Journal of Liquid Chromatography & Related Technologies, 33:1174–1207, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1082-6076 print/1520-572X online
DOI: 10.1080/10826076.2010.484371
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SAMPLE PREPARATION FOR TRACE ANALYSIS BY
CHROMATOGRAPHIC METHODS
Romeo-Iulian Olariu,1 Davide Vione,2 Nelu Grinberg,3 and Cecilia Arsene1
1
Department of Chemistry, Faculty of Chemistry, Laboratory of Analytical Chemistry,
‘‘Al. I. Cuza’’ University of Iasi, Iasi, Romania
2
Dipartimento di Chimica Analitica, Università di Torino, Torino, Italy
3
Boehringer Ingelheim Pharmaceuticals Inc., Ridgefield, Connecticut, USA
& The determination of trace analytes in complex natural matrices often requires extensive
sample extraction and preparation prior to chromatographic analysis. Correct sample preparation
can reduce analysis time, sources of error, enhance sensitivity, and enable unequivocal identification, confirmation, and quantification. This overview considers general aspects on sample preparation techniques for trace analysis in various matrices. The discussed extraction=enrichment
techniques cover classical methods, such as Soxhlet and liquid-liquid extractions along with more
recently developed techniques like pressurized liquid extraction, liquid phase microextraction
(LPME), accelerated microwave extraction, and ultrasound-assisted extraction. This overview also
deals with more selective methodologies, such as solid phase extraction (SPE), solid phase microextraction (SPME), and stir bar sorptive extraction (SBSE). The adopted approach considers the
equilibriums involved in each technique. The applicability of each technique in environmental,
food, biological, and pharmaceutical analyses is discussed, particularly for the determination of
trace organic compounds by chromatographic methods.
Keywords chromatographic analysis, enrichment techniques, gas-liquid, gas-solid
equilibriums, liquid-liquid, liquid-solid, sample preparation
INTRODUCTION
It has long been established that knowledge on complex chemical
systems in matrices of interest for the human beings, in various ways, is
critically dependent on chromatographic methods. Sensitive and robust
analytical methods, among which chromatography is quoted by far as the
most important, have been widely used during the past decades to investiAddress correspondence to Cecilia Arsene, Department of Chemistry, Faculty of Chemistry,
Laboratory of Analytical Chemistry, ‘‘Al. I. Cuza’’ University of Iasi, Carol I 11, 700506 Iasi, Romania.
E-mail: carsene@uaic.ro
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gate and identify various chemical compounds characterized by varying
degrees of structural complexity.
Modern analysis involves undertaking each single step of a complete
analytical diagram flow, beginning with the definition and outline of the
problem and ending with a detailed critical evaluation of the relevant analytical data which allows the presentation of the analytical result. Sample
preparation and the use of adequate analytical methods represent the
bridge between the two aforementioned steps, which will be largely dependent upon analyst experience.
In practical work, analysis of trace chemicals entails more than the mere
qualitative or quantitative detection and identification of a particular
element or chemical compound. It involves knowledge of the origin and
structure of the sample matrix, and also the analyst’s insight into analogous
problems from other disciplines to assure the plausibility of the questions
raised and to critically evaluate and interpret the results. It is suggested that
these preliminary observations are essential in trace analysis, where an
important focus is in sample preparation. Nevertheless, it is admitted that
tools, equipment, and methodological principles are common to both
general chemical analysis and modern trace analysis.[1]
The concentration levels of target analytes found in environmental,
biological, food, and drugs samples are generally too low to allow a direct
injection into a chromatographic system. Changing solvent, temperature,
pressure, phases, or volumes are among the main tools used by analysts
in order to solve a complex chromatographic problem. Most of the sample
preparation techniques rely on analytical steps including trapping of the
analytes of interest on various media, desorption and analysis (mainly by
chromatography). Poor sensitivity, the major problem in these procedures,
is presently overcome by including on-line combination of extraction with
liquid chromatography and injection of large volumes into the analytical
system (i.e., gas chromatography).[2]
The main goal of sample pretreatment is to make complex samples
suitable for chromatographic analysis.[3] This prerequisite is necessary to
reach detectable concentration of the target analyte and to isolate the
analytes from very complex matrices.
