Chapter 1: Introduction to Green Sample Preparation

An analytical technique typically consists of four steps, which are sample collection, sample preparation, test result analysis, and evaluation.¹  The sample preparation process includes converting analytes to more detectable forms, extracting them from sample matrices, and preconcentrating them for trace analysis. This step is considered to be a significant source of systematic errors due to the possibility of sample loss, contamination, or insufficient breakdown of analytes. It is often regarded as a difficult, costly, labor-intensive, and time-consuming step in the analytical process. Furthermore, large amounts of acids used for sample separation and organic solvents used for removal of interfering substances, concentration of analytes, and extraction and clean-up of samples, all of which return to the environment as toxic and harmful waste at this stage, make the process the most polluting step.² 

Environmental protection has been a significant priority in recent years. Conducting chemical processes is the task focused on in a more ecologically responsible manner. The notions of sustainable development and environmental conservation first appeared in the scientific community in recent decades. The primary aim is to minimize hazardous laboratory waste generation and maximize pollution avoidance. In response to growing concern about the environmental effects of chemical operations, Anastas and Warner proposed the 12 principles that constitute green chemistry in 1998.³  These principles are about reducing or eliminating the use of raw materials, products, byproducts, solvents, and reagents that could harm the environment or people's health. They also focus on using renewable resources, downsizing analytical procedures, and cutting back on sample preparation and extraction processes.

The focus of this chapter is on the processes of sample preparation, separation, and detection from a “green” standpoint. In particular, special attention is paid to microextraction techniques, which greatly revolutionized sample preparation methodologies, providing environmentally friendly alternatives to the methods used by reducing solvent and energy consumption. This work is expected to make a substantial contribution to the current literature and assist readers in understanding the principles and research behind green extraction.

As a result of the employment of poisonous reagents and solvents and a considerable amount of hazardous waste generated, there was a widespread concern about the adverse impacts of chemistry in the 1990s.⁴  It was in 1990, when the Pollution Prevention Act (PPA) was passed, that Green Chemistry began to take root in American politics, although its roots had been established in the 1970s and 1980s. The green chemistry movement, led by Paul T. Anastas,^3,5  resulted in the emergence of an ethical agreement between chemistry and the environment, although Cathcart⁶  was the first to adopt the term “green chemistry”. Figure 1.1 shows a summary of key milestones in green chemistry.⁷ 

Green chemistry is concerned with the reduction or removal of the use of hazardous compounds in chemical processes as well as the production of toxic intermediates or reaction products, including feedstock, reagents, and solvents. This manufacturing process also consists of the utilization of environmentally friendly raw materials and renewable energy sources.³  Green chemistry, as defined by the U.S. Presidential Green Chemistry Challenge in March 1995, refers to the development of chemical substances and methods that are more benign to the environment.⁸  Additionally, it provides new instruments for aspiring chemists to do chemistry in a more eco-friendly way.⁹  According to Linthorst,¹⁰  the green revolution was based on this, and it encompasses all facets of modern chemistry, from synthesis and analysis to engineering and other fields of study. Following the definition of green chemistry, Anastas and Warner, directors of the University of Massachusetts Green Chemistry Institute, proposed the twelve guiding principles of their approach to green chemistry. The suggested guidelines established a clear set of criteria for the field's continued development.^3,11–13 

The twelve principles of green chemistry:

1. Prevention – prevention of waste

2. Atom economy – design of synthetic methods

3. Less hazardous chemical syntheses – syntheses of less harmful chemicals

4. Designing safer chemicals – design of safer chemicals

5. Safer solvents and auxiliaries – use of safer solvents and auxiliaries such as separating agents

6. Design for energy efficiency – minimization of the energy requirements of a chemical process

7. Use of renewable feedstocks – use of renewable materials

8. Reduce derivatives – minimization of the derivatization process

9. Catalysis – use of catalytic reagents

10. Design for degradation – design of chemical products which can finally break down into innocuous degradation products

11. Real-time analysis for pollution prevention – real-time analysis for in-process monitoring

12. Inherently safer chemistry for accident prevention – safer chemistry to prevent accidents

The aforementioned principles have significant influences on current and future working practices.¹⁴  Although green chemistry is not a subfield of chemistry, it is concerned with adapting chemistry to these new external conditions. The topic of green chemistry is well-established, with several prominent applications in industry,¹⁵  and in this chapter, this topic is discussed in the light of present studies.

