Spatially and Temporally Resolved Detection of Arsenic in a Capillary

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Spatially and temporally resolved detection of Arsenic in a Capillary DBD by Hydride Generation High Resolved Optical Emission Spectrometry Sebastian Burhenn, Jan Kratzer, Milan Svoboda, Felix David Klute, Antje Michels, Damir Veza, and Joachim Franzke Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05072 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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Analytical Chemistry

Spatially and temporally resolved detection of Arsenic in a Capillary DBD by Hydride Generation High Resolved Optical Emission Spectrometry Sebastian Burhenn†, Jan Kratzer‡, Milan Svoboda‡, Felix David Klute†, Antje Michels†, Damir Veža§, Joachim Franzke†* †

Leibniz Institut für Analytische Wissenschaften – ISAS – e.V., Bunsen-Kirchhoff-Str. 11, 44139 Dortmund, Germany Czech Academy of Sciences, Institute of Analytical Chemistry, Veveří 97, 60200 Brno, Czech Republic § Department of Physics, Faculty of Science, University of Zagreb, Bijenicka 32, 10000 Zagreb, Croatia ‡

ABSTRACT: A new method for arsenic detection by optical emission spectrometry (OES) is presented. Arsine (AsH3) is generated from liquid solutions by means of hydride generation (HG) and introduced into a capillary dielectric barrier discharge (DBD) where it is atomized and excited. A great challenge in OES is the reduction of the recorded background signal because it negatively affects the limit of detection (LOD). In conventional DBD-OES methods the signal intensity of the line of interest, in this case arsenic, is integrated over a long timescale. However, due to the pulsed character of the plasma, the plasma on-time is only a small fraction of the integration time. Therefore a high amount of noise is added to the actual signal in each discharge cycle. To circumvent this, in the present study the emitted light from the DBD is collected by a fast gated iCCD camera which is mounted on a modified monochromator. The experimental arrangement enables to record the emission signal of arsenic in form of a monochromatic 2D-resolved picture. The temporal resolution of the iCCD camera in the nanosecond range provides the information at which point in time and how long arsenic is excited in the discharge. Using this knowledge it is possible to integrate only the arsenic emission by temporally isolating the signal from the background. With the presented method the LOD for arsenic could be determined to 93 pg mL-1 with a calibration curve linear over four orders of magnitude. As a consequence, the developed experimental approach has a potential to both, mechanistic studies of arsine atomization and excitation in DBD plasmas as well as routine applications in which arsenic determination at ultratrace levels is required.

The detection of arsenic has gained increasing importance in the recent years. The reason for this is the partly high content of arsenic in many foods such as rice1-4 and fish,5-7 as well as in drinking water.8 Arsenic chronic poisoning is a serious public health problem worlwide. In some areas with high chronic exposure to arsenic from groundwater, e.g. in Bangladesh, West Bengal and other Southeast Asian countries but also in United States and Europe, prevalences of diseases (e.g. skin lesions, diabetes, hypertension, or cancer) have reached epidemic proportions. Due to the significant differences in toxicity, mobility, bioavailability and other properties among various arsenic species, speciation analysis is nowadays often required rather than determination of total arsenic content.9 Arsenic and its inorganic compounds, As(III) and As(V), have been classified as carcinogenic to human by the International Agency for Research on Cancer (IARC).10 The combination of high-pressure liquid chromatography (HPLC) with subsequent inductively coupled plasma mass spectrometry (ICP-MS) detection is often used for arsenic speciation analysis.11-14 As an alternative, the coupling of HG15 to atomic absorption spectrometry (AAS)16,17 and atomic fluorescence spectrometry (AFS)18,19 as well as atomic emission spectrometry (AES)20,21 has established itself as a costeffective and time-saving method for the determination of the total arsenic content as well as its speciation analysis. In this work, arsenic hydride (arsine) has been selected as a model

