Journal of Chromatography A, 1217 (2010) 2383 2389
Contents lists available at ScienceDirect
Journal of Chromatography A
journal homepage: www.elsevier.com/locate/chroma
Review
Soxhlet extraction: Past and present panacea
M.D. Luque de Castro", F. Priego-Capote
Department of Analytical Chemistry, Annex C-3 Building, Campus of Rabanales, E-14071 Córdoba, Spain
a r t i c l e i n f o a b s t r a c t
Article history:
An overview of Soxhlet extraction, the advantages and shortcomings of this centenary technique as well
Available online 13 November 2009
as the attempts to improve its performance and achievements reached is here presented. Assistance of
high pressure, ultrasound or microwaves has decreased or minimized the negative characteristics of the
Keywords:
conventional extractor. Automation of Soxhlet performance opened the door to commercialization of
Soxhlet extraction
a number of different approaches. The evolution of Soxhlet extractor is here critically discussed, and
High-pressure Soxhlet extractors
the conclusion from this overview is that the adoption of new technologies to improve its performance
Automated Soxhlet extractors
converts Soxhlet extraction in almost a panacea in this field.
Ultrasound-assisted Soxhlet extractor
© 2009 Elsevier B.V. All rights reserved.
Microwave-assisted Soxhlet extractors
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2383
2. Conventional Soxhlet extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2384
3. High-pressure Soxhlet extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2384
4. Automated Soxhlet extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2385
5. Ultrasound-assisted Soxhlet extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2385
6. Microwave-assisted Soxhlet extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2385
6.1. Commercial microwave-assisted Soxhlet extractors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2385
6.2. The focused microwave-assisted Soxhlet extractor (FMASE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2386
6.2.1. First, simplest prototype: advantages and shortcomings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2387
6.2.2. Automated, flexible prototype overcoming the shortcomings of the first . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2387
6.2.3. A dual-operation automated prototype: advantages of the definitive prototype (commercial availability) . . . . . . . . . . . . . . . . . . . . . 2388
6.2.4. New incoming prototype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2388
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2388
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2389
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2389
1. Introduction ularly with solid samples. Solid samples are the most difficult to
process as most analytical instruments cannot handle them. There-
Sample preparation is most often a necessity as even the sim- fore, the first operation in the preparation of solid samples involves
plest samples are frequently unsuitable for direct analysis because transferring the target analytes to a liquid phase.
of excessive dilution or concentration of the target analytes or Solvent extraction of solid samples, which is commonly known
incompatibility with instrument operation procedures. For these as solid liquid extraction but should rather be named as leach-
reasons, sample preparation is, most times, the bottleneck of ana- ing or lixiviation to more strictly adhere to its physical chemical
lytical methodologies as it constitutes the principal source of error foundation, is one of the oldest techniques of solid sample prepa-
and remains as one of the most time-consuming steps [1], partic- ration. It serves, not only to remove and separate compounds of
interest from insoluble high-molecular-weight fractions, but also
from other compounds that could interfere with subsequent steps
of the analytical process. Classically, leaching has been widely car-
"
Corresponding author at: University of Cordoba, Department of Analytical
ried out by maceration, based on the correct choice of solvents and
Chemistry, Edificio Marie Curie Anexo, Campus Rabanales, E-14071 Cordoba, Spain.
the use of heat and/or agitation to increase the solubility of com-
Tel.: +34 957218615; fax: +34 957218615.
E-mail address: qa1lucam@uco.es (M.D. Luque de Castro). pounds and the rate of mass transfer. Despite the extensive use
0021-9673/$ see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.chroma.2009.11.027
2384 M.D. Luque de Castro, F. Priego-Capote / J. Chromatogr. A 1217 (2010) 2383 2389
Conventional Soxhlet extraction has some attractive advan-
tages. Thus, the sample is repeatedly brought into contact with
fresh portions of extractant, which facilitates displacement of the
transfer equilibrium. Also, the system remains at a relatively high
temperature by effect of the heat applied to the distillation flask
reaching the extraction cavity to some extent. In addition, no fil-
tration is required after leaching and sample throughput can be
increased by performing several simultaneous extractions in par-
allel, which is facilitated by the low cost of the basic equipment.