Fast-growing industrial technology has become inevitable in order to fulfill the ever-increasing demands of daily life. Growing industrial production causes a significant increase in the number of chemicals used in analytical techniques such as extraction, purification, and identification, as well as an increase in the chemicals used in the manufacturing process. It is vital to modify current analytical methods in order to include environmentally friendly techniques in order to reduce the generation of significant amounts of waste.¹⁶  Therefore, Green Analytical Chemistry (GAC), a subfield of Green Chemistry that applies the same philosophy and principles to the analytical sciences, is developing as a new area of research.¹⁷  It is possible to apply and refine the twelve principles of green chemistry in an analytical chemistry laboratory. Namiesnik's effort to identify the priorities of GAC may be an excellent first step.¹⁸  He focused on four options: (1) eliminating or drastically reducing the use of chemical reagents and solvents; (2) reducing emissions; (3) eliminating compounds with high toxicity and ecotoxicity from analytical processes; and (4) reducing labor- and energy-intensive steps, particularly analytical methods. These concepts have been improved into six main techniques for greening analytical procedures by Guardia and Garrigues:¹⁹ 

1. Direct examination of untreated material as much as feasible

2. Utilization of less polluting sample preparation methods

3. Utilization of automated and miniaturized approaches

4. Investigation into substitutes for reagents

5. Waste decontamination in real time

6. Optimization of energy consumption

The principles of GAC can be applied at every stage of the analytical procedure. Analytical procedures, in general, include sampling, sample preparation, separation, detection, and data analysis.²⁰  However, this chapter focuses on sample preparation, which is sometimes referred to as the “Achilles heel” of analysis. The preparation of samples aims at facilitating the separation of target analytes, reducing sample complexity, and removing the vast majority of matrix interferents.²¹  Preparing samples for analysis is a critical component of the procedure, which has a significant influence on the final result of analysis and requires a significant amount of time. The majority of the chemicals required to purify and separate the relevant components or fractions are consumed during sample preparation, which accounts for 60–80 percent of the analytical time. This is not surprising because sample pre-treatment is thought to be the most polluting part of the whole process. As a result, various attempts have been made to develop alternative sample preparation methods that may facilitate the process in a variety of ways, including compliance with GAC criteria and preservation or enhancement of the analytical performance of the developed method.²²  Recent advancements in sample preparation methods have mostly concentrated on automating, reducing, and simplifying the extraction operations themselves.²³ 

Traditional sample preparation procedures are the most time-consuming, are not automated, and require significant volumes of solvents. The sample preparation step is one of the most important stages in analytical processes.^24–32  With respect to it, numerous laboratories are researching ways to make sample preparation processes more environmentally friendly in accordance with GAC principles.²²  Among the other analytical process steps, sample pre-treatment or extraction is the most promising for green approaches, despite its numerous difficulties. A number of efforts have been made to make sample preparation more efficient by using less energy. These include minimizing the consumption of organic solvents, developing reusable extraction procedures, and increasing the speed of analysis.^33,34 

In sample preparation, the main goals are to decrease/remove interferences in/from the sample and to selectively preconcentrate target analytes from the complex matrix. Additionally, an interference-free sample matrix ensures straightforward target molecule separation, detection, and identification while protecting chromatographic system efficiency.³⁴ 

Liquid–liquid extraction (LLE) and solid-phase extraction (SPE) are general sample preparation procedures in different fields (chemistry, environmental engineering, etc.) for enriching analytes in complex samples via reversible physicochemical interactions. LLE involves the partitioning of an analyte between two immiscible solvents. The problems that LLE had in some areas inspired the development of SPE. SPE is a technique that uses a solid sorbent to adsorb solutes from an aqueous sample in a short column. After this, the analytes are extracted from the sorbent using tiny volumes of strong organic solvents, resulting in their enrichment. SPE is a technology used for separating a broad variety of organic analytes that is safe, efficient, and repeatable. Simple automated operation and selective analyte concentration are its main advantages.^35,36  Beyond these benefits, SPE is a multi-step, solvent-intensive method that takes time. Moreover, a small amount of solvent is used in the SPE process, which results in a few byproducts. Hazardous reagents and solvents used in preconcentration and extraction must be avoided or limited in accordance with GAC guidelines.

