Research Progress on Gas to Particle Conversion–Gas

ANALYTICAL SCIENCES JUNE 2018, VOL. 34 657

2018 © The Japan Society for Analytical Chemistry

† To whom correspondence should be addressed.E-mail: [email protected]

Research Progress on Gas to Particle Conversion–Gas Exchange ICP-MS for Direct Analysis of Ultra-trace Metallic Compound Gas

Masaki OHATA*† and Kohei NISHIGUCHI**

* Inorganic Standards Group, Research Institute for Material and Chemical Measurement, National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305–8563, Japan

** J-Science Lab Co., Ltd., 3-1 Hiuchigata, Kamitoba, Minami, Kyoto 601–8144, Japan

A novel gas to particle conversion–gas exchange technique coupled with inductively coupled plasma mass spectrometry (GPD-GED-ICP-MS) was recently proposed for the direct analysis of ultra-trace levels of metallic compound gases such as metal carbonyl and semiconductor gases as well as gaseous mercury (Hg) in ambient air. These metallic compound gases should reveal reactivity with respect to ozone and gas to particle conversion could be obtained in a gas to particle conversion device (GPD) through metal oxides by oxidation. The particles converted were separated from non-reactive gases such as nitrogen, oxygen, carbon dioxide in ambient air by a gas exchange device (GED) and the particles in argon, otherwise ICP cannot be maintained, were directly introduced and measured by ICP-MS. Since the technique detects the metallic compound gas directly without any sampling methods, it is expected to be applied to real-time monitoring. This article highlights the research progress and novelty on GPD-GED-ICP-MS for the direct analysis of ultra-trace metallic compound gas. It was also noted that the direct analysis of gaseous Hg at the concentration level of a few ng m–3 in ambient air mentioned in this article was world-first achieved by GPD-GED-ICP-MS. The research progress for multi-element analysis in suspended particulate matter by GED-ICP-MS was also mentioned since the GED was always used for GPD-GED-ICP-MS.

Keywords Gas to particle conversion device (GPD), gas exchange device (GED), inductively coupled plasma mass spectrometry (ICP-MS), metallic compound gas, gaseous mercury (Hg), direct analysis, real-time monitoring

(Received January 26, 2018; Accepted March 13, 2018; Published June 10, 2018)

1 Introduction 6582 GED-ICP-MS for Direct Multi-element

Analysis of SPM 658 2·1 Instrumental setup and concept of GED-ICP-MS 2·2 Application studies on GED-ICP-MS 2·3 Issue and future perspective of GED-ICP-MS3 GPD-GED-ICP-MS for Direct Analysis of

Metallic Compound gas 660 3·1 Discovery of gas to particle conversion for

GPD-GED-ICP-MS 3·2 Instrumental setup and reaction mechanisms

for GPD-GED-ICP-MS 3·3 Demonstration of ultra-trace level of

semiconductor gas by GPD-GED-ICP-MS 3·4 Particle size converted in GPD 3·5 Calibration curves and limits of detection 3·6 Direct determination of gaseous Hg by

GPD-GED-ICP-MS 3·7 Issue and future perspective of

GPD-GED-ICP-MS4 Conclusion and Future Remarks 6645 References 665

Masaki OHATA received his Ph.D. on March 2000 from Chuo University. Thereafter he worked at Faculty of Science and Engineering, Applied Chemistry, Chuo University as Technician and Assistant Professor, then moved to National Institute of Advanced Industrial Science and Technology, National Metrology Institute of Japan on April 2004. He has been working there as a senior researcher since 2011, and his continuous research interests are direct and simultaneous multi-element analysis

of solid and gas samples by plasma spectroscopy including laser technology.

Kohei NISHIGUCHI worked at Sumitomo Seika Chemicals Co., Ltd. (1998 – 2012), then moved to J-Science Lab Co., Ltd. on May 2012. He is a developer for both gas exchange device (GED) and gas to particle conversion device (GPD) described in this reviews. His continuous research interests are direct and simultaneous multi-element analysis of SPM as well as metallic compound gases especially used in industries by the analytical techniques described in this reviews, and recently he has started his extended studies as Ph.D.

student at Department of Environmental Engineering, Kyoto University.