Moreover, Soxhlet extraction is a very simple methodology that
requires little training, can extract more sample mass than most of
the latest alternatives (microwave-assisted extraction, supercriti-
cal fluid extraction, etc.) and seemingly subject to no matrix effects
this assertion is not strictly true as seen when Soxhlet extraction
is compared with supercritical fluid extraction of analytes strongly
bound to their matrix [3]. There is a wide variety of official methods
involving a sample preparation step based on Soxhlet extraction
[4 8].
The most serious drawbacks of Soxhlet extraction as compared
to other techniques for solid sample preparation are the long time
required for extraction and the large amount of extractant wasted,
which is not only expensive to dispose off, but also the source of
additional, environmental problems. Samples are usually extracted
at the solvent boiling point over long periods, which can result in
thermal decomposition of thermolabile target species. Also, a con-
ventional Soxhlet device provides no agitation, which would help to
Fig. 1. Conventional Soxhlet extractor.
expedite the process. In addition, the large amounts of extractant
used call for an evaporation concentration step after extraction.
of maceration, particularly for isolation of natural products, this is
Finally, the Soxhlet technique is limited by extractant and difficult
characterized by long extraction protocols with low efficiency.
to automate.
In 1879, von Soxhlet developed a new extraction system (Soxh-
Conventional Soxhlet extraction, with its advantages and short-
let extractor) which has for a long time been the most widely used
comings, has been used as starting point for the development of a
leaching technique [2]. In fact, Soxhlet extraction has been a stan-
variety of modifications intended to alleviate or suppress the latter
dard technique for over a century and the methods based on it
while keeping or even improving the former. Most of the modi-
remain the primary references against which performance in new
fications reported over the last few decades have been aimed at
leaching methods is measured. The advantages and shortcomings
bringing Soxhlet closer to that of the more recent techniques for
of Soxhlet extraction have been used as starting points for the
solid sample preparation, by shortening leaching times with the
development of a variety of modifications intended to alleviate or
use of auxiliary forms of energy and automating the extraction
suppress the latter while keeping or even improving the former.
assembly.
Most of the modifications reported over the last few decades have
been aimed at bringing Soxhlet closer to that of the more recent
3. High-pressure Soxhlet extraction
techniques for solid sample preparation, by shortening leaching
times with the use of auxiliary energies and automating the extrac-
Soxhlet extraction under a high pressure is achieved by plac-
tion assembly.
ing the extractor in a cyclindrical stainless-steel autoclave [9] or
The purpose of this review is both to outline the current posi-
by the use of either commercial or laboratory-made supercriti-
tion of Soxhlet extraction as a model to which the performance of
cal fluid-Soxhlet extractors [10]. The particularity of high-pressure
other extraction techniques is referred and offer an overview about
Soxhlet extraction is that the extractants do not reach supercrit-
the evolution of this technique with a discussion about the differ-
ical conditions. Examples of them can be low-boiling solvents
ent technical versions developed to accomplish a more competitive
or gases under normal pressure and temperature, but in liq-
extraction technique.
uid state under high pressure. The development of the Soxhlet
process under high pressure (1000 1500 psi) should short the
2. Conventional Soxhlet extraction time required and reduce solvents consumption. High-pressure
Soxhlet extraction has been used to isolate organochlorine pesti-
In its classical implementation, which was originally used to cides and polychlorinated biphenyls (PCBs) prior to determination
determine fat in milk [2], the sample is placed in a thimble- in certified potato, carrot, olive oil and lyophilized fish tissue
holder that is gradually filled with condensed fresh extractant samples. In this application, carbon dioxide was used as extrac-
(term used to refer to the solvent used for extraction) from a tant medium at the peak popularity of this extractant. The
distillation flask (see Fig. 1). When the liquid reaches the over- extraction set-up was immersed in a thermostated bath with a
ć%
flow level, a siphon aspirates the solute from the thimble-holder cooling water (0 C) pumping system to condense the extractant
and unloads it back into the distillation flask, thus carrying the [11].
