absorption in the infrared spectral region (between 0.7 and 25 lm). Near infrared (NIR, between 0.7 and 2.5 lm) spectroscopy techniques have benefited from developments in telecommunications where inexpensive and compact diode laser sources became available in the 1990s. In contrast, similar lasers were not available in the middle infrared (MIR, between 2.5 and 25 lm). Recent progress in semiconductor laser technology, in particular, the advent of the intersubband quantum cascade laser (QCL)7 and the interband

L

Received 13 April 2014; accepted 15 June 2014. * Author to whom correspondence should be sent. E-mail: ljs0625@126. com. DOI: 10.1366/14-00001

cascade laser (ICL),8 has provided new possibilities for highly sensitive and selective trace gas sensing utilizing MIR spectroscopy techniques in combination with multiple pass cells, modulation techniques, and cavity enhanced methods. Compared to some traditional MIR laser sources, QCLs overcome many drawbacks, for example, the cryogenic cooling requirement and low power of lead salt diode lasers, the lack of continuous wavelength tunability, the large size and weight of gas lasers (e.g., CO and CO2), as well as the complexity of coherent sources based upon difference frequency generation (DFG) and optical parametric oscillators (OPO). Amongst these MIR laser sources, newly developed QCLs offer the advantages of long lifetime, high power, compactness, and robustness, which make instruments based on these laser sources very suited for long-term in situ and online real-time measurements of atmospheric trace gases.3 Recently, QCLs have experienced rapid and dramatic improvements in power, efficiency, and wavelength range. By carefully designing the quantum wells, lasing has been achieved at wavelengths as short as 2.75 lm and as long as 3000 lm (i.e., 0.1 THz). The longer wavelength devices require cryogenic cooling still, but room temperature operation has been observed to at least 16 lm.9–12 In this review, we provide a brief overview of state-ofthe-art QCL techniques in applied laser spectroscopy and their current applications, beginning with a simple introduction to the key features of LAS and QCLs, as well as an overview of the most popular LAS techniques associated with the fundamental principles. Subsequent-

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FIG. 1. Concept of direct absorption spectroscopy.

ly, we focus on several selected applications of QCLbased modern spectroscopic techniques, in particular, molecular spectroscopy, industrial process control, combustion diagnostics, and medical breath analysis.

MODERN LASER SPECTROSCOPY TECHNIQUES Direct Absorption Spectroscopy. Direct absorption spectroscopy (DAS) is a simple, noninvasive, in situ technique for determining information about gas phase species, such as quantitative absolute concentration, temperature, pressure, velocity, and mass flux of the gas under observation.12–15 However, DAS suffers from a low sensitivity that limits its extension into several research fields (e.g., trace gas analysis). The basic principle is based on the Beer–Lambert law, as shown in Fig. 1. The transmission of laser light through an absorbing uniform gaseous substance with an optical path length of L can be expressed as Iðv Þ ¼ I0 ðv Þexpð-aðv ÞLÞ

ð1Þ

where I(v) and I0(v) is the transmitted and incident light intensity at frequency of v, respectively, and a(v) = r(v)N0C is the absorption coefficient of the sample with concentration of C. The Loschmidt constant N0 is the number density (molecules per unit volume), which is a function of temperature T and pressure P. Here, r(v) = /(v - v0)S(T) is the absorption cross-section of the absorbing species, S(T) and /(v - v0) are the line strength at temperature T and the line shape function for the particular absorption line transition, respectively, v0 is the central frequency of the absorption line. Semiconductor diode lasers were first developed in the mid-1960s16 and found immediate use in DAS for highresolution laser spectroscopy commonly referred to as tunable diode laser absorption spectroscopy (TDLAS).17,18 The wavelength of a diode laser is commonly tuned over a particular gas absorption line of interest; after traveling through the sample medium, the laser light intensity is attenuated and can be measured with a suitable detector. Therefore, the low sensitivity (detection of absorbance ~10-3) results from the fact that a small light attenuation has to be measured on top of a large background signal that in turn is proportional to the intensity of the light

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source. As a result, many experimental schemes have been proposed for improving the sensitivity, generally classified into two categories; that is, signal enhancement and noise reduction techniques, for instance, increasing the absorption path length using multi-pass absorption cells (e.g., White cell,19 Herriott cell,20 or astigmatic Herriott cell,21 Chernin cell,22 and integrating sphere23), and/or combining DAS with suitable modulation techniques and high-finesse optical cavity (discussed in the next section). From a practical application point of view, adaptive digital filtering is easy to implement since it requires no modifications or additions to the apparatus hardware and can be easily adapted to any experimental configuration. Mathematical filtering techniques for online noise reduction or offline data processing of recorded spectra are a better choice when temporal resolution is required.24–27 Modulation Spectroscopy. Modulation techniques are commonly used methods to increase spectral signal-to-noise ratio (SNR) by reducing the noise contribution in TDLAS. They make use of the fact that technical noise usually decreases with increasing frequency (which is why it is often referred to as 1/f noise) and improve the SNR by encoding and detecting the absorption signal at a high frequency, where the noise level is low. These techniques generally have a common principle, i.e., they transform the measured signal into a periodic signal by modulating a certain parameter, for example, the laser intensity or amplitude, emission wavelength, or frequency of a laser source, corresponding to intensity or amplitude modulation, wavelength modulation, and frequency modulation, respectively, as well as take advantage of the fact that a given electric dipole moment of a molecule interacts with an external electric field. Amplitude Modulation. The traditional modulation method for increasing the SNR in laser absorption spectrometry is to chop (or cut) the laser beam by a mechanical chopper and to amplify the laser and current in the detector with a lock-in amplifier. This technique has some shortcomings due to chopper frequency or when the background signal is low frequency and introduces additional mechanical vibration. Wavelength Modulation Spectroscopy. Currently, the most common modulation technique is wavelength

FIG. 2. Schematic diagram of a wavelength modulation spectroscopy system.

modulation spectroscopy (WMS), as depicted in Fig. 2. Wavelength modulation spectroscopy, also known as derivative spectroscopy,28 utilizes modulation frequencies x much smaller than the frequency of the absorption line of interest. Wavelength modulation spectroscopy has been used with tunable diode laser sources since the early 1970s.29–31 The basic theory of WMS involves a similar idea to DAS, with an additional fast sinusoid (at frequency f) modulation applied to the laser current. Traditionally, it performed at kilohertz frequencies, which is much smaller than the half-width of the absorbing feature, usually achieves sensitivities of 10-4 to 10-5 fractional absorption. The instantaneous laser frequency and output laser intensity can be expressed as v ðt Þ ¼ v0 þ a cosðxt Þ

ð2Þ

Iðt Þ ¼ I0 þ i0 cosðxt þ uÞ

ð3Þ

where a = mDv and i0 are the modulation amplitudes of laser frequency and intensity modulation, respectively, m is commonly termed the modulation index, Dv is the half-width at half-maximum of the emission line, while u is the phase shift between both modulation effects. Depending on the response of the laser source to the injection current, high-order nonlinear intensity–frequency modulation effect is possible, which results in the socalled residual amplitude modulation. This effect is not considered in this study. Due to the periodic even function characteristic, the time-dependent transmission coefficient s(v) can be expanded in a Fourier cosine series32 sðv0 þ a cosðxt ÞÞ ¼

n¼þ‘ X n¼0

Hn ðv0 ; aÞcosðnxt Þ

ð4Þ

where Hn(v0,a) is the nth component of the Fourier series, given by 8 Z p 1 > > sðv0 þ a coshÞd h ðn ¼ 0Þ > < 2p -p Z Hn ðv0 ; aÞ ¼ 1 p > > sðv0 þ a coshÞ  cosðnhÞd h ðn  1Þ > :p -p ð5Þ The modulated absorption signal on the photo-detector is then processed though a lock-in amplifier that demodulates the signal at the fundamental modulation frequency (first harmonic, 1f) and its integral multiples (two or higher harmonics), also known as the so-called phase sensitive detection. In practical applications, the second harmonic (i.e., 2f) is commonly utilized in WMS for improving the spectral signal-to-noise ratio (SNR).33,34 In the case of optically thin absorption (i.e., a(v)CL ,, 1),32 the transmission coefficient can be simplified as s ’ 1 - a(v)L = 1 - /(v - v0)S(T)N0CL. Therefore, the Fourier coefficient can be further simplified as Z SN0 CL p H2 ðv0 ; aÞ ¼ /ðv0 þ a coshÞ  cosð2hÞd h ð6Þ p -p Therefore, we can see that the amplitude of the 2f signal is linearly proportional to the concentration of the absorbing species at a given optical path length and molecular absorption cross section, assuming that the laser intensity is constant, i.e., I2f } I0 aCL

