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An improved approach has been developed for ultra-sensitive detection of the concentration of NO using Faraday Modulation spectrometry (FAMOS) combined with the strong electronic transition. By changing the modulating magnetic field attributing to linear absorption and refraction of gas sample, the polarized laser was rotated and absorbed by the complex refraction index of NO. We confirm the relation between the magnitudes of absorption and the optimum modulation magnetic field. Also, the accuracy and the precision of the technique have been evaluated at different pressures. It is shown that the system is capable of detecting NO concentration down to 0.34 ppb·m.
©2011 Optical Society of America
1. Introduction
Nitric oxide (NO) is an environmentally hazardous, reactive, and strongly toxic pollutant that contributes to ground-level ozone, causes smog and acid rain, and gives rise to direct health effects [1,2]. It is mainly anthropogenically produced by, for example, vehicle engines, power and incineration plants, and cigarette smoke [3]. On the other hand, NO is also an important intermediate in the chemical industry and an essential signaling molecule that participates in many physiological and pathological processes within the mammalian body [4]. All this makes it of importance to monitor and assess the presence of NO under various conditions. This requires, first of all, access to a detection technique with sufficient sensitivity. In addition, because gas samples from industrial processes, as well as from biological sources, e.g., combustive emissions and exhaled human breath, usually contain large amounts of water and carbon dioxide that often interfere with the detection of other constituents. This calls for a technique that allows for in situ detection of NO with a detectability in the low (or preferably also sub-) ppb·m range in the presence of significantly higher concentrations (often percentages) of concomitant species.
The NO molecule has its strongest vibrational absorption band in the mid-IR part of the spectrum at 5.2μm, whereas its first and second overtones appear in the IR region at approximately 2.65 and 1.8μm, respectively. Detection of NO at the two overtone bands therefore suffers from weak absorption cross-sections. This is manifested by the rather moderate detection limits that have been reported for detection of NO by TDLAS on the first and second overtones. For example, NO has been detected on its second overtone band by Sonnenfroh and Allen using an InGaAsP/InP DFB laser [5]. Interacting with a line strength of 3.7 × 10−4 cm−2/atm, the authors reported a detectability of NO in flames of 140 ppm·m. A complementary high-resolution study of NO in this wavelength region was performed by Mihalcea et al. with a line strength of 1.3 × 10−3 cm−2/atm [6]. Although they utilized a multipass cell with an effective length of 33m, they reported a detection limit of only 20 ppm. Detection of NO on its first overtone band (2.65μm) has been performed by Oh and Stanton using an AlGaAsSb/InGaAsSb laser [7]. The authors estimated the detectability of NO in air to be 30 ppm for an interaction path length of 0.5m with a line strength of 1.0 × 10−2 cm−2/atm. In recent years, there are also a few demonstrations of detection of NO on its fundamental vibrational band, using both lead salt lasers and quantum cascade (QC) lasers [8–12]. In addition, Sonnenfroh et al., who worked with the fundamental vibrational absorption band at approximately 5.4μm, reported the detection limit (0.5 ppm·m for a S/N≈1) was just barely in the sub-ppm·m region [8]. Multipass detection techniques have so far only occasionally been used for detection of NO. One example is the work by Nelson et al., who utilized a multipass Herriott cell together with a QC laser working at approximately 5.26μm to demonstrate the applicability for sensitive detection of NO, which is significantly better than what the corresponding nonextractive techniques have been able to achieve [9]. However, since multipass detection techniques require gas extraction, they do not have the same flexibility and versatility as conventional single pass TDLAS detection techniques and cannot be used for in situ assessments. In addition, they give rise to a longer response time and do not necessarily work well under conditions with significant amounts of particulates in the gas.