extracted analytes into the bulk liquid. This operation is repeated Another application was fractionation of low-molecular-weight
until extraction is complete. Operationally, Soxhlet extraction is polyethylene. In this study, liquid CO2 was found to be a suitable
thus a continuous discrete technique. In fact, since the extractant solvent for the lowest molecular weight hydrocarbons but failed
acts stepwise, the assembly operated as a batch system; however, to solubilize hydrocarbons with molecular weights higher than C-
extractant is recirculated through the sample, so the system also 40 C-50. Liquid pentane was found to be an effective solvent for
operates in a continuous manner somehow. hydrocarbons insoluble in liquid CO2 [12].
M.D. Luque de Castro, F. Priego-Capote / J. Chromatogr. A 1217 (2010) 2383 2389 2385
Fig. 2. Soxtec® System HT equipment initially commercialized by Tecator.
The main drawback accompanying to these extraction systems
Fig. 3. Experimental set-up designed for ultrasound-assisted Soxhlet extraction
is associated to their operational principles. The Soxhlet process
(adapted from Ref. [17], reproduced with permission of Elsevier).
should not be affected by its performance under high pressure,
which adds an extra level of complexity and reduces the robustness
of the extractors.
through which ultrasound is applied by means of an ultrasonic
probe, as shown in Fig. 3. The application of ultrasound to the sam-
ple cartridge provides results similar to, or even better than, those
4. Automated Soxhlet extraction
obtained by conventional Soxhlet leaching (official ISO method);
however, it enormously decreases the number of Soxhlet cycles
Automation of Soxhlet extraction was initially implemented on
needed in conventional procedures. But the most important result
the commercial equipment Soxtec® System HT (see Fig. 2), which
of ultrasound application is the decompaction effect it produces,
provided substantial savings in time and extractant [13]. This appa-
which avoids typical steps of grinding several times between Soxh-
ratus uses a combination of reflux boiling and Soxhlet extraction
let cycles to diminish the increased compactness produced by the
(both assisted by electrical heating) to perform two extraction steps
dropping extractant. Despite the reported oxidative effect of ultra-
(boiling and rinsing), followed by extractant recovery. Exchange
sound [18] under drastic conditions, the mild conditions used in
from one to other step is achieved by switching a lever. The Sox-
this extractor do not degrade the extracted oil.
tec counterpart B811 extractor, able to perform the same steps as
a Soxtec device, emerged to implement the possibility to operate
also as a conventional Soxhlet apparatus. The overall performance
6. Microwave-assisted Soxhlet extraction
of the B811 extractor is computer-controlled [14].
Similar systems, currently commercialized by Foss, are the auto- Between the attempts to improve Soxhlet performance, the
mated SoxtecTM 2050, the semiautomated SoxtecTM 2055, or the
most successful has been the use of microwaves, which has pro-
more economic versions SoxtecTM 2043 and 2045. One other ver- vided the wider variety of approaches. In fact, microwave-assisted
sion is the SoxCapTM 2047 that includes an acid hydrolysis step in
Soxhlet extraction remains the most interesting improvement of
the operation protocol used for total fat analysis [15]. Based on the
conventional Soxhlet extraction.
use of these devices there are about 80 thoroughly tested methods
Microwave-assisted Soxhlet extraction differs mainly in some
available in the form of Application Sub Notes within the agricul- or all of four aspects from other microwave-assisted extraction
tural, food and industrial sectors, ranging from total fat extraction
techniques, namely: (1) the extraction vessel is open, so it always
in meat to extraction of PCB in soil and sludge. Soxtec Systems have
works under normal pressure; (2) microwave irradiation is focused
been used in officially approved methods such as AOAC 2003.05 and
on the sample compartment; (3) the extraction step is totally or
2003.06 (crude fat in feed, cereal grain and forage using diethyl- partially performed as in the conventional Soxhlet technique (i.e.
ether and hexane extraction methods), AOAC 991.36 (fat crude in
with permanent sample fresh extractant contact); (4) no subse-
meat and meat products), ISO 1444:1996 (free fat content in meat
quent filtration is required. Therefore, these approaches retain the
and meat products) or EPA 3541 (extraction of PCBs in soil and
advantages of conventional Soxhlet extraction while overcoming
sludge). Despite the implementation of commercial extractors in
its limitations, as regards throughput, automatability and ability
reference analysis methods, their compact configuration does not
to quantitatively extract strongly retained analytes, as the most
improve the scarce versatility of the conventional Soxhlet device.
important.