ð7Þ

Indeed, WMS increases sensitivity by shifting to a

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focal point review higher frequency detection bandwidth and rejecting noise (e.g., 1/f laser noise) with lock-in amplifiers. Wavelength modulation spectroscopy is particularly useful for probing air-broadened and overlapping absorption features. Comparing to DAS, this calibration method removes the need to assign the chosen line(s) spectroscopically or to measure their integrated line strengths. The main drawback of WMS is the requirement of onsite calibration with a known gas concentration in some applications where in situ measurements are desired. This complicates WMS applications in harsh environments. To overcome this challenge, a calibrationfree WMS has been proposed by several research groups.34–38 Frequency Modulation Spectroscopy. Frequency modulation spectroscopy (FMS) is similar to WMS,39–41 but depends on much higher modulation frequencies, which are comparable to or larger than the half-width of the absorbing feature, generally between several hundreds of megahertz and gigahertz frequencies. Relevant information is shifted to radio frequencies, where lowfrequency noise is suppressed. In FMS, modulation results in the formation of two distinct sidebands shifted by xm with respect to the carrier x0. In principle, it is a heterodyne technique and determines the phase shift imposed on a probe beam by an atomic or molecular line rather than its typically very weak absorption signal. Therefore, the line-shapes obtained by FMS are dispersive, i.e., with a zero crossing on resonance and a linear slope, they are ideally suited as a lock signal for laser frequency stabilization. Depending on the number of modulation tones, the methods are referred to as single-tone FMS (i.e., standard FM) or two-tone FMS. A key limitation of standard FM techniques essentially arises from the fact that, in the presence of pressure broadened lines at atmospheric pressure environments, high modulation frequencies are required, and consequently the detection and processing electronics must have wide bandwidths. The two-tone technique combines the advantages of standard FM with the benefits of a considerable reduction in detection bandwidth, with an additional improvement in signal-to-noise ratio.42–44 This technique was first proposed in 1982 for a tunable diode laser with small modulation frequencies.45 In 1986, it was extended to much higher frequencies (up to 16 GHz)46 using a continuous wave (CW) dye laser source and an electro-optic modulator. Subsequently, high frequency single-tone and two-tone FM with a lead-salt diode laser was achieved,47 thus demonstrating the general validity of these techniques, offering the possibility of quantum noise limited detection so that laser source and detector limited sensitivities of the order of 10-7 to 10-8 are possible.48,49 Finally, it is worth noting that the two-tone FM signal is proportional to the square of the FM index rather than to the FM index as in conventional singletone FMS, and no phase information is encoded. In 2006 and 2005, Gagliardi et al.50–52 investigated the

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performance of QCLs at around 8.1 and 7.3 lm wavelength using single-tone and two-tone techniques for high-frequency modulation spectroscopy on lowpressure N2O and CH4 gases, respectively. Although some difficulties are encountered, they finally concluded that QCLs are suitable for this kind of spectroscopy in the mid-infrared region. Related technologies of tunable diode laser-based WMS, FMS, and two-tone FMS and their applications have been reviewed by Song and Jung.53 Therefore, they will not be addressed in this paper. Magnetic Rotation Modulation Spectroscopy. Another promising technique is the so-called magnetic rotation modulation spectroscopy, namely, the modulation of the absorption coefficient of a specific absorption line by means of the Zeeman, Stark, and Faraday effect using an external magnetic field (Fig. 3). Magnetic rotation spectroscopy was first reported in the 1980s54 and takes advantage of the fact that a given electric dipole moment of a molecule interacts with an external electric field because many important molecules such as ammonia, nitric acid and nitric oxide, formaldehyde, hydrogen peroxide, hydrogen fluoride, oxygen, OH radicals, and others show a permanent electric dipole moment,55 which is a prerequisite for interaction with an external electric field. This interaction causes a splitting and shift of the energy levels of the sample molecule. This enhances the specificity of detection of certain gases because it minimizes the effect of interfering absorbing species that have no electric dipole moment, such as water vapor. In 1984, Sasada56 reported that several combination band transitions for NH3 are observed by Stark modulation spectroscopy with a 1.23 lm semiconductor laser. In 1995, Smith et al.57 demonstrated frequency modulationenhanced magnetic rotation spectroscopy for detecting local concentrations of gaseous NO2, and a detection limit of 20 ppbv (parts per billion by volume) was achieved. Two years later, Brecha et al.58 presented the results of a study of molecular oxygen by magnetic rotation spectroscopy based on the use of a semiconductor diode laser operating near room temperature at 762 nm. In 2009, Sabana et al.59 employed a Faraday modulation spectroscopy technique based on a CW distributed feedback QCL operating near 5.4 lm for simultaneous detection of 14NO and 15NO. The detection limit (1r) of 6 ppb/=Hz for 15NO and 62 ppb/=Hz for 14 NO was achieved. The isotope ratio (d15N) was determined with a precision (1r) of 0.52% at 800 s averaging time for 100 parts per million (ppm) NO gas with a time resolution of 2 s. In 2011, a Faraday modulation spectrometer for sensitive and fast NO detection at 5.33 lm utilizing a room temperature continuous wave distributed feedback QCL was reported by Kluczynski et al.60 The spectrometer provided a detection limit of 4.5 ppb for a response time of 1 s. Moreover, most related work on the use of diode laser (VCSEL and QCL) based Faraday rotation spectroscopy

FIG. 3. Block diagram of a magnetic rotation modulation spectroscopy system.

for trace gases sensing (O2, NO, and OH radicals) was carried out by the Wysocki’s group at Princeton University.61–63 High-Finesse Optical Cavity Enhanced Spectroscopy. The high-finesse optical cavity enhanced spectroscopy technique, which takes advantage of long optical path length absorption in high-finesse optical cavities, is based on the observation of the decay rate of an injected laser beam stored in a cavity composed of ultrahigh reflective and low-loss dielectric mirrors. High-finesse optical cavities allow a large amount of light energy to build up in the cavity. The low-loss dielectric mirrors allow light to leak out of the optical cavity to characterize the absorption of gas inside the cavity. These kinds of cavity-based methods are generally classified into cavity

ring-down spectroscopy (CRDS), and its variants such as cavity enhanced absorption spectroscopy (CEAS) and integrated cavity output spectroscopy (ICOS). Cavity ring-down spectroscopy is also a direct absorption technique, which can be performed with pulsed or continuous light sources and has a significantly higher sensitivity than conventional absorption spectroscopy. The so-called CRDS technique was first experimentally demonstrated by O’Keefe and Deacon in 1988.64 This technique is based on the observation of the decay rate of an injected laser beam stored in a closed optical cavity composed of ultrahigh reflective spherical mirrors, as shown in Fig. 4. The advantage over traditional absorption spectroscopy results from the intrinsic insensitivity to light source intensity fluctuations

FIG. 4. Scheme of the experimental setup for CRDS.

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FIG. 5. Schematic diagram of the traditional photoacoustic spectroscopy system.

and the extremely long effective path lengths (many kilometers) that can be realized in short optical cavities (tens of centimeters). The sensitivity of CRDS is ultimately limited by the accuracy of the decay time measurement. Ten years later, two modifications of the CRDS technique, ICOS and CEAS, were proposed independently in 1998 by O’Keefe65 and Engeln et al.66 The concept of the three methods is similar, but there is a difference related to the alignment of a laser beam and the cavity as well as to the mode structure. One big advantage of ICOS and CEAS as compared with CRDS is that it can be used without limitations concerning ringdown time or mode matching between the laser frequency and the cavity free spectral range (FSR). Furthermore, the most promising approach to merge CEAS with the frequency modulation technique was obtained close to shot noise sensitivity of 1 3 10-14 cm-1 by Ye et al.67 and is known as noise-immune cavityenhanced optical-heterodyne molecular spectroscopy (NICE-OHMS). This sensitivity is superior to that achieved with CRDS, but the technical requirements are stringent. During the last decades, research on various CRDS detection schemes has been widely explored, as recently described in several review publications.68–70 Photoacoustic Spectroscopy. Photoacoustic spectroscopy (PAS) originates from the discovery of photoacoustic (PA) or optoacoustic effect by Bell in 1880.71 He found that when a beam of sunlight rapidly interrupted with a rotating slotted disk was focused onto thin diaphragms, sound was emitted. The absorbed energy from the light is transformed into kinetic energy of the sample by energy exchange processes. This results in local heating and the creation of a pressure wave or sound. A PA spectrum of a sample can be recorded by measuring the sound at different wavelengths. Photoacoustic spectroscopy can be applied to solids, liquids, and gases and is only sensitive to sample absorption, not scattering losses. Photoacoustic spectroscopy is a very promising technique for trace gaseous species detection. In the traditional PAS experiment (as shown in Fig. 5), sample materials in or sampled into a closed cell are irradiated

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using either a pulsed or a wavelength- or amplitudemodulated light source. The sample species absorb the light and are thermally excited, causing expansion and contraction of the gas within the cell in synchronization with the modulation frequency or laser pulse repetition frequency. The resulting dynamic motion of the gas creates an acoustic pressure front that can be monitored by a microphone or a piezoelectric crystal. The PA signal S in volts can be described as S ¼ Sm PCcell Ntot cm r