Due to the significant drawbacks of LLE and SPE, two new extraction methods, solid-phase microextraction (SPME) and liquid-phase microextraction (LPME), were developed in 1990 and 1996, respectively.³⁷  In accordance with the principles of green chemistry, the most important extraction methods include micro-solid-phase extraction (µ-SPE), dispersive micro-SPE (DMSPE), magnetic SPE (MSPE), microextraction by packed sorbent (MEPS), stir bar sorptive extraction (SBSE), pipette-tip micro-SPE (PT-µSPE), single-drop microextraction (SDME), hollow-fiber LPME (HF-LPME), and dispersive liquid–liquid microextraction (DLLME). These new techniques have been developed to make sample preparation faster, cheaper, and more easily automatable. Current trends in the field suggest that these techniques require fewer hazardous solvents and smaller amounts of samples to analyze and allow for the simultaneous removal of various target compounds from a single sample.³⁸  Due to the solvent-free nature of these approaches, they produce very little waste, expose the analyst to fewer dangerous chemicals, and simplify and speed up the sample preparation process, just to name a few advantages. It's also important to highlight that microextraction methods combine sampling and extraction with preconcentration and even derivatization in one step.⁴  The following section provides a brief overview of green sample preparation procedures based on microextraction.

Since solvents are so important to the process of increasing the recovery rate, a significant amount of them is used during the purification step in particular. Before the development of green chemistry, the effects of excessive solvent use on the environment were underappreciated.¹⁶  According to a study conducted in 2005 by Jiménez-González et al., the use of solvents has the greatest impact on a typical pharmaceutical product's environmental footprint.³⁹  By employing smaller sample/solvent volumes (a few µL or nL) or greener alternatives for analytical analysis, it is possible to limit the risk factors that might affect laboratory staff over time when they are exposed to potentially dangerous chemicals and/or the environment.⁴⁰  In order to decrease the use of solvents in small-scale analytical procedures, several approaches, such as accelerated extraction techniques, alternative solvent techniques, microextraction techniques, and SPE techniques, have been developed during the past decade.

Sorbent-based sampling methods reduce or even get rid of the need for organic solvents in the whole procedure, which results in a lower overall effect on the environment. Due to the adaptability of diverse sorbent materials, several sorbent-based microextraction methods have been developed in recent years. These methods include: (i) packing the sorbent material into small devices; (ii) dispersing the sorbent material along the sample matrix; and (iii) coating the sorbent material on a solid support.⁴¹  It is possible to distinguish two sorbent-phase modes, direct immersion (DI) and headspace (HS), based on the location of the sorbent material with respect to the sample.

When using DI-SPME, the analyte from the sample is directly transferred to the stationary phase on a fiber dipped in solution for HS absorption. In a recent study carried out by Rosa et al.,⁴²  a polydimethylsiloxane/divinylbenzene fiber was prepared that can be used for DI microextraction of 90 pesticides from groundwater samples from 30 distinct chemical groups. This approach makes it possible to identify pesticides in real samples using a simple, quick, and solvent-free multi-residue extraction process.

HS-SPME is the most commonly used method for analyzing alkylpyrazines in solid substrates like coffee powder.^43,44  However, owing to the high boiling temperatures of alkylpyrazines and their considerable water solubility, HS analyses have many disadvantages for liquid samples. On the other hand, SBSE, an innovative approach that does not require the use of solvents, was developed in 1999.⁴⁵  The lower amount of eluent prior to liquid chromatography (LC) analysis or solventless desorption prior to gas chromatography (GC) analysis is a green feature of this approach. Due to the robustness of this technique, it may be used as both a DI and HS extraction method.⁴⁶  This approach employs a tiny device consisting of a magnetic bar encased in a glass tube and coated with a sorbent, which in most instances is polydimethylsiloxane.⁴¹  MEPS is a miniature variant of SPE that works with samples as tiny as 10 µL. This new approach is promising since it's quick and easy, and needs a minimal sample volume to provide equivalent results to traditional SPE.⁴⁷  PT-µSPE is a portable and rapid sample pre-treatment technique in which a small amount of sorbent is packed inside the pipette tips to repeatedly extract analytes of interest. Small amounts of adsorbent in the SPE pipette tips result in a considerable decrease in sample and organic solvent consumption compared to conventional SPE cartridges.^48,49  Kandeh et al.⁴⁹  used the PT-µSPE method to successfully extract seven opioid analgesics (OAs) from biological samples using water-resistant electrospun poly(vinyl alcohol)–poly(acrylic acid)/carbon nanotube–cellulose nanocrystal (PVA–PAA@CNT–CNC) composite nanofibers, followed by HPLC–UV detection. Due to the low consumption of the nanofibers and organic solvents, the PT-µSPE approach based on the electrospun PVA–PAA@CNT–CNC nanofibers has greater potential for sample pre-treatment than the other methods. In addition, the proposed technique features low LODs, a broad linear range, significant reusability, and good extraction recovery as a sensitive and rapid approach. Figure 1.2 shows a schematic of the electrospun nanofiber preparation method and the PT-µSPE procedure.