The most commonly used non-extractive technique for in situ detection of molecular species in gas phase is based upon diode laser absorption spectrometry and referred to as Tunable Diode Laser Absorption Spectrometry (TDLAS) [13–16]. It incorporates most often Wavelength Modulation Spectrometry (WMS) for reduction of noise [17,18]. Although this technique is versatile and regularly used for detection of a variety of species [19–22], the detectability of the technique is not sufficient for in situ detection of NO in ppb concentrations, primarily caused by a combination of low transition probabilities of the most commonly accessed overtone band in the infrared (IR) region, a lack of diode lasers producing light in the wavelength regions of these bands, and significant spectroscopic interferences, primarily with water [23].
Based upon the fundamental fact that the linestrengths of electronic transitions in molecules are in general significantly larger than those of various types of vibrational transitions in the IR region [24,25], the present work constitutes a first evaluation of the performance of a prototype instrumentation for TDLAS with Faraday Modulation Spectroscopy (FAMOS) technique at the strongest electronic transition in the X2Π(υ” = 0)-A2Σ+(υ’ = 0) band of NO at ~226.8 nm. The transition probabilities for the electronic transitions are in general not only several orders of magnitude larger than those of the various vibrational overtones that are mostly used today, they are also significantly larger than those of the fundamental vibration. Electronic transitions therefore give rise to significantly stronger absorption signals whenever they can be used. In addition, they lie in wavelength regions less affected by spectral interferences than vibrational transitions. The increased transition strengths and free interferences therefore make absorption on electronic transitions an interesting alternative for sensitive detection of NO for practical application.
While FAMOS is another technique capable of enhancing the sensitivity of laser absorption spectroscopy by taking advantage of the fact that a transition in a paramagnetic molecule can alter the polarization state of incident linearly polarized light in the presence of a magnetic field [26–29]. The magnetic field breaks the magnetic degeneracy of the rotational states (Zeeman Effect). The resulting frequency shift of transitions is different for left-handed and right-handed circularly polarized light, giving rise to different refractive indices for these polarization components at a given radiation wavelength (circular birefringence). As a light beam, originally linearly polarized, propagates through the sample, this anisotropy leads to a rotation of the polarization axis. This magnetically induced birefringence in a longitudinal field and the related rotation of the polarization axis of linearly polarized light is called Faraday Effect. The rotation is detected by means of putting the sample between nearly crossed polarizers. In this way, laser amplitude noise is largely suppressed. Employing a static magnetic field B > 0 in combination with a tunable laser, the sensitivity of direct absorption spectroscopy can be improved by 2–3 orders of magnitude. This effectively avoids problems with background absorption from spectrally interfering diamagnetic compounds. In particular, gas samples obtained from biological sources, e.g., exhaled human breath, usually contain large amounts of water and carbon dioxide. Therefore, approaches using absorption spectroscopy often require extra effort to avoid interferences from water and other compounds. Thus, the outstanding sensitivity and specificity make Faraday modulation spectroscopy a most attractive method. In this paper, we demonstrated the alternative detection methodology for in situ detection of NO at sub-ppb range.
2. Theory
In Faraday modulation spectrometry (FAMOS), the presence of a an absorber will give rise to at least three effects affecting the total signal; ordinary absorption of light, circular dichroism, that converts the linearly polarized light to elliptically polarized, and circular birefringence, that rotates the plane of polarization. The Faraday rotation technique is based upon the latter of these. However, it is feasible that the measured signal is affect also by the other effects [30–32]. The signal is proportional to the birefringence of the sample medium. Thus the signal at a given frequency υ is given by the sum of the signals for all the different M transitions that make contributions, i.e.,
where M′ and M″ are M quantum numbers for the upper and lower states, respectively. For FAMOS configuration, p gives the selection rule on M: is the dispersion function for an individual M transition centered at a frequency is given bywhere χ(υ,,Γ) is line shape function, Γ is the linewidth, and the exponential term is the Boltzmann population factor, is the linestrength factor, given bywhere is integrated line intensity of the zero field transition. The line shape appropriate for this birefringence experiment is governed by the dispersion relation, or more precisely, by the frequency dependence of the real part of the complex refractive index. In case where pressure and Doppler contributions are relevant, the resulting line shape is [22]where NV is the scaling factor. The frequency of an individual M transition is given byυ0 is the zero field transition frequency, g′and g″ are factors for the upper and lower states in the transition, μ0 is the Bohr magnetron, and h is the Planck’s constant.3. Simulation
Using the simulation program, the transition frequencies, the Einstein coefficients, and the line strengths for all possible transitions in the 226.55-226.60 nm within the γ (0, 0) band region were calculated. Figure 1 shows the spectrum in the case when the dominating broadening mechanism is Doppler broadened. The transitions in the middle of the scan are the (partly) overlapping Q22 (10.5) and QR12 (10.5) transitions at 44135.22 cm−1 that was used for the present study.
