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Section II
The Analytical Process
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5
Greening Sampling Techniques
JosĂ© Luis GĂłmez Ariza and Tamara GarcĂa Barrera
Departamento De QuĂmica Y Ciencia De Los Materiales âProfesor JosĂ© Carlos VĂlchez MartĂnâ,
Universidad De Huelva, Huelva, Spain
Sampling is a very critical operation in analysis which can dramatically affect final results. Preservation and stabilization reagents such as acids, solvents and buffers, as well as filters, containers and refrigeration/cryogenic conditions, represent important environmentally detrimental toxic inputs. Since billions of samples are annually collected for environmental, health or food quality monitoring, the use of environmental friendly sampling practices can have important consequences. This paper reviews the more significant proposal for this purpose under the concept of Green Analytical Chemistry. The application of direct techniques of analysis that avoid sample collection, transport, pretreatment or preparation, such as ion-selective electrodes, portable X-ray fluorescence spectrometers or remote analysis techniques represent some examples of new trends in Green Analytical Chemistry. Other approaches are centered on solventless techniques for sample extraction, especially supercritical fluid extraction (SFE), membrane extraction systems, and solid phase extraction (SPE). The reduction of reagents and solvent volumes together with the increase of rate samples in flow-through solid phase spectroscopic, hollow-fiber GC/HPLC/CE and nanoparticle-based analytical approaches also represent relevant contributions in this field. Further options in green sampling is related the use of sensitive probes that work in streams, at in vivo samples and in-field analysis. Finally, automation of analysis contributes to labor and energy consumption and can be combined with the above approaches for green analysis and friendly environmental sampling, although many further studies are still required in this area.
5.1 Greening analytical chemistry solutions for sampling
The concept of green chemistry is closely related to the principles of sustainable chemistry and the clear trend towards their implementation in chemical plants and laboratories. From these premises guidelines have to be established for chemists and production engineers to make their activities less harmful towards the environment. This new way of action is supported by rules and principles [1,2]. A great number of analytical methods have been proposed to monitor environmental friendly activities, but these analytical activities have also been performed in a friendly way from the environmental point of view [3]. Therefore, any development towards green chemistry is not possible without the existence of Green Analytical Chemistry (GAC).
The features related to GAC consider the reduction of reagents, particularly organic reagents, from analytical procedures; the reduction in emissions of gases and vapors, liquid and solid waste produced in the analytical laboratories; the elimination and substitution of toxic reagents; and the reduction of energy necessary for analytical procedure development. Other important features in Green Analytical Chemistry are related to sampling. In routine analytical procedures, sample collection can be avoided by applying in-line systems that allow researchers to perform measurements directly in the sample, in-field analysis (e.g., analysis in a pipeline or a river). In this way, the problems related to sample preparation, transportation to the laboratory and storage can be avoided. The application of direct techniques of analysis that avoid sample transport, pretreatment or preparation, such as ion-selective electrodes for water analysis, portable X-ray fluorescence spectrometers for soil metal contamination assessment or remote analysis techniques (LIDAR or SODAR) to measure air pollution, represent some examples of new trends in Green Analytical Chemistry. Other approaches are centered on solventless techniques, especially for sample extraction. Several possibilities can be mentioned in relation to this stage of the analysis: supercritical fluid extraction, membrane extraction systems coupled directly (on-line) with measuring devices and solid phase extraction.
5.2 New green approaches to reduce problems related to sample losses, sample contamination, transport and storage
Sample interaction with wall sampling containers or transfer lines in on-line analytical arrangements can provoke significant losses or contamination of the analytes. In addition, problems related to sample transport and storage in batch analysis also affect significantly the precision of the results and require the use of considerable amounts of reagents and waste against GAC features. Direct analytical systems or analytical arrangements intending to reduce the minimum contact of analytical systems with the sample, and moreover to reduce or even eliminate sample transport and storage, have been proposed. However, ideal approaches based in direct analysis are difficult to achieve and alternative procedures based on a combination of simplified sample treatment methods, environmentally friendly, and high-rate sample devices based on the use of flow-through solid phase spectroscopy, hollow-fiber GC/HPLC/CE and nanoparticles have been proposed under the focus of Green Analytical Chemistry.
