João
Flávio da Silveira Petruci
ab,
Paula Regina
Fortes
bc,
Vjekoslav
Kokoric
b,
Andreas
Wilk
b,
Ivo Milton
Raimundo
Jr.
c,
Arnaldo Alves
Cardoso
a and
Boris
Mizaikoff
*b
aSão Paulo State University, Department of Analytical Chemistry, UNESP, CEP 14800-970, Araraquara, SP, Brazil
bUniversity of Ulm, Institute of Analytical and Bioanalytical Chemistry, 89081, Ulm, Germany. E-mail: boris.mizaikoff@uni-ulm.de
cUniversity of Campinas, Department of Analytical Chemistry, UNICAMP, Campinas, SP, Brazil
First published on 25th October 2013
Hydrogen sulfide is a highly corrosive, harmful, and toxic gas produced under anaerobic conditions within industrial processes or in natural environments, and plays an important role in the sulfur cycle. According to the U.S. Occupational Safety and Health Administration (OSHA), the permissible exposure limit (during 8 hours) is 10 ppm. Concentrations of 20 ppm are the threshold for critical health issues. In workplace environments with human subjects frequently exposed to H2S, e.g., during petroleum extraction and refining, real-time monitoring of exposure levels is mandatory. Sensors based on electrochemical measurement principles, semiconducting metal-oxides, taking advantage of their optical properties, have been described for H2S monitoring. However, extended response times, limited selectivity, and bulkiness of the instrumentation are common disadvantages of the sensing techniques reported to date. Here, we describe for the first time usage of a new generation of compact gas cells, i.e., so-called substrate-integrated hollow waveguides (iHWGs), combined with a compact Fourier transform infrared (FTIR) spectrometer for advanced gas sensing of H2S. The principle of detection is based on the immediate UV-assisted conversion of the rather weak IR-absorber H2S into much more pronounced and distinctively responding SO2. A calibration was established in the range of 10–100 ppm with a limit of detection (LOD) at 3 ppm, which is suitable for occupational health monitoring purposes. The developed sensing scheme provides an analytical response time of less than 60 seconds. Considering the substantial potential for miniaturization using e.g., a dedicated quantum cascade laser (QCL) in lieu of the FTIR spectrometer, the developed sensing approach may be evolved into a hand-held instrument, which may be tailored to a variety of applications ranging from environmental monitoring to workplace safety surveillance, process analysis and clinical diagnostics, e.g., breath analysis.
Next to its presence in various environmental, industrial, and clinical scenarios, H2S is a highly toxic, flammable, odorous, and corrosive gas.15 Corrosion of pipe walls (e.g., in petroleum and natural gas pipelines) is a serious issue in petroleum industries.16 Moreover, recent studies have attributed H2S as the second most common cause of death related to gas inhalation in workplace environments,17 in particular within confined spaces.18
The U.S. Occupational Safety and Health Administration has established a hydrogen sulfide concentration of 10 ppm as an 8 hour exposure limit.19 Furthermore, concentrations ranging from 15 to 100 ppm are considered to cause severe damage to human health including digestive upset, loss of appetite, olfactory paralysis, and eye and/or lung irritation.20 In terms of toxicity, the concentration of H2S is extremely more important than the exposure time.18 1000 ppm is sufficient to cause an immediate collapse within 1 or 2 breathing cycles. If compared with other inhaled toxic gases, this ‘knock down’ effect renders H2S a toxic gas with a minute margin of safety.21 From this point of view, reliable real-time monitoring of H2S is an essential demand.
Despite a variety of online monitoring options for gaseous hydrogen sulfide, its quantitative reliable determination still remains a challenge in the field of chemical sensors. The high reactivity and the ability to adsorb at many types of surfaces are the most common sources of errors during the detection of H2S.22 Gas chromatography is frequently used as the ‘gold standard’ for H2S quantification with remarkable sensitivity and precision.23 However, this type of analysis involves multiple sample preparation and analysis steps, and therefore, does not provide real-time results or short-term elevated exposures or concentration variations. Furthermore, the rather bulky dimensions and cost of instrumentation renders in situ monitoring via GC techniques less feasible. A variety of (physico-)chemical sensors providing real-time response are described in the literature, and are usually based on semiconducting metal-oxides,1,2 electrochemical sensing principles3,4 and optical sensing techniques.5,6,7 While these sensors have some apparent advantages including rapid response, high sensitivity, low cost, and ease of operation, their drawbacks include their limited selectivity, potential of interference under real-world measurement conditions, and effects of environmental parameters such as temperature and humidity.
Optical sensors appear particularly advantageous in terms of equipment cost, simplicity, ease of operation, and potential for miniaturization. Indirect chemical sensing methods, which are usually based on chemical reactions with dyes immobilized within polymeric matrices have shown remarkable sensitivity.24 However, extended response times render such devices less feasible for field usage. Direct optical sensing techniques including mid-infrared sensing concepts utilizing intrinsic optical properties of the analyte (e.g., fluorescence or absorption) are not widely described in the literature,25 whereas the near infrared (NIR) spectral range has been utilized to determine H2S at ppb levels using diode laser-based technology.26,27 This may be attributed to the rather weak absorption signature of H2S in the mid-infrared regime. A potential solution to mitigate this problem is the conversion of hydrogen sulfide to sulfur dioxide.