Reviews

658 ANALYTICAL SCIENCES JUNE 2018, VOL. 34

1 Introduction

It is already accepted that inductively coupled plasma mass spectrometry (ICP-MS) is widely applied for trace element analysis due to its high sensitivity, multi-element capability, and a wide linear dynamic range.1,2 Most samples are introduced as liquid or solid aerosols covered in argon (Ar) or mixtures of Ar and helium by conventional solution nebulization or direct solid sample introduction methods such as laser ablation (LA), respectively. Unfortunately, a high flow rate (several hundred milliliters per minute) of ambient air or other gases except for Ar, which is suitable to maintain an ICP, would extinguish the ICP immediately which would limit the direct analysis of elements in suspended particulate matter (SPM). In order to overcome this limitation, a gas exchange device (GED) has been proposed and its capabilities have been successfully demonstrated on a variety of applications.3–14 For example, the direct real-time multi-element analysis of airbone particulate matter (APM) has been examined and demonstrated.3–8,10,14 The GED has also been coupled to LA-ICP-MS for the direct atmospheric sampling of laser-generated aerosols for the determination of trace elements as well as the isotope ratio analysis, in particular for larger samples that cannot be placed in a commonly required ablation chamber.9,11,12 Because the GED consists of a porous silica membrane tube, it can not only be used for the exchange of ambient air but also for other gases. For example, the direct multi-element analysis of tobacco smoke, as one of the combustion gases, has also been demonstrated by an ICP-time of flight mass spectrometer (ICP-TOFMS) coupled with GED.13 From these demonstrations, the GED can be expected to be a useful sample introduction tool for ICP-MS to achieve the direct multi-element analysis of SPM in ambient air or other gases.

On the other hand, though SPM can be directly introduced into ICP-MS, trace amounts of gases such as metallic compound gas in ambient air or other gases are exchanged by Ar and therefore lost within the GED. In order to achieve the direct measurement of metallic compound gas, a gas to particle conversion–gas exchange technique coupled with ICP-MS (GPD-GED-ICP-MS) was newly proposed and its figures of merit were successfully demonstrated with respect to metallic compound gases such as metal carbonyl gas, semiconductor gas and gaseous mercury (Hg) in our previous studies.15–19 The reaction mechanism of the gas to particle conversion is based on either the oxidation of metallic compound gas by ozone (O3) or the agglomeration of metal oxide with ammonium nitrate (NH4NO3), which is generated by the reaction of O3 and ammonia (NH3) gases in a gas to particle conversion device

(GPD). In order to separate the reaction gases (remaining O3 and NH3) as well as non-reacted gases such as nitrogen (N2), oxygen (O2), carbon dioxide (CO2) in ambient air from the converted particles, GED3–14 was used and the particles in Ar, otherwise ICP cannot be maintained, were directly introduced and measured by ICP-MS. Since the technique detects the metallic compound gas directly without any sampling methods, it can be applied to the real-time monitoring.

This article highlights research progress on GPD-GED-ICP-MS for the direct analysis of ultra-trace metallic compound gas. The research progress for the multi-element analysis of SPM by GED-ICP-MS was also mentioned since the GED was always used for GPD-GED-ICP-MS.

2 GED-ICP-MS for Direct Multi-element Analysis of SPM

2·1 Instrumental setup and concept of GED-ICP-MS Figure 1 shows a schematic diagram of GED-ICP-MS as well

as one example of the experimental setup.13 The GED-ICP-MS consists of two units from GED (J-Science Labo. Co. Ltd., Kyoto, Japan) and ICP-MS as a sample introduction system and a detector, respectively. The sample gas containing SPM is introduced into the GED by an aspirator or a diaphragm pump which could be set at either entrance or exit of the GED, even though the aspirator is indicated at the exit of the GED in the case of Fig. 1.13 Because the GED consists of two concentric glass tubes with pores of 0.07 μm in diameter, it acts as a membrane. The introduced ambient air or other gases are exchanged by Ar introduced from the outer tube as sweep gas into the GED across the membrane. Thus, the particles stabilized in Ar were introduced directly into the ICP-MS. Due to the absence of ambient air or other gases that are not suitable for ICP, the ICP can be maintained stably and operated at parameters commonly used in ICP-MS. Photo 1 shows the GED-ICP-TOFMS developed at National Metrology Institute of Japan (NMIJ, Tsukuba, Ibaraki, Japan) as one example of GED-ICP-MS. If ICP-TOFMS is used for GED-ICP-MS, simultaneous multi-element analysis in one SPM, which is difficult by an ICP-quadrupole mass spectrometer (ICP-QMS) commonly used in the world, is expected.

2·2 Application studies on GED-ICP-MSThe GED-ICP-MS was developed by Nishiguchi and Utani,

and was firstly reported to industrial or scientific journal in 2008.4–6 They successfully demonstrated the real-time measurement of multi-elements in APM collected nearby their GED-ICP-MS through a PFA tube and also evaluated the

Fig. 1　Schematic diagram of gas exchange ICP-MS (GED-ICP-MS).