Quite often, the sample preparation in chromatographic methods is
representing a tedious, time-consuming, and error-prone step of an analytical procedure. Therefore, it is generally regarded as the rate-limiting step
in chemical analysis. It has been suggested that a large part of the time typically required to perform analytical tasks is spent on sample preparation.[4]
Indeed, sample pretreatment is frequently performed by off-line methods
(e.g., liquid-liquid extraction and solid-phase extraction). These procedures are usually performed manually, they are laborious and timeconsuming and sometimes lack precision and accuracy.
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THE STATE-OF-THE-ART IN CHROMATOGRAPHIC
INVESTIGATIONS TOWARD SAMPLE PREPARATION:
A GENERAL APPROACH
At the beginning of the chromatographic technique, insufficient detection limits, and the occurrence of many problems with sample preparation
and separation, preventing the analysis of real samples, were often acknowledged as major challenging aspects that chemists were facing in their
experimental work.[5] Recent progress in instrumental analytical chemistry
has resulted in the availability of methods that allow monitoring of various
chemicals at parts per trillion (ppt) and even parts per quadrillion (ppq)
range.
A number of important papers of fundamental and comprehensive
review brought the importance of choosing and using suitable analytical
techniques for the determination of trace residues and contaminants in
complex matrices to the attention of the scientific community. Presently,
a topic of great concern and interest is the analyses of the potential genotoxic impurities (PGIs) in pharmaceutical products. PGIs have received
increased consideration over the last years.[6] A threshold of toxicological
concern (TTC) value of 1.5 mg dayÿ1 has been developed as an acceptable
risk associated with the intake of a genotoxic impurity.[7] Analyzing the
PGIs, even at low ppm levels in active pharmaceutical ingredients (APIs),
is a challenging task, which presently can be performed through the use
of state-of-the-art technology. Currently, quantifying chemicals at such low
levels does not seem feasible in order to routinely control the level of the
impurities that might reside in active pharmaceutical products and, despite
the demand for sensitivity to detect trace concentrations, matrix interference and selectivity will represent important issues to overcome.[6] Modern hyphenated techniques, involving static headspace sampling (SHS)
coupled with capillary gas chromatography interfaced to mass spectrometry
(GC-MS) are presently available for the analysis of halides and haloalkenes,[8] while in situ derivatization by SHS-GC-MS is more suitable for the
analysis of aryl- and alkyl sulfonates.[9] Vanhoenacker et al. in 2009
proposed a sample preparation method where liquid chromatography
(LC) is preferred for the analysis of less volatile solutes.[6]
Presently, a number of reviews and original papers is available on the
state-of-the-art chromatographic methods for residue analysis of pharmaceuticals in samples of environmental concern,[10] for trace residues and
contaminants in foods and drinks,[11–13] for mycotoxins in biological
tissues[14], or for surfactants (e.g., alkylbenzene sulfonates, ethoxylated
nonionic surfactants, metabolites) in river water or wastewater.[15,16] Major
modern sample preparation techniques for the extraction and analysis of
medicinal plants were reviewed by Huie in 2002, and the author concludes
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that the solid-phase microextraction represents the most suitable
alternative for the sampling of volatile compounds before chromatographic
analysis.[17] Baltussen et al., in an excellent review regarding sorptive
sample preparation, concluded that this is a valuable strategy to overcome
the limitations of the adsorptive sampling. The technique is used in combination with thermal desorption for the analysis of very apolar analytes (i.e.,
alkanes, alkenes, and aromatics).[2]
As far as sample preparation is concerned, the hyphenation of various
techniques has been gaining importance over the past decades. Sample
pre-concentration and clean-up methods hyphenated with core analytical
techniques are acknowledged as powerful tools to accomplish the task of
low-level detection.[18] Hyphenated (coupled or hybrid) techniques, coupling chromatographic separation with sensitive and specific detectors
(usually mass spectrometry), has recently become one of the most powerful
instrumental tools in speciation analysis and the characterization of
complex samples.