Hydrogen sulfide may readily convert to sulfur dioxide and other sulfur compounds (e.g., sulfate) using oxidative compounds, e.g., for the removal of H2S from the atmosphere.28 Despite the fact that this reaction is not instantaneous (H2S has a resident time of approx. 24 hours under environment conditions), several conversion-based procedures are described in the literature using temperature, pressure, metal oxides, and UV-Vis radiation to catalyze this conversion process.29,30 Larsen et al.31 described an approach to determine H2S using FTIR spectroscopy after UV-assisted conversion to SO2, which has a very strong absorption within the 1400–1300 cm−1 band. Using an optical path length of 26 m, hydrogen sulfide was detectable at concentrations of 32 ppm. While this study does not provide the opportunity of continuous in-flow conversion along with rather bulky instrumentation, and without evaluation of the analytical-figures-of-merit, it certainly demonstrated the utility of MIR spectroscopy for the detection of H2S after conversion.
Hollow waveguides may fundamentally be considered highly miniaturized light-pipes with a coaxial hollow core usually made from dielectric materials that enable radiation propagation by reflection at the inside wall.32 If gaseous samples are injected into the hollow core, the hollow waveguide (HWG) may simultaneously act as an extremely low volume gas cell with a well-defined optical absorption path length facilitating miniaturized MIR gas sensing strategies.33 The utility and adaptability of HWGs coupled to FTIR spectrometers operating at the 3–20 μm band for gas sensing applications have extensively been described in the literature.34–36
The so-called substrate-integrated hollow waveguide (iHWG) represents a fundamentally new generation of hollow waveguides pioneered by the research team of Mizaikoff and collaborators.37 The compact dimensions of the waveguide substrate (75 × 50 × 12 mm; made from aluminum), the adaptable (i.e., designable) optical path length via integration of meandered hollow waveguide structures at virtually any desired geometry into an otherwise planar substrate, the minute volume of the gas sample required for analysis, and the short transient time of such small volumes (few hundred microliters) through the active transducer region facilitating real-time monitoring are the most remarkable features of this innovation.
As previously demonstrated, iHWG-based sensing devices provide a compact, robust, and simply assembled system enabling real-time monitoring of relevant gas phase constituents.38 In the present study, we show for the first time the potential for real-time online analysis of hydrogen sulfide at low ppm concentration levels using a compact FTIR spectrometer coupled to the iHWG. H2S is quantitatively converted to SO2 using UV irradiation within a flow-through cell without any delay of the sensor response time, thus providing an excellent alternative to conventional analysis techniques for H2S reported to date.
H2O + HO˙ → HS˙ + H2O | (1) |
HS˙ + O2 → SO + HO˙ | (2) |
SO + 1/2O2 → SO2 | (3) |
The flow-through conversion system was operated using a H2S concentration of 100 ppm (in synthetic air), a gas flow rate within the glass tubing wrapped around a UV lamp of 5 mL min−1, and a UV radiation exposure path length of 95 cm. 50 spectra were averaged for each IR measurement at a spectral resolution of 4 cm−1 at a measurement interval of 3.5 seconds between each measurement. Thus obtained SO2 spectra are exemplarily illustrated in Fig. 1 along with a comparison of a simulated sulfur dioxide spectrum calculated using the HITRAN simulator.40 The most intense band within the SO2 spectrum relates to the asymmetric SO stretching vibration, and is located in the spectral range 1400–1300 cm−1. A typical signal-to-noise ratio (SNR) obtained during such measurements is 45.
SO2 + hv(<218 nm) → ˙SO + O˙, | (4) |
SO2 + HO˙ → ˙HSO3 | (5) |
˙HSO3 + O2 → SO3 + ˙HO2 | (6) |
SO3 + H2O → H2SO4 | (7) |
Fig. 2 Evaluation of the effect of the gas sample flow rate through the UV-assisted conversion device (H2S ⇒ SO2). |
In the present study, synthetic air containing trace amounts of water was used as the background matrix. Therefore, the flow rate was optimized for the present sample conditions. In the case of samples containing higher levels of moisture, which may adversely affect the conversion time31 the flow rate must accordingly be optimized. Baseline fluctuations were previously described at extended irradiation times, and relate to the formation of elemental sulfur and sulfate particles.29 However, this behavior was not observed during the experiments reported here, which is again beneficially attributed to the flow-through conversion. For all further experiments, a conversion flow rate of 10 mL min−1 was therefore used.
Fig. 3 Evaluation of the effect of the synthetic air composition for preparation of H2S gas samples at a concentration of 100 ppm. |
The capabilities of the developed iHWG-IR sensor system for online monitoring were evaluated by continuously monitoring the sample after conversion. For this purpose, the UV lamp was switched off at intervals of 2 min for instantaneously interrupting the H2S–SO2 conversion, and then switched back on after 2 min again enabling the conversion. After 80 seconds, the maximum signal was again obtained, which results from the dead volume of the conversion device. The obtained results are shown in Fig. 4 confirming excellent reproducibility of the peak height, and of the transient signal for 100 ppm of hydrogen sulfide.
Parameter | Value |
---|---|
Limit of detection (3 × SD of blank) | 3 ppm |
Limit of quantification (10 × SD of blank) | 11 ppm |
Correlation coefficient | 0.9905 |
Conversion repeatability | 2.21% |
Linear range | 10–100 ppm |
Regression equation | A = 0.0057 [H2S] – 0.0056 |
In future, a significant reduction of the physical dimensions of the developed sensing device is anticipated by replacing the FTIR spectrometer with a suitable tunable quantum cascade laser (QCL) light source.41,42 Furthermore, the usage of appropriate solid-phase preconcentration techniques is currently investigated for additionally improving the sensitivity, which is relevant for analyzing environmentally and clinically relevant H2S levels in the ppb concentration range.
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