Green sample preparation of complex matrices: towards sustainable separations of organic compounds based on the biorefinery concept

Introduction

Especially after the industrial revolution, analytical chemistry has been considered central in all research areas, manufacture sectors, regulatory agencies and human activities, enabling the qualitative and quantitative understanding of natural and artificial complex systems. For such challenging matrices, not only the amount and identity of the components, but also how they are distributed in space and time as well as the best ways to measure them are fundamental data to be known, which include the design of the necessary analytical instruments and approaches [1, 2]. In spite of the present concern about the socio-environmental impacts caused by the chemical practices from lab scale to production processes over the last decades, most of the conventional analytical methods employed for the determination of organic compounds in several matrices show negative effects, mainly due to the high quantities of hazardous substances involved in multiple steps of the whole procedure. In many contexts, these procedures can generate a larger amount and variety of toxic residues which can be more deleterious than the studied analytes [3].

From the 1960s onwards, when processes and chemicals showed adverse effects well beyond their intended use, the society began to realise that catastrophic implications could result from conventional chemicals and practices, which, among other initiatives, contribute to generate laws and regulations to address the problems derived from chemistry in diverse contexts. As a result, a philosophy known as green chemistry started and got much attention in the 1990s worldwide and is defined as the utilisation of a “set of principles that reduces or eliminates the use or generation of hazardous substances in design, production and application of chemicals” [4].

According to US Environmental Protection Agency (EPA), the efforts to speed the adoption of this philosophy have led to significant environmental benefits, innovation and a strengthened economy, emphasising that green chemicals will save industry $65.5 billion by 2020 [5]. The RD&I in green chemistry have grown significantly over the last 15 years and, as can be seen in Fig. 1, a total of 9774 papers have been published during this period, among which 1330 manuscripts focus on green analytical chemistry (ISIS Web of Knowledge; Thomson Reuters) (Fig. 1). Not surprisingly, one of the papers that has the highest number of citations was written by Anastas and Kirchhoff (>880), pointing out that among the main topics studied in green chemistry, analytical method development has always occupied a primordial place in this wide research field [6, 7].

Fig. 1:

Number of papers containing the keywords (a) “green chemistry” and (b) “green analytical chemistry” by ISIS Web of Knowledge (January 2000 to August 2015).

Green analytical chemistry can be understood as the development and application of safer techniques and methods that generate less hazardous waste. As expected, some principles are more directly related to analytical chemistry, as the prevention of waste, the use of ideally innocuous reagents and renewable materials, omission of derivatisation steps, real-time analysis, the application of practices that minimise accidents and design for energy efficiency. In fact, no analytical procedure is completely benign, but it is necessary to follow this ideal to make a method as environmentally friendly as possible. Therefore, operating conditions associated with adequate quantity and low toxicity of reactants, products and by-products are considered as valuable as other parameters of critical analytical quality, such as selectivity, sensitivity, accuracy, precision, linearity and robustness [8]. Additionally, another important dimension that should be taken into account is the cost of a method. Nowadays, increased greenness and efficiency do not always equate with overall reduced cost as a number of factors need to be considered, e.g., price of reagents, energy used in the system, disposal of by-products, etc. [9]. However, a more comprehensive economical understanding of the greening efforts in method development will be one of the most attractive aspects of green analytical chemistry in the near future. New devices and systems are available in the market at a competitive cost, providing analytical determinations as fast-as-possible where, in many cases, time is also a matter of business (Fig. 2) [10].

Fig. 2:

Balance between the benefit and cost dimensions through green analytical chemistry [Adapted from 10].

In general, analytical processes encompass sample collection, transportation, storage, followed by sample preparation and final determination. Overall, sample preparation is considered the most demanding stage, consuming around 60–80% of the analysis time and reagents for the treatment and separation of the components or fractions of interest. This step is the primary source of error, and is most often a necessity since the simplest samples are frequently inadequate for direct determination, which arise from excessive dilution of the analytes, the high number of interferents or from the incompatibility with the standard operational requirements for some equipment [11].