Liquid or gas chromatographic methods coupled with mass spectrometric detection play an increasingly important role in environmental
analysis, especially in the aquatic environment and in water treatment.[19,20] Hydrophilic interaction chromatography (HILIC) hyphenated
with mass-spectrometry (MS) is a potentially powerful technique in the
quantitative analysis of drugs and drug metabolites.[21] High-performance
liquid chromatography stability-indicating methods are particularly attractive for the determination of active substances (e.g., ascorbic acid) and for
the quantification of potentially occurring degradates in pharmaceutical=
cosmetic preparations, developed as oil-in-water emulsion and aqueous
gel.[22] Such a method presents convenience, rapidity, and the ability to
separate substances quantitatively without pre-derivatization.
There is also an important number of reports which refer to fast, simple, sensitive, and efficient sample preparation methods prior to analytical
detection of a wide range of persistent organochlorine pollutants,[23] polychlorinated biphenyls in soils,[24] iodixanol used as a contrasting agent,[25]
sodium azide used as a preservative,[26] pharmaceuticals, drugs, anesthetics,
and metabolites,[27–36] and even adjuvants (epichlorohydrin) in paper and
pharmaceutical industries.[37] Presently, also in the enantioselective analysis of various drugs marketed as racemic mixtures, tedious sample preparation methods turned towards more rapid and feasible procedures.[38]
It is generally agreed that the works that are mainly serum=plasma
related are especially important for assessment studies of health issues
related to human exposure.[23] However, matrices of physiological concern,
such as plasma, serum, and biological tissues, were previously assigned as
being particularly complex, with numerous sample preparation
problems.[27] Major problems that should be addressed in the sample
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preparation step are the presence of interference masking the analytes of
interest, of non-analytes progressively reducing the performance of the
analytical column, and the variability between the samples induced by
the multitude of the non-analytes.
Presently, limits of the chromatographic methods to simultaneously
detect several water-soluble vitamins in complex matrices require identification and use of separate assay methods. Sample preparation, sensitivity
of the detection method that is used, and equipment costs are among
the main problems limiting the use of chromatographic methods in routine analysis of, for example, ascorbic acid. Therefore, it is suggested that
chromatographic methods be replaced by flow injection analysis (FIA)
based on spectrophotometric or electroanalytical detectors, which is a
more suitable tool to solve analytical problems characterized by
time-consuming procedures of extraction, reaction, and analysis, or when
only one analyte has to be determined in a large number of samples.[39]
However, in 2003, Iwase supplied excellent preparative aspects for the routine chromatographic analysis of ascorbic acid in food.[40] Vinci et al., in
1995, claimed that, by improving the chromatographic conditions and
the sample pretreatment operations, it is possible to optimize and make
easier the overall procedure of analysis of ascorbic acid in fruits with high
nutritional value, which contain generally high levels of hydrosoluble
vitamins.[41]
In samples of environmental concern, the methods used to isolate trace
volatiles for gas chromatographic analysis may have profound effects on the
resultant chromatograms.[42] Per se injection was the usual method used in
the past for the analysis of samples containing low-boiling petroleum fractions or essential oils. Additional problems may occur with samples containing large amounts of water, alcohol, or nonvolatile materials (including
most food products), or samples containing volatile compounds as dilute
vapor systems (e.g., air or headspace gases).