ð8Þ

where the microphone sensitivity Sm is in units of millivolts per pascal; the optical power of light radiation source P is in watts; the PA cell response constant Ccell has units of pascal per inverse centimeters per watt; Ntot is the total number density of molecules (molecule/cm3); and coefficients cm and r are the concentration and absorption cross-section of the analyte, respectively. The PA cell constant Ccell is a scaling factor depending on the PA cell geometry, on the modulation frequency, and on the measurement conditions. It usually can be determined experimentally from measurements with known gas absorption and certified concentration. On the other hand, we can see that PAS is a laser power dependent technique. Photoacoustic spectroscopy as a calorimetric spectroscopy technique has gradually matured in utility with significant performance improvements in light sources and modulators and in PA signal transducers. This technique has the potential for compact, robust, highly selective, highly sensitive, large dynamic range, and relatively low-cost trace gas analysis. Furthermore, because the acoustic properties of the PA system (e.g., microphone responses) do not depend on the spectral distribution of the absorbed radiation, PA spectrometers are optically broadband devices; that is, wavelength independent. From the technique point of view, several promising methods of improving the sensitivity of PAS have been proposed, for instance, acoustic cell resonant photoacoustic spectroscopy (R-PAS),72–76 wavelength modulation photoacoustic spectroscopy (WM-PAS),77–80 multi-

pass enhanced photoacoustic spectroscopy (MEPAS),81,82 cavity-enhanced photoacoustic spectroscopy (CE-PAS).83,84 Since the early 2000s, significant development and improvement has occurred to the sensing element. Novel sensing methods for PAS have been proposed, such as quartz-enhanced photoacoustic spectroscopy (QEPAS)85–88 and cantilever-enhanced photoacoustic spectroscopy (CEPAS).89–94 For an in-depth review of PAS techniques, the reader is referred to other publications.95–97 A comprehensive review of the recent development and progress of infrared PAS techniques to 2011 was reported by Li et al.98

SELECTED APPLICATIONS OF QUANTUM CASCADE LASER-BASED SPECTROSCOPY TECHNIQUES Molecular Spectroscopy. Molecular spectroscopic parameters are the essential elements needed in laser absorption spectroscopy techniques. Especially for direct absorption techniques, highly accurate trace gas concentration measurements rely on the molecular absorption line intensity. For atmospheric applications, therefore, a good knowledge of line parameters (i.e., positions, strengths, pressure-broadening, and shift coefficients) of the spectral lines is required to very high precision and accuracy. Important work on developing a QCL absorption spectrometer for MIR molecular spectroscopy study was carried out by the Groupe de Spectrome´trie Mole´culaire et Atmosphe´rique (GSMA) of Reims University (France). Water vapor in the atmosphere is an important greenhouse gas and is thereby a key component of Earth’s climate system. Most terrestrial water vapor is located in the troposphere. A better understanding of the mechanisms involved in the injection of tropical tropospheric H2O is of importance. The measurement of water vapor isotopes can be helpful to address this issue. Therefore, the GSMA reported a laboratory study of H216O, H218O, and HDO line intensities between 1483 to 1487 cm-1,99 which is suitable for in situ laser sensing of these isotopologues in the atmosphere using a continuous wave distributed feedback quantum cascade laser (CWDFB-QCL) near 6.7 lm. Line intensities and self-broadening coefficients were measured in the m1 band of SO2 and 34SO2, as well as m1 þ m2–m2 bands of 32SO2 with a CWDFB-QCL at 9.1 lm between 1088 and 1090 cm-1.100,101 Furthermore, five lines of the m1 band of N2O at 7.9 lm (spectral region ranging from 1275 to 1280 cm-1) have been studied.102 The results of intensity measurements, air-broadening coefficients, and their variation in temperature from -58 8C up to ambient temperature are compared with previous determinations and available databases. Recently, McCall et al.103,104 observed the v8 vibrational band of methylene bromide (CH2Br2) and the v2 bending region of D2O using a QCL-based cavity ring-down spectrometer. A supersonic free-jet spectrum of the m4 band of CF3Cl from 1215.8 to 1220.6 cm-1 was measured

using a liquid nitrogen cooled QCL and a 2.8 m White cell.105 In addition, Tonokura et al.106,107 demonstrated the spectral line parameters (line strength and N2-broadening coefficients) measurements of the v3 band of hydroperoxyl (HO2) radical in the mid-infrared region at around 1065 cm-1 using CW-DFB-QCL spectroscopy. Two of the strongest formaldehyde (H2CO) m6 band transitions around 8 lm are assigned as (1, 1, 1) (2, 0, 2) and (10, 1, 9) (9, 2, 8) centered at 1252.11231 and 1253.14392 cm-1 by Wang and Sharples.108 Pressure-induced broadening and shift coefficients for these transitions with a variety of gases (He, Ne, Kr, Ar, N2,O2, and CO2) was also determined using pulsed DFB-QCL-based absorption spectroscopy. Lee et al.109 determined the effective line strengths of the trans conformer of nitrous acid (HONO) near 1275 cm-1 and of the cis conformer at 1660 cm-1, both at a spectral resolution of 0.001 cm-1 by utilizing CW-QCL-based differential absorption spectroscopy. The strong m2 vibrational band of ammonia (NH3) between 9–12 lm has a high absorption strength needed for sensing trace concentrations. Within this band, the 1103.46 cm-1 feature is one of the strongest and has minimal interference from CO2 and H2O. However, the six rotational transitions that make up this feature have not been studied previously with absorption spectroscopy due to their small line spacing ranging from 0.004 to 0.029 cm-1. A tunable CW-QCL was used to accurately study these six NH3 transitions between 1100.4 and 1108.2 cm-1 by Owen et al.110 Most recently, the state of the art of high resolution terahertz spectroscopy with QCLs was reviewed by Hu¨bers et al.111 Industrial Process Control and Combustion Diagnostics. The High Temperature Gas Dynamics Laboratory at Stanford University has been active in combustion diagnostics for over 40 years. This group has published over 400 scientific papers, dealing with advances in shockwave physics and chemistry, laser spectroscopy, advanced optical diagnostics and sensors, as well as chemical kinetics, combustion science, and advanced propulsion. Nitric oxide (NO) is an important atmospheric constituent of the oxides of nitrogen (NOx), and the majority of this pollutant is formed from the combustion of fossil fuels. As a result, the electric-power industry has a growing need to develop reliable control systems to suppress the release of NOx effluent from combustiondriven generation of electricity. Nitric oxide has transitions in three different vibrational bands, two overtone bands v3 and v2 near 1.8 and 2.7 lm, respectively, and the fundamental band v1 near 5.2 lm. The fundamental band holds the most promising candidate transitions in terms of their strong absorption (about 100 times stronger line strengths than in the first overtone band) and relatively weaker interference transitions of the other combustion species. In 2011, a novel external cavity (EC)-QCL-based MIR absorption sensor for in situ detection of NO in combustion exhaust gases was developed and demonstrated for temperatures up to 700 K.112 A NO detection limit of ,60 ppb m (Hz)-1/2 was

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focal point review obtained with a laboratory combustion exhaust rig and a 1.79 m constant temperature, line-of-sight path. The sensor system was modified with calibration-free WMS (i.e., 1f-normalized WMS with second-harmonic detection strategy, WMS-2f/1f) for improving the sensor performance. A 0.3 ppm-m detection limit was estimated using the R15.5 transition near 1927 cm-1 with 1 s averaging.113 Another key feature of DAS and WMS is the capacity for temperature measurements by using the ratio of absorption peaks or line shape integrated absorption of two neighboring molecular absorption lines. Carbon monoxide (CO) is also a typical pollutant resulting from the incomplete combustion of carbon-based fuels that are widely used for power generation, industrial heating, petrochemical refining, and propulsion. The strongest absorption of the CO fundamental band is at ~4.6 lm, providing orders-of-magnitude greater sensitivity than the overtone bands accessible with telecommunications lasers. A high-speed (up to 1–2 kHz) scanned-wavelength absorption sensor utilizing a thermoelectrically cooled CW-QCL source and both WMS and DAS techniques for CO detection at 4.6 lm and hightemperature thermometry between from 900 to 4000 K was developed.114,115 The high-temperature measurements of CO mole fraction and temperature agreed with the post-reflected shock conditions within 61.5 and 61.2% for DAS (or 61.9% for WMS), respectively. A method for measuring the temporal temperature and number density in a rapid compression machine (RCM) using QCL absorption spectroscopy near 7.6 lm was developed by the Sung group.116,117 The ratios of H2O absorption peaks at 1316.55 and 1316.97 cm-1 were used for measurements. For RCM temperatures between 1000 and 1200 K and pressures between 10 and 20 bars, the measured temperature was found to be within 65 K of the calculated values. For the measured temporal number density of H2O, an accuracy of 1% was obtained. Hu¨bner et al.118 reported gas temperature measurements (ranging from about 300 K up to about 500 K) in a pulsed DC air plasma admixed with 0.8% NO at 1900 cm-1 (5.26 lm) using pulsed quantum cascade laser absorption spectroscopy (QCLAS) with a pulse repetition frequency of 30 kHz, leading to a time resolution of 33 ls. Plasma diagnostics were reported by researchers at the Leibniz Institute for Plasma Science and Technology (IPN) Greifswald (Germany). The recent development of quantum cascade lasers (QCLs) for the monitoring and control of industrial plasma processes were summarized successively by Ro¨pcke et al.119–122 at IPN. The current state-of-the-art terahertz QCL technology and applications focusing on plasma diagnostics was reported in the work of Mahler et al.123 Most recently, Yumii et al.124 demonstrated NO2 concentration or density measurements in a small plasma of a nitrogen oxide (NOx) treatment reactor using sensitive QCL absorption spectroscopy. The high sensitivity of spectroscopy is achieved by the amplitude-to-time conversion technique, and the observation is consistent with that of an earlier

