Analytical atomic spectrometry: an active research area in China

China is among the countries with active research in the field of analytical atomic spectrometry during the last decade. This editorial provides a rough general picture of the present status of analytical atomic spectrometry in China. The 4th Asia-Pacific Winter Conference on Plasma Spectrochemistry (2010 APWC) will be held in November 26–30, 2010, in Chengdu, Sichuan Province, China. Therefore, it is good timing to write such an editorial now to welcome you to China for the conference. Also, congratulations to Dr Guibin Jiang for being elected as an academician of the Chinese Academy of Sciences.

As illustrated in Fig. 1, the number of total publications in the period of 1999–2008 has been growing rapidly, and it was doubled in 2006. Major reasons may include that more and more Chinese scholars have returned to China from overseas in recent years, and they tend to publish their research results in international journals; and that funding for fundamental research from the Chinese government has been growing steadily and fairly heavily. It should also be pointed out that the general R&D capability of Chinese scientists has been improved in recent years. The publications in Fig. 1 mainly include research works in atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma-atomic (optical) emission spectrometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS). Those publications in non-SCI-indexed journals are not included in Fig. 1.

Fig. 1 Number of publications in analytical atomic/atomic mass spectrometry by Chinese authors in the period of 1999–2008. Data from ISI Web of Science.

While publications with traditional AAS and ICP-OES are still among the majority and grow steadily, studies in AFS and ICP-MS develop much faster in China. The successful commercialization of hydride generation AFS started as early as in 1980s in China partially explains its fast growth in the AFS publications by Chinese scholars. In addition to the extremely high detection capability of ICP-MS, one more fact is that more and more organizations in China can now afford relatively more expensive ICP-MS instruments for their research. Additionally, it is interesting to note that more than 60% of the publications are from 20 major organizations, which are fairly evenly distributed in the whole of China except the northwest part. Those organizations, mainly including Wuhan University, Xiamen University, Nankai University, China University of Geosciences (Wuhan), Jilin University, Hebei University, Sichuan University, and the Research Center for Eco-Environmental Sciences of the Chinese Academy of Sciences, publish at least four papers per year on average. It should be mentioned that some other universities or institutes, including Northeast University, Zhejiang University, University of Science and Technology of China, Tsinghua University, Nanjing University and the Institute of High Energy Physics of the Chinese Academy of Sciences, have become more and more active in the field of analytical atomic spectrometry in recent years. Their diverse research interests span from sample introduction, separation/preconcentration, speciation analysis, atomization/excitation, instrumentation, and novel applications of analytical atomic spectrometry especially in biological and environmental studies. Its unique applications in metallomics and clinical chemistry have opened a new research area for modern analytical atomic spectrometry.

1. ICP-MS based immunoassay

The synergy of elemental and biomolecular mass spectrometry is increasingly desired for qualitative and quantitative analysis in life sciences. In 2001, Xinrong Zhang's group reported the pioneering work of ICP-MS based immunoassay for TSH and T4 quantitative determination by using Eu³⁺ as an elemental label linked to an antibody.^1,2 The method’s sensitivity was enhanced by using gold nanoparticle tags instead of lanthanide ions in 2002.³ They achieved simultaneous determination of AFP and hCGβ in clinical human serum samples by using Eu³⁺-labeled anti-AFP and Sm³⁺-labeled anti-hCGβ monoclonal antibodies in 2004.⁴ They also used laser ablation to introduce multi-elemental tags of antibodies immobilized on a microarray to ICP-MS directly.⁵ Furthermore, they proposed a highly sensitive immunoassay based on single-particle mode detection by ICP-MS in a time-resolved analysis mode with Au-NPs serving as model tags.⁶ Prof. Zhang has paved the way of the ICP-MS based immunoassay and is still bringing milestones in the development of this area.

Prof. Xinrong Zhang, Tsinghua University

Ph.D., Ghent University, Belgium, 1997

2. Marriage of atomic spectrometry to chromatography for speciation analysis

The marriage of the high detection power of atomic spectrometry to the high separation capability of chromatography produces the best applications in speciation analysis. Many Chinese groups have been very active and made great contribution to this research area. Xiu-Ping Yan's group extensively explored capillary electrophoresis (CE) to develop a series of hyphenated techniques for speciation analysis, including CE-AFS,^7,8 CE-flame furnace AAS,⁹ CE-ETAAS,^10,11 chip CE-AFS¹² and short CE-ICP-MS.¹³ These hyphenated techniques are expected to find wide applications in small laboratories lacking expensive and sophisticated instruments to carry out speciation analysis and to study the interactions between metal species and biomolecules.¹⁴