arsenic species since As(III) and As(V) compounds are converted to arsine in the hydride generator. Arsenic detection by means of HG-AAS, HG-AFS and HGAES requires the conversion of arsine into free atoms. This can be achieved in different ways, but externally heated quartz tube atomizers (QTA)22-24 are predominantly used in AAS, whereas diffusion flames (DF) prevail in AFS.25 In HG-AES, plasma-based sources such as conventional ICP or microwave induced plasma can be routinely used for effective arsine atomization and subsequent excitation of free atoms. QTA, DF and ICP mentioned above are well established for hydride atomization/excitation, but they all require high temperature environment. However, the high temperature of the device hinders the miniaturization of the setup. In order to avoid this, dielectric barrier discharges (DBD) of various designs can be universally used as hydride atomizers in AAS and AFS as well as atomization/excitation devices in OES.26-29 The DBD have proven to be a versatile tool in the field of analytical chemistry, especially as preliminary stage for mass spectrometry. It can serve as ionization or desorption source for the detection of larger molecules, for example amino acids30 or explosives.31,32 Moreover, a significant signal enhancement due to the combination of a DBD with chemical vapor generation techniques has been reported in the field of trace element analysis.33 The advantages of DBDs are low manufacturing costs, small size, good optical accessibility, low

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operating temperatures and the stability of discharge operation. Using a noble gas like helium and applying a highfrequency alternating voltage to the electrodes separated by a dielectric, a plasma can be ignited. By introducing AsH3 into the plasma the hydride molecule is dissociated and free arsenic atoms are produced. Many studies focused on the detection of elements by means of DBD-OES were published resulting in excellent LODs indicating the applicability of this combination.34-37 In addition, Li et al. introduced a portable DBD-OES spectrometer for multielement analysis taking advantage of the small size of the DBD.38 A common approach is to guide the light emitted from the discharge through a monochromator and detect it with a photomultiplier tube (PMT). However, due to the pulsed character of the DBD caused by the dielectric barrier between the electrodes, light is only emitted during a small time period in the range of a few hundred nanoseconds up to microseconds mainly dependent on the applied frequency and voltage. Therefore, by integrating the emission over a long time period undesired background signal is added to the actual analyte signal within each discharge cycle, which adversely affects the LOD. In addition, by using a PMT on the exit slit of the monochromator the spatial information about the emission signal is lost. In the present study, arsenic hydride generated by HG is fed into a capillary DBD which is mounted in front of a modified monochromator. The slits of the monochromator are removed and the PMT is replaced by a fast gated iCCD-camera. This allows to take two dimensional images of the DBD with additional spectral resolution providing the information where arsenic is excited in the discharge. Moreover, the high temporal resolution of the camera in the range of a few nanoseconds enables to determine the point in time at which arsenic is excited. With the knowledge of these two parameters the arsenic signal can be temporally and spatially separated from the background. Therefore, by means of the presented method, the separated arsenic signal can be integrated over many discharge cycles and still preserving a high signal-to-noise-ratio resulting in significant improvements of the LOD in comparison to common time integrated OES methods.

EXPERIMENTAL SECTION The experimental arrangement is illustrated in Figure 1.

tions were prepared by serial dilution of this stock solution in 1 mol L-1 HCl (Merck, Germany). The reductant solution contains 1% NaBH4 (Sigma Aldrich, Germany) in 0.1 % KOH (Lach-Ner, Czech Republic) and was prepared daily. If not stated otherwise, 12 mg L-1 As standard solution in 1 mol L-1 HCl was used. Hydride Generator. An in house made continuous flow hydride generation system (see Figure 1) was used. It was based on a peristaltic pump (Ismatec, Switzerland) and a 3 ml inner volume gas-liquid separator (GLS) with a forced outlet. Using a three-way valve, either the As standard or a blank (HCl) stream (4 mL min-1) was merged with reductant (NaBH4) flow (1.2 mL min-1) in a 1 m long (1 mm i.d.) PTFE reaction coil. A flow of 60 mL min-1 He as carrier gas was admixed upstream the GLS. A dryer realized by a polypropylene cartridge (100 mm long, 15 mm i.d.) filled with solid NaOH beads (diameter ≥2 mm) was inserted downstream the GLS to prevent aerosol and droplets to enter the DBD plasma. The gaseous phase leaving the GLS contains, apart from the carrier gas and arsine, 15 mL min-1 H2 produced as a side product from NaBH4 decomposition. The gas mixing chamber (50 ml inner volume) downstream the GLS enables homogeneous mixing of the gaseous phase from the GLS with 400 mL min-1 He discharge stream as well as compensates for any pressure fluctuations in the system. Gas flows were controlled by mass flow controllers (Omega Engineering, USA). Atomizer and electrical setup. A capillary dielectric barrier discharge was used as atomizer. It consists of a quartz capillary with an inner diameter of 1 mm and a wall thickness of 0.25 mm. Two 15 mm wide electrodes, separated by a 10 mm gap, are soldered directly onto the quartz capillary in order to realize an optimal electrical contact. Helium was used as carrier gas, since the discharge was most stable in helium. The plasma is ignited by applying to the electrodes a 7 kV square wave voltage with a frequency of 20 kHz. It is provided by two interconnected in-house built generators with a maximum voltage of 3.5 kV at short rise-times. Procedure. The measurements were carried out in the direct transfer mode, in which arsine was continuously generated in the hydride generator and immediately introduced into the capillary DBD discharge. It takes arsine about 30 seconds to reach the discharge after switching the three way valve from blank back to the arsenic standard solution. All the measurements performed in this work have been performed under the conditions in which the DBD discharge was supplied by a constant flow of arsine, i.e. every time when the three-way valve was switched from blank to arsenic standard the signal measurement started not sooner than 40 s after changing from blank to arsenic standard solution. Optical design and measuring principle.