Fig. 1 Simulation of an absorption spectrum in the 226.55 – 226.61 nm region of the γ(0,0) band of NO
The laser supplied to us produced radiation around 226.58 nm, near the overlapped Q22(10.5) and QR12(10.5) transition of NO in X2Π(υ” = 0)-A2Σ+(υ’ = 0) band. Faraday modulation signals associated with this transition has 42 contributions with ΔM = ± 1 to the signal for the transitions. For X2Π3/2(υ” = 0)-A2Σ+(υ’ = 0), the excited state is the A2Σ+ state, because of the lack of ρ type doubling (i.e., no displacement of satellite lines from corresponding main branch lines) indicates weak coupling of the spin to the molecule through the magnetic field produced by end-over-end rotation. The doublet splitting in the A2Σ+ state was the only displacement assumed for the energy levels belonging to Ms = ± 1 for the same value of J′, which gives the g′ factor is 2.14 [33]. For 2Π3/2 ground state, the magnetic moments and molecular gJ′ factor calculated by previous publications [23,34]. Let us consider the Q22 (10.5) and QR12 (10.5) transition of NO, the values of g″ factor is more less than g′ which is only −2.47*10−3 [28]. From Eq. (6), it gives that magnetic modulation sensitivity (dυ/dB) is 5.1*10−4cm−1/G.
Figure 2 shows simulated FAMOS signals within the range of the 44135 to 44136 cm−1 range from Doppler contribution pressure for different magnetic from 100 to 2000 G. Each spectrum is calculated as the sum of all the different M transition with the overlapped Q22 (10.5) and QR12 (10.5) transition of NO in X2Π(υ” = 0)-A2Σ+(υ’ = 0) band, calculated according to (1) and (3). As can be seen from the figure, the signals of the FAMOS increase with magnetic modulation depth, but more importantly, that the FAMOS signal starts to appear two peaks when Zeeman splitter is too large at the highly magnetic modulation field. Figure 3 shows the simulated amplitude of the FAMOS signal as a function of magnetic modulation depth. From the figure, it shows the optimal magnetic modulation field is about 1020 G for this overlapped transitions with an assumed linewidth of 0.04965 cm−1 (HWHM), corresponding to magnetic modulation index ()is 1.03 which is agreed with the previous experimental findings by Ganser [29]. The simulations predict that the amplitude of the FAMOS signals from the overlapped transitions has a certain dependence on modulation magnetic field and this dependence is non-linear.
Fig. 2 Simulated FAMOS signals of the overlapped Q22(10.5) and QR12(10.5) transitions of NO in γ(0,0) band with assumed linewidth (HWHM) is 0.04965 cm−1. The 8 curves correspond to magnetic field amplitudes of 100, 200, 400, 800, 1000, 1200, 1500 and 2000G, respectively.
Fig. 3 The amplitude of FAMOS signals under Doppler contribution on resonance as a function of magnetic modulation amplitude from 100 to 3000 Gauss
4. Experiment and process
Here, we focus on nitric oxide detection via Faraday rotation spectroscopy with alternating magnetic fields. The experimental set up is schematically illustrated in Fig. 4 . It consists of a fully-diode-laser-based UV laser system (UV-DLS), two nearly crossed Rochon polarizers with extinction ratio about 10−5, a 10 cm long fused silica cell with wedged windows, a UV sensitive detector, a lock-in amplifier (LIA), a magnetic field with power amplifier (PA), and a computer with a 16-bits A/D card. The system also incorporates a wavelength meter (WM) for wavelength calibration.