5.2.1 Methods based on flow-through solid phase spectroscopy
The immobilization of the species of interest (either the analyte or any derivative product) on an appropriate solid support (usually microbeads from either a polymeric or a non-polar material) and the direct measurement of the analytical property (typically absorbance or luminescence) on the solid is the base of Flow-Through Solid-Phase Spectroscopy (FSPS), in which retention (sampling), preconcentration, and detection processes are all performed simultaneously in the flow cell. These systems allow increasing of both sensitivity and selectivity to be reached, together with a drastic decrease of reagent consumption. This instrumental implementation is called optosensor, flow-through optosensor or, simply, sensor [4]. An interesting alternative has been the use of sequential injection analysis (SIA) instead of flow injection analysis (FIA) [5] providing strong reduction of both reagents and waste. Therefore, the combination of multicommutation principles with SPS detection [6] has become a significant contribution to the GAC concept, since each reagent is operated independently in multicommutated optosensors and only the required volume for each sample is introduced in the analytical system which represents a drastic minimization of both reagents and waste. Therefore, FSPS can be envisaged as a variant of analysis based on the use of immobilized reagents.
Tables 5.1 and 5.2 summarize several typical applications of solid phase spectroscopy in GAC, as well as sample treatments proposed in these approaches which intend to reduce or avoid the amount of waste and the use of toxic reagents.
Simpler optosensors use monochannel devices in which the carrier solution also acts as an eluent and use intrinsic properties of the analytes for sensing (Table 5.1). These systems provide increased sensitivity in relation to analogous approaches in homogeneous solution, possibly due to the higher concentration of the analyte in the solid phase, this support can be reused which represents a considerable reduction of solid waste. In addition, the amount of sample is drastically reduced and the only reagent necessary is the carrier solution, all these characteristics represent valuable contributions in relation to GAC. Similar benefits can be mentioned in multiparameter sensors.
Sequential injection analysis in combination with solid phase spectrometry (SPS) (Table 5.2) is another interesting alternative in GAC based in optosensors, since a remarkable reduction in reagents and waste is achieved as well as a higher automation level of analysis.
Multicommutation devices allow us to handle micro-volumes of samples and reagents in simple and complex instrumental manifolds, minimizing both reagents and waste production, therefore the combination of multicommutation with SPS has become a good implementation in the optosensor field [17].
An important point to be considered is that samples in typical case-studies that use flow-through solid phase spectroscopic systems (Tables 5.1 and 5.2) are mainly liquids (waters, serum, urine, syrup) which require very little preparation. However, in pharmaceuticals excipients, such as tablets or suppositories and especially condiments such as pepper, sample handling is more complex with many steps that can contribute to analyte losses and lower accuracy of results. In addition, these treatments involve the use of reagents and solvents reducing the greening character of these procedures. Therefore, further studies on solid sample treatment that complement the green possibilities of flow-through solid phase spectroscopic sensors are necessary.
5.2.2 Methods based on hollow-fiber GC/HPLC/CE
The use of porous hydrophobic membranes has been proposed for air, headspace and aqueous phase analysis. These systems allow a direct analysis of volatile compounds from air or water (headspace extraction) and quasi-direct analysis of polar compounds in liquid samples, which reduce the problems associated to sampling and sample storage and treatment. Due to its low cost, the hollow-fiber extraction device can be disposed after a single extraction which eliminates the possibility of carry over effects. In addition, because a small volume of organic solvent is required and little waste generated, these approaches are environmentally friendly and compatible with the âGreen Chemistryâ concept.
A poly(dimethylsiloxane) (PDMS) hollow-fiber membrane has proved useful to perform surface-water monitoring using an especially designed cap [18]. The extraction cap was placed on the aqueous surface in a glass water bath container which contained aqueous sample to a 1 cm depth. The pressure of the headspace was ambient. For underwater monitoring, the extraction cap was positioned in an aqueous sample at a depth of 25 cm. The pressure of the headspace was higher than the room pressure because of the height of the water. To allow a 1 cm depth of water inside the cap, the headspace pressure was adjusted by opening and closing a pressure release switch. During the sampling/extraction, the position of the extraction cap was fixed using a clamp to hold the neck of the cap, and the cap was tightly in contact with the bottom of the bath container in each extraction. This contact ensures the proper sealing of the aqueous sample inside the extraction cap and prevents the sample inside from exchanging with the solution outside of the cap. The monitoring of VOCs (benzene, toluene, ethylbenzene, o-xylene, and trichloroethylene) was performed using 4 min of trapping and 2 s of thermal desorption for further analysis by gas chromatography with flame ionization detection (FID). Compared with d...