Figure 1. Experimental arrangement of the hydride generator, the DBD, the optical parts of the monochromator and the iCCDcamera. The CCD chip of the camera is located in the optical plane of the monochromatic 1:1 image of the discharge.

Reagents. An arsenic stock solution of 1000 mg L-1 was prepared from solid As2O3 (Lach-Ner, Czech Republic) by dissolving 33 mg of As2O3 in 2.5 mL 10% NaOH (Lach-Ner, Czech Republic) and dilution of this mixture by deionized water to a final volume of 25 mL. The working arsenic solu-


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Figure 2. Schematic measuring principle of the time resolved camera measurement. Shifting the gate by increasing the delay enables to scan the emission in a discharge cycle.

The light emitted by the DBD is analyzed by a monochromator (GCA/McPherson Instruments, f=1 m model M2015), which is used to select the wavelength of the species to be observed. The monochromator was adapted to the new requirements of the experiment. For this purpose, both the entrance and exit slits were removed. The discharge was placed vertically in the optical plane of the entrance slit. Therefore, the grating and the concave mirrors inside the monochromator create a monochromatic 1:1 image of the discharge in the optical plane of the exit slit (see Figure 1). At this position the CCD chip of a fast-gated iCCD camera (Andor iStar DH 720 18F-03) with a spectral range from 180 nm to 850 nm at a peak quantum efficiency of 15 % at 440 nm was placed. The temporal resolution of the device corresponds to the minimal optical gate width of 5 ns and the dynamic range at the selected gain setting is 70 dB. By rotating the grating of the spectrograph, it is possible to record spectrally resolved 2D images of the discharge. In order to determine the point in time at which a certain species is excited, a high temporal resolution is necessary. However, by decreasing the exposure time of the camera, the number of photons reaching the detector is also decreasing. This means that the measured signal can no longer be distinguished from the background caused by thermal noise of the CCD or CCD read noise. To circumvent this, the periodicity of the discharge can be utilized as follows. The in house built square wave generator is capable of sending a TTL trigger pulse on each rising edge of the square wave voltage signal. The trigger output is connected to the external trigger input of the iCCD camera defining the starting point for opening the camera gate. This ensures a synchronization of the camera with the voltage supply. Since emission from the discharge occurs periodically a few 100 ns later after the rising edge of the voltage, the gate has to be shifted as shown in Figure 2. This shift is provided by a digital delay generator integrated into the camera head. Therefore, the camera is capable of taking images of the plasma at a fixed point in time during each discharge cycle. The signals of several periods are directly integrated on the chip and read out afterwards. In order to observe the temporal evolution of the emission of the discharge the delay between the trigger pulse and the gate can be increased stepwise in the nanosecond range, which provides a scan through the discharge period.

The signal-to-noise ratio can be further improved by means of a subsequent software-based averaging. For the time resolved measurements, a gate width of 5 ns with 50 accumulations was selected. Prerequisite for the application of this method is the temporal periodicity of the observed phenomenon which is fulfilled by using the stable capillary discharge. The amount of measured data could be reduced by converting the monochromatic 2D picture into a monochromatic 1D picture. This was achieved by defining a vertical channel on the CCD chip with a certain width corresponding to the plasma channel. The pixels in each horizontal column of the channel are binned which results in a 1D image of the plasma with spectral information in each pixel. The CCD chip was cooled to -10 °C during the measurements to reduce thermal noise. For preliminary measurements, when explicitly stated the detection of arsenic emission was performed with a commercial broadband spectrometer (Ocean Optics USB 4000). This spectrometer does not allow time and spatially resolved measurements.