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study on NO decomposition by the same type of plasma reactor.125 Medical Breath Analysis. Among the numerous applications of QCL trace gas sensors, the analysis of human breath for (early) diagnosis of diseases has one of the highest potential impacts on public health and quality of life. In clinical medicine (such as in critical care and operating room settings), breath gas analysis is a promising new field of medicine and medical instrumentation. Potentially, breath gas analysis can offer noninvasive, real-time, and point-of-care disease diagnostics and metabolic status monitoring with minimal risk and negligible discomfort for patients. Employing QCL-based spectroscopy technologies to perform concentration measurements of different molecular species in breath gas were reported by the Laser Spectroscopy and Sensing Lab at ETH Zurich led by Markus Sigrist, the Life Science Trace Gas Research Group at Radboud University led by Frans Harren, and the Laser Science Group at Rice University led by Frank Tittel. The research activities of the Sigrist group focused on fundamental aspects and applications in monitoring and analysis of trace gases (from atmospheric chemistry, industrial process surveillance, agriculture to medical diagnostics) by employing laser spectroscopic techniques, such as PAS and CRDS. New research areas concerning noninvasive glucose detection in human tissue and the analysis of drugs in saliva have recently been carried out by this group.126,127 The Harren group focused on trace detection of gases of interest in plant physiology, post-harvest research, soil science, microbiology, ecology, molecular biology, medicine, and human health involving DAS with multi-pass cell, WMS and FMS, PAS, CRDS, CEAS and ICOS, NICE-OHMS, as well as FRS. These applications include such gases as C2H4, C2H6, CH4, NO, CO, CO2, water vapor, aldehydes, alcohols, ketones, acids, terpenoids, and many other hydrocarbons, all at or below the parts per billion by volume concentration level. This work was summarized in Refs. 128–134. The research at Tittel’s group focused on the further development of advanced mid-infrared trace gas sensors for applications in four technical areas: environmental monitoring, medical diagnostics, and the life sciences, industrial process analysis and control, as well as nuclear security. This research will take advantage of recent, significant advances in the commercial availability of high performance mid-infrared QCL and interband cascade laser (ICL) devices in the 3 to 11 lm spectral range and current laser spectroscopy techniques, such as QEPAS, ICOS, and CRDS. The recent trends and developments of laser-spectroscopybased techniques for applications in breath analysis to 2006 and to 2010 was successively presented by this group.135–137 Considering ammonia (NH3) inherent reactivity, factors influencing breath ammonia determination in both breath instrumentation and the breath collection process were investigated by their collaborator.138 Additional review articles on this topic from other groups

can be found in Refs. 139–145. Here, we will review recent developments in laser spectroscopy for breath gas analysis utilizing novel QCL-laser sources, which are not summarized in previous reviews. Recent studies show that numerous diseases are associated with oxidative stress, e.g., atherosclerosis, Parkinson’s, Alzheimer’s, and different inflammations of the respiratory tract. Carbon monoxide (CO) in human breath is another focus of research because of the potential use of CO as a biomarker molecule for the abovementioned diseases. The researchers at the Institut fu¨r Lasermedizin (Germany) reported the online analysis of exhaled CO using a cavity leak-out spectroscopy system. A time resolution of less than 1 s and a detection limit of 7 ppb Hz-1/2 were obtained,146 and different spectroscopic techniques for CO quantifications were compared.147 In 2011, Rubin et al.148 used a pulsed DFB-QCL in the spectral region around 2300 cm-1 in a flow-through system for 12CO2/13CO2 isotope ratio determination in human breath. The analysis was based on a Lorentzian fit of single rotational-vibrational lines for 12 CO2 and 13CO2 within a spectral window of 2 cm-1 providing parts per billion sensitivity for the determination of the absolute 13CO2 concentration. Most recently, Wo¨rle et al.149 demonstrated an EC-QCL (operating in the spectral range from 2150 to 2450 cm-1) with a miniaturized hollow waveguide gas cell for quantitatively determining the 12CO2/13CO2 ratio in the exhaled breath of mice. Highly accurate determinations of the isotope ratio in breath samples collected from a mouse research unit validated via hyphenated gas chromatography-mass spectrometry, which confirmed the viability of the ECQCL sensing technique for isotope-selective exhaled breath analysis. In addition, acetone at sub-ppm levels was reported by Ciaffoni et al.150 in a breath sample with a precision of 0.17 ppm (1r) by combining a CW-DFBQCL operating at 8.2 lm with sensitive CEAS. Laser absorption spectroscopy is widely recognized for applications where continuous monitoring of targeted exhaled gases with sensitivity, selectivity, and fast response are required, such as in critical care and operating room settings. Currently, a number of biomarker molecules have been identified in human breath gas that could be used to predict disease and disease progression. Single-molecule exhaled breath LAS sensors are commercially available. For example, an exhaled nitric oxide sensor (Breathmeter) is available from Ekips Technologies, Inc., and an exhaled ammonia sensor (Nephrolux) is available from Pranalytica, Inc. The availability of real-time, portable monitors will represent a breakthrough for clinical diagnosis.

CONCLUSIONS AND OUTLOOK The aim of this paper was to review recent developments and applications of laser absorption spectroscopic techniques utilizing novel QCLs as light sources, with particular emphasis on molecular spectroscopy, industrial process control, combustion diagnostics, and

medical breath analysis. Quantum-cascade-laser-based gas analyzers are usually for online and real-time gas concentration monitoring and offer many benefits over conventional sensors, such as low-power consumption, high specificity, high sensitivity, fast response, and provide enough accuracy and precision to fulfill different performance requirements. The current challenge is to extend the capabilities of QCLs toward longer wavelengths to cover the far infrared and terahertz regions of the spectrum and the realization of their operation at room temperature. We can expect that QCL-based sensors will enable new science and will lead to many exciting discoveries in molecular physics as well as in astronomy and planetary physics. ACKNOWLEDGMENTS The authors are grateful for the financial support from Tianjin Application Fundamental and Frontier Technology Research projects (Grant No. 14JCYBJC17100) and Anhui University Personnel Recruiting Project of Academic and Technical Leaders (Grant No. 10117700014). The authors gratefully acknowledge the anonymous reviewers and the editor for their valuable comments and suggestions to improve the quality of the paper.

1. I. Linnerud, P. Kaspersen, T. Jaeger. ‘‘Gas Monitoring in the Process Industry Using Diode Laser Spectroscopy’’. Appl. Phys. B: Lasers Opt. 1998. 67: 297-305. 2. M. Lackner. ‘‘Tunable Diode Laser Absorption Spectroscopy (TDLAS) in the Process Industries: A Review’’. Rev. Chem. Eng. 2007. 23(2): 65-147. 3. J.S. Li, W. Chen, H. Fischer. ‘‘Quantum Cascade Laser Spectrometry Techniques: A New Trend in Atmospheric Chemistry’’. Appl. Spectrosc. Rev. 2013. 48(7): 523-559. 4. J. Hodgkinson, R.P. Tatam. ‘‘Optical Gas Sensing: A Review’’. Meas. Sci. Technol. 2013. 24(1): 012004. doi:10.1088/0957-0233/24/ 1/012004. 5. T.D. Rapson, H. Dacres. ‘‘Analytical Techniques for Measuring Nitrous Oxide’’. TrAC, Trends Anal. Chem. 2014. 54: 65-74. 6. G.R. Kirchhoff, M. Planck. ‘‘Abhandlungen ueber Emission und Absorption’’, Ostwald’s Klassiker der exakten Wissenschaften, Nr. 100. Leipzig: Akademische Verlagsgesellschaft, 1898. Pp. 1-74. 7. J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho. ‘‘Quantum Cascade Laser’’. Science. 1994. 264(5158): 553-556. 8. R.Q. Yang. ‘‘Infrared Laser based on Intersubband Transitions in Quantum Wells’’. Superlattices Microst. 1995. 17(1): 77-83. 9. M. Razeghi, S.Y. Jae, A.J. Evans, S. Slivken, S.R. Darvish, J.E. David, J. Nguyen, B. Gokden, S. Khosravani. ‘‘Quantum Cascade Laser Progress and Outlook’’. Proc. SPIE. 2004. 5617: 221-232. 10. A. Lambrecht. ‘‘Quantum Cascade Lasers, Systems, and Applications in Europe’’. Proc. SPIE. 2005. 5732: 122-133. 11. C. Gmachl, F. Capasso, D.L. Sivco, A.Y. Cho. ‘‘Recent Progress in Quantum Cascade Lasers and Applications’’. Rep. Prog. Phys. 2001. 64: 1533-1601. 12. F. Capasso. ‘‘High-Performance Midinfrared Quantum Cascade Lasers’’. Opt. Eng. 2010. 49(11): 111102. 13. L.C. Philippe, R.K. Hanson. ‘‘Laser Diode Wavelength-Modulation Spectroscopy for Simultaneous Measurement of Temperature, Pressure, and Velocity in Shock-Heated Oxygen Flows’’. Appl. Opt. 1993. 32(30): 6090-6103. 14. M.S. Zahniser, D.D. Nelson, J.B. McManus, P.L. Kebabian, D. Lloyd. ‘‘Measurement of Trace Gas Fluxes Using Tunable DiodeLaser Spectroscopy’’. Philos. Trans. R. Soc., A. 1995. 351(1696): 371-382. 15. M.P. Arroyo, R.K. Hanson. ‘‘Absorption Measurements of WaterVapor Concentration, Temperature, and Line-Shape Parameters

APPLIED SPECTROSCOPY OA

1103

focal point review 16. 17.

18.