Qiuquan Wang's group developed a novel photochemical vapor generation (photo-CVG) in which electrons at the conduction band of nano-TiO₂ generated by using UV illumination are used for pre-reduction of Se species and direct cold vapor generation of Hg species. Accordingly, they made a photocatalysis reduction device to hyphenate HPLC and AFS for Se¹⁵ and Hg¹⁶ speciation analysis. Meanwhile, they developed time-resolved thiourea (TU) pre-reduction system¹⁷ and a photo-CVG system using mercaptoethanol,¹⁸ which were also successfully used for online interfacing HPLC and AFS for Se and Hg speciation in environmental and biological samples. Furthermore, they designed and constructed a thermodiffusion interface to couple GC with ICP-MS for fast simultaneous speciation of organic and inorganic Pb and Hg species.¹⁹ These approaches are useful in monitoring the occurrence, pathways, toxicity, and/or biological effects of these compounds in the environment and in organisms.²⁰ More importantly, they developed element-coded affinity tag strategies for peptide/protein screening and quantification, in which native element-tag (Se) and foreign element-tags (Hg and lanthanides) have been investigated. Quantified peptides and/or proteins in biological samples can be obtained by determination of the tagged elements using HPLC-AFS with photo-CVG and HPLC-ICP-MS.^21,22 Xinrong Zhang's group made a survey of arsenic species in Chinese seafood by using anion and cation exchange HPLC coupled to ICP-MS for food safety evaluation.^23,24 It was found that a major share of arsenic in seafood is organic arsenic with a low toxicity and does not pose a risk to human health. Bin Hu's group combined liquid phase microextraction with GC-ICP-MS for selenoamino acids,²⁵ butyltin compounds²⁶ and polybrominated diphenyl ethers (PBDEs)²⁷ analysis. With self-prepared stir bar sorptive extraction coating, they developed a headspace sorptive extraction-GC-ICP-MS method for the analysis of volatile organo seleno compounds in biological samples.²⁸ Various microextraction techniques have been heavily involved in their works of ETV-ICP-OES/MS and graphite furnace AAS.^29–31

Guibin Jiang's group made great contributions to the hyphenated techniques, especially for the speciation analysis of mercury-, arsenic-, selenium-, and tin-compounds by using chromatographic separation coupled with atomic spectrometric detection.^32,33 In recent years, his group developed a series of methods based on AFS detection coupled with GC or HPLC for speciation analysis of mercury,^34,35 arsenic³⁶ and selenium.³⁷ Based on a novel photo-CVG with formic acid instead of the conventional K₂S₂O₈/KBH₄ system, they developed a simple and environment-friendly HPLC-AFS method for mercury speciation.³⁸ By using HPLC-HG-AFS, they provided a new, non-toxic and effective extraction method for arsenic speciation in rice straw with water-ethanol mixture and microwave-assisted extraction.³⁶ They also developed an on-line coupled HPLC-HG-ICP/MS system for rapid and direct speciation of methyltins in seawater.³⁹ Their distinguished work made contributions to the development of hyphenated techniques and speciation analysis in China.

