5.2
The Role of Extraction in Environmental Analysis
The effects of human activity on the environment have raised an increasing concern
since the 1970s, and international policy frameworks have been developed and
implemented over the past decades to control and mitigate this impact (ISDR
2007). Nowadays, our environment is constantly being sampled and analysed for a
variety of purposes, including for the assessment of the quality of drinking water,
groundwater, river and seawater, wastewaters, effluents and sewage sludge,
sediments, soil and biota. In order to be able to detect even very low concentrations
of pollutants in the ecosystems, it is necessary to follow a series of steps.
The overall process of data gathering, analysis and interpretation starts with
sampling. This step involves collecting samples that faithfully represent a waste or
element of the environment from which they are taken. Samples must not be
contaminated during collection and handling, and analyte concentrations must not
change between the time of collection and analysis (EPA Victoria 2009). If an
unrepresentative sample is obtained due to choosing an inappropriate technique or
cross-contamination, even the careful laboratory analysis that follows cannot ensure
delivery of accurate and reliable data.
Prior to analysis performed by instrumental methods that dominate modern
analytical chemistry, most samples require preparation. This may involve mechanical operations such as crushing, sieving and blending, airÀ/freezeÀ/ovenÀ/microwave-drying and different operations that fall under three main categories of
chemical and physico-chemical sample pretreatment: (1) isolation and clean-up,
(2) sample preconcentration and (3) sample derivatization. The isolation of the
chemical specie(s) of interest (i.e. analyte) from sample matrix (air, water, sediment,
soil, biota, etc.) is achieved by various separation methods, such as (co)precipitation,
filtration, adsorption on a sorbent, decantation, centrifugation, magnetic separation,
evaporation, distillation, dissolution, digestion and, by one of the leading sample
preparation techniques in trace element analysis, extraction. Some analytes also need
to be cleaned up from co-extracted, non-target substances and/or increased in
concentration prior to analysis or detection. Besides being thought of as a separation
technique, extraction is also being frequently used for both of these purposes,
i.e. sample purification and preconcentration.
Occasionally, it is also necessary to transform a chemical compound (educt) into
another similar compound (derivative) by altering one or more of its functional
groups (definition given by the Royal Society of Chemistry). This can be done for
various reasons: (1) to enhance extraction efficiency, (2) to boost detection sensitivity or (3) to make compounds more amenable to a particular analysis mode
(Kudlejova et al. 2012). For instance, if the analyte is not volatile enough to be
conveniently determined via headspace extraction (Jeannot 2007), it can be
derivatized to improve its volatility. The derivatizing agent can be added to the
sample before, during or after sample extraction. The first two options may lead to
enhanced sensitivity and selectivity of both extraction and detection and, in case of
solid-phase microextraction, to determination of analytes that are normally not
amenable by this method. Regarding post-extraction derivatization, it can only
5 The Recent Strategies Employed in Chemical Analysis of Contaminated Waters,. . .
133
The Role of Extraction in Environmental Analysis
The effects of human activity on the environment have raised an increasing concern
since the 1970s, and international policy frameworks have been developed and
implemented over the past decades to control and mitigate this impact (ISDR
2007). Nowadays, our environment is constantly being sampled and analysed for a
variety of purposes, including for the assessment of the quality of drinking water,
groundwater, river and seawater, wastewaters, effluents and sewage sludge,
sediments, soil and biota. In order to be able to detect even very low concentrations
of pollutants in the ecosystems, it is necessary to follow a series of steps.
The overall process of data gathering, analysis and interpretation starts with
sampling. This step involves collecting samples that faithfully represent a waste or
element of the environment from which they are taken. Samples must not be
contaminated during collection and handling, and analyte concentrations must not
change between the time of collection and analysis (EPA Victoria 2009). If an
unrepresentative sample is obtained due to choosing an inappropriate technique or
cross-contamination, even the careful laboratory analysis that follows cannot ensure
delivery of accurate and reliable data.
Prior to analysis performed by instrumental methods that dominate modern
analytical chemistry, most samples require preparation. This may involve mechanical operations such as crushing, sieving and blending, airÀ/freezeÀ/ovenÀ/microwave-drying and different operations that fall under three main categories of
chemical and physico-chemical sample pretreatment: (1) isolation and clean-up,
(2) sample preconcentration and (3) sample derivatization. The isolation of the
chemical specie(s) of interest (i.e. analyte) from sample matrix (air, water, sediment,
soil, biota, etc.) is achieved by various separation methods, such as (co)precipitation,
filtration, adsorption on a sorbent, decantation, centrifugation, magnetic separation,
evaporation, distillation, dissolution, digestion and, by one of the leading sample
preparation techniques in trace element analysis, extraction. Some analytes also need
to be cleaned up from co-extracted, non-target substances and/or increased in
concentration prior to analysis or detection. Besides being thought of as a separation
technique, extraction is also being frequently used for both of these purposes,
i.e. sample purification and preconcentration.
Occasionally, it is also necessary to transform a chemical compound (educt) into
another similar compound (derivative) by altering one or more of its functional
groups (definition given by the Royal Society of Chemistry). This can be done for
various reasons: (1) to enhance extraction efficiency, (2) to boost detection sensitivity or (3) to make compounds more amenable to a particular analysis mode
(Kudlejova et al. 2012). For instance, if the analyte is not volatile enough to be
conveniently determined via headspace extraction (Jeannot 2007), it can be
derivatized to improve its volatility. The derivatizing agent can be added to the
sample before, during or after sample extraction. The first two options may lead to
enhanced sensitivity and selectivity of both extraction and detection and, in case of
solid-phase microextraction, to determination of analytes that are normally not
amenable by this method. Regarding post-extraction derivatization, it can only
5 The Recent Strategies Employed in Chemical Analysis of Contaminated Waters,. . .
133