In some cases, the development of appropriate preparation steps is also
a crucial prerequisite for studies intended at understanding the separation
process at the molecular level, where both experimental observation and
theory must be put forth.[43–47]
High sensitivity with no column overloading and adequate resolution is
challenging tasks that can be solved nowadays. On-line on-column derivatization at controlled temperature is regarded as a useful method for the
analysis of an active aldehyde,[48] while on-line column-switching devices
combined with advanced separation media technologies is regarded as a
suitable technique for the analysis of complex matrices (e.g., mixtures of
enantiomers specific for various cardiovascular drugs).[49] Presently, ultra
high pressure liquid chromatography is largely used for fast enantiomeric
separation of chiral molecules,[50,51] the absolute configuration of the
Trace Analysis by Chromatographic Methods
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enantiomeric analytes being assessed with the help of the vibrational
circular dichroism (VCD). VCD is a technique capable of solving problems
of absolute stereochemistry[52] under specific experimental conditions.[53]
Lowering the temperature at which the separation occurs is a suitable
alternative to studying active compounds or unstable molecules. Extensive
in-column cyclization of an analyte, occurring at room temperature, can be
significantly diminished when working at sub-ambient temperature.[54]
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THE CHOICE OF A SUITABLE SORBENT PHASE
Various materials are successfully used for the selective adsorption of
the analytes of interest from complex matrices, although undesired effects
(incomplete desorption, artifact formation) may also occur.[2] The sorbent
materials can be adopted as stationary phases in chromatography or
employed in the step of sample preparation for extraction and clean-up
purposes. Many research groups have attempted to prepare suitable sorbents for the separation of various compounds. Production of novel
capillary GC stationary phases based on persubstituted cyclodextrins have
attracted a great deal of attention during the past decades, especially for
their potential application in the separation of chiral silicon compounds.[55]
Fundamental studies on intermolecular interactions influencing solute
retention on novel carbon surfaces prepared by vapor deposition on
porous zirconia microspheres proves that these carbon sorbents may considerably improve the chromatographic separation.[56]
The introduction of polar embedded-phases, containing polar moieties
within an alkyl chain, involves changing the chemistry of the stationary
phase itself in order to set-up parameters to improve the selectivity or
reproducibility. Modeling studies have proved that reduced peak tailing
can be obtained with the use of stationary phases with embedded polar
groups, compared with conventional alkylsilane phases.[57]
Strong cation-exchange supports are suitable extractors for the determination of Triton-X 100, a surfactant used in reaction mixtures in order
to increase the solubility of various compounds and to provide homogeneous reaction environments. For instance, it is used as a surfactant in
the presence of quinoline derivatives from the leukotriene D4 class, which
is a therapeutic agent with a potentially important role in the etiology of
various diseases. Development of flow injection methods with on-line
solid-phase extraction offered the most suitable solutions to solve practical
aspects related to the instrumental maintenance, which also enabled the
chromatographic columns to operate for longer times. The method has
also solved a complex problem regarding the analysis of a surfactant that
appears as a mixture of various oligomers, with important implications in
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its quantification. Flow injection analysis with on-line solid phase
extraction represent a simple, rapid, and accurate method for Triton-X
100 determination.[58,59]
On a C18 hybrid stationary phase, using pure water as a mobile phase at
temperature above 100 C, a temperature range where the solvation properties of pressurized hot water changes, it is possible to separate complex
mixture of organic constituents in a short time period.[60,61] Under such
conditions, it is possible to separate at least 12 anilines in less than
10 min,[61] to be compared with a total analysis time of about 80 min as
reported by Gennaro et al.[62]
METHODS OF ANALYTE ISOLATION AND CONCENTRATION/
ENRICHMENT TECHNIQUES
The choice of the suitable extraction=enrichment techniques for the
recovery of trace chemicals from various samples (biological, drugs,
environmental, food, and drinks) must take into account the sensitivity,
selectivity, and separation capabilities of the selected analytical method,
the complexity of the sample, and, last but not least, the chemical and
physical characteristics of the analytes.[15]
In the last few years, on-line dialysis has been successfully applied to the
LC determination of several drugs in biological fluids and especially in
plasma. The sample preparation is normally carried out using the ASTED
(Automated Sequential Trace Enrichment of Dialysates) system connected
on-line with an LC system. Chiap et al. described such an automated
procedure for the chromatographic determination of various chemicals
(i.e., sotalol and human anesthetics) in human plasma. The method
involves on-line dialysis, enrichment of the dialysate on a precolumn that
has been prepacked with a strong cation-exchange material, and subsequent LC analysis using UV detection. The studies described are among
the first experiments where a combination of dialysis with the enrichment
of the dialysate on a cation-exchange sorbent was used.[33,34]
However, prior to the chromatographic analysis, complex samples may
require multiple preparation techniques. For biological matrices, the complex of techniques may consist of deproteinization of the plasma samples,
liquid–liquid extraction after alkalinization followed by back extraction in
an acidic medium, as well as solid phase extraction on disposable cartridges
after deproteinization or alkalinization. These are off-line procedures that
are often performed manually and, therefore, are laborious and timeconsuming. Under these circumstances, when the number of samples to
be analyzed is particularly large, the automation of sample preparation
often becomes a necessity. On-line automated LC procedures based on a
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1181
column-switching technique or on-line sample preparation involving dialysis and trace enrichment on cation-exchange pre-columns have been
recently developed.[33,34] The trace enrichment system is incorporated to
overcome the dilution of the sample caused by dialysis and to improve
method selectivity.
