iCONVERT: An Integrated Device for the UV-Assisted Determination of

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Technical Note

iCONVERT: An Integrated Device for the UV-Assisted Determination of H2S via Mid-Infrared Gas Sensors João Flávio da Silveira Petruci, Arnaldo Alves Cardoso, Andreas Wilk, Vjekoslav Kokoric, and Boris Mizaikoff Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b02731 • Publication Date (Web): 14 Sep 2015 Downloaded from http://pubs.acs.org on September 16, 2015

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Analytical Chemistry

TECHNICAL NOTE

iCONVERT: An Integrated Device for the UV-Assisted Determination of H2S via Mid-Infrared Gas Sensors

João Flavio da Silveira Petruci1,2, Arnaldo Alves Cardoso1, Andreas Wilk2, Vjekoslav Kokoric2 and Boris Mizaikoff2*

1

São Paulo State University, Department of Analytical Chemistry, UNESP, CEP 14800-970, Araraquara, SP, Brazil

2

University of Ulm, Institute of Analytical and Bioanalytical Chemistry, 89081, Ulm, Germany

ACS Paragon Plus Environment


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ABSTRACT In this technical note, we describe an integrated device platform for performing in-flow gaseous conversion reactions based on ultraviolet (UV) irradiation. The system combines - using the same footprint – an integrated UV-conversion device (iCONVERT), a preconcentrator unit (iPRECON), and a new generation of mid-infrared (MIR) gas cell simultaneously serving as photon conduit, i.e., so-called substrate-integrated hollow waveguide (iHWG) optically coupled to a compact Fourier transform infrared (FTIR) spectrometer. The iCONVERT is assembled from two blocks of aluminum (dimensions: 75 x 50 x 40 mm; L x W x D) containing 4 miniaturized UV-lamps (47 x 6 x 47 mm each). For the present study, the iPRECON-iCONVERT-iHWG sensing platform has specifically been tailored to the determination of H2S in gaseous samples. Thereby, the quantitative UV-assisted conversion of the rather weak IR-absorber H2S into the more pronouncedly responding SO2 is used for hydrogen sulfide detection. A linear calibration model was established in the range of 7.5 to 100 ppmv achieving a limit of detection at 1.5 ppmv using 10 min of sample preconcentration (onto Molecular Sieve® 5A) at a flow rate of 200 mL min-1. When compared to a conventional UV-conversion system, the iCONVERT revealed similar performance. Considering the potential for system miniaturization using, e.g., dedicated quantum cascade lasers (QCL) in lieu of the FTIR spectrometer, the developed sensing platform may be further evolved into a hand-held device.


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INTRODUCTION Within recent decades, monitoring of hydrogen sulfide has been identified as a highly relevant analytical tasks in a variety of monitoring scenarios. H2S is a flammable, toxic, corrosive, and odorous gas with its emissions into the atmosphere related to a series of natural and anthropogenic activities including bacterial reduction of sulfur constituents, petroleum extraction, fuel burning, and processes associated with paper production1–4. Concerning the toxicity, H2S contamination has been attributed the second most common cause of death in workplace environments5. Hence, much lower exposures limits for H2S have been recently been adopted by regulatory agencies6. For example, these thresholds limit values constitute an 8 h time-weighed average (TWA) of 1 ppmv, and a 15 min short-time exposure limit (STEL) of 5 ppmv6. In non-polluted environments, H2S is usually found at trace concentration levels (i.e., < 0.1 ppmv)7. However, workers have been exposed to concentration higher than 50 ppmv in workplaces such as waste management, petroleum and natural gas extraction8. Likewise, natural catastrophic episodes such as volcanic eruptions are responsible for releasing up to six thousand times more H2S into the atmosphere9. The protocol of H2S determination usually requires a sample preconcentration step for enhancing the analytical signal prior to the laboratory analysis10. Adsorption at solid sorbents at room temperatures has been considered among the most efficient and straightforward methods for the enrichment of volatile sulfur constituents (VSCs)11. Gas chromatography (GC)

is

considered

the

standard

measurement


technique

for

H2S


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determination

at

trace

and

ultra-trace

sensitivity12.