performance of GED in detail with respect to the exchanging rate of gases from air to Ar as well as the penetration efficiency of particles into ICP-MS. The real-time multi-element monitoring of APM is possible at the required monitoring site, if the GED-ICP-MS can be transported to there. However, it is not so easy since ICP-MS is not movable easily and it should be located at a fixed position normally due to its size and weight as well as needed facilities such as electricity, an exhaust duct and Ar gas. The calibration method was also required and it was not established at the beginning of the study. They proposed the metal standard gas generator (MSG2),3–7 which is now commercially available from J-Science Lab Co., Ltd. (Kyoto, Japan), to calibrate the sensitivity of elements for GED-ICP-MS. The MSG2 is at least convenient to optimize the operating conditions of GED-ICP-MS such as the gas flow rate of the aspirator as shown in Fig. 1.13 Another group also examined the GED-ICP-MS for the quantitative real-time monitoring of multi-elements in APM in 2010 – 2012.8,10 In order to evaluate the concentration of elements in APM, they proposed a calibration method using ultra-sonic nebulization as well as the MSG2 to calibrate the GED-ICP-MS in advance, even though the method was not a direct one. They also built a sampling line that was ca. 80 m away from their GED-ICP-MS to introduce their required APM into their instrument. As a result, they examined direct multi-element analysis of APM; however, the analytical values obtained by GED-ICP-MS should be corrected by those obtained with filter-collection data since incomplete ionizations of elements in ICP, which was dependent on the particle size as well as their chemical forms such as oxides, were found.10 The correction method was based on the premise that the ionization efficiency of APM during the measurement was always same. Since the GED could also exchange the ambient air to not only Ar but also other gases such as He, it was also applied to LA-ICP-MS as LA-GED-ICP-MS to measure large samples that could not be placed in the usually used airtight ablation cell with limited space.9,11,12 They also developed a quasi-closed cell as an ablation chamber and examined the enhancement of the uptake rate of laser-generated particles through GED from large samples.11 Though a loss of sensitivity was observed, the quasi-closed cell developed was expected to be useful for large sample analysis by LA-GED-ICP-MS. The LA-GED-ICP-MS was also demonstrated with respect to isotope ratio measurements and it

was expected to be attractive for the direct analysis of large and precious samples, even though future development was still required to improve the analytical performance.12 The multi-element analysis of other gases including tobacco smoke by GED-ICP-TOFMS was carried out in our previous study,13 since the GED also could exchange not only ambient air but also other gases such as combustion smoke to Ar. Figure 2 shows one of our results on multi-element analysis obtained from car exhaust gas by GED-ICP-TOFMS. At that time, we used a sampling technique by Tedlar bag as shown in Photo 2, in order to bring the SPM in the sample gas to the GED-ICP-TOFMS for measurement, since the instrument, especially for ICP-TOFMS, was not movable easily from the once located position in the laboratory due to its size and weight as well as needed facilities such as electricity, an exhaust duct and Ar gas. As can be seen in Fig. 2, platinum (Pt) which were expected to be originated from catalyst in the engine of a car could be observed. The Pt was considered to be contained in SPM generated from car exhaust gas since the signal was observed by ICP-TOFMS coupled with GED which could be expected to exchange all gases to Ar. Figure 3 shows one of the analytical results obtained from tobacco smoke by GED-ICP-TOFMS in our previous study.13 The different intensity ratios of lead to thallium (Pb/Tl) in SPM could be found between the mainstream smoke and sidestream one. Though a loss of SPM occurred due

Photo 1　Gas exchange ICP-time of flight mass spectrometer (GED-ICP-TOFMS) developed at National Metrology Institute of Japan (NMIJ, Tsukuba, Japan).

Fig. 2　Mass spectra for both car exhaust and outside air obtained by GED-ICP-TOFMS; elements and their mass numbers (in parenthesis) are indicated.

Photo 2　Sampling manifolds for gas as well as suspended particulate matter (SPM).


to adsorption in the Tedlar bag during gas sampling, the intensity ratios of elements such as Pb/Tl could be compared between the different gases since they were not influenced on the SPM adsorption largely even though their signal intensities could never be compared directly. Recently, GED-ICP-MS for the measurement of 90Sr in APM was reported.14 In order to obtain a 90Sr signal, the authors used an ICP-tandem mass spectrometer and examined the operating conditions of the collision-reaction gases to reduce any isobaric and polyatomic ion interferences of 90Zr and 89Y1H, respectively. It can be considered that the ICP-tandem mass spectrometer will be an effective tool for GED-ICP-MS to measure the elements such as lighter elements of Al, Si, P, Cl, K, Ca as well as Fe, which are concerned with polyatomic ion interferences.

2·3 Issue and future perspective of GED-ICP-MSAs we mentioned above, since GED-ICP-MS can achieve

direct multi-element analysis of SPM in many gases such as ambient air, car exhaust and combustion gases, it is expected to be attractive for environment monitoring as well as relevant industries and societies related to gas analysis. However, if a real-time multi-element measurement in SPM is needed at required site, the GED-ICP-MS should be transported to there and it is not so easy since ICP-MS is not movable easily due to its size and weight as well as needed facilities such as electricity, exhaust duct and Ar gas. If the sample gas cannot be obtained near by the GED-ICP-MS, any sampling techniques should be needed. However, the sampling technique using a Tedlar bag is not suitable for the quantitative analysis of elements since the loss of SPM always occurred during gas sampling even though it can be used for qualitative analysis such as the intensity ratios of elements.13 Though an indirect calibration method using ultra-sonic nebulization and MSG2 was proposed, it was not still sufficient since the introduction efficiency of an elemental standard solution from ultra-sonic nebulization to ICP should be evaluated in advance. In addition, the introduction efficiency of ultra-sonic nebulization is 20 – 40%, which is not closed to 100%, and the efficiency may be changing between different analysis days. If the introduction efficiency is closed to 100%, the estimated uncertainty or bias of analytical results from the calibration method will be smaller and constantly obtained; however, a lower introduction efficiency may result in a larger uncertainty and error with respect to the analytical results. Therefore, the introduction efficiency should be evaluated or calibrated daily to guarantee the accuracy and precision for the quantitative result since it might be changing between different

days. It is considered that suitable sampling and direct calibration methods are at least needed for GED-ICP-MS to expand its versatility and utility.

