Solid-phase microextraction for the determination of iron organic compounds in seawaters and soils by gas chromatography coupled to microwave-induced plasma with atomic emission detection spectrometry
Introduction
The environmental impact of organometallic compounds is mainly a consequence of their wide uses [1]. Ferrocene (Fc) and ferrocene derivatives are an important class of compounds with applications in very different areas. Several publications have highlighted their uses in areas such as material development, catalysis, industry, agriculture, medicine and biology [2], [3], [4]. The biological activity of these metallocenes as antitumor, antimalarial, antineoplastic and analgesic agents, among other properties, explains their role in medicine. In fact, many drugs for cancer and malaria treatment contain mixtures of ferrocenyl derivatives in their formulations, because of their enzyme inhibitory activity. Their application in the development of novel materials, including magnetic materials, polymers and liquid crystals, and as catalysts because of their electronic behavior, give them an added value [2], [3], [4].
Regarding agricultural applications, since the patent published more than forty years ago 1977 [5] for ferrocenyl derivatives, they have been used as agrochemicals to prevent iron deficiency diseases in plants and even for regulating plant growth [6,7]. In addition, their qualities as selective colorimetric and electrochemical chemosensors are opening up a new application field [8,9].
Bearing in mind the potential transport of iron organic compounds from agricultural soils to water sources, and their possible toxic effect for aquatic species [10], rapid, sensitive and reliable analytical methods for their determination in environmental samples is clearly of interest for the protection of human and environmental health. Nevertheless, in spite of the wide and well-known uses of these compounds, to the best of our knowledge, very few analytical methods have been described in the literature for their determination [11,12]. Among the methods proposed are the quenching of fluorescence [11] and electrochemical analysis [12]. In addition, gas chromatography with mass spectrometry (GC–MS) has been used for monitoring Fc and its derivatives to evaluate their potential degradation by bacteria [13]. The literature reports different detection systems coupled to GC for the determination of organometallic compounds such as organotin, organomercury, organolead and organoarsenic species. Thus, MS and atomic emission detection (AED) are the most commonly used methods for organometallic compounds [1]. The latter, particularly, is a well-established tool for element-specific detection. AED is based on monitoring the intensity of the atomic emission lines of the different elements, opening up the possibility of multielemental detection, given that selectivity and sensitivity are its main advantages.
Miniaturized preconcentration techniques have demonstrated their capacity to greatly enhance the sensitivity of analytical methods that fall within the Green Analytical Chemistry guidelines. In this respect, solid-phase microextraction (SPME), which was developed in the early 1990s, is a good choice for the determination of organometallic compounds [14,15], overcoming the drawbacks of classic sample treatments by minimizing the consumption of sample and organic solvents. Especially in the case of GC analyses, reproducibility is improved as SPME provides the opportunity to implement an on-line analytical procedure. SPME is based on the partition equilibrium of the analytes between an organic polymeric phase coated on to a fused-silica fiber and the sample matrix, providing high preconcentration power. The extensive application of SPME for the analysis of different sample matrices, such as environmental samples, has demonstrated its usefulness [16,17].
This paper describes for first time a method for the determination of Fc and five derivatives (1,1′-dimethylferrocene (FcMe2), ferrocenecarboxaldehyde (FcCOH), acetylferrocene (FcCOMe), ferroceneacetonitrile (FcCN) and benzoylferrocene (FcCOPh)) in seawater and soil samples based on SPME applied in the direct immersion mode followed by GC-MIP-AED.
Section snippets
Reagents and solutions
Ferrocene (Fc), 1,1′-dimethylferrocene (FcMe2), ferrocenecarboxaldehyde (FcCOH), acetylferrocene (FcCOMe), ferroceneacetonitrile (FcCN) and benzoylferrocene (FcCOPh) were purchased from Aldrich (Steinheim, Germany). All the compounds were in the 95–98% range purity. Individual stock solutions (500 µg mL−1) of each compound were prepared using HPLC grade methanol (Fluka, Buchs, Switzerland). Working solutions were freshly prepared by diluting the stock solutions in methanol. All the solutions
Chromatographic and AED parameters
To optimize the chromatographic separation and detection steps, 0.2 µL of a methanolic standard solution containing the analytes at a concentration level of 1 µg mL−1 were injected in the splitless mode, with the solvent venting step switched on for 1 min immediately after injection into the GC. Separation was carried out at a 4 mL min−1 helium constant flow-rate, since this value reduced the analysis time and increased the sensitivity compared with lower flow-rates. Higher flow-rate values are
Conclusions
Solid-phase microextraction was seen to act as an interesting preconcentration approach, since it avoids the use of organic solvents in agreement with the goals of green analytical chemistry and allows high preconcentration efficiencies of Fc and its derivatives in only 15 min. Furthermore, the simplicity of the DI-SPME-GC-AED combination, as well as the excellent selectivity of AED, are other advantages of the described method. There was no matrix effect for the seawater samples, but, due to
CRediT authorship contribution statement
Rosa Peñalver: Investigation, Resources, Writing - original draft. Natalia Campillo: Methodology, Validation. Ignacio López-García: Formal analysis, Visualization. Manuel Hernández-Córdoba: Project administration, Supervision.
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.
Acknowledgements
The authors acknowledge the financial support of the Comunidad Autónoma de la Región de Murcia (CARM, Fundación Séneca, 19888/GERM/15), the Spanish MICINN (PGC2018-098363-B-100) and the European Commission (FEDER/ERDF).
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