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However,

such

instrumentation is of limited utility for integration into portable systems or for providing on-site sensing capabilities. Mid-infrared (MIR; 3-20 µm) absorption spectroscopy techniques provide highly discriminatory information at a molecular level due to the excitation of inherently specific fundamental vibrational and vibro-rotational transitions13. Using compact FTIR spectrometers has enabled in-field H2S determination, however, based upon gas cells with short absorption path lengths, and thus, limited sensitivity, i.e., not suitable for addressing the low ppmv and/or ppbv concentration range14. Hydrogen sulfide has a rather weak absorption signature in the MIR providing a ν2 bending fundamental band15 in the range of 1000 – 1500 cm-1, and a stretching vibration feature in the range of 3600 – 4000 cm-1. Illuminating H2S at 185 nm in the presence of oxygen induces the quantitative conversion of H2S into SO2, which provides a significantly more pronounced IR-signature (symmetric and asymmetric stretch band in the range of 1395 – 1321 cm-1, and 1165 – 1135 cm-1). Consequently, we have already suggested the conversion of SO2 into H2S as an innovative strategy for IR-based H2S monitoring16,17. In the present study, we therefore complement the previously developed iHWG16,18–20 and iPRECON21,22 technology with a UV-conversion device of similar footprint – the iCONVERT. Coupling a preconcentrator with an integrated UV-conversion device enables enhancing the analytical signal for H2S such that quantification at the required ppmv level is enabled. Thus amplified gaseous samples are then injected into substrate-integrated hollow


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waveguides (iHWGs), which are either coupled to a FTIR spectrometer or – alternatively - to a laser light source such as a tunable quantum cascade laser (QCL)23–25. iHWGs are innovative alternatives for conventional multi-pass gas cells minimizing the device footprint while maximizing the absorption path length within a smartly structured yet robust substrate. Hence, the developed iPRECON-iCONVERT-iHWG concept provides a modular sensing toolbox based on a uniform device footprint facilitating tailorable mid-infrared gas sensing solutions that require short sample transient times, probing of minute sample

volumes,

inherent molecular

selectivity

and

multi-component

capability, and a high degree of device portability.

EXPERIMENTAL Hydrogen sulfide, synthetic air, and nitrogen were obtained from MTI Industriegase AG (Neu-Ulm, Germany). A gas mixing system based on massflow controllers developed by the Institute of Analytical and Bioanalytical Chemistry @ University of Ulm, and Lawrence Livermore National Laboratory (LLNL; Livermore, USA) was used for preparing and delivering hydrogen sulfide samples in nitrogen at various concentrations at a flow rate of up to 200 mL min-1. The preconcentrator contained Molecular Sieve® 5A (SigmaAldrich, St Louis, USA) at a packing length of 40 mm serving as the solid sorbent material. Thermal desorption was performed via resistive heating using a voltage/current controller (Basetech, BT-305, Germany). Figure 1 shows a 3D rendered model of the iDEVICES components. The H2S-SO2 conversion was performed by exposing H2S/synthetic air mixtures to UV radiation at 185 nm. The device was assembled using two



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blocks of aluminum (75 x 50 x 20 mm each; similar footprint as iPRECON and iHWG as evident in figure 1), and 4 miniaturized UV-lamps (UV-C, Rexim LLC, Watertown, MA, USA) emitting @ 185 mm (47 x 6 x 47 mm; length x width x depth). Each lamp is supplied with a voltage of 5V, and a current of 30 mA, thus providing an energy output of 49 µW cm-2 @ 185 nm. The gas inand outlet were located at the top plate of the device using M5-threaded stainless-steel Luer Lock® adapters facilitating a direct connection option to iPRECON and iHWG at minimal dead volumes. The two plates were sealed using epoxy-based glue. The UV lamps were inserted into the device and sealed using rubber O-rings. During the experiments reported herein, iCONVERT was coupled to the preconcentrator and the iHWG using polytetrafluoroethylene tubes with Luer Lock® adapters; in future it is anticipated providing direct stackable connections between these modules. Three-way valves were used for selecting the gas flow pathway through the system. All measurements were performed using a compact FTIR spectrometer (Alpha OEM, Bruker Optics Inc., Ettlingen, Germany) equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector (FTIR–22.1.00, Infrared Associates, Stuart/FL, USA). A straight-line iHWG made from polished aluminum providing an integrated hollow waveguide channel (i.e., absorption path length) of 7.5 cm at device dimensions of 75 x 50 x 25 mm (length x width x depth) was coupled to the spectrometer using gold-coated off-axis parabolic mirrors with a focal length of 1” (Thorlabs, Dachau, Germany). The IR spectra were recorded in the wavelength range of 4000-650 cm-1 at a spectral resolution of 4 cm-1; 10 spectra were averaged per measurement unless stated otherwise. The OPUS 7.2 software package


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(Bruker Optics Inc, Ettlingen, Germany) was used for data acquisition. All components of the experimental setup were positioned at a Thorlabs breadboard with dimensions of 450 x 300 mm. Figure 2 shows a schematic of the entire IR gas sensing system.