The UV laser system is an external cavity diode laser (ECDL) in an external grating configuration producing cw light at 907 nm. The light passes an optical isolator before it is fed into an external cavity containing a KNbO3 crystal, which produces light at approximately 453 nm. This light is fed into a second external cavity containing a BBO crystal, that in general produces light at approximately 226.6 nm UV light. The UV light was passed through a Rochon polarizer and then fed through a detection cell, which is inside a copper wire coil for the application of a magnetic field. A second Rochon polarizer behind the cell (called the analyzer) is set to the almost crossed position with a slight offset angle (φ = 12°). The transmitted light is detected by the GaP UV-enhanced photodiode (Roithner, EPD-150-0/3.6). The output voltage is sent to a lock-in amplifier for demodulation. The demodulated signal is recorded by a personal computer equipped with an A/D card and LabVIEW software.
For each experiment, using a wavemeter (Burleigh, WA-1500), the center wavelength of the laser was first tuned to the vicinity of 906.30 nm to produce UV radiation around the targeted NO transitions at 226.58 nm. The UV light was then scanned back and forth across the selected transitions by modulating the piezo crystal transducer (PZT) in the ECDL laser in saw function by a function generator at a rate of 1 Hz. The scanning range was typically 38 GHz (1.3 cm−1) in UV, which is slightly more than ten times the Doppler width of the NO transition. The magnetic field is sinusoidally modulated with a frequency of ~3 kHz. A typical value for the applied alternating magnetic field B is 1.1 × 10−2 Tesla, which is limited by the largest power of PA in our lab, which results in a periodic Zeeman shift in the order of 5.6 × 10−2 cm−1. The data acquisition rate was 6 kHz. A gas mixture of 100 ± 10 ppm NO in N2 was used at various pressures for the experiments.
5. Results and discussion
A FAMOS spectrum from the overlapping Q22 (10.5) and QR12 (10.5) lines at around 226.58 nm from 4.2 Torr of the 100 ppm NO mixture (which corresponds to a concentration path length of 55 ppb·m) is shown by the solid line in the upper panel of Fig. 5 . Also shown in the same panel (as the dotted line) is a fitted curve. As can be seen from the figure, there is a good agreement between the spectrum and the fit. The residual has a peak-to-peak excursion that is below 2 × 10−2, which is assumed to originate from etalon effects in the sample cell as well as random fluctuations and drifts of the laser light power. The standard deviation of the residual is 0.006 corresponding to a concentration of 1.8 ppb.m. The good fit, and the lack of structure in the residual, indicate that there is an almost complete overlap between the Q22(10.5) and QR12(10.5) lines.
Fig. 5 Upper panel: measured and fitted FAMOS signal for the combined Q22(10.5) and QR12(10.5)transition at around 226.577 nm for 4.2 Torr of 100 ppm of NO in N2, corresponding to a concentration of 55 ppb.m. Lower panel: the residual which the standard deviation σ of the fit coefficients corresponds to 1.8 ppb.m
In order to assess the sensitivity of the technique for detection of NO on the combined Q22 (10.5) and QR12 (10.5) transition, absorption signals were recorded for a set of pressures (from 1.3 × 10−3 to 0.197 atm, corresponding to 1 to 150 Torr) of the premixed 100 ppm NO/N2 gas mixture. Each measurement was repeated 10 times and the amplitude of the FAMOS signal was retrieved and plotted in Fig. 6 . As can be seen from the figure, the absorption line broadens substantially due to collision broadening (with N2) and the peak position shifts as the pressure is increased.