In 2007, in an excellent review, Ridgway et al. treated many aspects of
sample preparation.[12] They referred to the determination of trace residues and contaminants in complex matrices, such as food, which often
requires extensive sample extraction and preparation prior to instrumental
analysis. The idea was to offer analysts with an excellent background in
selecting suitable extraction and concentration methods, which should
move toward more environmentally friendly techniques, using less solvent
and smaller sample sizes. In 2009, Nerin et al. are critically reviewing all
recent developments in solventless techniques for the extraction of analytes
in different areas.[63]
TECHNIQUES AVAILABLE TO INCREASE SELECTIVITY AND
SENSITIVITY
A wide range of sample preparation techniques are presently available
for the analysis of the three states of matter (gas, liquid, and solid). Modern
sample preparation techniques were developed and they gained more
importance over conventional methods due to their major advantages
(i.e., reduction in organic solvent consumption, improved clean-up procedures and concentration steps before chromatographic analysis, increases
in extraction efficiency and selectivity). In this context, analysts’ skills are
completed by a full understanding of the theoretical aspects of equilibriums in liquid-liquid, liquid-solid, liquid-gas, and gas-solid systems.
In the present paper, the sample preparation techniques for trace analysis
by chromatographic methods have been classified based on two equilibrium
types: liquid-liquid or liquid-solid equilibriums, and gas-liquid or gas-solid
equilibriums. This review considers most of the aspects of sample preparation
for trace analysis by chromatographic methods. It covers general extraction
techniques, such as liquid-liquid extraction; Soxhlet and pressurized liquid
extraction; microextraction techniques, such as liquid phase microextraction
(LPME); and more selective techniques, such as solid phase extraction (SPE);
solid phase microextraction (SPME); and stir bar sorptive extraction (SBSE),
including their most recent developments and applications.
The theory of the extraction process is not covered in this review as this
aspect is the subject of several books that treat comprehensive theoretical
and practical aspects concerning sample preparation techniques in
different research areas.[64–66]
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Derivatization
Derivatization is a chemically driven process usually incorporated into
an analytical method to facilitate chromatographic separation to increase
selectivity and to improve the limit of detection. Although numerous
methods have been reported and several books cover the technique, only
a few reactions are widely used in routine analysis.[67,68]
Most derivatization methods for gas chromatography involve esterification or etherification. For example, an analytical method has been
developed to identify compounds containing one or more carbonyl, carboxy, and hydroxy functional groups in atmospheric samples. In the
method, –C¼O groups are derivatized using O-(2, 3, 4, 5, 6- pentafluorobenzyl) hydroxy amine (PFBHA), and ÿCOOH and ÿOH groups are
derivatized using the silylation reagent N,O-bis(trimethylsilyl)- trifluoroacetamide (BSTFA).[69–72] Derivatization can also be performed on fiber=
coatings before, during, or after sorptive extractions.[73]
Derivatizations for HPLC are designed mainly to improve the limit of
detection, permitting the use of highly sensitive or selective detectors
inapplicable to the analytes themselves. Enhanced absorption of UV-visible
light is achieved by the introduction of chromophoric groups. Analytes can
also be rendered fluorescent by the introduction of fluorophoric groups.