5. Ultrasound-assisted Soxhlet extraction 6.1. Commercial microwave-assisted Soxhlet extractors
An extractor based on the physical chemical principles of Soxh- The most used commercial microwave-assisted extractor is the
let by taking advantage of ultrasound effects [16] was designed, Soxwave-100 apparatus, which was patented and made commer-
constructed and applied by the authors team to the extraction cially available by Prolabo (Paris, France). The principle behind the
of total fat from oil seeds such as sunflower, rape and soyabean Soxwave-100 is similar to Kumagawa extraction and its operation
[17]. The approach uses the conventional Soxhlet glasware, but similar to that of the Soxtec® System HT [13], the process involving
has the Soxhlet chamber accommodated in a thermostatic bath extraction in three steps: a first step where the sample is immersed
2386 M.D. Luque de Castro, F. Priego-Capote / J. Chromatogr. A 1217 (2010) 2383 2389
Fig. 4. (A) First FMASE prototype (adapted from Ref. [29], reproduced with permission of Elsevier). (B) Reverse configuration of the FMASE (adapted from Ref. [30], reproduced
with permission of Royal Society of Chemistry). (C) Coupling of the extraction unit with a dynamic manifold to monitor the leaching process: (1) controlled FMASE; (2) flow
injection manifold to interface extraction with detection (adapted from Ref. [31], reproduced with permission of American Chemical Society). (D) Complete automatic
configuration of the first prototype (adapted from Ref. [32], reproduced with permission of American Chemical Society).
in the boiling extractant, followed by lifting of the cartridge over through the sample and are condensed on arrival at the condenser.
the solvent and continuous dropping of the condensate on the Then, the condensate is dropped down onto the sample by adjust-
cartridge. In the first step, a matrix extractant partitioning equilib- ing a 3-way valve. Obviously, this operation is not based on the
rium of the extractable species is established while the microwave Soxhlet principle that exploits contact between the sample and
radiation acts on both the sample and extractant. In the second step, fresh extractant in each leaching cycle; therefore, displacement
the partitioning equilibrium is displaced to extraction completion of the partitioning equilibrium to complete extraction is impos-
by effect of the sample coming into contact with fresh extractant sible. Extraction must be inevitably followed by filtration in order
in the absence of microwave irradiation. to separate the remaining solid matrix from the extract. Despite
The Soxwave-100 extractor uses a single heating source (viz. the name used by the authors, the device does not integrate Soxh-
focused microwaves), which acts on both the sample and solvent. let and microwaves. This extractor has been used to isolate lipids
This fact makes the dielectric constant of the solvent used as extrac- from foods [27] and oily seeds [28].
tant of paramount importance in this extractor; therefore, polar
solvents are more efficient here than non-polar and low-polar sol- 6.2. The focused microwave-assisted Soxhlet extractor (FMASE)
vents. Because the amount of energy required by the solvent is
different from that required to remove the target analytes from This extractor was designed by the authors group and its first
the sample, a compromise must inevitably be made in this respect. prototype was also constructed by Prolabo (see Fig. 4A). Contrar-
The Soxwave-100 has retained its original commercial design ily to the Soxwave-100, the FMASE works like a conventional
and its uses have been restricted to Prolabo application sheets Soxlet apparatus; thus, it performs a series of cycles where the
(namely, environmental [19 21], polymer [22], drug [23] and food extractant is completely renewed but the sample is irradiated with
samples [24 26]). microwaves for a preset time each cycle. It uses two energy sources
It is worth mentioning here the attempts by the Chemat team (microwaves for sample irradiation and electrical heating of the
to develop a microwave-assisted extractor (the name they give extractant), which leads to the following behaviour: (i) extractant
to the device is microwave-integrated Soxhlet extractor ), which heating is non-dependent on the solvents polarity; (ii) the energy
they consider to be similar to a Soxhlet extractor but in fact dif- for solvent heating and that required to remove the target analytes
fers markedly from it in operational terms [27]. Thus, there is no from the sample can be optimized independently at each tempera-
contact of the sample with fresh extractant and no siphoning of ture; (iii) in FMASE, clean extractant and microwave irradiation are
the extract; also the extractant is heated by microwaves (similarly simultaneous, which facilitates mass transfer and shortens extrac-
to the Soxwave-100), and a filtration step is required. Low-polar tion times as a result [29].
