Highly Sensitive Determination of Arsenic and Antimony based on an

Subscriber access provided by The University of British Columbia Library

Article

Highly Sensitive Determination of Arsenic and Antimony based on an Interrupted Gas Flow Atmospheric Pressure Glow Discharge Excitation Source Chun Yang, George C.-Y. Chan, Dong He, Zhifu Liu, Qisi Deng, Hongtao Zheng, Shenghong Hu, and Zhenli Zhu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03944 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

Highly Sensitive Determination of Arsenic and Antimony based on an Interrupted Gas Flow Atmospheric Pressure Glow Discharge Excitation Source Chun Yang,† George C.-Y. Chan,§ Dong He,† Zhifu Liu,† Qisi Deng,† Hongtao Zheng,‡ Shenghong Hu,† Zhenli Zhu*,† †State

Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, 430074, China §Lawrence

Berkeley National Laboratory, Berkeley, CA 94720, USA

‡Faculty

of Materials Science and Chemistry, China University of Geosciences, Wuhan, 430074, China Corresponding Author *Tel.: +86 27 67883455; Fax: +86 27 67883456; e-mail: [email protected]. ABSTRACT: A novel interrupted gas flow (IF) technique has been proposed for highly sensitive determination of ultratrace levels of arsenic and antimony in water samples by atmospheric pressure glow discharge (APGD) excitation source coupled with HClKBH4 hydride generation (HG). It is demonstrated that the gas flow interruption technique provides a dramatic and reproducible enhancement of emission signals of 1 to 2 orders of magnitude for As and Sb over conventional continuous gas flow (CF) in APGD. The enhanced analyte emission sensitivities in IF-APGD were investigated from the viewpoint of changes in plasma excitation temperature and analyte density. With eight As lines as the thermometric probe, no measurable change in excitation temperature was found, suggesting that the enhancement is caused by an increase in analyte number density in the plasma immediately following the gas-flow interruption. Furthermore, the enhancement factor was found to increase with the time interval in

ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

between the gas interruption, supporting an analyte adsorption (or trap)-release mechanism hypothesis. Under optimized conditions, the detection limits (DLs) of IFAPGD mode for As and Sb were calculated to be 0.02 and 0.003 μg L-1, which are respectively about 27 and 120-fold improved compared to CF-APGD mode. The linearity of calibration for both As and Sb reached R2> 0.999 in the 0.1 μg L-1 to 5 μg L-1 range. The accuracy of the proposed method was validated by the determination of certified reference materials (CRMs) and the results agreed well with the certified values.


Page 2 of 37

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Micro-plasma excitation sources developed for atomic emission spectrometry (AES) have attracted considerable attention because of their small size, reduced gas and power consumption, and relatively low manufacturing cost.1-3 In recent years, numerous miniaturized excitation sources have been reported such as dielectric barrier discharge (DBD),4-7 solution cathode glow discharge (SCGD),8-12 solution anode glow discharge (SAGD),13-14 capacitively coupled plasma (CCP),15-16 microwave induced plasmas (MIP),17-18 and atmospheric pressure glow discharge (APGD).19-23 For example, in our recent work, we have demonstrated that the APGD source can be used for the determination of As and nitrite ion by coupling with chemical vapor generation.24,25 However, the sensitivity of these miniaturized plasma excitation sources is not as efficient as conventional inductively coupled plasma (ICP) atomic emission spectrometry, especially for the determination heavy metals at ultratrace levels. To address this deficiency, efforts have been focused on enhancing the sensitivity and the response of microplasma sources. Extraction techniques including cloud point extraction (CPE),26 headspace solid-phase microextraction (HS-SPME),27 liquidliquid extraction (LLE),28 and solid-phase extraction (SPE)29 have been shown to improve the sensitivity of microplasmas because of analyte pre-concentration. However, these methods suffer from being time-consuming and requiring large volumes of solvents. Besides extraction methods, the use of surfactants and low molecular weight (LMW) compounds as additives have been investigated, and are



regarded as preferred means, and thus frequently applied, to modify the composition of the liquid electrode discharge to enhance analyte response.2 Nevertheless, addition of LMW compounds or surfactants are mainly used to improve the sensitivity of elements with relatively low excitation potentials (e.g., Li, Na, K, Ca, Mg, Cs, Ag, and Sr) but rarely utilized for the determination of elements with high excitation potentials, e.g., As or Sb.30,31 Recently, Burhenn et al.32 presented a sensitive method for As detection based on spatially and temporally resolved spectra acquisition, in which the reported limit of detection for As is improved to 0.09 μg L-1. However, this method needs a complex experimental setup and the use of an expensive detector (an ICCD, intensified charge coupled device), which is not ideal for the development of a miniaturized instrument. Based on our recent work,24 an interrupted gas flow atmospheric pressure glow discharge (IF-APGD) excitation source for highly sensitive atomic emission determination of As and Sb was proposed. To the best of our knowledge, this is the first report of a discontinuous gas flow used to enhance atomic emission signals in an APGD microplasma excitation source. It was found that disturbing the APGD source by simple gas-flow interruption significantly affected the plasma appearance and behavior and resulted in enhancement of atomic emission signals. Under optimized conditions, the detection sensitivity for As and Sb of the interrupted gas flow APGD were reliably improved by more than one order of magnitude compared to continuous gas flow