There are three primary kinds of LPME techniques: SDME, HF-LPME, and DLLME. A common characteristic of many LPME variants is that the analytes of interest are extracted into at least one liquid solvent, often organic. Additionally, the tiny volume of this solvent (usually 1–100 µL) is a common feature.⁵⁰  This contributes to the reduction of solvent consumption and the greening of the methods. SDME, which was first reported by Jeannot and Cantwell in 1996,⁵¹  is the most straightforward method for performing LPME. After exposing a drop of organic solvent to the sample for a period of time, it is collected and analyzed. The method in which a drop of organic solvent is immediately immersed in the sample liquid is referred to as DI-SDME, whereas the method in which a drop of organic solvent is additionally exposed to the HS above the sample is referred to as HS-SDME. A solvent-free concentrating extraction method, SPME, has been extensively used in the determination of many different types of samples, including biological,^52–54  environmental,^55,56  and food samples.^57,58  HF-LPME, which was first described by Pedersen-Bjergaard and Rasmussen in 1999,⁵⁹  has been used by many researchers because it reduces the consumption of organic solvents and eliminates the problems of SDME such as droplet instability, contamination, and low reproducibility. This technique is based on the use of polypropylene hollow fibers that contain liquid extractant (several microliters) within the fiber lumen. This method can be categorized into two modes: a two-phase mode and a three-phase mode.⁶⁰  As a green alternative, HF-LPME has been utilized for microextraction of various analytes, such as Cr(vi) from water samples,⁶¹  dinitrophenols from urine and saliva samples,⁶²  cocaine derivatives from hair samples,⁶³  pharmaceuticals from wastewater,⁶⁴  and volatile organic compounds (VOCs) from water.⁶⁵  DLLME, first reported by Rezaee et al., is a relatively recent addition to the family of LPME methods. This method has been widely used throughout the years to identify a wide range of compounds in a variety of different matrices because of its simple operation, speed, low cost, good recovery, and low consumption of organic solvents. The typical procedure includes rapidly injecting a water-immiscible extraction solvent and a water-miscible dispersion solvent into an aqueous sample. Thus, it generates a very fine emulsion, allowing for the fast transfer of analytes to the solvent. The separation of the extraction phase by centrifugation is followed by the determination of the enriched analytes in the precipitation phase (Figure 1.3).^50,66  Generally, DLLME is preferred for basic matrix samples. Due to its poor selectivity and sample clean-up efficiency, water samples are widely researched.^67–69  In recent years, DLLME has been most frequently used for extraction of analytes from more complex matrices, including biological,⁷⁰  pharmaceutical,⁷¹  and food⁷²  samples.

Solvents define a significant portion of the environmental performance of a process and also influence safety and health concerns. The idea of “green” solvents indicates the desire to reduce the environmental effect of solvent consumption in chemical synthesis.⁷³  Due to the fact that organic solvents used as solvents and extractants in several microextraction techniques are often toxic, their replacement with more suitable, environmentally friendly agents may have positive results in terms of the green approach.⁷⁴  As an example, hexane might be exchanged with heptane, which is a more benign solvent. Since both these solvents are straight chain alkanes, it is likely that there is not much difference in extraction selectivity. In addition, the potentially mutagenic and carcinogenic chloroform, dichloroethane, or carbon tetrachloride, commonly used in many liquid-based extraction methods, can be replaced with dichloromethane, which can provide similar extraction yields.⁷⁵  Obviously, it would be insufficient to categorize a solution simply as green or not. Naturally, the topic of how to determine how “green” a solvent is, is emerging. Categorizing solvents based on the “Environmental, Health, and Safety” (EHS) characteristics necessitates the use of a comprehensive selection tool. Likewise, the National Environmental Methods Index (NEMI) is one of the most comprehensive and straightforward approaches to assessing the eco-friendliness of analytical processes. A variety of methods and procedures, based on the constraints imposed by “Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH)”, a list of “substances of high concern”, and so on, are used for the selection of solvents. It is important to take into account the performance of the solvent, as well as the energy requirements for its manufacture and use, while switching from traditional solvents to more environmentally friendly bio-based organic solvents.^73,76  Figure 1.4 combines cumulative energy demands (CEDs) and EHS ratings to highlight the effect of solvents.