Carboxylic acids can be transformed into esters that absorb UV or visible light by reacting with 1-naphthyldiazomethane[74] or bromophenacyl
bromides.[75] a-Keto acids (e.g., glycolic, glyoxylic acids) are detectable with
UV light after derivatization with 2,4-dinitrophenylhydrazones.[76] Fluorescent compounds are obtained by reacting carboxylic acids with
4-bromomethyl-7-methoxycoumarin[77] or 4-hydroxymethyl-7-methoxycoumarin.[78] Analytes containing hydroxyl groups, such as phenols,
glycols, and alcohols, can be converted with 3,5-dinitrobenzoyl chloride
into compounds that absorb UV or visible light.[79] Fluorescent derivatives
can be obtained with 7-[(chlorocarbonyl)methoxy]-4-methylcoumarin.[80]
Derivatizations for HPLC purposes are accomplished either off-line or
on-line. An on-line process may involve either precolumn or postcolumn
reaction, depending on the analyte under consideration and the adopted
instrumentation. In the case of pre-column derivatization, it is essential
to check its compatibility with the separation process.
Separation and Enrichment Techniques Driven by Liquid-Liquid
or Liquid-Solid Equilibriums
Liquid-Liquid Extraction
Liquid-liquid extraction is one of the most common methods of
extraction, particularly for organic compounds from aqueous matrices. It
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is a simple, manual, and off-line extraction procedure, used almost
exclusively in the 1980s. It involves extraction of analytes in solution or
liquid samples by direct partitioning with an immiscible solvent. Repeated
extractions might ensure the complete partitioning of the interest analyte
into the required phase. Clean-up or analyte enrichment=concentration
steps, prior to instrumental analysis, may help for trace level analysis and
improve the selectivity of the whole method.[81,82]
The major disadvantage of liquid-liquid extraction is the need for large
volumes of organic solvents; the formation of emulsions may represent a
relatively frequent problem.[15] In liquid-liquid extraction, there is a tendency for compounds to adsorb on all phase boundaries, which can lead to
the formation of emulsions and prevent a complete phase separation. In
some cases, to avoid emulsions, salt may be added and centrifugation or
freezing can be used if necessary.[12,83]
The demixing=microextraction approach is very appropriate for GC-MS
analysis, as it is a procedure that avoids the imprecise solvent evaporation
steps. It can be applied to water-ethanol mixtures (e.g., wine) and consists
of the separation between water and ethanol, achieved by addition of salts,
followed by microextraction of the analytes from the ethanolic phase.
Extraction is performed at laboratory temperature, ultra high purity
solvents are not required, and the final extract can be cleaned enough if
the extraction conditions are correctly chosen.[11]
Within the liquid-liquid extraction process, the decisive parameter is the
distribution coefficient for the analyte between the particular phases involved.
If the distribution coefficient is sufficiently large, the simplest approach to
liquid-liquid extraction is shaking the sample with an appropriate amount of
an organic solvent. The distribution coefficient can be pH dependent, dividing
the sample into strongly or weakly acidic, neutral, or basic fractions.[84]
With smaller distribution coefficients or large sample volumes, continuous extraction or countercurrent extraction is required to achieve a complete separation. The apparatus for continuous extraction causes a liquid
immiscible with the sample solution to circulate continuously through
the sample.[85] Extracted analytes are concentrated by distillation at
appropriate times between individual extraction cycles.
More recently, classical liquid-liquid extractions have been replaced by
modern, efficient and versatile microextraction techniques. The time
needed to reach equilibrium and the volume of solvent needed for the
quantitative recovery of analytes switch the preference toward more modern methods. They are more and more frequently adopted both in organic
synthesis laboratories and for the separations of metal complexes, metal
chelates, and=or ion-pairing reagents.[65]
Although old in fashion, a variant of liquid-liquid extraction takes
advantage of a liquid phase immobilized on a solid sorbent such as
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kieselguhr, Celite, Chromosorb W, or Chromosorb P.[86] In this method,
the immobilized phase may be either aqueous or non-aqueous.