3 GPD-GED-ICP-MS for Direct Analysis of Metallic Compound gas

3·1 Discovery of gas to particle conversion for GPD-GED-ICP-MS

It was issued that the analysis of metal carbonyl gases such as Ni(CO)6 and Fe(CO)6 in CO and other gases was very difficult because the gas sampling method such as the impinger method using acids could not trap them as sufficiently as necessary for quantification.20–25 In order to overcome the limitation, an effective sampling method that could trap the metal carbonyl gas in hydrochloric acid (HCl) in a vacuum cylinder was developed.20,21,25 The ozone (O3) gas was required to oxidize the metal carbonyl gas to a metal oxide which was then absorbed in HCl. When the required oxidation process was carried out, a white fume was found immediately due to the reaction between metal carbonyl gas and O3, which anticipated the conversion from metal carbonyl gas to particles of metal oxide. On the basis of this observation and the advanced knowledge about the possibility of direct analysis of 0.1 μm particles using GED-ICP-MS,3–14 we studied several gas reactions for gas to particle conversion in more detail and found the reaction using O3 and NH3 gases was the most efficient to generate particles from the metal carbonyl gas.

From the research progress, a gas to particle conversion device (GPD), which could be applied to the gas to particle conversion-gas exchange technique for the direct analysis of metallic compound gas by ICP-MS (GPD-GED-ICP-MS), was developed and the working principle, the reaction mechanism and the figures of merit of GPD-GED-ICP-MS have been discussed in our previous studies.15–19 It is noted that the metallic compound gas as an analytical target should be reactive with respect to O3 gas for gas to particle conversion.

3·2 Instrumental setup and reaction mechanisms for GPD-GED-ICP-MS

Photo 3 shows the GPD-GED-ICP-MS at J-Science Lab Co., Ltd. (Kyoto, Japan) as one example of GPD-GED-ICP-MS. It can be seen that the system consists of three units. Both GPD and GED are commercially available from J-Science Lab Co., Ltd., and the ICP-MS used here is ICP-QMS which is available from several manufactures. If ICP-TOFMS is used, simultaneous multi-element detection originated from several metallic compound gases is expected. However, the simultaneous detection is not always necessary for GPD-GED-ICP-MS since the concentration of metallic compound gas does not change so rapidly compared to that of SPM which reveals transient signals. Figure 4 shows a schematic diagram of GPD-GED-ICP-MS. In order to remove any SPM from the sample gas, a quartz fiber filter was set at the entrance of GPD as shown in Fig. 4. The reaction mechanism of the gas to particle conversion is based on either the oxidation of metallic compound gas by O3 or the agglomeration of metal oxide with NH4NO3 which is generated by the reaction of O3 and NH3 gases in GPD.15–19 The oxidation of arsine (AsH3), as an example, by O3 for the gas to particle conversion in the GPD can be considered as follows:17

2AsH3 (g) + 2O3 (g) ⎯→ As2O3 (s) + 3H2O (l)

The NH3 gas supplied was also considered to generate particles

Fig. 3　Mass spectra for both the sidestream and the mainstream smokes obtained by GED-ICP-TOFMS; elements and their mass numbers (in parenthesis) are indicated.


of NH4NO3 in the GPD when reacting with O3 as described below.

2NH3 (g) + 4O3 (g) ⎯→ NH4NO3 (s) + H2O (l) + 4O2 (g)

The sample gas containing metallic compound gas is introduced into the reaction chamber in GPD where it is mixed with O3 and NH3. Though the oxidation efficiency may be different for different metallic compound gases since their reaction rates in the gas-phase reaction with respect to O3 are slightly different, almost 100% oxidation efficiency is expected if the concentration of O3 as well as the reaction time in GPD is sufficient. The 3% O3 is supplied by an ozonator that is equipped with the latest GPD. The required NH3 gas was obtained from normally ca. 2% NH3 solution prepared from 20% one (e.g., Ultrapur, Kanto Chemical Co. Inc., Japan) by bubbling Ar through a PFA bottle. The reaction with O3 and NH3 provokes the formation of particles which are online transported to the ICP-MS through GED. The particles converted are expected to be almost completely decomposed and ionized in ICP since they mainly consist of NH4NO3. The introduced air and the remaining gases from the reaction (O3 and NH3) are exchanged by Ar introduced as sweep gas into the GED across the membrane. Thus, the particles converted by GPD and stabilized in Ar were introduced directly into the ICP-MS (see Fig. 4). Due to the absence of ambient air, O3 and NH3, the ICP can be maintained stably and operated at parameters commonly used in ICP-MS. The mixed metal carbonyl gases of Cr(CO)6, Mo(CO)6 and W(CO)6 are