and non-polar extractants are heated to their boiling points by Three prototypes have been designed and constructed since
using microwaves while stirring with a Weflon magnetic stirrer to focused microwave-assisted Soxhlet extraction was first proposed
absorb microwave radiation. In this way, solvent vapours penetrate as a sample preparation approach in 1998. The prototypes were
M.D. Luque de Castro, F. Priego-Capote / J. Chromatogr. A 1217 (2010) 2383 2389 2387
sequential improvements (particularly as regards efficiency and pressure pump is used to transfer the distilled extractant from
flexibility) over their previous incarnations. Therefore, each proto- the reservoir in condenser 1 to the quartz vessel and, following
type had new advantages and some disadvantages over its older contact between the sample and extractant, to lead the extract
siblings. Below are described the most salient features of each. to its reservoir.
(b) The dynamic FI manifold for on-line monitoring of the extrac-
6.2.1. First, simplest prototype: advantages and shortcomings tion process consists of a low-pressure pump and injection
The prototype (Fig. 4A) was constructed by Prolabo (Paris, valve, a flow-cell located in a fluorimeter and transport tubes
France) in 1998 and consisted of a modified Microdigest A301 to lead the effluents from the outlets of the injection valve and
focused microwave digestor (200 W maximum power) where a flow-cell to the distillation flask.
hole was made at the bottom of the irradiation zone to connect
the cartridge compartment with the distillation flask through the Each cycle involves filling the quartz vessel containing the
siphon. This adaptation allowed the cartridge compartment of a cartridge and sample with fresh extractant from the distillate reser-
conventional Soxhlet unit to be accommodated in the irradiation voir, irradiation with focused microwaves and unloading of the
zone of the microwave oven. Operationally, the extractor is iden- extract in its reservoir, the graduation in which allows measure-
tical to a conventional Soxhlet apparatus except that it affords ment of the aspirated extract volume.
irradiation of the cartridge with focused microwaves for a preset Simultaneously with the start of each cycle, the channel of the
time during each extraction cycle while fresh extractant (con- FI pump, which is used to aspirate the extract from the previous
densed vapours from the distillation flask) is dropped on and passed cycle, is enabled to have the stream circulate through the 500- l
through the solid sample. In this way, breaking of analyte matrix loop of the injection valve IV onto the distillation flask. The outlet
bonds is facilitated by application of the appropriated energy. A Pro- of the flow-cell in the fluorimeter also reaches the distillation flask,
lab Megal 500 thermometer was used to monitor the extraction thus avoiding losses of the extract used for monitoring.
temperature. Also, two controllers were used for the microwave This combined system fulfills the following objectives: (1) oper-
unit and thermometer, and an electrical isomantle furnished with ation as a screening system (yes/no answer); (2) monitoring of
a rheostat was used as heating source for the distillation flask. The extraction kinetics; (3) semi-quantitation of the analytes in routine
operational variables amenable to optimization in the FMASE are analyses when the sample composition is approximately known.
the irradiation power, irradiation time and number of cycles. It is also very useful with a view to establishing the refractivity
The device retains the advantages of conventional Soxhlet of samples without times losses, and overcomes the most signifi-
extraction while overcoming restrictions such as its long extraction cant limitation of extraction techniques in general when the yields
times, non-quantitative extraction of strongly retained analytes of specific compounds to be extracted are dependent on the bulk
which is enabled by easier cleavage of analyte matrix bonds by composition of the sample (matrix effects).