Page 4 of 37

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conditions. In addition, the proposed method was successfully applied to the determination of CRMs and a tap water sample. EXPERIMENTAL SECTION Instrumentation. The interrupted gas flow APGD system, detailed in Figure 1, consists of a pinch valve and an Ar-APGD source. The pinch valve is employed to control the gas flow here and the APGD used is similar to the one previously reported.24 The APGD source mainly consists of a quartz tube (o.d. 6 mm, i.d. 4 mm, length 50 mm), a titanium (Ti) tube (o.d. 4 mm, i.d. 3 mm) as the anode electrode and a tungsten cathode. The cathode compartment is comprised of a stainless steel tube (o.d. 3 mm, i.d. 2 mm) with an inserted tungsten rod (length 8 mm, diameter 2 mm) of which the 4 mm tapered end extends from the tube. Close to the end of the stainless steel tube with the contained tungsten cathode, four evenly spaced holes are drilled to allow the passage of Ar gas and hydrides. A discharge current of 15-35 mA is stabilized by a 10 kΩ ballast resistor, and the discharge is powered by a Kepco (Flushing, NY) BHK 2000-0.1MG high-voltage power supply operated in constant-current mode with an output voltage around 800 V. A pinch valve (Beion Scientific Instrument Co., Ltd., Shanghai, China) is connected between the continuous flow HG system and APGD excitation source to switch the gas flow in on and off cycles for gas flow interruption. The distance between the pinch valve and the APGD was about 20 mm. A in-house made timer is employed



to control the pinch valve and trigger the microspectrometer for data acquisition. The mixed gas (e.g., Ar, hydrides, and H2) from the HG system is connected to the two-way pinch valve with plastic tubing (o.d. 2 mm, i.d. 1 mm). When the power supply of pinch valve is set to “ON”, channel 1 is open and channel 2 is closed, and the gas flow is introduced into the APGD source at a steady rate named GFON (cf. Figure 1). On the contrary, when the power supply of pinch valve is set to “OFF” (a condition named GFOFF), no gas reaches APGD. In conventional CF-APGD mode, the GFON condition is continuously applied; a normal and stable glow discharge is sustained and steady emission signal is obtained. In this new IF-APGD mode, the gas flow is interrupted by “GFON-GFOFF-GFON” cycle. In the schematic diagram shown in Figure 1, the gas flow from the HG system is first swept into APGD source to sustain the discharge (GFON) at a setting time of TON. Then the power supply of the pinch valve is turned to “OFF” position at the end of TON with a duration of TOFF, during which the gas flow into the APGD is halted (GFOFF) and directed into channel 2. During this GFOFF period, no Ar gas reaches the APGD and the appearance of the plasma changes significantly. Immediately at the end of TOFF, the gas flow is resumed by switching the pinch valve back to the GFON position, and the APGD gradually returns to its normal bright appearance. The emission signal is measured along the axial direction of the discharge axis through the center of the quartz tube by a Maya 2000 Pro microspectrometer (wavelength range 180-325 nm, Ocean Optics Inc., USA.) through


Page 6 of 37

Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


an optical fiber. Unless otherwise specified, integration time was 50 ms per spectral scan. Reagents and samples. The details of the reagents and samples are shown in the Supporting Information. RESULTS AND DISCUSSION Operation of the APGD with an interrupted gas flow produces interesting behavior compared to operation under continuous gas flow. In our preliminary experiment, we observed that the Ar-APGD plasma could be self-sustained for 1-2 seconds after turning off the gas supply, because of residual argon in the APGD cell. As argon diffuses from the APGD source and is replaced by air, the APGD plasma becomes less stable and eventually extinguishes if the Ar supply remains off. This phenomenon appears similar to a discontinuous helium flowing atmospheric-pressure afterglow (FAPA) desorption ion source,33,34 which could be sustained for up to several minutes after the helium flow was halted. In our present study, we set TOFF to less than 1 s to avoid plasma extinction. The APGD plasma was observed to return smoothly to normal operation within tens of seconds after the gas flow resumed. It was surprising to observe a significant increase in the emission signal during and immediately after the period of gas flow interruption (Figure 2a). In the conventional CF-APGD mode, the generated hydrides (AsH3 and SbH3 in our case) were continuously introduced into a stable, diffuse APGD source for excitation, and thus a steady emission signal was attained. In contrast, in the IF-APGD