also obtained by a metal standard gas generator (MSG2, J-Science Lab Co., Ltd., Kyoto, Japan)3–7 which can be used for the sake of optimization of the support gas for ICP-MS as shown in Fig. 4.16–18 Since the technique detects the metallic compound gas directly without any sampling methods, it can be applied to the real-time monitoring.

3·3 Demonstration of ultra-trace level of semiconductor gas by GPD-GED-ICP-MS

A semiconductor is widely used for electric and electronics equipment in our living tools such as computers and cellular phones, and the market of the semiconductor industry is huge. The ultrapure semiconductor gases such as AsH3, phosphine (PH3) and so on, are well known to be used in the synthesis of semiconducting materials related to microelectronics and solid-state lasers. On the other hands, these gases show serious toxicity with respect to human beings. For example, exposure of AsH3 may lead to renal failure and is a recognized carcinogen.26–29 From toxicity points of view, these gases should be monitored not only at working environments in semiconductor related industries but also in living environments at sites nearby such as coal-fired power plants, coal-fired boilers for industrial use, waste incineration facilities, volcanos and natural gas fields. Therefore, we applied the GPD-GED-ICP-MS to demonstrate the direct analysis of semiconductor gases of AsH3 and PH3 in ambient air.17

Figure 5 shows the time-resolved signal of 75As, as one example, obtained by GPD-GED-ICP-MS with a continuous introduction of 1 nL L–1 AsH3 in air filtered and stable signals could be observed with a smaller fluctuation of ca. 2.5% relative standard deviation (RSD). Slightly severe memory effects were

Photo 3　Gas to particle conversion–gas exchange ICP-MS (GPD-GED-ICP-MS) developed at J-Science Lab Co., Ltd. (Kyoto, Japan).

Fig. 4　Schematic diagram of gas to particle conversion–gas exchange ICP-MS (GPD-GED-ICP-MS) for direct analysis of metallic compound gas.

Fig. 5　Time-resolved signal of 75As obtained by the introduction of 1 nL L–1 AsH3 in ambient air through GPD-GED-ICP-MS. The RSD of 2.5% was calculated from the data obtained between 100 and 200 s.


also observed which required 200 s washout time or more. Figure 6 shows the time-resolved signal of 95Mo obtained by the direct introduction of Mo(CO)6 in Ar into the ICP-MS, in order to compare the signal stability and repeatability between the semiconductor gas converted by GPD and the metal carbonyl gas directly introduced into ICP-MS. As can be seen in Fig. 6, the flat and steady signal with smaller fluctuation of 1.7% RSD was observed for Mo(CO)6 in Ar compared to 2.5% RSD for AsH3 (see Fig. 5). The slightly larger signal fluctuations of AsH3 could be due to the ionization fluctuation of particles in the ICP. The efficiency of the gas to particle conversion of AsH3 was expected to be 50 – 60% since the operating conditions of GPD were not sufficient to achieve 100% conversion efficiency compared to those examined for metal carbonyl gases in our previous study.16 Although either the optimization of operating conditions or the design of reaction chamber of GPD might increase the conversion efficiency and reduce the fluctuation as well as the memory effect, the GPD-GED-ICP-MS could be used for the measurement of trace level of semiconductor gas such as AsH3 in ambient air since RSDs observed sufficiently small.

3·4 Particle size converted in GPDThe size distribution of the converted particles, which were

introduced into the ICP-MS, was also evaluated by scanning mobility particle sizer, GRIMM SMPS + C systems (GRIMM Aerosol Technik GmbH & Co. KG).17,30 The SMPS comprises a condensation particle counter (CPC) as a detector as well as a differential mobility analyzer (DMA), and is designed to measure particle size distribution in a range from 11 to 1078 nm sequentially. In the case of GPD-GED-SMPS measurement, N2 gas was introduced into the GED as a sweep gas for SMPS measurement. Figure 7 shows the size distributions of the particles converted for either the mixed semiconductor gas or the blank one (filtered air).17 The size distribution of particles in a range from 20 to 110 nm as well as the highest particle concentrations at around 60 nm were observed for either the mixed semiconductor gas or the air filtered and the different size distribution as well as the number of particles between them could not be observed. From these results, NH4NO3 is considered to be the main particle produced in GPD by the reaction with O3 and NH3. Though it was not distinctly identified the oxidation step indicated in equations mentioned previously, the converted particles were considered to be agglomerated with NH4NO3 particles and provided for the

measurement by ICP-MS. From the main component of particle as well as the particle size points of views, the ionization efficiency of the converted particle was expected to be almost 100% because the particle was an agglomerated one by mostly NH4NO3 with a size of ca. 60 nm.