Fig. 6 Measured FAMOS spectra for a set of total pressures of the 100 ppm NO / N2 gas mixture. The various curves represent total pressures of 1, 2, 4.2, 8, 10, 20, 30, 50, 60, 69.6, 80, 90, 90, 100, 110, 118 128, and 149.5 Torr, respectively.
Figure 7 displays the peak-to-peak values SF of fits to the FAMOS as a function of total pressure of the premixed NO shown in Fig. 6. It is convenient to define a sensitivity of the technique, ξ, in terms of the signal magnitude per unit of partial pressure of NO, i.e., as ∂SF/∂pNO. The sensitivity of the transition is therefore given by the slope of the curve in Fig. 7. It was found, however, that the curve has a weak nonlinearity, i.e., a slightly less-than-linear dependence on pressure for high pressures (the solid curve, represents a fit of a second-order polynomial to the data). However, taking the linear part of the curve for low and intermediate pressures as a good representation of the sensitivity of the technique implies that the sensitivity for this particular pair of transitions and for this special premixed NO with 100 ppm could be assessed to 0.018V/Torr, which means the sensitivity for partial NO is 180V/Torr.
Fig. 7 A set of FAMOS signal magnitude from measurement according to different premixed 100 ppm NO in N2. The relatively large error bars originate from temperature fluctuations in the NO cell. The solid curve is a two order polynomial curve fit is obtained from the measured amplitudes and known NO concentration levels.
The limit of detection can conveniently be defined as the partial pressure of NO, for which the signal to noise is equal to 3. Defining the noise as the standard deviation of FAMOS signal, σN = 0.0008V, evaluated from fits to a number of empty cell measurements (each consisting of the average of 10 consecutive scans), implies that (pNO)LOD can be expressed as 3σN/ξ, where ξ is the sensitivity of the transition as defined above. An analysis of signals from measurements from an evacuated cell was therefore performed using the procedure described as Fig. 5. Such a procedure provides a standard deviation of the FAMOS signal, σN. Making use of the sensitivity of the transition from above implies that (pNO)LOD is 13.3 μTorr. This corresponds to a minimum detectable concentration of NO of 17 ppb. Using the fact that the interaction region was 10 cm, this implies that the system is capable of detecting 1.7 ppb·m NO. From simulation described above, the FAMOS signal can achieve 5 times better if takes the optimal magnetic modulation which means the limitation of detection can achieve 0.34 ppb·m.
In addition to the evaluation of the accuracy of the methodology, step concentration measurements of NO are depicted in Fig. 8 . Each measurement was repeated 10 times and the concentration (X) of NO by fitted the amplitude of FAMOS and the sensitivity described above, i.e. X = S/ξ. The particle NO concentrations were assessed using a calibration according to the procedure described as Fig. 7. The mean value of the assessed concentration in the first region was found to be 33 ppb·m, which STD is 1.28 ppb·m shows that the precision of the technique is < 3.9%. While the mean value of the assessed concentration at 578 ppb.m gives the precision of the technique is satisfactory (0.8%). It also shows that the technique is fully capable of being an in situ monitor of NO.
5. Conclusions
Detection of nitric oxide at Torr pressures of a premixed gas with 100 ppm NO in N2 has been performed on an overlapped electronic transition in γ (0, 0) band at 226.577 nm by the use of fully-diode-laser-based ultraviolet absorption spectrometry utilizing Faraday rotation modulation technique. Over an absorption path length of 10 cm, the minimum detectable partial pressure of NO was found to be 2.66 µTorr at the optimal alternating magnetic field, corresponding to a relative concentration of 3.4 ppb. This corresponds to a concentration path length of 0.34 ppb·m, which clearly indicates the large potential of UV TLDAS for in situ detection of NO. The detectability of the FAMOS spectrometer in terms of gas concentration can be swiftly improved using a longer absorption path-length.
Acknowledgments
This work is supported by the Natural Science Foundation of China (NO.60702078) and Zhejiang province Science and Technology Foundation (NO.2010C33162).
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