In recent years, green solvents such as subcritical water (SW), deep eutectic solvents (DESs), and ionic liquids (ILs) have gained popularity in sample preparation and comply with GAC principles. The term “subcritical water” is defined as high temperature and high pressure liquid water. Some of the most important aspects of SW are that it is low-cost, eco-friendly, safe, non-toxic, and waste-free. Another advantage is that SW has temperature-adjustable density, ion product, and dielectric constant values, making it possible to perform selective extractions.⁷⁷  SW has been used to extract a broad variety of organic compounds, including phenolics,⁷⁸  flavoring compounds,⁷⁹  flavonoids,⁸⁰  polysaccharides,⁸¹  and pigments.⁸²  ILs with a salt structure have emerged as a class of extremely useful chemicals with excellent features such as chemical stability, thermal stability, and non-volatile properties at low vapor pressure, resulting in little environmental emissions. Because of other positive properties such as tunable hydrophobicity, polarity, and selectivity with respect to the organic solvents used for extraction, as well as their environmentally friendly properties, ILs have been successfully used in the extraction of a variety of substances, including metal ions,⁸³  polycyclic aromatic hydrocarbons (PAHs),⁸⁴  biomolecules,⁸⁵  and pesticides.⁸⁶  Recent years have seen a rise in the use of biodegradable solvents, which are less hazardous and simpler to prepare than ILs.⁸⁷  One of the most promising new developments in the field is the development of DESs, which have the potential to overcome some of the major drawbacks of the current generation of ILs, including high toxicity, non-biodegradability, complex synthesis, and high cost. Abbot et al. proposed the first definition of DESs in 2001.⁸⁸  DESs, a subclass of ILs, are typically mixtures of two or three cost-effective and non-hazardous components. These components have the ability to self-associate, most frequently through the formation of hydrogen bond interactions, in order to produce a eutectic mixture that has a lower melting point than that of each individual component. To this point, research on DESs has been conducted in the fields of catalysis, organic synthesis, biomass degradation, electrochemistry, materials chemistry, and so on.⁸⁹  Some studies on their use as green solvents in the extraction process have been done on sulphonamides in fruit juices,⁹⁰  oxyprenylated phenylpropanoids in vegetable oils,⁹¹  organic pollutants in aqueous solutions,⁹²  PAHs in liquid and solid foods,⁹³  herbicides in oils,⁹⁴  antioxidants,⁹⁵  Co(ii) from electrode materials,⁹⁶  etc.

Reducing and even preventing waste generation is one of the most important principles of green chemistry. So it is extremely important to avoid the waste generated in an analytical process. An analytical procedure's greenness can be evaluated using the AGREE metric, which gives the highest possible score if the waste generated is less than or equal to 0.1 g mL.⁹⁷  In the process of sample preparation, the primary contributors to the generation of chemical waste are the solvents, acids, bases, and derivatization agents that are used in various processes, such as extraction, digestion, and/or derivatization. There are different approaches that can be used to eliminate or reduce the amount of solvent used. The extraction of analytes could be accomplished using non-hazardous materials (green solvents) or by employing techniques that are based on microextraction. When combined with thermal desorption, SPME has the potential to be a technology that does not need the use of a solvent. In a similar manner, Esfandiarnejad and Sereshti developed a method⁹⁸  in 2019 that eliminates the use of desorption solvent, known as absolutely solvent-free MSPE (ASF-MSPE), for the extraction and determination of chlorophenols (CPs). Here, the CPs on magnetic graphene oxide as a sorptive phase were then thermally desorbed without the use of any organic solvents. Figure 1.5 shows a schematic illustration of the ASF-MSPE procedure.