3·5 Calibration curves and limits of detectionFigure 8 shows a calibration curve observed for 75As as a

function of different concentrations of AsH3 in filtered air.17 The coefficient of determination (r2) was better than 0.999 in a concentration range of 0 – 5 nL L–1, as shown in Fig. 8. The limit of detection (LOD) estimated was 1.5 pL L–1 (4.9 pg L–1). The LODs from 4 to 400 pg L–1 were expected for AsH3 by ICP-MS coupled with a cryogenic trap.31 The LOD observed for AsH3 in the present study was similar to that of the reported ones, though GPD-GED-ICP-MS could measure AsH3 in ambient air directly without any sampling methods. Since the LODs obtained by GPD-GED-ICP-MS revealed sufficiently lower values than the concentrations of 0.5 nL L–1 for AsH3; which was one order of magnitude lower values of TLV-TWA revealed in ACGIH,26–29 the GPD-GED-ICP-MS could be useful for direct and highly sensitive semiconductor gas analysis.

3·6 Direct determination of gaseous Hg by GPD-GED-ICP-MS

It is well known that mercury (Hg), especially methylmercury

Fig. 6　Time-resolved signal of 95Mo obtained by the direct introduction of Mo(CO)6 in Ar (ca. 5 nL L–1 from MSG2) into ICP-MS. The RSD of 1.7% was calculated from the data obtained between 110 and 300 s.

Fig. 7　Size distribution of particles converted by gas to particle conversion device (GPD). The mixed semiconductor gas that contained 1 nL L–1 AsH3 and PH3 in ambient air was introduced into GPD-GED-SMPS.

Fig. 8　Calibration curve of 75As obtained by GPD-GED-ICP-MS as a function of different concentrations of AsH3 in ambient air. The bars indicated are standard deviations obtained by time-resolved signals (see Fig. 5 for 1 nL L–1 AsH3).


(MeHg) and dimethylmercury (DMHg), is a highly toxic pollutant that poses a serious threat to human health, such as causing Minamata disease,32–36 as well as wildlife.37–39 Gaseous elemental Hg (GEM, Hg0), reactive gaseous Hg (RGM, Hg2+) and particulate Hg (Hgp) are known as atmospheric Hg and the former two exist mainly (>95%) in the gaseous phase.36–45 GEM is the dominant form of gaseous Hg in the atmosphere with background levels reported to be from 1.0 to 1.7 ng m–3 and from 0.5 to 1.2 ng m–3 in the Northern and the Southern Hemisphere, respectively.46–52 Whilst both RGM and Hgp are more reactive and readily exchange between surfaces and the atmosphere, GEM is very stable with a residence time from 6 months to 2 years.36–39 This enables GEM to be transported over a long range and makes it well-mixed on a global scale and it can be a predominant source of Hg in pristine ecosystems in remote areas.

In Japan, since 1996, the guideline values for hazardous air pollutants have been set to reduce health risks, and that for Hg and its compounds is less than 40 ng m–3 for the annual average. In addition, in October 2013 in Minamata city, Kumamoto prefecture, Japan, the Minamata convention on Hg was unanimously adopted and signed by 92 nations including Japan.53 The convention imposes the implementation of measures to reduce atmospheric emissions according to BAT/BET (Best Available Technique and Best Environmental Practice), etc. at the sites of specific sources of emissions of Hg and its compounds into the atmosphere, such as coal-fired power plants, coal-fired boilers for industrial use, smelting and heating processes used in the production of non-ferrous metals, waste incineration facilities, cement clinker production facilities. Thus research into the behavior of Hg concentrations during the disposal process and the continuous monitoring of final exhaust emissions can be expected to become increasingly necessary. From these points of view, atmospheric gaseous Hg (GEM and RGM) should be measured frequently to monitor the concentration of Hg in the atmospheric environment as well as in working and living environments. In order to estimate the risk to working and living environments due to gaseous Hg, direct and highly sensitive analysis, as well as real-time monitoring of gaseous Hg in ambient air is ideally required.

From this point of view, we applied the GPD-GED-ICP-MS to the direct analysis of gaseous Hg in ambient air and succeeded in the direct detection of gaseous Hg at a concentration level of a few ng m–3 in ambient air for the first time in the world.18,19

The oxidation of gaseous Hg to mercury oxide (HgO) particles by O3 in the GPD can be considered as follows:18,54

Hg (g) + O3 (g) ⎯→ HgO (s) + O2 (g)