RESULTS AND DISCUSSION Before carrying out the time-resolved measurements, the emission of the discharge was recorded time-integrated with the previously described commercial Ocean Optics broadband spectrometer in the range of 180-740 nm (Figure 3). In order to investigate the influence of arsenic on the spectrum of the discharge, the emission signal of a 12 mg L-1 arsenic standard solution (red) was compared to a blank sample (black).

Figure 3. Spectrum of the discharge operating in helium with hydride generator switched on running the blank (black) and 12 mg L-1 arsenic standard (red). The As 228 nm line is located on top of the H2-continuum from 200-450 nm.

The arsenic lines, which appear in the measurement of the standard sample in the range below 300 nm, are clearly visible. The As 228 nm line is particularly intense. Below the arsenic lines there is a broad continuum extending between 200 nm and 450 nm. This continuum is related to the dissociation of molecular hydrogen39 and occurs at high concentrations of hydrogen in the discharge. The 2sσ 3Σ+g state of the hydrogen molecule is excited by electron impact. This state relaxes back to the repulsive 2pσ 3Σ+u state by emitting light and hydrogen is dissociated. Due to the slope of the 2pσ 3Σ+u state, the radiation is continuous. Since the arsenic line is located on top of the continuum it can no longer be distinguished from



the background at low peak heights, making a detection of small concentrations of arsenic difficult. For the spatially and temporally resolved detection, a series of monochromatic two-dimensional single images of the discharge were recorded covering 650 ns after the voltage rise of the generator. The temporal resolution was defined by the gate width of the CCD-chip, which was set to 5 ns. After each picture the delay between trigger and starting the measurement was increased by 5 ns. The resulting pictures show the distribution of the arsenic emission in the DBD evolving over time steps of 5 ns. Applying the previously described binning method for data reduction on each image, a series of one dimensional graphs showing the signal intensity along the discharge axis was created. These graphs can be merged together resulting in a contour plot showing the signal intensity of the arsenic line over 500 ns depending on the point in time and the position in the plasma channel (Figure 4). Beside the arsenic signal, the signal of the hydrogen dissociation continuum was obtained by measuring the blank sample at 228 nm without the addition of arsenic to the system with the same procedure. The resulting contour plot for the hydrogen continuum was subsequently subtracted from the arsenic signal, resulting in a background corrected contour plot. The prerequisite for this is that the addition of arsenic to the discharge does not severely influence the plasma in the range of the measuring tolerance. This was verified before by comparing the signals of hydrogen and helium during the measurement of the blank and arsenic standard respectively. There were no changes in the temporal and spatial evolution of the discharge. Apart from the rise of the arsenic lines, the spectrum in Figure 3 shows no significant change in the line intensities of other species after arsenic was added. The contour plots in Figure 4 show that no significant emission from arsenic or hydrogen can be detected before 215 ns on the 228 nm line. The maximum for the emission of the H2 continuum is located in vicinity of the negative electrode close to the inlet at 255 ns whereas the maximal emission of the broader arsenic signal occurs 10 ns later at 265 ns. Since the emission of the H2 continuum originates from the relaxation of the excited hydrogen molecule to the repulsive dissociative state, a bright signal is linked with an increased production of atomic hydrogen radicals which are mainly responsible for the atomization of AsH3. This fits very well with the observations made by Zhu et al. in a planar DBD design,26 where no atomic absorption signal from arsenic could be detected employing a helium discharge in the absence of hydrogen. Only when hydrogen was artificially added to the discharge, the arsenic signal was observed again. The reason is that the hydrogen radicals in the discharge atomize AsH3 in a reductive threestep process. The atomized arsenic can then be excited by electron impact collision, causing the shift of the maximum of the arsenic signal compared to the emission of the H2 dissociation continuum. The main difference between both contour plots is a weaker second emission peak with a maximum at 390 ns which is only visible in the arsenic plot. This temporally broad peak has a long lifetime and is still detectable after 580 ns. Both peaks are separated by a narrow dark region. This indicates an additional excitation mechanism, which is presumably a collision of arsine with helium metastables. This collision leads to a simultaneous atomization of arsine and excitation of arsenic. The long emission signal is due to the extended lifetime of the

metastables, which can be greater than a few hundred nanoseconds. Therefore the observed emission signal corresponds to an overlap of the two different mechanisms.