19. 20. 21. 22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

Using a Tunable InGaAsP Diode Laser’’. Appl. Opt. 1993. 32: 61046116. E.D. Hinkley. ‘‘High-Resolution Infrared Spectroscopy with a Tunable Diode Laser’’. Appl. Phys. Lett. 1970. 16(9): 351-354. P. Werle. ‘‘A Review of Recent Advances in Semiconductor Laser Based Gas Monitors’’. Spectrochim. Acta, Part A. 1998. 54(2): 197-236. P. Werle, F. Slemr, K. Maurer, R. Kormann, R. Mucke, B. Janker. ‘‘Near- and Mid-Infrared Laser-Optical Sensors for Gas Analysis’’. Opt. Lasers Eng. 2002. 37(2-3): 101-114. J.U. White. ‘‘Long Optical Paths of Large Aperture’’. J. Opt. Soc. Am. 1942. 32(5): 285-288. D.R. Herriott, H.J. Schulte. ‘‘Folded Optical Delay Lines’’. Appl. Opt. 1965. 4(8): 883-891. S.M. Chernin, E.G. Barskaya. ‘‘Optical Multipass Matrix Systems’’. Appl. Opt. 1991. 30(1): 51-58. J.B. McManus, P.L. Kebabian, M.S. Zahniser. ‘‘Astigmatic Mirror Multipass Absorption Cells for Long-Path-Length Spectroscopy’’. Appl. Opt. 1995. 34(18): 3336-3348. D. Masiyano, J. Hodgkinson, R.P. Tatam. ‘‘Gas Cells for Tunable Diode Laser Absorption Spectroscopy Employing Optical Diffusers. Part 2: Integrating Spheres’’. Appl. Phys. B: Lasers Opt. 2010. 100(2): 303-312. P. Werle, R. Mucke, F. Slemr. ‘‘The Limits of Signal Averaging in Atmospheric Trace Gas Monitoring by Tunable Diode Laser Absorption Spectroscopy’’. Appl. Phys. B: Lasers Opt. 1993. 57(2): 131-139. P. Werle, P. Mazzinghi, F.D. Amato, M. De Rosa, K. Maurer, F. Slemr. ‘‘Signal Processing and Calibration Procedures for In Situ Diode-Laser Absorption Spectroscopy’’. Spectrochim. Acta, Part A. 2004. 60(8-9): 1685-1705. R.S. Disselkamp, J.F. Kelly, R.L. Sams, G.A. Anderson. ‘‘Signal-toNoise Enhancement Techniques for Quantum Cascade Absorption Spectrometers Employing Optimal Filtering and Other Approaches’’. Appl. Phys. B: Lasers Opt. 2002. 75(2-3): 359-366. J.S. Li, B. Yu, W. Zhao, W. Chen. ‘‘A Review of Signal Enhancement and Noise Reduction Techniques for Tunable Diode Laser Absorption Spectroscopy’’. Appl. Spectrosc. Rev. 2014. 49(8): 666-691. P.G. Craven, S.A. Fairhurst, L.H. Sutcliffe. ‘‘A Simple Approach to Derivative Spectroscopy’’. Spectrochim. Acta, Part A. 1988. 44(5): 539-545. K.L. Shaklee, J.E. Rowe. ‘‘Wavelength Modulation Spectrometer for Studying the Optical Properties of Solids’’. Appl. Opt. 1970. 9(3): 627-632. C.Y. Fong, M.L. Cohen, R.R.L. Zucca, J. Stokes, Y.R. Shen. ‘‘Wavelength Modulation Spectrum of Copper’’. Phys. Rev. Lett. 1970. 25(21): 1486-1490. T.C. O’Haver. ‘‘Wavelength Modulation Spectroscopy’’. In: D.M. Hercules, G.M. Hieftje, L.R. Snyder, M.A. Evenson, editors. Contemporary Topics in Analytical and Clinical Chemistry, Vol. II. New York: Springer Press, 1978. Pp. 1-28. A. Farooq, J.B. Jeffries, R.K. Hanson. ‘‘Sensitive Detection of Temperature Behind Reflected Shock Waves Using Wavelength Modulation Spectroscopy of CO2 Near 2.7 lm’’. Appl. Phys. B: Lasers Opt. 2009. 96(1): 161-173. I.D. Lindsay, P. Gross, C.J. Lee, A. Adhimoolam, K.J. Boller. ‘‘MidInfrared Wavelength- and Frequency-Modulation Spectroscopy with a Pump-Modulated Singly-Resonant Optical Parametric Oscillator’’. Opt. Expr. 2006. 14(25): 12341-12346. X. Chao, J.B. Jeffries, R.K. Hanson. ‘‘Wavelength-Modulation Spectroscopy for Real-Time, In Situ NO Detection in Combustion Gases with a 5.2 lm Quantum Cascade Laser’’. Appl. Phys. B: Lasers Opt. 2012. 106(4): 987-997. J. Henningsen, H. Simonsen. ‘‘Quantitative Wavelength-Modulation Spectroscopy Without Certified Gas Mixtures’’. Appl. Phys. B: Lasers Opt. 2000. 70(4): 627-633. C.S. Goldenstein, C.L. Strand, I.A. Schultz, K. Sun, J.B. Jeffries, R.K. Hanson. ‘‘Fitting of Calibration-Free Scanned-WavelengthModulation Spectroscopy Spectra for Determination of Gas Properties and Absorption Lineshapes’’. Appl. Opt. 2014. 53(3): 356-367.

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37. Z.M. Peng, Y.J. Ding, L. Che, X.H. Li, K.J. Zheng. ‘‘Calibration-Free Wavelength Modulated TDLAS Under High Absorbance Conditions’’. Opt. Expr. 2011. 19(23): 23104-23110. 38. Y. Zakrevskyy, T. Ritschel, C. Dosche, H.-G. Lo¨hmannsro¨ben. ‘‘Quantitative Calibration- and Reference-Free Wavelength Modulation Spectroscopy’’. Infrared Phys. Technol. 2012. 55(2-3): 183-190. 39. S. Schilt, L. The´venaz, P. Robert. ‘‘Wavelength Modulation Spectroscopy: Combined Frequency and Intensity Laser Modulation’’. Appl. Opt. 2003. 42(33): 6728-6738. 40. F.S. Pavone, M. Inguscio. ‘‘Frequency- and Wavelength-Modulation Spectroscopies: Comparison of Experimental Methods Using an AlGaAs Diode Laser’’. Appl. Phys. B: Lasers Opt. 1993. 56(2): 118-122. 41. J.M. Supplee, E.A. Whittaker, W. Lenth. ‘‘Theoretical Description of Frequency Modulation and Wavelength Modulation Spectroscopy’’. Appl. Opt. 1994. 33(27): 6294-6302. 42. N.-Y. Chou, G.W. Sachse. ‘‘Single-Tone and Two-Tone AM–FM Spectral Calculations for Tunable Diode Laser Absorption Spectroscopy’’. Appl. Opt. 1987. 26(17): 3584-3587. 43. V.G. Avetisov, P. Kauranen. ‘‘Two-Tone Frequency-Modulation Spectroscopy for Quantitative Measurements of Gaseous Species: Theoretical, Numerical, and Experimental Investigation of Line Shapes’’. Appl. Opt. 1996. 35(24): 4705-4723. 44. C.S. Gudeman, M.H. Begemann, J. Pfaff, R.J. Saykally. ‘‘ToneBurst Modulated Color-Center-Laser Spectroscopy’’. Opt. Lett. 1983. 8(6): 310-312. 45. D.T. Cassidy, J. Reid. ‘‘Harmonic Detection with Tunable Diode Lasers: Two-Tone Modulation’’. Appl. Phys. B: Lasers Opt. 1982. 29(4): 279-285. 46. G.R. Janik, C.B. Carlisle, T.F. Gallagher. ‘‘Two-Tone FrequencyModulation Spectroscopy’’. J. Opt. Soc. Am. B. 1986. 3(8): 1070-1074. 47. D.E. Cooper, R.E. Warren. ‘‘Frequency Modulation Spectroscopy with Lead-Salt Diode Lasers: A Comparison of Single-Tone and Two-Tone Techniques’’. Appl. Opt. 1987. 26(17): 3726-3732. 48. P. Werle, F. Slemr, M. Gehrtz, C. Brduchle. ‘‘Quantum-Limited FMSpectroscopy with a Lead-Salt Diode Laser: A Comparison of Theoretical and Experimental Data’’. Appl. Phys. B: Lasers Opt. 1989. 49(2): 99-108. 49. C.B. Carlisle, D.E. Cooper, H. Prier. ‘‘Quantum Noise-Limited FM Spectroscopy with a Lead-Salt Diode Laser’’. Appl. Opt. 1989. 28(13): 2567-2576. 50. G. Gagliardi, F. Tamassia, P. De Natale, C. Gmachl, F. Capasso, D.L. Sivco, J.N. Baillargeon, A.L. Hutchinson, A.Y. Cho. ‘‘Sensitive Detection of Methane and Nitrous Oxide Isotopomers Using a Continuous Wave Quantum Cascade Laser’’. Eur. Phys. J. D. 2002. 19(3): 327-331. 51. G. Gagliardi, S. Borri, F. Tamassia, F. Capasso, C. Gmachl, D.L. Sivco, J.N. Baillargeon, A.L. Hutchinson, A.Y. Cho. ‘‘A FrequencyModulated Quantum-Cascade Laser for Spectroscopy of CH4 and N2O Isotopomers’’. Isot. Environ. Health Stud. 2005. 41(4): 313-321. 52. S. Borri, S. Bartalini, P. De Natale, M. Inguscio, C. Gmachl, F. Capasso, D.L. Sivco, A.Y. Cho. ‘‘Frequency Modulation Spectroscopy by Means of Quantum-Cascade Lasers’’. Appl. Phys. B: Lasers Opt. 2006. 85(2-3): 223-229. 53. K.S. Song, E.C. Jung. ‘‘Recent Developments in Modulation Spectroscopy for trace Gas Detection Using Tunable Diode Lasers’’. Appl. Spectrosc. Rev. 2003. 38(4): 395-432. 54. G. Litfin, C.R. Pollock, R.F. Curl, F.K. Tittel. ‘‘Sensitivity Enhancement of Laser-Absorption Spectroscopy by Magnetic Rotation Effect’’. J. Chem. Phys. 1980. 72: 6602-6605. 55. P. Werle, S. Lechner. ‘‘Stark-Modulation-Enhanced FM-Spectroscopy’’. Spectrochim. Acta, Part A. 1999. 55(10): 1941-1955. 56. H. Sasada. ‘‘Stark-Modulation Spectroscopy of NH3 with a 1.23-lm Semiconductor Laser’’. Opt. Lett. 1984. 9(10): 448-450. 57. J.M. Smith, J.C. Bloch, R.W. Field, J.L. Steinfeld. ‘‘Trace Detection of NO2 by Frequency-Modulation-Enhanced Magnetic Rotation Spectroscopy’’. J. Opt. Soc. Am. B. 1995. 12(6): 964-969. 58. R.J. Brecha, L.M. Pedrotti, D. Krause. ‘‘Magnetic Rotation Spectroscopy of Molecular Oxygen with a Diode Laser’’. J. Opt. Soc. Am. B. 1997. 14(8): 1921-1930.