mode, significant signal enhancement was observed during the “GFON-GFOFF-GFON” cycle (cf. Figure 2a). Figure 2b shows the emission spectra of As and Sb acquired in CF-APGD and IF-APGD modes; it clearly demonstrates the substantial increase in As and Sb emission in IF-APGD measurement. It was found that the maximum emission signals of analytes occurred reproducibly almost synchronously with the gas flow interruption (around the GFOFF period, Figure 2a). It should be noted that during the period of gas-flow interruption, the generated hydrides bypassed the APGD and were swept to another channel (channel 2, cf. Figure 1). Therefore, the emission signal enhancement is not likely a result of increased introduction of generated hydrides. As will be discussed in detail in the next section, because the physical appearance of the APGD was observed to change markedly during the gas-flow interruption, suitable candidates for the explanation of this increase in As and Sb emission are increased residence time, increased atomic As and Sb densities, and change in excitation temperature of the APGD source. Whatever the mechanism, these results indicate that gas-flow interruption provides a simple and efficient way to improve the sensitivity of the APGD source. Characterization of the interrupted gas flow atmospheric pressure glow discharge To further characterize the behavior of the interrupted gas-flow APGD source, the time-dependent variation of both the discharge appearance and the emission signal were investigated under a higher temporal resolution. Photographs of the APGD were taken with a digital single-lens reflex camera (Nikon D810, Nikon, Japan). The net peak


Page 8 of 37

Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


height of As (193.7 nm) and Sb (206.8 nm) atomic emission were monitored during the “GFON-GFOFF-GFON” cycle. Figure 3a shows the temporal As and Sb emission during the “GFON-GFOFFGFON” cycle. Both the spectrometer and pinch valve were triggered by the cycle timer. The GFOFF time (TOFF) and the spectrometer acquisition time were set to 200 ms and 10 ms, respectively. A significant enhancement of As (193.7 nm) and Sb (206.8 nm) emission lines are clearly observed in the IF-APGD mode. During the GFOFF period, the emission signals of As and Sb both increase after closing the pinch valve with a delay of about 40 ms and then levels off for the remaining GFOFF period. This time delay may be related to the volume between the pinch valve and the APGD source, which acts as a buffer and slows down the response of the APGD to the interrupted gas flow. Immediately following the resumption of the gas flow, the emission of As and Sb further increase momentarily and sharply with very little delay. Immediately after the spike, the signal exhibits a rapid decay and then slowly returns to steady-state value comparable with CF-APGD mode. In Figure 3b, a series of images is presented to show how the discharge appearance of the IF-APGD changes with time. The GFOFF time (TOFF) was 200 ms. During continuous gas-flow operation (GFON), a steady, bright and diffuse plasma (Figure 3b1) identical to CF-APGD mode was observed. When the pinch valve was turned off (GFOFF), the appearance of plasma did not change significantly (Figure 3b-2) for approximately 40 ms. Shortly afterwards, the plasma became slim and dimmed (Figure



3b-3, 100 ms after the gas-flow was interrupted) because of the air entrainment. The plasma continued to shrink in size until the Ar flow was restored (Figure 3b-4), gradually regained its brightness and diffuse appearance (Figure 3b-5) before it returned smoothly to its normal condition within approximately 60 s following gas flow restoration (Figure 3b-6). This behavior is similar to the temporal progression of the discharge appearance in FAPA after interruption of He flow.33 Although detailed mechanistic study for signal enhancement is beyond the scope of the current work, two experiments were performed to narrow down the candidates. In general, common causes for signal enhancement in atomic emission source are raise in excitation temperature, longer residence time for the analyte, and increased atomic number densities of the analyte. In the first experiment, the excitation temperature of the plasma was compared in CF-APGD and IF-APGD measurement modes. The most straightforward approach is to measure the excitation temperature by means of Boltzmann plot and compare the temperature values in these two measurement modes. However, absolute temperature measurement is difficult with the current setup because such measurement requires a calibration of the spectral response of the whole optical detection system. Therefore, a different approach based on a variant of Boltzmann plot was used to monitor the change of excitation temperature (if any). In the absence of self-absorption and under the assumption that excited states of an atomic system (e.g., As or Sb atoms) are populated according to Boltzmann distribution, the measured intensity, I, of an emission line in the plasma is governed by


Page 10 of 37

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


equation 1: ― 𝐸𝑞

( )

𝐼 = 𝐶 × 𝑛 × 𝑒𝑥𝑝

(1)

𝑘𝑇

where C is a combined constant consisting of various spectroscopic constants as well as physical parameters of the plasma and the optical detection system; n is the number of atoms or ions; Eq is the excitation potential of the upper energy level of the measured optical transition; k is the Boltzmann constant; and T is the excitation temperature. From equation 1, it is clear that an increase in emission, I, could be a result of an increase in analyte density, n, and/or an increase in excitation temperature, T. The relationship of a change in analyte emission (i.e., dI) to changes in analyte density (dn) and excitation temperature (dT) can be represented by equation 2: 𝑑𝐼 𝑑𝑛 𝑑𝑇 = + 𝐸𝑞 × 2 𝐼 𝑛 𝑘𝑇

(2)