In order to make sample preparation greener, a number of approaches for the determination of organic compounds have been described in the literature, e.g., techniques based on miniaturised liquid-liquid extraction (LLE), solid-phase extraction (SPE), solid-phase microextraction (SPME), headspace (HS) analysis, membrane techniques or the use of alternative solvents [10].

It is noteworthy that the increasing amount of data obtained with the use of instrumentation for high throughput screening has also driven the development of tools for data processing, with special attention to chemometric methods. Also, another important and relatively incipient approach is the use of metrics, which enable a meaningful assessment of green processes corresponding to the principles of green chemistry. In real-world scenarios, these metrics can provide part of the story of a process, i.e., encompassing “shades of green” [10, 11].

Green metrics cover topics as resources, processing, materials, cleaning and renewability. Amongst them, life cycle assessment (LCA) is a methodology that explores the environmental impact assessment known as a cradle-to-grave evaluation, in which the resource consumption, pollutants generated, and their environmental impacts are taken into account and assessed at each stage [12].

As can be noted, the trends in analysis and monitoring indicate that progress has been particularly prominent in the development of those methods and techniques ensuring compliance with green analytical chemistry principles, being solvent-free sample preparation techniques of particular interest [13]. Based on current achievements [14], and on the personal author’s experience and views on present and future issues related to sample preparation of complex matrices, a new perspective about sustainable separations of organic compounds related to the biorefinery concept will be discussed.

Trends and challenges in green sample preparation: demands for plural point of view

Ideally, the best sample preparation is “no sample preparation”, but this can be a far-away dream mainly because of the complexity of the samples and the concentration of the analytes [15]. However, depending on the case, some approaches can be of great interest. For instance, the direct determination of rutin in green tea infusions was successfully performed using square-wave voltammetry with a rigid carbon-polyurethane composite electrode. It was observed that for tea infusions the electroanalytical method with deconvolution procedure could be applied without previous separation, showing that from 75.4 to 91.3×10^–6 mol L^–1 of this flavonoid was present in the commercial teas investigated. In addition, very satisfactory reproducibility (3.5%) and LOD values for rutin determinations were found, similar to the chromatographic ones (2.5×10^–6 mol L^–1) [16].

However, despite the current degree of development of the instrumentation used for final determinations, this ideal approach cannot be fully accomplished for the majority of real samples. Concepts like miniaturisation, hyphenation, integration and simplification became key ones that have already been shown to effectively contribute to face a number of drawbacks of conventional sample preparation methods and that, in some studies involving size-limited samples, can probably be considered the best, if not the only, analytical alternatives [3, 8, 17, 18]. Aiming at illustrating some of the sample preparations to determine organic compounds proposed by the author, examples of those will be presented, which can reduce, replace or eliminate the use of hazardous solvents, as well as minimise the number of steps of the whole method, which guarantee all the necessary analytical quality parameters for the given cases. It is important to highlight that these case studies can be considered greener and simpler alternatives to solve (or contribute to face) real problems related to socio-environmental studies, quality control of phytomedicines, healthy food and food security, among other topics of ultimate importance all over the world.

Sustainable separations: biorefinery concept as a starting point

First of all, it is essential to decide what we want and/or need to separate in a complex system. This is also about our capacity of discrimination, i.e., the establishment of the difference between one thing and another and the best way to materialise it. Separation is one of the fundamental processes in chemistry and chemical engineering, involving multiphase and/or multicomponent distinctions. Nearly all reactions generate co-products or residues that usually require extraction and/or purification steps; the same occurs with natural products or raw materials before their use, transformation or removal. For instance, a bottleneck for the lignocelluloses industrialisation is the lack of an efficient and robust separation technique [26]. If the components of the lignocellulosic feedstock cannot be separated adequately, the control of the quality and characteristics of the final products can be impossible. Besides not playing an important role in improving economic efficiency, the lignocellulose industry has also become a source of waste. It is estimated that 350 million tons of lignocellulose waste is available per year only in Brazil, showing that the country has not reached its full potential yet, especially important for the sugarcane bagasse [27].