An ‘‘in-line’’ liquid-liquid extraction (LLE) system is created in hydrophilic
interaction chromatography (HILIC), where the mobile phase forms a
water-rich layer on the surface of the highly polar stationary phase. The mechanism involves distribution of the analytes between the water-rich stationary
layer and the mobile phase with mostly organic content. The analytes possessing higher polarity will have a higher affinity to the stationary aqueous layer
than the analytes possessing weaker polarity. As HILIC requires a high-organic
and low-aqueous mobile phase, which are favorable conditions for MS in terms
of sensitivity, HILIC appears to be a preliminary preparative step in MS analysis.[21] A technical and cost effective method for a therapeutic drug monitoring
program of ribavirin (a synthetic purine analogue of guanosine, used in the
standard treatment of chronic hepatitis C virus) proposes hyphenation of a
liquid=liquid extraction method coupled with HPLC-UV measurements.[87]
Soxhlet Extraction
Soxhlet extraction, a liquid-solid equilibrium technique, has application in sample preparation prior to chromatographic analysis. It is basically a leaching technique based on two processes: 1) reflux boiling of a
solvent, and 2) a siphon procedure. This technique has already been
reviewed.[88,89] Recent developments have included the use of focused
microwave-assisted extraction; ultrasonic extraction has been used to
improve extraction efficiencies.[90,91]
The large volume of solvent that is needed for the sample extraction, the
extra step required to concentrate the sample after solvent evaporation, the
lack of thermal stability, the volatility of some sample analytes, and the interference from contaminants in the extraction thimbles (requiring a blank
extraction prior to sample extraction) limit the application of this technique.
Although exhaustive, the Soxhlet technique is not selective and further
clean-up is necessary. Due to the temperatures involved, Soxhlet extraction
can degrade thermally labile compounds.[89] Most applications of Soxhlet
extraction are for environmental samples, such as soil, but it has been used
for the analysis of food followed by further clean-up.[88,92,93]
Automated Soxhlet extraction systems are available, which claim to
greatly reduce extraction times and perform boiling, rinsing, and solvent
recovery automatically. Up to 6 samples can be extracted simultaneously
and lower volumes of solvent can be used.[88]
Ultrasound-Assisted Extraction (USE)
Ultrasound-assisted extraction (USE) is among the easiest and most
reliable of the wide range of available extraction techniques.[94] Ultrasound
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assistance is a growing trend in analytical chemistry.[95] The technique is
performed statically and utilizes energy in the form of acoustic sound waves
to accelerate mass transport from a solid sample immersed in a solvent. The
extraction setup is uncomplicated. Normally, an ultrasonic bath filled with
water and a number of extraction vessels, together with a relatively strong
solvent or mixture with appropriate properties for the targeted analytes
and matrix, can be selected to obtain maximum extraction efficiency and
required selectivity.[96] This is a fast technique but efficiency is not as high
as with other techniques. Low concentrations of analytes in samples require
multiple extractions. Several extractions can be performed simultaneously.
The technique is relatively inexpensive compared to most modern extraction methods, because no specialized laboratory equipment is required.
One important disadvantage of ultrasound-assisted extraction is that it is
not suitable for volatile analytes.
Herrera and Luque De Castro in 2005 used an ultrasound-assisted
extraction technique followed by HPLC for the analysis of phenolic compounds from strawberries,[97] and Rezic et al., the same year, used
ultrasound-assisted extraction and thin-layer chromatography for the
determination of pesticides in honey.[98] Kimbaris et al. performed a
comparison of distillation and ultrasound-assisted extraction methods
for the isolation of aroma compounds from garlic.[99] Other applications
of the USE technique include extraction of polycyclic aromatic hydrocarbons (PAHs) from lichen samples,[100] determination of polyphenols
in tobacco,[101] determination of butyltin and phenyltin species in sediments,[102] determination of organophosphorus pesticides in sludge,[103]
and determination of triazine herbicide residues in horticultural products.[104] However, as both selectivity and sample enrichment capabilities are limited, further clean-up and=or concentration steps are
usually required for the determination of trace analytes in several
matrices.[104,105]
Microwave-Assisted Extraction (MAE)
In recent years, microwave-assisted extraction (MAE) has attracted
growing attention as it allows rapid extraction of solutes from solid
matrices, with extraction efficiency comparable to that of the classical techniques.[106,107] Accelerated dissolution kinetics is produced in MAE as a
consequence of the rapid heating processes that occur when a microwave
field is applied to a sample. Microwave-assisted extraction gained enlarged
attention due to its applicability to a wide range of sample types, and
because the selectivity can be easily manipulated by altering solvent
polarities.[108] There are studies suggesting that MAE affords a lower
solvent consumption than pressurized liquid extraction (PLE, vide infra).[109]
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An overview of the different microwave-based devices used for solid
sample pretreatment has been published in 2003.[110] The authors
described multi-mode and focused microwave devices, as well as closed
and open systems. Special open systems, such as a microwave-ultrasound
combined reactor, a focused microwave-assisted Soxhlet extractor, a
microwave-assisted dryer, and a microwave-assisted distiller were discussed.