Prof. Guibin Jiang, Research Center for Eco-Environmental Sciences

Ph.D., Chinese Academy of Sciences, Beijing, 1991

Academician, Chinese Academy of Sciences, 2009

Prof. Qiuquan Wang, Xiamen University

Ph.D., Gunma University, Japan, 1998

3. Sample introduction

Sample introduction remains a hot research topic in analytical atomic spectrometry. Sample introduction via chemical vapor generation (CVG) frequently enjoys many critical advantages, such as increased detection power, elimination of interference, and high transport efficiency. Traditional CVG (HG, hydride generation) is still popular, especially in its hyphenation with atomic fluorescence spectrometry. Xiu-Ping Yan's group found room temperature ionic liquids (RTIL) could enhance the vapor generation efficiency of Cu, Ag, Au and Ni,^40,41 and a mechanistic study showed that RTILs can interact with metal cations to provide more effective precursors for CVG. Recently, the variety of elements amenable to CVG has been increased significantly from the classical group IVA to VIA elements and Hg to a number of transition and noble metals. Especially, the use of UV radiation has been explored for enhancing the vapor generation efficiency and to expand detectable element coverage. This newly emerging research area may provide a powerful alternative to conventional schemes due to its simplicity, versatility, greener nature, and cost effectiveness, as summarized in a recent review by Xiandeng Hou and Ralph Sturgeon.⁴² Applications of photo-CVG comprise processes to pre-reduce valence states of various species prior to use of conventional one, for example, the reduction of a number of selenium species prior to hydridization with tetrahydroborate;¹⁵ and direct photo-CVG which gives rise to the formation of volatile metal containing species, such as volatile selenium hydride, carbonyl or methyl- and ethylated-derivatives, depending on the experimental conditions. By use of photo-CVG, Xiandeng Hou's group achieved simple speciation analysis without chromatographic separation,^43,44 and a green analytical purpose.⁴⁵ Photo-CVG with low molecular weight organic alcohol, aldehyde or carboxylic acid will have a great future, especially for the determination of mercury.^46,47 On top of their excellent work on separation and preconcentration, Bin Hu's group made great contributions to electrothermal vaporization for sample introduction into ICP-OES and ICP-MS. They used polytetrafluoroethylene (PTFE) slurry as fluorinating reagent to promote the volatilization of refractory elements and carbide-forming elements from the graphite furnace. The vaporization and transport efficiencies were improved and the detection limits were one to two orders of magnitude lower.⁴⁸ Xiandeng Hou's group used tungsten-coil electrothermal vaporization to expand the detectable element coverage for traditional hydride generation-AFS instruments,⁴⁹ and the same device can be used to introduce samples into flame furnace AAS.^50,51 This can be useful for direct solid sampling.

Prof. Xiandeng Hou, Sichuan University

Ph.D., University of Connecticut, USA, 1999

Prof. Bin Hu, Wuhan University,

Ph.D., Wuhan University, China, 1992

4. Advanced materials for separation and preconcentration

Besides the great achievement on the studies of hyphenated techniques, Xiu-Ping Yan's group devoted much of their effort to the study of unique separation and preconcentration methodologies. For example, they coupled flow injection (FI) knotted reactor/minicolumn preconcentration with atomic spectroscopic techniques for improved detection of trace elements in complicated matrices.^52,53 Particularly, several advanced materials, including ion-imprinted functionalized silica gel,⁵⁴ small carboxylic acid grafted polytetrafluoroethylene fiber,⁵⁵ biomolecule-functionalized carbon nanotube,⁵⁶ and macrocycle immobilized silica gel,⁵⁷ were fully investigated as the packing materials for the minicolumn. They also developed a displacement sorption protocol by using the stability difference of the complexes formed between metal ions and complexing agents to improve the selectivity by 2–3 orders of magnitude.^58–60 Bin Hu's group also paid great attention to the advanced materials for separation and preconcentration. They explored various advanced materials as solid-phase extraction materials, such as nanometer sized TiO₂, Al₂O₃ and ZrO₂, mesoporous TiO₂ and Al₂O₃, magnetic nanoparticles, as well as ion-imprinted materials, and established novel analytical methods based on flow injection on-line/off-line micro-column separation/preconcentration-ICP-OES/MS for trace element analysis or speciation analysis.^61–64 Xiandeng Hou's group explored the surface chemistry of nano-TiO₂ for automated speciation analysis of Cr(III) and Cr(VI) in drinking water using flow injection and ETAAS detection.⁶⁵

Prof. Xiu-Ping Yan, Nankai University

Ph.D., Chinese Academy of Sciences, Beijing, 1993

5. Flow based sample pretreatments hyphenated with atomic spectrometry

In the development of flow injection/sequential injection configuration, the introduction of the lab-on-valve (LOV) system is one of the most intriguing innovations, which not only provides a more flexible approach for flow manifold design but also opens a promising avenue for the miniaturization of analytical instrumentation. It has wide potential applications in miniaturized sample pretreatment. Jianhua Wang's group has been dedicating to the study of miniaturized separation and preconcentration protocols for trace metal species with the so-called “meso-fluidic lab-on-valve system”. By introducing a renewable micro-column, they employed octadecyl immobilized silica beads and other solid phase extraction substrates as packing materials for ultra-trace elemental separation and preconcentration in a lab-on-valve system,^66,67 followed by graphite furnace AAS and AFS detection. They have recently exploited the potential applications of live cells in the separation and speciation of metal species. Speciation of inorganic arsenic can be achieved when incorporating live HeLa cells immobilized Sephadex G-50 beads into a lab-on-valve system as preconcentration and differentiation medium.⁶⁸