The HgO particles then formed agglomerates with NH4NO3 particles which were also generated in the GPD and measured by ICP-MS through GED.16–19 In order to remove any SPM from the sample gas, the same quartz fiber filter as previously shown in Fig. 4 was also set at the entrance of GPD. The absorption or adsorption of gaseous Hg was not observed since Hg signals obtained by ICP-MS were not decreasing during the measurements. If the fiber filter is not used, the total Hg measurement in ambient air is possible because not only gaseous Hg but also Hgp can be introduced into ICP-MS through GPD-GED. Though the concentration of Hgp is considered to be different for different measurement sites, its concentrations observed at Cape Hedo at the northermost site in Okinawa main island are at a few pg m–3 level in recent years which is almost three orders of magnitude lower than those of gaseous Hg.55 Therefore, it can be considered that the GPD-GED-ICP-MS can

be used for total Hg measurements in ambient air if the concentration of Hgp is very low at the measurement site. In order to obtain a background signal of gaseous Hg in ambient air by GPD-GED-ICP-MS, a blank gas, which does not contain gaseous Hg, such as high-purity N2 gas, must be used since gaseous Hg exists in ambient air. The blank gas could also be obtained from ambient air by removing gaseous Hg using a charcoal filter (e.g., Charcoal tube, Jumbo type, Shibata Scientific Technology Ltd., Saitama, Japan). In order to prepare the calibration standards of gaseous Hg for GPD-GED-ICP-MS, the standard gas box MB-1 (Nippon Instruments Co., Tokyo, Japan) which provides saturated gas of Hg can be used. The different concentrations of gaseous Hg can be prepared by the dilution of the saturated Hg gas, collected using a gastight micro-syringe (e.g., Hamilton), with high-purity N2 gas in a Tedlar bag (e.g., GL Sciences Inc., California, USA).

Figure 9 shows time-resolved signals of all Hg isotopes obtained by GPD-GED-ICP-MS with a 1000-ms dwell time. Though 196Hg showed a small signal, all Hg isotopes from gaseous Hg in filtered room air could be detected directly by GPD-GED-ICP-MS. The flat and steady signals with 2 – 3% RSD could be observed for major Hg isotopes. As can be seen in Fig. 9, it was also considered that a washout time of ca. 200 s using a blank gas such as high-purity N2 was at least necessary to obtain the original background level of the signal (e.g., ca. 300 cps for 202Hg). The washout time could be due to stagnation of the agglomerates in the GPD. The original background could be attributed to Hg gas contamination from the ozonator in the GPD and Ar gas in both the GED and ICP-MS. From these results, it was evaluated that the GPD-GED-ICP-MS is expected to be useful for the direct analysis of gaseous Hg in ambient air and a longer dwell time such as 1000 ms for ICP-MS provided more accurate analytical results since smaller RSDs could be obtained, even though a slightly longer washout time and reduction of the background level should be areas to improve. We also estimated the gas to particle conversion efficiency of

Fig. 9　Time-resolved signals for (a) seven Hg isotopes and (b) 196Hg obtained by GPD-GED-ICP-MS with 1000 ms of dwell time on ICP-MS.


gaseous Hg by GPD. Since the concentration of O3 from ozonator used was 3% which was expected to improve the efficiency compared to that of 1% and the Hg signals obtained by ICP-MS were not improved if the flow rates of both O3 and NH3 increased to more than 25 mL, the efficiency was considered to be almost 100%.18 Figures 10 and 11 show time-resolved signals of 202Hg isotope observed for different concentrations of gaseous Hg prepared from a standard gas box with tedlar bags and a calibration curve of gaseous Hg observed for 202Hg by GPD-GED-ICP-MS with 1000 ms of dwell time for ICP-MS as a function of different concentrations of gaseous Hg, respectively. The coefficient of determination (r2) was better than 0.9999 in a concentration range of 0 to 10 ng m–3, as shown in Fig. 11. The concentration of gaseous Hg in filtered room air was evaluated to be ca. 2 – 3 ng m–3 by the calibration curve. The LOD of gaseous Hg estimated was ca. 0.12 ng m–3.18 According to a manual from the Ministry of the Environment (MOE) for the Government of Japan, the analytical technique indicated is CVAAS with gold amalgam Hg collection which needs a long sampling time for gaseous Hg, such as 24 h.56,57 The LOD of Hg by CVAAS is ca. 1 pg. Another known technique is CVAFS with gold amalgam Hg collection whose sensitivity is higher than that of CVAAS, resulting in a shorter sampling time of gaseous Hg. The LOD of Hg by CVAFS is ca. 0.1 pg.58–60 Even though CVAFS is used for the analyzer, a sampling time of at least 5 min is necessary for measurements of gaseous Hg at a concentration level of a few ng m–3 in

ambient air. On the other hand, the GPD-GED-ICP-MS could directly detect gaseous Hg at a concentration level of 2 – 3 ng m–3 with a short dwell time such as 1000 ms for ICP-MS, as shown in Figs. 9 – 11. If the concentration of gaseous Hg is 2 – 3 ng m–3, it could be evaluated that GPD-GED-ICP-MS with a 1000 ms dwell time detected 7 – 10 fg of gaseous Hg directly (e.g. 2 ng m–3 × 200 mL min–1 = ca. 6.6 fg s–1).18 As mentioned above, since the LOD of gaseous Hg obtained by the GPD-GED-ICP-MS is ca. 0.12 ng m–3, the LOD in 1000 ms of the dwell time could be estimated to be ca. 0.4 fg Hg. The LOD is at least three or two orders of magnitude lower than those obtained by CVAAS or CVAFS, respectively. Since the LODs obtained by GPD-GED-ICP-MS revealed sufficiently lower values than those of the conventional analytical methods, it could be useful for highly sensitive, direct and real-time monitoring of gaseous Hg in ambient air as well as in working and living environments.