In this context, sustainable separations can be understood in the scope of biorefinery concept, conceived as an integrated unity for the conversion of biomass into multiple value-added products including energy, chemicals and materials. According to this model, the circular system can create flexible, zero waste networks, which are applicable to a variety of low value local feedstocks or, ideally, biomass residues (such as forest and agro-industrial residues, food waste, etc.) [28]. Based on our experience, biorefineries are more than promising approaches that are able to generate energy, solvents and biobased chemicals, but also unities that can either produce and/or separate (extracted/purified) high-value compounds using the energy and materials manufactured in the same system. These are the cases of CO₂, solvents or biofuels (bioethanol), catalysts or even carbonaceous mesoporous material employed as sorbents [17, 28] (Fig. 3).

Fig. 3:

The integrated biorefinery as a source of chemicals, energy and materials also used to perform sustainable separations in the same system (see detached arrows) [Adapted from 28].

For instance, from an orange peel based-biorefinery it is possible to produce fibre, essential oil, essences, d-limonene, ethanol, seed oil, limonoids as well as pectin that can be transformed and employed as solid phase to separate endogenous citrus compounds that were extracted in the same processing unity [26]. Based on this circular model, the pectin derived from citrus peel can be used as starting material for the preparation of mesoporous carbon chromatographic columns which, in turn, allows the determination of high-value natural compounds present in the same fruits or their wastes. Pectin aerogel can be prepared in a three-step process consisting of gelation in water, addition of TBA to the desired TBA-water ratio, and freeze-drying, followed by a pre-treatment to be used as a sorbent [29]. In fact, the method based on solid phase extraction (SPE) using residual derived-pectin as sorbent was successfully applied to the determination of nine flavonoids commonly found in citrus waste from Brazil, showing similar accuracy and reproducibility of those determined by conventional C₁₈-SPE, which recoveries from 80.4 to 118.4% (RSD from 0.6 to 10.5%) at 10 ng mL^–1 level [17, 30].

The tuneable properties of mesoporous polysaccharide-derived materials from waste can lead to the production of a set of sustainable sorbents that can fit a wide variety of applications. This holistic approach can aggregate analytical methods from lab to large scale, including the biochemical, thermochemical, extraction and purification processes with internal reuse/recycle of materials (as gases, water) and energy. In addition, to guarantee as much as possible sustainable processes that are not only dependent on the “stuffs”, but mainly responsible decision-makers, it is important to perform interdisciplinary studies involving contents from chemistry, engineering, bioeconomy, social science, anthropology fields, etc. [26, 31–34]. However, such interdisciplinary studies associated to the establishment of a biorefinery model that allows to identify possible valuable components derived from biomass waste with a maximum efficiency and the least steps possible is a challenging task yet.

Conclusion

Indubitably, green analytical methods are of fundamental importance to reduce socio-environmental impacts derived from chemical analyses, especially of sample preparation in manufacturing activities, research or regulation, from public or private institutions. Among them, miniaturisation and solvent elimination are of particular interest currently. As has already been stated, extraction of valuable natural products before their transformation during biochemical and thermal treatments can increase the overall financial returns significantly, but it requires a prospective interdisciplinary study involving professionals from other fields. The transition from an instrumental to a more holistic science model that demands a responsible participation to better understand and decide what, how and why to separate the components (or fractions) of a complex mixture is in progress.

Thus, biorefinery is a new perspective beyond the principles of green chemistry so that, based on a comprehensive design, a number of so-called wastes derived from agro-industrial biomass can be employed to prepare solvents and other materials especially useful in making alternative tools, equipment as well as proposing new techniques and methods for the determination of a given compound in different samples, normally of high complexity. As an example, pectin extracted from citrus peel can be used as starting material for the preparation of mesoporous carbon chromatographic columns for SPE, allowing the extraction and purification of high-value natural products present in the same wastes, as is the case of flavonoids. This new circular approach constitutes a real opportunity to bring together sustainable separations and an integrated web of systems that use biological resources, maximise value creation and, as far as possible, work in close conjunction in terms of raw materials, water, by-products, costs and energy supported by interdisciplinary research teams.