The term (dI / I) is the fractional change of the emission, which is related to signal enhancement factor, R = IIF-APGD / ICF-APGD, via the relationship dI / I = (IIF-APGD − ICF-APGD) / ICF-APGD = R − 1. Thus, a plot of R vs. Eq would clearly pinpoint the role of excitation temperature of the plasma in the observed emission enhancement ratio. Specifically, an unambiguous positive slope in the R vs. Eq plot corresponds to an increase in excitation temperature. Likewise, a negative slope concludes a decrease in excitation temperature; and a flat slope or a slope that does not differ statistically from zero indicates that there is no experimental evidence, within experimental uncertainty, that there is a change in excitation temperature. Furthermore, the y-intercept of the R vs. Eq plot gives the relative number densities of the atomic species. The advantage of



this variant approach for Boltzmann plot is that only the relative changes (or enhancement ratios) of the selected spectral lines are required for the diagnosis, and no knowledge of other spectroscopic constants except the upper energy levels of the transitions, Eq, are needed. In addition, different lines can be measured with different spectrometric gain and there is no need to calibrate the spectrometric response of the spectrometer-detector system. The drawback is that this method allows one to follow only the relative change in the excitation temperature (which is sufficient for our purpose for this study), but does not provide information on the absolute excitation temperature. Figure 4a shows the enhancement ratios of eight As emission lines against their excitation energies. The spectroscopic constants are given in Table 1.35 The enhancement ratios were determined from separate CF-APGD and IF-APGD measurements. In the CF-APGD mode, 300 replicated spectra were collected and averaged as one CF-APGD measurement spectrum, whereas the IF-APGD measurement spectrum is the one containing the signal spike (the highest analyte signal) among the 100 collected spectra during the “GFON-GFOFF-GFON” cycle (TOFF=200 ms). As clearly depicted in Figure 4a, the slope of the R vs. Eq plot is flat indicating that there is no measurable change in excitation temperature, T. It should be stressed that the signal spike (i.e., measurement time for IF-APGD) was observed almost immediately after the pinch valve was cycled back to its “ON” position (cf. Figure 3a). Thus, technically speaking, the gas-flow is no longer interrupted at the moment during


Page 12 of 37

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the signal spike. The no-measurable change in excitation temperature does not contradict with the change in plasma appearance during the TOFF period (cf. Figure 3b); it reveals that if there is a change in temperature during the TOFF period, the temperature is quickly restored once the gas flow is resumed. The fact that no measurable change in excitation temperature was observed for IFAPGD suggests that other mechanism(s) is responsible for the signal enhancement. It was found that the enhancement strongly depended on the duration of the GFON time (TON) (i.e., the time interval between subsequent gas-flow interruptions). Figure 4b shows the effect of GFON time on the relative sensitivities of As and Sb emission in IF-APGD measurements. The concentrations of As and Sb were both at 10 μg L-1. Surprisingly, both the As and Sb emission were found to increase monotonically with the GFON time, initially in a linear fashion and then at a reduced rate. The observation that the enhancement is a strong function on the time interval in-between gas-flow interruptions with an initial linear trend suggests a hypothesis involving an analyte adsorption (or trap)-release mechanism for the signal enhancement. To further pinpoint the cause, we analyzed the inner surface of the APGD quartz tube housing for As species. In this work, the APGD was operated with an introduction of 5 μg L-1 As standard solution (as hydride) at CF mode for 10 minutes, and the quartz tube was dissembled and placed into the nitric acid to dissolve any trapped/adsorbed species. Arsenic species was found at the inner surface of the quartz tube, supporting the adsorption (or trap)-release hypothesis for the signal enhancement. However, the detail



mechanism is not clear at the present stage. Optimization of the Working Conditions. A few important working conditions affecting the performance of IF-APGD measurement, including discharge current, flow rates of KBH4 and Ar, and GFOFF time (TOFF), were optimized to achieve the maximum signal. In IF-APGD mode, every “GFON-GFOFF-GFON” cycle requires tens of seconds before the plasma becomes stable again, so the GFON time TON was fixed at 20 s in the following experiment. Unless otherwise specified, the relative intensities of the atomic emission at 193.7 nm (As) and 206.8 nm (Sb) were monitored and normalized to the maximum signal at each set of the studied parameters. Because the APGD is a semi-closed system, the ambient air might be introduced into the system which could extinguish the discharge, the TOFF is required to be controlled to less than 2 s. The As and Sb emission signals influenced by the TOFF duration were evaluated with TOFF at 0.2, 0.4, 0.6, 0.8 and 1.0 s (cf. Figure S1a). It should be noted that each TOFF was repeated 5 times in this test. As shown in Figure S1a, the relative emission signals of Sb and As decreases with increasing TOFF. A possible reason is that an extended TOFF period might cause more argon discharge gas be replaced by air. Consequently, the inclusion of too much air consumes the energy of the discharge and affects the dissociation and excitation of hydrides. Based on these observations, a TOFF time of 0.2 s was adopted in the subsequent tests. The effect of the discharge current on the signal enhancement during IF-APGD was evaluated (cf. Figure S1b). Increasing the discharge current from 15 to 35 mA