Figure 4. Spatially and temporally resolved evolution of the emission of the a) H2 dissociation continuum and b) arsenic at 228 nm during the positive half period of the discharge cycle. The signal of the H2 dissociation continuum was measured by running the blank. The discharge dimensions as well as the electrodes, the polarity and the gas flow are indicated schematically on the left side. For Figure b) the concentration of the arsenic standard was 12 mg L-1.

This knowledge can then be used to detect and quantify arsenic more efficiently. In the presented setup, the problem with the detection of arsenic by OES is that the arsenic line is located on top of the H2 continuum. Thus there are two superimposing factors contributing to the total noise of the signal, which are the noise of the hydrogen continuum and the noise of the arsenic signal itself. This leads to a poor signal-to-noise ratio in conventional optical methods with a time integrating spectrometer. However, the accuracy of the measurement can be significantly improved by the knowledge about the temporal distribution of the signal. By choosing a suitable gatewidth of the camera, one can selectively integrate over areas from the contour plots where the emission of arsenic is dominant. This can also be done periodically with a fixed delay, which enables to integrate the signal on the CCD chip over many discharge cycles in combination with software based accumulations. Therefore, it is possible to only integrate over the long-lasting arsenic signal after the occurrence of the maximum of the H2 continuum, which separates the signal from the background. The set delay on the camera was 350 ns, which means that the camera starts a measurement immediately after the emission from the H2 continuum has extinguished and the second arsenic peak appears. By choosing a suitable gate width of the camera, the area of the second arsenic peak


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Analytical Chemistry can be isolated. The counts in each pixel of this area are added up which results in a total number of counts proportional to a certain concentration of arsenic. By means of this method a calibration curve can be measured varying the concentration of the arsenic sample over approximately four orders of magnitude as depicted in Figure 5. In order to obtain a reasonable value for the LOD of this method the evaluation of the noise has to be taken into account. Due to the suppression of the hydrogen dissociation background and its contributing noise, the total noise depends mainly on the stability of the discharge and the statistical noise of the CCD. The latter can be reduced by a high number of accumulations. However, this results in a higher measurement time in which the discharge has to perform temporally stable. The optimum conditions were 3000 accumulations causing a measurement time of several minutes. The noise signal σ was determined by measuring the blank and integrating the counts in the same area as in the measurement with arsenic. The LOD corresponds to the concentration at a signal of 3σ and is 93 pg mL-1. As a comparison to the presented method a time integrated calibration curve measured with the previously described Ocean Optics spectrometer is added in Figure 5. Due to the fact, that the signal is located on top of the hydrogen continuum, a small signal cannot be distinguished from the background, resulting in a LOD of 74 ng mL-1. The presented spatially and temporally resolved method exceeds this value by approximately three orders of magnitude.

Figure 5. Calibration curves of the presented spatially and temporally resolved method (red) and an Ocean Optics spectrometer for arsenic in a helium capillary DBD (black). The 3σ LODs are indicated by the dashed line, respectively.

CONCLUSIONS An improved method for the detection of arsenic using a specially adapted setup for OES was presented. The knowledge about the point in time at which arsine is excited enables to integrate the arsenic emission signal temporally separated from the H2 dissociation background enhancing the signal-to-noise ratio. By means of this method the LOD of arsenic was determined to 93 pg mL-1 in a linear calibration over four orders of magnitude in concentration. The experimental approach developed and described in this work has been proven to have potential to both, mechanistic studies of

arsine atomization and excitation in DBD plasmas of various designs as well as routine applications in which arsenic determination at ultratrace levels is required. It can be easily applied to mechanistic studies of dissociation/excitation of hydride molecules of other analytically important elements (Se, Sb, Pb, Bi) in DBD plasmas of various designs (planar, cylindrical, etc.).

AUTHOR INFORMATION Corresponding Author *Phone: +49 231 1392-174 E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The financial support by the Ministerium für Innovation, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, the Senatsverwaltung für Wirtschaft, Technologie und Forschung des Landes Berlin, the Bundesministerium für Bildung und Forschung, and the Deutsche Forschungsgemeinschaft is gratefully acknowledged. This research has been supported by the Czech Science Foundation under contract 17-04329S and by the Czech Academy of Sciences, Institute of Analytical Chemistry, v. v. i. (Institutional Research Plan no. RVO: 68081715).

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Spatially and Temporally Resolved Detection of Arsenic in a Capillary

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