measurement mode. Moreover, an appropriate method for a target
analyte may not be good for comprehensive screening of compounds.
In recent years, sample-preparation methodologies evolved from
hard strategies to soft methods based on room-temperature,
ultrasound-assisted leaching [19–21] or microwave-assisted digestion
using closed systems [22–24], so providing a fast, safe
methodology, especially for sample digestion and sample dissolution.
Analyte extraction has the double purpose of matrix isolation
and analyte preconcentration, and the appropriate selection of solvents
and reagents and the control of the preconcentration process
are absolutely necessary in order to:
• separate quantitatively the target analyte from the matrix; and,
• increase the concentration level of the target analyte in the final
solution to be measured.
Sometimes, solid samples are difficult to analyze due to the need
to transfer the target analytes to a liquid phase. Leaching the analyte
(i.e., solid–liquid extraction or lixiviation) is one of the easiest, most
widely used sample treatments. Classically, leaching has been widely
carried out by maceration, based on the correct choice of solvents
and the use of room temperature or controlled temperature and/
or agitation to increase the solubility of compounds and the rate
of mass transfer. In general, heating the system increases the solubilization
power of the reagents or solvents used, but involves
environmental side-effects (i.e., energy consumption). Despite the
extensive use of leaching, it is characterized by long extraction protocols
with low efficiency.
In 1879, Franz von Soxhlet developed Soxhlet extraction, which
is the most widely used leaching technique [25]. Soxhlet extraction
is a primary reference against which performance in new
leaching methods is measured. It is still an attractive option for
routine analysis because of its general robustness and relatively low
cost. The Soxhlet system is simple and easy to use, and it enables
the use of a large amount of sample (i.e., 1–100 g). However, the
main drawbacks are long extraction times and large amounts of
solvent required, which also mean that the solvent must be evaporated
to concentrate the analytes before their determination [26].
However, when samples are water or aqueous solutions with a
complex matrix (e.g., wastewater or seawater, body fluids or juices)
it can be necessary to move from the original solution to a new phase
using immiscible solvents or solid phases, suitable to extract the
analytes selectively.
In short, there are several strategies proposed in the literature
for analyte extraction [27,28], involving liquid-liquid extraction (LLE)
[29–31] and solid-phase extraction (SPE) [32,33]. LLE and SPE are
the most widely used techniques for the extraction of liquid samples.
In the first type, an appropriate selection of the extraction solvent
permits removal of the analyte from the original solution to the new
phase, directly or after a previous derivatization.
From our point of view, SPE is probably the best option to improve
analyte concentration and to separate it from a complex matrix. Generally,
SPE consists of four steps:
• column conditioning;
• sample loading, which implies analyte retention into the solid
phase;
• column post-wash; and,
• analyte elution from the solid phase using an appropriate solvent.
The most common design applied in SPE is the polypropylene
cartridge with placed sorption phase, which varies in size from
micro-sized disks in 1 mL syringes to 6 mL syringes. SPE can manage
relatively high volumes of samples for analyte preconcentration being
suitable to be eluted on-line with microliters of an appropriate
solvent to do their determination.
The sample-preparation step can be performed off-line, at-line
or on-line. At-line procedures are performed with a robotic system
or autosampler and no manual preparation is required, which is the
case for off-line systems. On the other hand, on-line procedures
combine directly the sample-preparation step with the measurement
mode, usually via a multiport valve.
4. Green extraction solutions
Fig. 3 shows, as a scheme, the different variables to be considered
on greening the extraction steps which, in short, involve the
nature and the amount of reagents used and the reduction of the
energy employed for extraction.
As it has been mentioned above, leaching of the analyte from a
solid sample is one of the easiest and most widely used sample treatments.
As a consequence, a variety of sample-preparation methods