Page 14 of 37

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


results in an approximately linear decline in the As and Sb signals. This observation is in marked contrast to conventional CF-APGD, in which higher discharge current always offers higher sensitivity. It suggests that higher discharge current (presumably also higher gas temperature of the plasma) is not favorable for the adsorption (or trap)release process and degrades the signal enhancement. It was noted that at lower current, the APGD plasma was less robust and extinguished more easily during gas flow interruption. Thus, a discharge current of 15 mA was employed in the subsequent tests. The flow rate of KBH4 affects the generation efficiency of hydrides (AsH3, SbH3) and the amount of co-generated H2 that introduced into the APGD source. As shown in Figure S1c, the flow rate of KBH4 was evaluated from 0.5 to 1.5 mL min-1 with a concentration fixed at 0.5% (m/v). It was found that the As and Sb relative intensities decreased significantly with an increase in KBH4 flow rate. However, at flow rates lower than 0.5 mL min-1, the diffuse and stable discharge changed to filamentous appearance and caused poor analytical performance. From these results, an optimal KBH4 flow rate of 0.5 mL min-1 was selected in the subsequent tests. The influence of the Ar flow rate on the emission signal of As and Sb in IF-APGD system was also studied over the range 100 to 300 mL min-1 (cf. Figure S1d). It was found that As emission signal increased with Ar flow rate increasing from 100 to 200 mL min-1 and then decreased, which was similar to the CF mode. However, Sb signal behaved differently and rose significantly with increasing Ar flow rate from 100 to 250 mL min-1; after which the increasing trend slowed down. It should be noted that the



trend of Sb measured by the CF mode was a bit difference from the IF mode. For Sb measured by the CF mode, signal increased from 100 to 200 mL min-1 and then decreased. This observation indicates that the behavior of As and Sb are different even though other trends are similar. At lower Ar flow rate, both As and Sb have poor sensitivity, which might be caused by inefficient transport of the analytes into the APGD. On the basis of these observations a flow rate of 250 mL min-1 was selected for the remaining experiments. Analytical performance and figures of merit. Under the optimized conditions, the analytical performance of the IF-APGD, with a GFON time of 240 s, was evaluated. The relative standard deviation (RSD) of five measurements from a 1 μg L-1 As standard solution was 4.2% (Figure 5a). The As and Sb emission signals at 193.7 nm and 206.8 nm were monitored with an integration time of 50 ms. The calibration curves obtained from standard As and Sb solutions all provided linear correlation coefficients (R2) better than 0.999 in the concentration range from 0.1 to 5 μg L-1. For comparison, the calibration curves under conventional CF-APGD mode were also determined and presented in Figure 5b. It was observed that the sensitivities of As and Sb were improved by factors of 68 and 150, respectively, with IF-APGD. The DLs of As and Sb were calculated to be 0.02 and 0.003 μg L-1 with IF-APGD, which improved 27- and 120-fold, respectively, compared with conventional CF-APGD. Table 2 compares HGIF-APGD-AES with other relevant methods for As and Sb detection. The attainable DLs for As and Sb are improved by at least one order of magnitude compared to other


Page 16 of 37

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


miniaturized plasma sources like APGD,24 SCGD,12 PD,36 CCP,16 and DBD.37 Compared to the DBD-AES32 with ICCD as detector for the determination of As, IFAPGD-AES extends lower DL despite the fact that a handheld micro-spectrometer was utilized. These results demonstrate that the developed method offers high sensitivity and it provides a novel simple and cost-effective way to boost the sensitivity of the APGD excitation source. The analytical performance of the proposed interrupted gas flow APGD source were also evaluated by the determination of certified reference materials (simulated natural water GBW08605, GSB 07-1376-2001), and a tap water sample (Table 3). Due to the absence of reference materials with suitably low concentrations, the As and Sb CRMs were diluted by 100 and 15 times, respectively. The results for As and Sb determination in GBW08605 and GSB 07-1376-2001 by IF-APGD-AES were 0.502±0.022 mg L-1 and 29.6±1.0 μg L-1 respectively, which agreed well with the certified reference values (0.500±0.001 mg L-1 and 29.8±1.5 μg L-1, respectively). The As and Sb determination in tap water were 0.99±0.08 and 0.86±0.05 μg L-1, respectively. To further assess the accuracy of the method, recovery efficiency of As and Sb was also determined; a 2 μg L-1 As and Sb standard solution was spiked into the tap water sample. The recoveries for As and Sb were found to be 93% and 99%, respectively. These results demonstrate that the new IF-APGD technique provides good analytical accuracy. CONCLUSIONS