Prof. Jianhua Wang, Northeast University

Ph.D., Technical University of Denmark, Denmark, 2002

6. DBD in analytical atomic spectrometry

Dielectric barrier discharge (DBD) features simplicity, compactness, low cost, low power consumption, low temperature, and more importantly, highly energetic electrons (1–10 eV). In recent years, Xinrong Zhang's group developed a hydride atomizer for AAS using atmospheric pressure DBD,⁶⁹ and used as an HPLC detector for arsenic speciation. Using the DBD-AAS system, they were able to measure Se, Sb, and Sn down to the nanogram per milliliter level via hydride generation for sample introduction.⁷⁰ The DBD atomizer can also be used in AFS, for example, for As, Se, Pb, Sb and Bi determination.^71–73 Jianhua Wang et al. developed a miniaturized AFS system based on a DBD atomizer in a lab-on-valve,⁷⁴ providing similar sensitivity as bulky atomic fluorescence spectrometer. Furthermore, the DBD has very recently been applied as a radiation source for optical emission spectrometry (OES) for the determination of mercury by Xinrong Zhang et al. collaborating with Gary M. Hieftje⁷⁵ and Jianhua Wang et al.;⁷⁶ and by Xiandeng Hou's group for chemiluminescence and molecular emission spectrometry for the detection of organic species separated by GC.^77,78 The characteristics of DBD source make it attractive in potential portable AAS, AFS and OES systems for field analytical chemistry.

7. Instrumentation

Flame AAS is one of the most widespread traditional analytical techniques for the determination of trace elements, but it often suffers from low sensitivity. A successful attempt to improve the sensitivity of FAAS is the development of the flame furnace AAS (FF-AAS) atomizer by Harald Berndt. Xiandeng Hou's group constructed the FF-AAS atomizer and enhanced the performance of FF-AAS by using cloud point extraction and precipitation-dissolution in knotted reactor as preconcentration methods.^79,80 Integrating on-line atom trapping with FF-AAS, they realized complete introduction of aerosol sample into FF-AAS and on-line preconcentration, thus further improving the sensitivity of FF-AAS (for cadmium, 730-fold over conventional flame AAS).⁸¹ The on-line atom trapping FF-AAS has also been successfully applied for the quantification of cadmium in high-salinity samples after CVG for complicated matrix elimination.⁸² As aforementioned, DBD is also a very promising new type of atomizer for AAS and AFS.

Besides, as an electrothermal vaporizer, tungsten coil can also be used as atomizer for AAS. By using a W-coil atomizer and/or a hand held CCD detector, a portable W-coil electrothermal AAS instrument was constructed^83,84 by Xiandeng Hou's group for the determination of trace metals in environmental water. Besides previous commercialized scanning microwave plasma torch (MPT) spectrometers, Qinhan Jin's group has developed a miniaturized simultaneous MPT spectrometer recently.^85,86 The preliminary results demonstrate that such a simultaneous spectrometer has sample- and time-saving, and low-cost advantages.

8. Nuclear analytical techniques and more

Although we do not plan to include traditional X-ray fluorescence spectrometry here, we cannot miss the use of synchrotron radiation X-ray fluorescence (SRXRF) especially in metallomics studies.^87,88 In recent years, Zhifang Chai's group has been very active in using SRXRF in the study of toxicological effects of heavy metals as well as nanomaterials in living system. To evaluate the neurotoxicological effects of the exposure of rare earth elements, nondestructive and multi- elemental microbeam SRXRF (μXRXRF) was used to map the alteration of many elements, such as Ca, Fe and Zn, in rat brain section after lanthanum exposure.⁸⁹ In recent years, they performed μXRXRF in the study of transportation and toxicological effects of nanomaterials (titanium dioxide, ferric oxide and copper nanoparticles) in animal models.^90–92 For example, they imaged the biodistribution of elements in a model organism, Caenorhabditis elegans, after exposure to copper nanoparticles by μXRXRF.⁹¹ Metalloproteins have been further studied by conventional separation methods combined with SRXRF. In their group, Hg-, Se- and As-containing proteins in liver tissues of big head carp and grass carp sampled from a mercury-polluted area of Guizhou Province, China were separated by thin-layer isoelectric focusing, and the relative content of Hg, Se, and As in protein bands was measured by SRXRF. The results provided important data for the metabolic studies. Other excellent works include the unique applications of laser ablation-based analytical atomic spectrometry in the study of geosciences by Shan Gao's and Shenghong Hu's groups of China University of Geosciences.

Prof. Zhifang Chai, Institute of High Energy Physics

Academician, Chinese Academy of Sciences, 2007

Xiandeng Hou

College of Chemistry and Analytical & Testing Center, Sichuan University

Chengdu, Sichuan, 610064, China

houxd@scu.edu.cn

Acknowledgements

X. D. Hou acknowledges the financial support from the National Natural Science Foundation of China (No.20835003).

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