59. H. Sabana, T. Fritsch, M.B. Onana, O. Bouba, P. Hering, M. Mu¨rtz. ‘‘Simultaneous Detection of 14NO and 15NO Using Faraday Modulation Spectroscopy’’. Appl. Phys. B: Lasers Opt. 2009. 96: 535-544. 60. P. Kluczynski, S. Lundqvist, J. Westberg, O. Axner. ‘‘Faraday Rotation Spectrometer with Sub-Second Response Time for Detection of Nitric Oxide Using a CW DFB Quantum Cascade Laser at 5.33 lm’’. Appl. Phys. B: Lasers Opt. 2011. 103(2): 451-459. 61. S.G. So, E. Jeng, G. Wysocki. ‘‘VCSEL Based Faraday Rotation Spectroscopy with a Modulated and Static Magnetic Field for Trace Molecular Oxygen Detection’’. Appl. Phys. B: Lasers Opt. 2011. 102(2): 279-291. 62. W. Zhao, G. Wysocki, W. Chen, E. Fertein, D. Le Coq, D. Petitprez, W. Zhang. ‘‘Sensitive and Selective Detection of OH Radicals Using Faraday Rotation Spectroscopy at 2.8 lm’’. Opt. Expr. 2011. 19(3): 2493-2501. 63. Y. Wang, M. Nikodem, G. Wysocki. ‘‘Cryogen-Free HeterodyneEnhanced Mid-Infrared Faraday Rotation Spectrometer’’. Opt. Expr. 2013. 21(1): 740-755. 64. A. O’Keefe, D.A.G. Deacon. ‘‘Cavity Ringdown Optical Spectrometer for Absorption Measurements Using Pulsed Laser Sources’’. Rev. Sci. Instrum. 1988. 59(12): 2544-51. 65. A. O’Keefe. ‘‘Integrated Cavity Output Analysis of Ultra-Weak Absorption’’. Chem. Phys. Lett. 1998. 293(5-6): 331-336. 66. R. Engeln, G. Berden, R. Peeters, G. Meijer. ‘‘Cavity Enhanced Absorption and Cavity Enhanced Magnetic Rotation Spectroscopy’’. Rev. Sci. Instrum. 1998. 69(11): 3763-3769. 67. J. Ye, L.-S. Ma, J.L. Hall. ‘‘Ultrasensitive Detections in Atomic and Molecular Physics: Demonstration in Molecular Overtone Spectroscopy’’. J. Opt. Soc. Am. B. 1998. 15(1): 6-15. 68. G. Berden, R. Peeters, G. Meijer. ‘‘Cavity Ring-Down Spectroscopy: Experimental Schemes and Applications’’. Int. Rev. Phys. Chem. 2000. 19(4): 565-607. 69. K.K. Lehmann, G. Berden, R. Engeln. ‘‘An Introduction to Cavity Ringdown Spectroscopy’’. In: G. Berden, R. Engeln, editors. Cavity Ringdown Spectroscopy Techniques and Applications. Chichester, UK: John Wiley and Sons, 2009. Pp. 1-26. 70. P. Maddaloni, M. Bellini, P. De Natale. Laser-Based Measurements for Time and Frequency Domain Applications: A Handbook. Boca Raton, FL: CRC Press/Taylor and Francis, 2013. Pp. 1-764. 71. A.G. Bell. ‘‘On the Production and Reproduction of Sound by Light: The Photophone’’. Am. J. Sci. 1880. 20(118): 305-324. 72. R.S. Quimby, P.M. Selzer, W.M. Yen. ‘‘Photoacoustic Cell Design: Resonant Enhancement and Background Signals’’. Appl. Opt. 1977. 16(10): 2630-2632. 73. O. Nordhaus, J. Pelzl. ‘‘Frequency Dependence of Resonant Photoacoustic Cells: The Extended Helmoltz Resonator’’. Appl. Phys. 1981. 25(3): 221-229. 74. S. Barbieri, J.P. Pellaux, E. Studemann, D. Rosset. ‘‘Gas Detection with Quantum Cascade Lasers: An Adapted Photoacoustic Sensor Based on Helmholtz Resonance’’. Rev. Sci. Instrum. 2002. 73(6): 2458-2461. 75. J.S. Li, X.M. Gao, L. Fang, K. Liu, W.J. Zhang, H. Cha. ‘‘Resonant Photoacoustic Detection of Trace Gas with DFB Diode Laser’’. Opt. Laser Tech. 2007. 39(6): 1144-1149. 76. J.M. Rey, M.W. Sigrist. ‘‘Simultaneous Dual-Frequency Excitation of a Resonant Photoacoustic Cell’’. Infrared Phys. Tech. 2008. 51(6): 516-519. 77. S. Schilt, L. The´venaz. ‘‘Wavelength Modulation Photoacoustic Spectroscopy: Theoretical Description and Experimental Results’’. Infrared Phys. Tech. 2006. 48(2): 154-162. 78. J.S. Li, X.M. Gao, W.Z. Li, Z.S. Cao, L.H. Deng, W.X. Zhao, M.Q. Huang, W.J. Zhang. ‘‘Near-Infrared Diode Laser Wavelength Modulation-Based Photoacoustic Spectrometer’’. Spectrochim. Acta, Part A. 2006. 64(2): 338-342. 79. J. Li, K. Liu, W. Zhang, W. Chen, X. Gao. ‘‘Carbon Dioxide Detection Using NIR Diode Laser Based Wavelength Modulation Photoacoustic Spectroscopy’’. Opt. Appl. 2008. 38(2): 341-352. 80. J. Saarela, J. Toivonen, A. Manninen, T. Sorvaja¨rvi, R. Hernberg. ‘‘Wavelength Modulation Waveforms in Laser Photoacoustic Spectroscopy’’. Appl. Optic. 2009. 48(4): 743-747.