A novel, simple and highly sensitive method for the determination of ultratrace As and Sb was established based on an interrupted gas flow APGD excitation source. It was found that when the gas flow entering an APGD excitation source was interrupted, dramatic enhancement in the analytes’ emission signals was observed during GFOFF, and particularly immediately following the gas restoration after interruption. Under optimized conditions, the detection limits for As and Sb were found to be 0.02 and 0.003 μg L-1, respectively, by the IF-APGD-AES system, and are improved by more than one order of magnitude compared to conventional CF-APGD-AES systems. Although the detailed mechanism for signal enhancement is not clearly understood at the present time, our experimental results do not contradict with an adsorption (or trap)release hypothesis. The linearity of calibration for both As and Sb reached R2 > 0.999 in the 0.1 μg L-1 to 5 μg L-1 range. The validation of the proposed method was demonstrated by the successful determination of certified reference materials (GBW08605 and GSB 07-1376-2001). The spiked recoveries for As and Sb in tap water samples were all also close to 100%. All results demonstrate that the gas-flow interruption technique provides a relatively simple and efficient method to improve the performance of APGD source. Furthermore, some others hydride-forming elements (e.g. Bi, Cd, and Se) were also observed to show considerable enhancement in the IFAPGD-AES system. It is interesting to consider the prospect of applying the interrupted gas-flow technique to other microplasma sources to achieve highly sensitive atomic emission determination of heavy metals in the future.


Page 18 of 37

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information. Reagents and samples; Figure S1. the optimization of working conditions

ACKNOWLEDGEMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21822405, 41673014, and 41521001), National Key Research and Development Program (2017YFD0801202), and Nature Science Foundation of Hubei Province (2016CFA038).



REFERENCES (1) Karanassios, V. Microplasmas for chemical analysis: analytical tools or research toys? [J]. Spectrochim. Acta, Part B 2004, 59, 909-928. (2) Pohl, P.; Jamroz, P.; Swiderski, K.; Dzimitrowicz, A.; Lesniewicz, A. Critical evaluation of recent achievements in low power glow discharge generated at atmospheric pressure between a flowing liquid cathode and a metallic anode for element analysis by optical emission spectrometry [J]. TrAC, Trends Anal. Chem. 2017, 88, 119-133. (3) Broekaert, J. A. C. The development of microplasmas for spectrochemical analysis [J]. Anal. Biocanal. Chem. 2002, 374, 182-187. (4) Yu, Y.; Du, Z.; Chen, M.; Wang, J. Atmospheric-pressure dielectric-barrier discharge as a radiation source for optical emission spectrometry [J]. Angew. Chem. Int. Ed. 2008, 47, 7909-7912. (5) Zhu, Z.; Chan, G. C.-Y.; Ray, S. J.; Zhang, X.; Hieftje, G. M. Microplasma source based on a dielectric barrier discharge for the determination of mercury by atomic emission spectrometry [J]. Anal. Chem. 2008, 80, 8622-8627. (6) Han, B.; Jiang, X.; Hou, X.; Zheng, C. Dielectric barrier discharge carbon atomic emission spectrometer: universal GC detector for volatile carbon-containing compounds [J]. Anal. Chem. 2013, 86, 936-942.


Page 20 of 37

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Brandt, S.; Schütz, A.; Klute, F. D.; Kratzer, J.; Franzke, J. Dielectric barrier discharges applied for optical spectrometry [J]. Spectrochim. Acta, Part B 2016, 123, 6-32. (8) Webb, M. R.; And, F. J. A.; Hieftje, G. M. Compact glow discharge for the elemental analysis of aqueous samples [J]. Anal. Chem. 2007, 79, 7899-7905. (9) Davis, W. C.; Marcus, R. K. Role of powering geometries and sheath gas composition on operation characteristics and the optical emission in the liquid sampling-atmospheric pressure glow discharge [J]. Spectrochim. Acta, Part B 2002, 57, 1473-1486. (10) Huang, C.; Li, Q.; Mo, J.; Wang, Z. Ultratrace determination of Tin, Germanium, and Selenium by hydride generation coupled with a novel solution-cathode glow discharge-atomic emission spectrometry method [J]. Anal. Chem. 2016, 88, 1155911567. (11) Schwartz, A. J.; Wang, Z.; Ray, S. J.; Hieftje, G. M. Universal anion detection by replacement-ion chromatography with an atmospheric-pressure solution-cathode glow discharge photometric detector [J]. Anal. Chem. 2013, 85, 129-137. (12) Greda, K.; Jamroz, P.; Jedryczko, D.; Pohl, P. On the coupling of hydride generation with atmospheric pressure glow discharge in contact with the flowing liquid cathode for the determination of arsenic, antimony and selenium with optical emission spectrometry [J]. Talanta 2015, 137, 11-17.