It should be highlighted that automated sample preparation can extend the life of solvents and reagents via online waste treatment and recycling. Instead of external waste treatment, online decontamination of analytical wastes, such as recycling, degradation, and passivation, should be conducted.⁹⁹  Furthermore, it should be highlighted that biodegradable materials should be used if effective treatment of generated waste is not feasible. It is important to note that biodegradability is one of the deciding elements when selecting materials for the construction of microfluidic devices.¹⁰⁰ 

Finally, method optimization studies with the use of chemometric tools are an alternative waste reduction strategy, as they drastically reduce the number of tests required and make a smart use of resources by carefully planning studies.^101,102 

Miniaturization of analytical systems is a new trend in analytical chemistry, which is an important factor in determining the method's environmental impact. It is essential to examine the miniaturization, automation, and portability of the sample preparation, separation, and detection phases of the analytical process. It is widely believed that downscaling of sample preparation procedures evolved as a result of attempts to miniaturize both separation and detection systems. Other interconnected factors such as economy, speed, analytical performance, sensitivity, energy efficiency, and reduction of samples, solvents, and waste have encouraged scientists to miniaturize analytical techniques. In addition, the production of such micro-analytical systems may assist in the elimination of hazards other than chemicals, such as temperature, fumes, noise, high voltages, and ultrasonic waves.

After the introduction of SPME as the first miniature sample preparation technique by Arthur and Pawliszyn,¹⁰³  many new miniature sample preparation techniques such as DMSPE, SBSE, MSPE, MEPS, LPME, SDME, and DLLME have emerged over time as a result of the application of innovative design and materials. Required information on their features and application areas is provided in Section 1.4.1. The following are some of the common features of these procedures that enable a “green” approach to sample preparation: (1) easy downsizing, (2) high sensitivity, (3) simple automation, (4) quick extraction times, (5) cost-effectiveness, and (6) easy connection with chromatographic equipment. Furthermore, certain sample preparation approaches allow selective separation of target analytes through the use of appropriate derivatization materials. The ability of miniaturized sample preparation methods to simultaneously extract and derivatize targets is an additional benefit of these methods. The integration of these two procedures not only optimizes the sample throughput but also minimizes the number of analysis steps.

On the other hand, it has been noted that the chemicals and materials industry is one of the greatest energy consumers, and separation operations such as isolation, purification, and other processes account for a considerable amount of this energy used.¹⁰⁴  Green sample preparation also attempts to reduce sample preparation energy consumption as a means of contributing to long-term sustainability objectives. When properly constructed, systems meant to promote “self-separation” may reduce energy consumption and material utilization.¹⁰⁵  In this respect, the MSPE method, in which the analyte is separated from the large volume of sample solution using magnetic sorbents and an external magnetic field, has been shown to be an interesting technique. The primary benefit of MSPE is the easy and fast phase separation of the sorbent, with the use of magnets, eliminating intermediate steps such as filtration and centrifugation.¹⁰⁶ 

Until now, use of green sample preparation approaches has been reported in several publications and reviews. This chapter presented a description of extraction techniques and their theoretical bases, as well as their most recent applications, which are extensively employed, particularly among green sample preparation methods, in order to provide a better understanding of the field.

Sample preparation, the most important step in an analytical process, is essential for the determination of low concentration analytes, as conventional procedures are not particularly sensitive. This is accomplished by the isolation and preconcentration of the analyte prior to the quantitative step. Recently, analytical chemists in particular have focused on the simplicity and automation of analytical processes, as well as the development of new procedures aimed at preventing waste, or, in short, being environmentally friendly. As a result, new and more efficient microextraction methods derived from green sample preparation techniques, such as LPME and conventional SPME variants, have gradually replaced earlier and less efficient approaches like LLE and SPE. Green microextraction is a fast, simple, generally solvent-free, sensitive, reliable, and cost-effective technique that simplifies, miniaturizes, and minimizes the sample size of the analytical process. This approach also improves the accuracy and sensitivity of chromatographic analysis by reducing sample preparation errors.

The growing interest in GAC principles is expected to result in the development of new eco-friendly extraction techniques in the future, such as the production of new sorbent materials, the discovery of less toxic solvents, greater miniaturization, and automation of existing extraction techniques.