3·7 Issue and future perspective of GPD-GED-ICP-MSAs mentioned above, since GPD-GED-ICP-MS can achieve

the direct analysis of ultra-trace metallic compound gas at least in ambient air, it is expected to be attractive for relevant industries and societies related to environment monitoring including working and living environments as well as gas analysis. It is also expected that the GPD-GED-ICP-MS can be applied to the other gases except for ambient air such as car exhaust and combustion gases. However, if the real-time monitoring of metallic compound gas is needed at required sites such as monitoring sites, the GPD-GED-ICP-MS should be transported to there and it is not so easy since ICP-MS was not movable easily due to its size and weight as well as needed facilities such as electricity, exhaust duct and Ar gas which is the same issue mentioned for GED-ICP-MS. If the sample gas can not be obtained near by the GPD-GED-ICP-MS, any sampling techniques should be needed. In the case of GPD-GED-ICP-MS, the sampling technique using a Tedlar bag is expected to be the one for the analysis of metallic compound gases since it is already used widely for organic gas analysis such as volatile organic compounds (VOCs). If the standard gas of metallic compound one as calibration standards can be available or prepared, e.g., for gaseous Hg from a saturated gas generator introduced in our previous study,18 both qualitative and quantitative analyses are possible. If the standard gas with SI traceability can be available, the quantitative analytical results will be traceable to SI units. The MSG2 commercially available from J-Science Lab Co., Ltd. (Kyoto, Japan) can be also used as the device to generate calibration standards for some metal carbonyl gases. The further studies to explore the applicability of other metallic compound gases such as sulfur and halogen related gases including the availability of their calibration standards as well as other matrix gases except for ambient air such as combustion gases from e.g., coal-fired power plants and boilers for industrial use, which is also expected to contribute to Minamata convention on Hg, should be necessary to expand its versatility and utility for GPD-GED-ICP-MS.

4 Conclusion and Future Remarks

This article highlights the research progress and novelty on GPD-GED-ICP-MS for the direct analysis of ultra-trace metallic compound gas. The research progress on GED-ICP-MS which can provide direct multi-element analysis of SPM is also mentioned since it is necessary for GPD-GED-ICP-MS. Both

Fig. 11　Calibration curve of gaseous Hg obtained by GPD-GED-ICP-MS with 1000 ms dwell time. Only 202Hg mono-isotopic measurement was carried out by ICP-MS.

Fig. 10　Time-resolved signals of 202Hg observed for different concentrations of gaseous Hg by GPD-GED-ICP-MS with 1000 ms dwell time.


GPD and GED are sample introduction systems for ICP-MS and the analytical methods mentioned in this article are attractive for direct and real-time measurements of gas including SPM which can not be carried out easily by ICP-MS with only conventional sample introduction systems. Since ICP-MS is used, high sensitive analysis of multi-elements can be achieved which also results in the direct analysis of gaseous Hg at concentration levels of a few ng m–3 in ambient air for the first time in the world. Though both analytical methods are attractive for direct and real-time measurements, transportation of instruments is one of the issues since ICP-MS is not movable easily due to its size and weight as well as needed facilities such as electricity, exhaust duct and Ar gas. These issues limit to expand their versatility and utility, e.g., for real-time monitoring at required sites. The sampling methods are expected to compensate the limitation for at least GPD-GED-ICP-MS. In the case of GED-ICP-MS, a suitable sampling method has not been found at this moment. The calibration method for GED-ICP-MS is still an issue to improve even though an indirect one has already been proposed. From these points of view, it is considered that suitable sampling and direct calibration methods are at least needed for GED-ICP-MS to expand its versatility and utility. On the other hand, a sampling method using a tedlar bag is expected to be potentially useful for GPD-GED-ICP-MS. Though the candidate standard gases as calibration standards are considered to be not sufficient, the standard gas of a metallic compound one can be available from e.g., MSG2 for some metal carbonyl gases, a saturated gas generator for gaseous Hg, and some gas companies for e.g., semiconductor gases; therefore, it may be considered that the space to improve for GPD-GED-ICP-MS is smaller than that of GED-ICP-MS. Further studies to explore the applicability of other metallic compound gases such as sulfur and halogen related gases including the availability of their calibration standards as well as other matrix gases except for ambient air such as combustion gases from e.g., coal-fired power plants and boilers for industrial use should be needed to expand its versatility and utility for GPD-GED-ICP-MS.

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