RESULTS AND DISCUSSION Conversion efficiency In our related study17, we have readily demonstrated sample preconcentration and thermal desorption of H2S followed by UV-based conversion into SO2 using a benchtop-style conversion system comprising a quartz tube coiled around a conventional UV lamp (length: 210 mm). Here, we have evaluated the efficiency of H2S-SO2 conversion of the integrated iCONVERT

system

the

initial

laboratory

set-up.

Hence,

the

same

preconcentration parameters as applied in the related study have been used: (i) sampling time: 10 min; (ii) flow rate: 200 mL min-1; (iii) desorption temperature: 270 ºC; (iv) desorption period: 3 min. The desorption flow is the crucial parameter for adapting the UV irradiation time such that quantitative conversion conditions are achieved. Hence, a variety of gas flow rates were evaluated using a sample with a concentration of 75 ppm H2S, and synthetic air as the desorption matrix. Figure 3 shows the performance of the iCONVERT vs. the laboratory-style UV conversion system evaluating the obtained IR signal vs. the gas flow rate. Evidently, the iCONVERT revealed maximum efficiency of 63.9% conversion rate at a sample flow of 40 mL min-1. Hence, despite the much more compact device dimensions and transient



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volumes the iCONVERT provides a highly efficiency in UV-assisted conversion of H2S into SO2 for subsequent IR analysis.

Analytical figure-of-merit Calibration functions were established enabling quantitative data analysis based on the evaluation of the peak area with integration boundaries ranging from 1395 to 1321 cm-1 vs. the H2S concentration. For each concentration, the mean value of three replicate measurements was calculated. The proposed method revealed excellent linearity across the concentration range of 5 - 100 ppmv H2S, which was selected for the present study. The calculated limits of detection (LOD) were considered three times the standard deviation of the blank signal, and were determined at 1.5 ppmv. The obtained results are summarized in table 1.

CONCLUSIONS In this study, we have described a novel compact device – the iCONVERT - for efficient UV-based conversion of H2S to SO2 @ 185 nm, which complements our previously reported iPRECON and iHWG technology. Coupling the iCONVERT to preconcentrator device and an IR gas cell provides a modular, compact, and robust device platform that may flexibly be adapted to almost any kind of molecular gas sensing and monitoring scenario. The performance of iCONVERT favorably compared to a conventional laboratory-based UV-conversion system and confirmed similar conversion performance. The developed sensing platform enables the determination of low ppmv H2S concentrations relevant to a wide variety of workplace safety


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scenarios at a sampling time of only 10 min. Given its modularity, it is anticipated that the iPRECON-iCONVERT-iHWG mid-infrared gas sensing toolbox will find applications in a wide range of gas analysis scenarios ranging from environmental monitoring to process analysis and clinical diagnostics such as exhaled breath analysis.

ACKNOWLEDGMENTS JFSP thanks FAPESP (2014/23974-3) for financial support during a research stay at IABC-UULM. BM acknowledges partial support of this study by the project APOSEMA funded by the German BMBF within the M-Era.net program. This work was performed in part under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344. This project was funded under LLNL sub-contract Nos. B598643 and B603018.



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Figure 1. (a) 3D model of the bottom substrate plate of iPRECON, iCONVERT, and iHWG components, and (b) 3D model of each assembled iDEVICE.


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Figure 2. Scheme of the H2S gas-sensing platform setup.



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Figure 3. iCONVERT H2S-SO2 conversion efficiency evaluation for the determination of 75 ppmv of H2S at optimized preconcentration conditions. The label of the y-axis (AREA) refers to the peak area of the SO2 absorption with integration boundaries ranging from 1395 to 1321 cm-1.


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Figure 4. IR spectra obtained after sample preconcentration of hydrogen sulfide standards at concentration of 50 and 7.5 ppm recorded using the iPRECON-iCONVERT-iHWG sensor at a spectral resolution of 4 cm-1.



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Table 1. Analytical-figures-of-merit obtained for the iPRECONiCONVERT-iHWG infrared hydrogen sulfide gas sensing system. Parameter

Value

Limit of detection (3*SD of blank)

1.5 ppm

Limit of quantification (10*SD of blank)

8 ppm

Correlation coefficient

0.9915

Linear range

7.5 – 100 ppm

Regression equation

A = 0.033 [H2S] – 0.122


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TOC (Full Paper) “Online Sensing of H2S and SO2 using Advanced Preconcentration- UV-Assisted–iHWG Mid-Infrared Chemical Sensors” (J. Petruci, A. Wilk, A. Cardoso, B. Mizaikoff*)

TOC (Technical Note) “iCONVERT: An Integrated Device for the UV-Assisted Determination of H2S via Mid-Infrared Gas Sensors” (J. Petruci, A. Cardoso, V. Kokoric, A. Wilk, B. Mizaikoff*)


iCONVERT: An Integrated Device for the UV-Assisted Determination of

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