81. J.M. Rey, D. Marinov, D.E. Vogler, M.W. Sigrist. ‘‘Investigation and Optimization of a Multipass Resonant Photoacoustic Cell at High Absorption Levels’’. Appl. Phys. B: Lasers Opt. 2005. 80(2): 261-266. 82. M. Na¨gele, M.W. Sigrist. ‘‘Mobile Laser Spectrometer with Novel Resonant Multipass Photoacoustic Cell for Trace-Gas Sensing’’. Appl. Phys. B: Lasers Opt. 2000. 70(6): 895-901. 83. A. Rossi, R. Buffa, M. Scotoni, D. Bassi, S. Iannotta, A. Boschetti. ‘‘Optical Enhancement of Diode Laser-Photoacoustic Trace Gas Detection by Means of External Fabry-Perot Cavity’’. Appl. Phys. Lett. 2005. 87(4): 041110-3. 84. M. Hippler, C. Mohr, K.A. Keen, E.D. McNaghten. ‘‘CavityEnhanced Resonant Photoacoustic Spectroscopy with Optical Feedback CW Diode Lasers: A Novel Technique for Ultratrace Gas Analysis and High-Resolution Spectroscopy’’. J. Chem. Phys. 2010. 133(4): 044308-8. 85. A.A. Kosterev, Y.A. Bakhirkin, R.F. Curl, F.K. Tittel. ‘‘QuartzEnhanced Photoacoustic Spectroscopy’’. Opt. Lett. 2002. 27(21): 1902-1904. 86. A.A. Kosterev, F.K. Tittel, D.V. Serebryakov, A.L. Malinovsky, I.V. Morozov. ‘‘Applications of Quartz Tuning Forks in Spectroscopic Gas Sensing’’. Rev. Sci. Instrum. 2005. 76(4): 043105-9. 87. F.K. Tittel, G. Wysocki, A. Kosterev, Y. Bakhirkin. ‘‘Semiconductor Laser Based Trace Gas Sensor Technology: Recent Advances and Applications’’. In: M. Ebrahim-Zadeh, I.T. Sorokina, editors. MidInfrared Coherent Sources and Applications. NATO Advanced Research Workshop on Mid-Infrared Coherent Sources. Barcelona, Spain: 2005. Pp. 467-493. 88. R.F. Curl, F. Capasso, C. Gmachl, A.A. Kosterev, B. McManus, R. Lewicki, M. Pusharsky, G. Wysocki, F.K. Tittel. ‘‘Quantum Cascade Lasers in Chemical Physics’’. Chem. Phys. Lett. 2010. 487(1-3): 1-18. 89. K. Wilcken, J. Kauppinen. ‘‘Optimization of a Microphone for Photoacoustic Spectroscopy’’. Appl. Spectrosc. 2003. 57(9): 1087-1092. 90. J. Kauppinen, K. Wilcken, I. Kauppinen, V. Koskinen. ‘‘High Sensitivity in Gas Analysis with Photoacoustic Detection’’. Microchem. J. 2004. 76(1-2): 151-159. 91. T. Laurila, H. Cattaneo, V. Koskinen, J. Kauppinen, R. Hernberg. ‘‘Diode Laser-Based Photoacoustic Spectroscopy with Interferometrically Enhanced Cantilever Detection’’. Opt. Expr. 2005. 13(7): 2453-2458. 92. V. Koskinen, J. Fonsen, K. Roth, J. Kauppinen. ‘‘Cantilever Enhanced Photoacoustic Detection of Carbon Dioxide Using a Tunable Diode Laser Source’’. Appl. Phys. B: Lasers Opt. 2007. 86(3): 451-454. 93. J. Lu, T. Ikehara, T. Kobayashi, R. Maeda, T. Mihara. ‘‘Quality Factor of Micro Cantilevers Transduced by Piezoelectric Lead Zirconate Titanate Film’’. Microsyst. Technol. 2007. 13(11-12): 1517-1522. 94. V. Koskinen, J. Fonsen, K. Roth, J. Kauppinen. ‘‘Progress in Cantilever Enhanced Photoacoustic Spectroscopy’’. Vib. Spectrosc. 2008. 48(1): 16-21. 95. G.A. West, J.J. Barrett, D.R. Siebert, K.V. Reddy. ‘‘Photoacoustic Spectroscopy’’. Rev. Sci. Instrum. 1983. 54: 797-817. 96. A. Miklo´s, P. Hess, Z. Bozo´ki. ‘‘Application of Acoustic Resonators in Photoacoustic Trace Gas Analysis and Metrology’’. Rev. Sci. Instrum. 2001. 72(4): 1937-1955. 97. T. Schmid. ‘‘Photoacoustic Spectroscopy for Process Analysis’’. Anal. Bioanal. Chem. 2006. 384(5): 1071-1086. 98. J.S. Li, W.D. Chen, B.L. Yu. ‘‘Recent Progress on Infrared Photoacoustic Spectroscopy Techniques’’. Appl. Spectrosc. Rev. 2011. 46(6): 440-471. 99. L. Joly, B. Parvitte, V. Ze´ninari, D. Courtois, G. Durry. ‘‘A Spectroscopic Study of Water Vapor Isotopologues H216O, H218O, and HDO Using a Continuous Wave DFB Quantum Cascade Laser in the 6.7 lm Region for Atmospheric Applications’’. J. Quant. Spectrosc. Radiat. Transf. 2006. 102(2): 129-138. 100. L. Joly, V. Ze´ninari, B. Parvitte, D. Weidmann, D. Courtois, Y. Bonetti, T. Aellen, M. Beck, J. Faist, D. Hofstetter. ‘‘Spectroscopic Study of the m1 Band of SO2 Using a Continuous-Wave DFB QCL at 9.1 lm’’. Appl. Phys. B: Lasers Opt. 2003. 77(6-7): 703-706.

APPLIED SPECTROSCOPY OA

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focal point review 101. V. Ze´ninari, L. Joly, B. Grouiez, B. Parvitte, A. Barbe. ‘‘Study of SO2 Line Parameters with a Quantum Cascade Laser Spectrometer Around 1090 cm-1: Comparison with Calculations of the m1 and m1 þ m2–m2 Bands of 32SO2 and the m1 Band of 34SO2’’. J. Quant. Spectrosc. Radiat. Transf. 2007. 105(2): 312-325. 102. A. Grossel, V. Ze´ninari, B. Parvitte, L. Joly, D. Courtois, G. Durry. ‘‘Quantum Cascade Laser Spectroscopy of N2O in the 7.9 lm Region for the In Situ Monitoring of the Atmosphere’’. J. Quant. Spectrosc. Radiat. Transf. 2008. 109(10): 1845-1855. 103. B.E. Brumfield, J.T. Stewart, B.J. McCall. ‘‘High-Resolution Spectroscopy of the m8 Band of Methylene Bromide Using a Quantum Cascade Laser’’. J. Mol. Spectrosc. 2011. 266(1): 57-62. 104. J.T. Stewart, B.J. McCall. ‘‘Additional Bands of the Ar-D2O Intramolecular Bending Mode Observed Using a Quantum Cascade Laser’’. J. Mol. Spectrosc. 2012. 282: 34-38. 105. J.F. Kelly, A. Maki, T.A. Blake, R.L. Sams. ‘‘Supersonic Free-Jet Quantum Cascade Laser Measurements of m4 for CF335Cl and CF337Cl and FTS Measurements from 400 to 1260 cm-1’’. J. Mol. Spectrosc. 2008. 252(1): 81-89. 106. S. Miyano, K. Tonokura. ‘‘Measurements of Nitrogen-Broadening Coefficients in the v3 Band of the Hydroperoxyl Radical Using a Continuous Wave Quantum Cascade Laser’’. J. Mol. Spectrosc. 2011. 265(1): 47-51. 107. Y. Sakamoto, K. Tonokura. ‘‘Measurements of the Absorption Line Strength of Hydroperoxyl Radical in the m3 Band Using a Continuous Wave Quantum Cascade Laser’’. J. Phys. Chem. A. 2012. 116(1): 215-222. 108. L. Wang, T.R. Sharples. ‘‘Intrapulse Quantum Cascade Laser Spectroscopy: Pressure Induced Line Broadening and Shifting in the m6 Band of Formaldehyde’’. Appl. Phys. B: Lasers Opt. 2012. 108(2): 427-435. 109. B.H. Lee, E.C. Wood, J. Wormhoudt, J.H. Shorter, S.C. Herndon, M.S. Zahniser, J.W. Munger. ‘‘Effective Line Strengths of transNitrous Acid Near 1275 cm-1 and cis-Nitrous Acid at 1660 cm-1’’. J. Quant. Spectrosc. Radiat. Transf. 2012. 113(15): 1905-1912. 110. K. Owen, E. Essebbar, A. Farooq. ‘‘Measurements of NH3 Linestrengths and Collisional Broadening Coefficients in N2, O2, CO2, and H2O Near 1103.46 cm-1’’. J. Quant. Spectrosc. Radiat. Transf. 2013. 121: 56-68. 111. H.-W. Hu¨bers, R. Eichholz, S.G. Pavlov, H. Richter. ‘‘High Resolution Terahertz Spectroscopy with Quantum Cascade Lasers’’. J. Infrared, Millimeter, Terahertz Waves. 2013. 34(5-6): 325-341. 112. X. Chao, J.B. Jeffries, R.K. Hanson. ‘‘In Situ Absorption Sensor for NO in Combustion Gases with a 5.2 lm Quantum-Cascade Laser’’. Proc. of the Combustion Institute. 2011. 33: 725-733. 113. X. Chao, J.B. Jeffries, R.K. Hanson. ‘‘Wavelength-ModulationSpectroscopy for Real-Time, In Situ NO Detection in Combustion Gases with a 5.2 lm Quantum-Cascade Laser’’. Appl. Phys. B. 2012. 106(4): 987-997. 114. J. Vanderover, M.A. Oehlschlaeger. ‘‘A Mid-Infrared ScannedWavelength Laser Absorption Sensor for Carbon Monoxide and Temperature Measurements from 900 to 4000 K’’. Appl. Phys. B: Lasers Opt. 2010. 99(1-2): 353-362. 115. J. Vanderover, W. Wang, M.A. Oehlschlaeger. ‘‘A Carbon Monoxide and Thermometry Sensor Based on Mid-IR QuantumCascade Laser Wavelength-Modulation Absorption Spectroscopy’’. Appl. Phys. B: Lasers Opt. 2011. 103(4): 959-966. 116. M. Uddi, A.K. Das, C.-J. Sung. ‘‘Temperature Measurements in a Rapid Compression Machine Using Mid-Infrared H2O Absorption Spectroscopy Near 7.6 lm’’. Appl. Opt. 2012. 51(22): 5464-5476. 117. A.K. Das, M. Uddi, C.-J. Sung. ‘‘Two-Line Thermometry and H2O Measurement for Reactive Mixtures in Rapid Compression Machine Near 7.6 lm’’. Combust. Flame. 2012. 159(12): 3493-3501. 118. M. Hu¨bner, D. Marinov, O. Guaitella, A. Rousseau, J. Ro¨pcke. ‘‘On Time Resolved Gas Temperature Measurements in a Pulsed DC Plasma Using Quantum Cascade Laser Absorption Spectroscopy’’. Meas. Sci. Technol. 2012. 23(11): 115602-7. doi:10.1088/ 0957-0233/23/11/115602. 119. J. Ro¨pcke, G. Lombardi, A. Rousseau, P.B. Davies. ‘‘Application of Mid-Infrared Tuneable Diode Laser Absorption Spectroscopy to

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135.