effect of interactions with focused microwave energy , difficult of This configuration can be modified in order to couple extrac-
automation which is made easier by replacing glassware with tion to other steps of the analytical process such as derivatization,
pumps and the large volumes of organic solvent that are wasted. pre-concentration, clean-up or any type of high-resolution separa-
Unlike a conventional Soxhlet extractor, the microwave-assisted tion (e.g. gas or liquid chromatographs, capillary electrophoresis)
Soxhlet system allows up to 75 85% of the total extractant volume or detection.
to be recycled by evaporation collection of most of the extractant Automatic configuration: This configuration is similar to part A
volume. Electrical heating of the extractant, the efficiency of which of the previous one (see Fig. 4D). In this case, two single-channel
is independent of the extractant polarity, is also crucial for this step. piston pumps equipped with flexible tubes are used to aspirate
This prototype, which is especially flexible, has been the subject the extractant and replace the siphon. In this way, more strict
of the following modifications and/or combinations: control of the contact time between sample and fresh solvent is
Reverse configuration: The name given to this configuration achieved by aspirating the latter at preset intervals and introduc-
comes from the sample location, which is not the cartridge, but tion of fresh extractant into the cartridge at the preset flow-rate is
rather the extraction vessel (see Fig. 4B). This modification enables facilitated. Operationally, the process consists of a number of cycles
the use of acid extractants, which can destroy the cartridge upon each involving the following three steps: (1) filling of the extraction
contact with them under microwave irradiation. Thus, the cellu- vessel with fresh extractant delivered by pump 1; (2) microwave
lose cartridge is used as a filter rather than as a sample container. irradiation; (3) unloading of the extraction vessel and delivery of
The main drawbacks of this configuration are the inability to in situ the extract to the distillation flask with the help of pump 2 [32].
recycle of the extractant hence it is only applicable to aqueous As stated above, the main drawback of the first FMAS prototype
extractants, and the dilution effect on analytes; in any case, it is was the difficulty of using high-boiling extractants. This excluded
very useful for removal of metals from coal [30]. green applications based on the use of water as extractant. This
Coupling for monitoring extraction: A flow-injection (FI) inter- shortcoming could have been circumvented by replacing the glass-
face between the FMASE (in modified form) and an appropriate ware with piston pumps and Teflon tubing; however, it promoted
detector allows the extraction process to be independent of the the development of a new prototype intended to expand its scope
sample matrix [31]. Fig. 4C shows the overall system, which com- of application with other types of extractant [33].
prises the following parts:
6.2.2. Automated, flexible prototype overcoming the
(a) The extractor, which is connected to the distillation flask in shortcomings of the first
addition to the Microdigest A301 with the orifice at the bot- This second prototype was constructed by SEV (Puebla, Mexico)
tom, and the quartz sample container in which the Megal 500 and called MIC II (Fig. 5). It is based on the same principles as the
thermometer is inserted. The distillation flask is connected to previous FMAS extractor and consists of a single unit where short-
condenser 1, with a reservoir for condensed vapour, from which ening of the distillation glassware allows reception of the extractant
fresh extractant is pushed to the quartz sample container. A vapour on a condenser connected to the top of the sample cartridge
second condenser (number 2) directly connected to the quartz vessel with minimal losses in the way, its condensation, and drop-
container condenses vapour from it. No siphon is used and the ping on the solid sample. The siphon has been replaced with a valve
extract is led from the orifice at the bottom of the quartz vessel that allows filling of the vessel to the desired level or its draining
to a graduated reservoir after each cycle. A two-channel low- to the distillation flask. The short glassware distillation path used
2388 M.D. Luque de Castro, F. Priego-Capote / J. Chromatogr. A 1217 (2010) 2383 2389
Fig. 5. Scheme of the MIC II FMASE (adapted from Ref. [34], reproduced with per-
mission of American Chemical Society).
affords the use of water or other high-boiling extractants. Since
unloading of the extract from the sample vessel can be controlled
via a switching valve, a new operational variable named delay
time (viz. interval during which the sample is in contact with the
solvent after microwave irradiation and before draining from the
irradiation vessel) can also be optimized for improved extraction
Fig. 6. Automatic FMASE prototype (adapted from Ref. [36], reproduced with per-
[34,35]. The device operates at a microwave power between 100
mission of Elsevier).
and 400 W with irradiation time control ranging from 1 s to 1 h.