(13) Liu, X.; Zhu, Z.; He, D.; Zheng, H.; Gan, Y.; Belshaw, N. S.; Hu, S. H.; Wang, Y. Highly sensitive elemental analysis of Cd and Zn by solution anode glow discharge atomic emission spectrometry [J]. J. Anal. At. Spectrom. 2016, 31, 1089-1096. (14) Greda, K.; Swiderski, K.; Jamroz, P.; Pohl, P. Flowing liquid anode atmospheric pressure glow discharge as an excitation source for optical emission spectrometry with the improved detectability of Ag, Cd, Hg, Pb, Tl, and Zn [J]. Anal. Chem. 2016, 88, 8812-8820. (15) Anghel, S. D.; Simon, A.; Frentiu, T. Characterization of a very low power argon CCP [J]. J. Anal. At. Spectrom. 2005, 20, 966-973. (16) Frentiu, T.; Butaciu, S.; Ponta, M.; Darvasi, E.; Senila, M.; Petreus, D.; Frentiu, M. Simultaneous determination of As and Sb in soil using hydride generation capacitively coupled plasma microtorch optical emission spectrometry-comparison with inductively coupled plasma optical emission spectrometry [J]. J. Anal. At. Spectrom. 2014, 29, 1880-1888. (17) Engel, U.; Bilgiç, A. M.; Haase, O.; Voges, E.; Broekaert, J. A. C. A microwaveinduced plasma based on microstrip technology and its use for the atomic emission spectrometric determination of mercury with the aid of the cold-vapor technique [J]. Anal. Chem. 2000, 72, 193-197. (18) Matusiewicz, H.; Ślachciński, M. Simultaneous determination of As, Bi, Sb, Se and Sn by microwave induced plasma spectrometry using a quadruple-mode microflow


Page 22 of 37

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ultrasonic nebulizer for in situ hydride generation with internal standardization [J]. Microchem. J. 2017, 131, 70-78. (19) Broekaert, J. A. C.; Reinsberg, K. G. Spectrochemical analysis with DC glow discharges at atmospheric pressure [J]. Spectrochim. Acta, Part B 2015, 106, 1-7. (20) Eijkel, J. C. T.; Stoeri, H.; Manz, A. A dc microplasma on a chip employed as an optical emission detector for gas chromatography [J] Anal. Chem. 2000, 50, 2547-2552. (21) Andrade, F. J.; Wetzel, W. C.; Chan, G. C. Y.; Webb, M. R.; Gamez, G.; Ray, S. J.; Hieftje, G. M. A new, versatile, direct-current helium atmospheric-pressure glow discharge [J]. J. Anal. At. Spectrom. 2006, 21, 1175-1184. (22) Gielniak, B.; Fiedler, T.; Broekaert, J. A. C. Study of a new direct current atmospheric pressure glow discharge in helium [J]. Spectrochim. Acta, Part B 2011, 66, 21-27. (23) Meng, F.; Duan, Y. Nitrogen microplasma generated in chip-based ingroove glow discharge device for detection of organic fragments by optical emission spectrometry [J]. Anal. Chem. 2015, 87, 1882-1888. (24) Yang, C.; He, D.; Zhu, Z.; Peng, H.; Liu, Z.; Wen, G.; Bai, J.; Zheng, H.; Hu, S.; Wang, Y. Battery-operated atomic emission analyzer for waterborne arsenic based on atmospheric pressure glow discharge excitation source [J]. Anal. Chem. 2017, 89, 36943701.



(25) Zheng, H.; Guan, X.; Mao, X.; Zhu, Z.; Yang, C.; Qiu, H.; Hu, S. Determination of nitrite in water samples using atmospheric pressure glow discharge microplasma emission and chemical vapor generation of NO species [J]. Anal. Chim. Acta 2018, 1001, 100-105. (26) Shekhar, R.; Madhavi, K.; Meeravali, N. N.; Kumar, S. J. Determination of thallium at trace levels by electrolyte cathode discharge atomic emission spectrometry with improved sensitivity [J]. Anal. Methods 2014, 6, 732-740. (27) Zheng, C.; Hu, L.; Hou, X.; He, B.; Jiang, G. Headspace solid-phase microextraction coupled to miniaturized microplasma optical emission spectrometry for detection of mercury and lead [J]. Anal. Chem. 2018, 90, 3683-3691. (28) Manjusha, R.; Reddy, M. A.; Kumar, S. J. Determination of cadmium in Zircaloys by electrolyte cathode discharge atomic emission spectrometry (ELCAD-AES) [J]. Anal. Methods 2014, 6, 9850-9856. (29) Li, Q.; Zhang, Z.; Wang, Z. Determination of Hg2+ by on-line separation and preconcentration with atmospheric-pressure solution-cathode glow discharge atomic emission spectrometry [J]. Anal. Chim. Acta 2014, 845, 7-14. (30) Greda, K.; Jamróz, P.; Pohl, P. The improvement of the analytical performance of direct current atmospheric pressure glow discharge generated in contact with the smallsized liquid cathode after the addition of non-ionic surfactants to electrolyte solutions [J]. Talanta 2013, 108, 74-82.


