Elsevier

Analytica Chimica Acta

Volume 1130, 15 September 2020, Pages 10-19
Analytica Chimica Acta

Monitoring of adsorption and transfer of organochlorines in soybean seeds and sprouts with mass spectrometric imaging

https://doi.org/10.1016/j.aca.2020.07.011Get rights and content

Highlights

  • Monitoring the transfer and bio-accumulation of hazardous compounds.

  • Mass spectrometric imaging of molecular distribution in soybean seeds.

  • Structural interpretation with fragment ions.

  • Ionization of neutral molecules by exothermic photoelectron capture.

  • Fragmentation through electron-initiated bond cleavages or hole oxidization.

Abstract

Development of analytical techniques that can monitor the adsorption, transfer and in-situ distribution of environmental pollutants in agricultural products is essential to ensure the implementation of stringent food safety standards for consumer protection. A mass spectrometric imaging approach is described herein to investigate the dynamic changes and spatial distributions of 4, 4′-DDT (dichlorodiphenyltrichloroethane) in soybean seeds and sprouts during the growth. Soy beans seeds incubated in DDT containing water were sliced in every 20 μm and directly blotted on the surface of a compressed thin film of (Bi2O3)0.07(CoO)0.03(ZnO)0.9 nanoparticles. Endogenous molecules and exogenous DDT compounds in soy bean seeds were ionized and dissociated by photoelectrons that are generated on surfaces of semiconductor nanoparticles upon the irradiation of the 3rd harmonic (355 nm) of Nd3+:YAG laser. Structural identification is achieved by the interpretation of fragment ions resulting from electron-initiated specific bond cleavages or hole oxidization. Mass spectrometric images reveal increased quantities of DDT residues in soy bean seeds and sprouts during the growth. It provides an in situ way without extensive sample preparation to monitor the transfer and distribution of exogenous pollutants as well as the possible impacts on plant growth.

Introduction

There are increasing concerns globally on persistent, bio-accumulative environmental pollutants that may have impact on ecosystems and human health. People has been seeking various techniques that can not only be used to eliminate insects, weeds and other plant pests but also to control bacterial, fungal and viral infections [[1], [2], [3], [4], [5]]. With the development of advanced organic synthetic chemistry, chemicals for agricultural usage have been changed from simple sulfur, vinegar or whale oil to more targeted and effective organic compounds [6,7]. In particular, chlorinated compounds such as dichlorodiphenyl trichloroethane (DDT) have ever been used extensively as insecticides worldwide [[8], [9], [10], [11], [12], [13]]. The insecticidal properties of DDT are ascribed to its impact on the transmission of nerve impulses and the dedicate balance of sodium and potassium in neuron cells [14]. Slowing sodium (Na+) influx and inhibiting potassium (K+) outflow result in excess intracellular K+ in the neuron which cause depolarization of cells and damage peripheral nerves and brains [[15], [16], [17]]. Currently, it is still used in South Africa in order to fight against mosquitoes that transmit a serious global human disease malaria, as well as yellow fever [[18], [19], [20]]. The high efficiency in controlling a wide range of insects but low acute mammalian toxicity makes it become a popular agricultural and forestry pesticide [[21], [22], [23]]. More than four billion pounds of DDT have been used throughout the world since 1940 [[24], [25], [26]]. However, the non-degradability and poor solubility in water cause the bioaccumulation of DDT in soils, river sediments and eventually transfer to food chains [[27], [28], [29]]. The usage of DDT was banned in most countries by Stockholm convention for ecological considerations [[30], [31], [32], [33]].

Currently DDT residues can still be detected not only in environmental soils, sediments and ecosystems but also in milk, human blood or tissue samples [[34], [35], [36], [37]]. It has been reported that DDT is most prevalent and persistent among known organochlorine pesticides (OCPs) [[38], [39], [40]]. Detected quantities of DDT range from several hundreds of ng/g to sub-μg/g in soil samples collected from the surrounding of waste disposal sites [[41], [42], [43]]. Like other environmental pollutants, DDT are generally analyzed with ensemble-averaged approaches that are focused on the analysis of overall quantities, instead of dynamic changes at different times and different locations. In these approaches, environmental samples are usually subjected to sample preparation steps such as solvent extraction or solid phase enrichment followed with chromatographic separation and structural identification with spectroscopic techniques [[44], [45], [46], [47]]. In order to investigate how environmental pollutant enters food chains, it is necessary to establish analytical techniques that can monitor the dynamic processes of the transfer and transformation of those compounds in the ecosystem.

A mass spectrometric imaging technique [[48], [49], [50]] that does not need extensive sample preparation is described herein to show the transfer of exogenous compounds from the environment into soy beans, changes of quantities in different growth stages and the distribution of pesticide residues and metabolites in different parts. Compared with spectroscopic techniques, mass spectrometry can unambiguously identify structures of unknown molecules based on accurate masses of molecular ions and fragment ions. Then it can be applied to reveal the bio-transformation reactions that may produce unknown ions or molecules. In particular, the capability to simultaneously detect co-existed endogenous metabolites in a full scan manner makes it possible to investigate the effects of exogenous compounds on biological processes. This technique provides a new way to detect hazardous compounds and facilitates the in situ studies of the toxicity.

Section snippets

Reagents and apparatus

Nanoparticles of bismuth cobalt zinc oxide (Bi2O3)0.07(CoO)0.03(ZnO)0.9 (<100 nm BET or <50 nm XRD) was purchased from Sigma-Aldrich (MO, USA). In order to remove background peaks of trace organic contamination, nanoparticles have been thermally treated at 350 °C for 2 h in a muffle furnace (Hubei, China) before use. All standard fatty acids (including C4:0, C6:0, C8:0, C10:0, C12:0, C14:0, C16:0, C18:0, C20:0 and C22:0) and 4, 4′- dichlorodiphenyl trichloroethane (DDT) were purchased from

Photoelectron-directed ionization and dissociation of 4, 4′-DDT on surfaces of semiconductor nanoparticles

Mass spectrometry is based on the detection of mass-to-charge (m/z) ratios of gaseous ions resulting from the ionization and dissociation of neutral molecules. In this work, samples were analyzed under a high vacuum condition that is set in a static electric field as shown in Fig. 1 (A) which eliminates the contamination to the environment and avoids the expose risks. The absorption spectrum of 4, 4′-DDT in Fig. 1 (B) shows that there are strong absorptions in the region from 200 nm to 250 nm.

Conclusion

In order to ensure the implementation of stringent food safety standards, the simple analysis of static quantities of environmental pollutant is not able to monitor the dynamic transfer, distribution, bio-accumulation and bio-transformation of hazardous compounds as well as the quantitative assessment of health risks. Mass spectrometric imaging provides a new way for in situ investigation of exogenous and endogenous molecules. Samples were analyzed in the high vacuum condition with the proposed

Author contribution

Hongying Zhong has worked on conceptualization, formal analysis, funding acquisition, methodology, project administration, supervision, writing-draft and writing-review & editing. Xiaojie Yang and Xiebin Leng have worked on data curation and investigation. Yinghua Qi, Juan Zhang and Ruowei Jiang have been involved in data curation, investigation and data analysis. Weidan Li has been involved in data analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Authors thank the financial support from National Natural Science Foundation of China (NSFC 81761128005, 21575046), Research Funds of Central China Normal University from the Ministry of Education. The Program of Introducing Talents of Discipline to Universities of China (111 program, B17019), International Joint Research Center for Intelligent Biosensing Technology and Health, Hubei International Scientific and Technological Cooperation Base of Pesticide and Green Synthesis.

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    These authors contribute equally to this work.

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