The main limitation of this prototype is that the extraction process
cannot be completely automated; thus, the valve must be switched
diation. In this way, the temperature of the leaching process can be
by hand, and so must microwave irradiation. One other limita-
effectively controlled, which can be indispensable for applications
tion is inability to recycle the extractant, which is desirable with
involving thermolabile compounds.
extractants other than water. Complete automation and extrac-
tant recycling were thus two objectives to be fulfilled with a new
prototype.
7. Conclusions
6.2.3. A dual-operation automated prototype: advantages of the Soxhlet extraction has for more than a century demonstrated
definitive prototype (commercial availability) its advantages, which have surpassed in most cases its shortcom-
A fully automated focused microwave-assisted Soxhlet extrac- ings. The latter have been more or less successfully overcome in the
tor was designed and constructed also by SEV. This extractor following ways:
(Fig. 6), called MIC V, uses two extraction units, which allow the
simultaneous processing of two samples for replicated extraction.
(1) By increasing the pressure into the sample cartridge, thus
Automation is accomplished by using an optical sensor, a solenoid
favouring extractant penetration into the solid and shorten-
valve and control via microprocessor software. An 18-cm long
ing the extraction time as a result; and also decreasing the
siphon houses the optical sensor, which is positioned at a given
extractant volume. Nevertheless, working at high pressure
siphon height to have the magnetron start irradiation of the sample
complicates the experimental set-up.
when the solvent reaches the preset level. The higher the position
(2) By automating extraction using different approaches that have
of the optical sensor along the siphon is, the higher the extractant
given place to a number of commercial extractors with different
volume that is brought into contact with the target sample in each
characteristics but with a common denominator: shortening of
cycle. The solenoid valve, inserted at the bottom of the siphon, is
the extraction time, decreasing of the extractant volume and
automatically switched at the end of the irradiation step to empty
providing simultaneous extraction of several samples. Maybe
the sample vessel. One parameter related with extractant volume,
the most significant shortcomings of these devices are relatively
and hence dependent on the position of the optical sensor, is the
high acquisition costs and lack of versatility.
unloading time, which is the time during which the solenoid valve
(3) By assisting extraction with auxiliary energies. There is at
remains in its unload position. This prototype can be coupled to
present no commercial extractors based on this principle. Nev-
other steps of the analytical process via an appropriate FI interface
ertheless, the use of ultrasonic energy and, mainly that of
by introducing a Teflon tube in the distillation flask. This final pro-
microwaves, looks very promising and is, in the authors opin-
totype overcomes the limitations of its predecessors and affords
ion, the best alternative so far to surpass Soxhlet shortcomings.
fully automatic extraction of two samples at once [36,37].
6.2.4. New incoming prototype It is clear that conventional Soxhlet has for long time been the
A new, more compact prototype called Accesox (Barcelona, best leaching alternative. Improvement of the conventional extrac-
Spain) has recently been developed to reach a wider market. This tor by incorporation of present technologies allows its adaptation
device has the additional choice of the maximum temperature to be to the present necessities; so, it can be said that Soxhlet extraction
reached in the sample extractant medium during microwave irra- has been, and it is, almost a panacea in this area.
M.D. Luque de Castro, F. Priego-Capote / J. Chromatogr. A 1217 (2010) 2383 2389 2389
Acknowledgement [20] J. Dean, Prolabo Application Book Organic Extraction , Application Sheet No.
14.
[21] D. Stalling, Prolabo Application Book Organic Extraction , Application Sheet
The Spanish Ministerio de Ciencia e Innovación (MICINN) is
No. 18.
thanked for financial support (Project CTQ2009-07430). [22] J. Dean, Prolabo Application Book Organic Extraction , Application Sheet No.
300.
[23] S. Kingston, Prolabo Application Book Organic Extraction , Application Sheet
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