Page 24 of 37

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Zhang, Z.; Wang, Z.; Li, Q.; Zou, H.; Shi, Y. Determination of trace heavy metals in environmental and biological samples by solution cathode glow discharge-atomic emission spectrometry and addition of ionic surfactants for improved sensitivity [J]. Talanta 2014, 119, 613-619. (32) Burhenn, S.; Kratzer, J.; Svoboda, M.; Klute, F. D.; Michels, A.; Veža, D.; Franzke, J. Spatially and temporally resolved detection of Arsenic in a capillary dielectric barrier discharge by hydride generation high-resolved optical emission spectrometry [J]. Anal. Chem. 2018, 90, 3424-3429. (33) Storey, A. P.; Zeiri, O. M.; Ray, S. J.; Hieftje, G. M. Helium conservation by discontinuous introduction in the flowing atmospheric-pressure afterglow source for ambient desorption-ionization mass spectrometry [J]. J. Anal. At. Spectrom. 2015, 30, 2017-2023. (34) Storey, A. P.; Zeiri, O. M.; Ray, S. J.; Hieftje, G. M. Use of interrupted helium flow in the analysis of vapor samples with flowing atmospheric-pressure afterglowmass spectrometry [J]. J. Am. Soc. Mass. Spectrom. 2017, 28, 263-269. (35) Kramida, A.; Ralchenko, Yu.; Reader, J.; NIST ASD Team. NIST Atomic Spectra Data-base (version 5.6.1) [Online], National Institute of Standards and Technology: Gaithersburg, MD, 2018; https://physics.nist.gov/asd.



(36) Li M.; Deng Y.; Zheng C.; Jiang X.; Hou X. Hydride generation-point discharge microplasma-optical emission spectrometry for the determination of trace As, Bi, Sb and Sn [J]. J. Anal. At. Spectrom. 2016, 31, 2427-2433. (37) Zhu Z.; He H.; He D.; Zheng H.; Zhang C.; Hu S.; Evaluation of a new dielectric barrier discharge excitation source for the determination of arsenic with atomic emission spectrometry [J]. Talanta, 2014, 122, 234-239.


Page 26 of 37

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures and Tables Captions Figure 1. A schematic diagram of the experiment setup of the interrupted gas flow atmospheric pressure glow discharge system. Figure 2. The temporal emission intensities of Sb 206.8 nm line (a), and the emission spectra of As and Sb measured with CF-APGD and IF-APGD modes (b). (TOFF=200 ms, TON=10 s, integration time of 50 ms, Ar flow rate of 200 mL min-1). Figure 3. The temporal emission profile of As 193.7 nm and Sb 206.8 nm lines with TOFF=200 ms (a) and the photographs showing the temporal variations of the plasma appearance during the “GFON-GFOFF-GFON” cycle (b). (1) Plasma with 200 mL min1

continuous Ar flow (GFON), (2) 40 ms after Ar flow is turned off (GFOFF), (3) 100

ms after Ar flow is turned off (GFOFF), (4) 20 ms after Ar flow is restored (GFON), (5) 100 ms after Ar flow is restored (GFON), (6) 60 s after Ar flow is restored and plasma become stable. Figure 4. The enhancement factor with different As emission lines, and the error bars represent the standard deviations through error propagations from eleven CF-APGD and IF-APGD measurements (a). The effect of GFON time on As and Sb emission signals (b). Figure 5. The temporal profile of the As 193.7 nm signals from five repetitive measurements with 1 μg L-1 As standard solution (a) and the calibration curves of As 193.7 nm and Sb 206.8 nm emission lines measured by IF-APGD and CF-APGD modes (b).



Table 1. Arsenic spectral lines and their upper energy levels35 for the variant Boltzmann plot. Table 2. The determination of As and Sb in the certified reference materials and tap water by IF-APGD-AES. Table 3. Comparison of HG-IF-APGD-AES with other relevant methods for As and Sb detection.


Page 28 of 37

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1



Figure 2


Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3



Figure 4


Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5



Table 1 As line wavelength / nm

Eq / cm-1

189.042

52897.9

193.759

51610.2

197.262

50693.8

199.035

60834.8

200.334

60815.0

228.812

54605.3

234.984

53135.6

237.077

60815.0


Page 34 of 37

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Sample

Determined value

Certified value

Recovery

GBW08605 (As)

0.502±0.022 mg L-1

0.500±0.001 mg L-1

--

GSB 07-1376-2001 (Sb)

29.6±1.0 μg L-1

29.8±1.5 μg L-1

--

Tap water (As)

0.99±0.08 μg L-1

--

93%

Tap water (Sb)

0.86±0.05 μg L-1

--

99%



Page 36 of 37

Table 3 DLs (μg L-1)

RSD (%)

Methods

Ref. As

Sb

As

Sb

HG-APGD-AES a

0.25

--

2.5

--

24

HG-SCGD-AES b

4.2

1.2

--

--

12

7

5

4.2

0.7

36

HG-CCP-AES a

0.2

0.18

--

--

16

HG-DBD-AES b

4.8

--

2.8

--

37

HG-DBD-AES c

0.09

--

--

--

32

HG-CF-APGD-AES a

0.54

0.37

2.2

1.8

This work

HG-IF-APGD-AES a

0.02

0.003

4.2

3.5

This work

HG-PD-AES a

a

Handheld microspectrometer

b

Monochromator with a photomultiplier tube (PMT)

c

Monochromator with an Intensified Charge-Coupled Device (ICCD)


Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For TOC only


Highly Sensitive Determination of Arsenic and Antimony based on an

Recommend Documents