136.

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Plasma Diagnostics: A Review’’. Plasma Sources Sci. Technol. 2006. 15(4): S148-S168. S. Welzel, F. Hempel, M. Hu¨bner, N. Lang, P.B. Davies, J. Ro¨pcke. ‘‘Quantum Cascade Laser Absorption Spectroscopy as a Plasma Diagnostic Tool: An Overview’’. Sensors. 2010. 10(7): 6861-6900. N. Lang, J. Ro¨pcke, H. Zimmermann, A. Steinbach, S. Wege. ‘‘In Situ Monitoring of Plasma Etch Processes with a Quantum Cascade Laser Arrangement in Semiconductor Industrial Environment’’. J. Phys.: Conf. Ser. 2009. 157(1): 012007-8. N. Lang, J. Ro¨pcke, S. Wege, A. Steinbach. ‘‘In Situ Diagnostic of Etch Plasmas for Process Control Using Quantum Cascade Laser Absorption Spectroscopy’’. Eur. Phys. J.: Appl. Phys. 2010. 49(1): 13110-3. L. Mahler, A. Tredicucci, M.S. Vitiello. ‘‘Quantum Cascade Laser: A Compact, Low Cost, Solid-State Source for Plasma Diagnostics’’. J. Instrum. 2012. 7: C02018-10. T. Yumii, N. Kimura, S. Hamaguchi. ‘‘Quantum Cascade Laser Absorption Spectroscopy with the Amplitude-To-Time Conversion Technique for Atmospheric-Pressure Plasmas’’. J. Appl. Phys. 2013. 113(21): 213101-4. T. Yumii, T. Yoshida, K. Doi, N. Kimura, S. Hamaguchi. ‘‘Oxidation of Nitric Oxide by Atmospheric Pressure Plasma in a Resonant Plasma Reactor’’. J. Phys. D. 2013. 46(13): 135202-7. J. Kottmann, J.M. Rey, J. Luginbu¨hl, E. Reichmann, M.W. Sigrist. ‘‘Glucose Sensing in Human Epidermis Using Mid-Infrared Photoacoustic Detection’’. Biome. Opt. Expr. 2012. 3(4): 667-680. J. Kottmann, U. Grob, J.M. Rey, M.W. Sigrist. ‘‘Mid-Infrared FiberCoupled Photoacoustic Sensor for Biomedical Applications’’. Sensors. 2013. 13(1): 535-549. S. Cristescu, J. Mandon, D. Arslanov, J. De Pessemier, C. Hermans, F.J.M. Harren. ‘‘Current Methods for Detecting Ethylene in Plants’’. Annals of Botany. 2013. 111(3): 347-360. S.M. Cristescu, D. Marchenko, J. Mandon, K. Hebelstrup, G.W. Griffith, L.A.J. Mur, F.J.M. Harren. ‘‘Spectroscopic Monitoring of NO Traces in Plants and Human Breath: Applications and Perspectives’’. Appl. Phys. B: Lasers Opt. 2013. 110(2): 203-211. D. Marchenko, J. Mandon, S.M. Cristescu, P.J.F.M. Merkus, F.J.M. Harren. ‘‘Quantum Cascade Laser-Based Sensor for Detection of Exhaled and Biogenic Nitric Oxide’’. Appl. Phys. B: Lasers Opt. 2013. 111(3): 359-365. S.M. Cristescu, J. Mandon, F.J. Harren, P. Merila¨inen, M. Ho¨gman. ‘‘Methods of NO Detection in Exhaled Breath’’. J Breath Res. 2013. 7(1): 017104-11. S.M. Cristescu, J. Mandon, D. Arslanov, J. De Pessemier, C. Hermans, F.J.M. Harren. ‘‘Current Methods for Detecting Ethylene in Plants’’. Ann. Bot. 2013. 111(3): 347-360. B.W.M. Moeskops, H. Naus, S.M. Cristescu, F.J.M. Harren. ‘‘Quantum Cascade Laser-Based Carbon Monoxide Detection on a Second Time Scale from Human Breath’’. Appl. Phys. B: Lasers Opt. 2006. 82(4): 649-654. J. Mandon, M. Ho¨gman, P.J.F.M. Merkus, J. van Amsterdam, F.J.M. Harren, S.M. Cristescu. ‘‘Exhaled Nitric Oxide Monitoring by Quantum Cascade Laser: Comparison with Chemiluminescent And Electrochemical Sensors’’. J. Biomed. Opt. 2012. 17(1): 017003-7. T.H. Risby, F.K. Tittel. ‘‘Current Status of Mid-Infrared Quantum and Interband Cascade Lasers for Clinical Breath Analysis’’. Opt. Eng. 2010. 49(11): 111123. doi: 10.1117/1.3498768. M.R. McCurdy, Y. Bakhirkin, G. Wysocki, R. Lewicki, F.K. Tittel. ‘‘Recent Advances of Laser-Spectroscopy-Based Techniques for Applications in Breath Analysis’’. J. Breath Res. 2007. 1(1): 014001. R. Lewicki, A.A. Kosterev, D.M. Thomazy, T.H. Risby, S. Solga, T.B. Schwartz, F.K. Tittel. ‘‘Real Time Ammonia Detection in Exhaled Human Breath Using a Distributed Feedback Quantum Cascade Laser Based Sensor’’. Proc. of SPIE. Conference on Quantum Sensing and Nanophotonic Devices VIII. San Francisco, CA: 2011. January 23–27, 2011. 7945: 79450K. S. Solga, T. Schwartz, M. Mudalel, L. Spacek, R. Lewicki, F.K. Tittel, C. Loccioni, T. Risby. ‘‘Factors Influencing Breath Ammonia Determination’’. J. Breath Res. 2013. 7(3): 037101-6. S. Svanberg. ‘‘Medical Applications of Laser Spectroscopy’’. Phys. Scr. 1989. T26: 90-98.

140. T.H. Risby, S.F. Solga. ‘‘Current Status of Clinical Breath Analysis’’. Appl. Phys. B: Lasers Opt. 2006. 85(2-3): 421-426. 141. M. Mu¨rtz, P. Hering. ‘‘Online Monitoring of Exhaled Breath Using Mid-Infrared Laser Spectroscopy’’. In: M. Ebrahim-Zadeh, I.T. Sorokina, editors. Mid-Infrared Coherent Sources and Applications. NATO Science for Peace and Security Series. Dordrecht: Springer, 2008. Pp. 535-555. 142. C. Wang, P. Sahay. ‘‘Breath Analysis Using Laser Spectroscopic Techniques: Breath Biomarkers, Spectral Fingerprints, and Detection Limits’’. Sensors. 2009. 9(10): 8230-8262. 143. K.M. Paschke, A. Mashir, R.A. Dweik. ‘‘Clinical Applications of Breath Testing’’. F1000 Medicine Reports. 2010. 2: 1-6. 144. J. Wojtas, Z. Bielecki, T. Stacewicz, J. Mikołajczyk, M. Nowakowski. ‘‘Ultrasensitive Laser Spectroscopy for Breath Analysis’’. Opto-Electronics Review. 2012. 20(1): 26-39. 145. L. Ciaffoni, R. Peverall, G.A.D. Ritchie. ‘‘Laser Spectroscopy on Volatile Sulfur Compounds: Possibilities for Breath Analysis’’. J. Breath Res. 2011. 5(2): 024002-12.

146. M. Sowa, M. Mu¨rtz, P. Hering. ‘‘Mid-Infrared Laser Spectroscopy for Online Analysis of Exhaled CO’’. J. Breath Res. 2010. 4(4): 047101-6. 147. J.A. Nwaboh, S. Persijn, K. Heinrich, M. Sowa, P. Hering, O. Werhahn. ‘‘QCLAS and CRDS-Based CO Quantification as Aimed at in Breath Measurements’’. Int. J. Spectrosc. 2012. 2012: 89484110. 148. T. Rubin, T. von Haimberger, A. Helmke, K. Heyne. ‘‘Quantitative Determination of Metabolization Dynamics by a Real-Time 13CO2 Breath Test’’. J. Breath Res. 2011. 5(2): 027102-6. 149. K. Wo¨rle, F. Seichter, A. Wilk, C. Armacost, T. Day, M. Godejohann, U. Wachter, J. Vogt, P. Radermacher, B. Mizaikoff. ‘‘Breath Analysis with Broadly Tunable Quantum Cascade Lasers’’. Anal. Chem. 2013. 85(5): 2697-2702. 150. L. Ciaffoni, G. Hancock, J.J. Harrison, J.P.H. van Helden, C.E. Langley, R. Peverall, G.A.D. Ritchie, S. Wood. ‘‘Demonstration of a Mid-Infrared Cavity Enhanced Absorption Spectrometer for Breath Acetone Detection’’. Anal. Chem. 2013. 85(2): 846-850.

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