Finally, there are brief comments on microwave-assisted robotic methods,
and closed and open microwave systems are compared.
Because of its applicability to solid, semi-solid, and liquid matrices,
microwave-assisted (MAE) extraction has emerged as a powerful sample
preparation technique. It is only applicable to thermally stable compounds
due to the increase in temperature during extraction. Although MAE can
be used also for leaching purposes, nowadays, its power in sample solubilization is mostly used for samples dissolution=digestion.
The main applications of MAE are as an alternative to Soxhlet extraction as good extraction efficiencies can be achieved using less solvent
and shorter extraction times.[111] Most publications to date have been for
environmental applications, although Hermo et al. present the comparison
between two analytical methods used for the determination of quinolones
in pig muscle.[112] The procedures involve the extraction of the quinolones
from the tissues by traditional extraction and using microwave assisted
extraction (MAE), a step for clean-up and preconcentration of the analytes
by solid phase extraction, and subsequent liquid chromatographic separation with UV absorbance detection. In that study,[112] microwave-assisted
extraction (MAE) has proven to be an alternative to classical extraction
because less interfering substances were observed and cleaner extracts were
obtained. As with Soxhlet extraction, further extraction or clean-up steps
such as solid phase extraction (SPE) are generally required, particularly
for the determination of trace contaminants.[113]
Accelerated Solvent Extraction (ASE)
Accelerated solvent extraction (ASE), sometimes referred to as pressurized liquid extraction (PLE) or pressurized fluid extraction (PFE),
may be used for solid and semi-solid samples. The elevated temperatures
and pressures used in these techniques are causing reduction in dipole
interactions and hydrogen bonds, increasing the surface wetting. ASE
has the advantage that water may also be used as solvent, if it is below
the critical point.
Often, due to a large number of samples that need to be analyzed,
methods to speed up the extraction process have been widely examined.
ASE involves extraction with liquid solvents but at elevated temperatures
and pressures. In ASE, the sample is heated in the presence of an
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extraction solution at high pressures, up to 2000 psi. Like the closed-vessel
microwave approach, this technique utilizes the fact that liquids at elevated
pressure can be heated to temperatures above their respective boiling
points without transition to the gaseous phase. Several other names have
been used for this technique, including pressurized fluid extraction
PFE), high-pressure solvent extraction (HPSE), high-pressure, high
temperature solvent extraction (HPHTSE), pressurized hot solvent extraction (PHSE), and subcritical solvent extraction (SSE). Carabias-Martinez
et al. reviewed the distinct advantages of this technique exploited in several
areas, including biology and the pharmaceutical and food industries.[114] A
relatively new variant of ASE switches the usual procedure to superheated
water extraction when water is used as a solvent. A review of the technique,
including several applications, was given recently by Smith.[115]
ASE provides faster extractions than conventional Soxhlet techniques,
because of the accelerated desorption of analytes from the matrix and
the more rapid kinetic processes for dissolution.[116,117] In the case of most
organic solvents, diffusion rates increase exponentially with temperature.
Due to the lower viscosity and higher diffusivity of the solvent, mass transfer
into the extraction solvent is faster. The higher temperatures also make it
easier for the solvent to overcome intermolecular interactions of the
analyte and matrix effects.
The nature of the extraction procedure in ASE is both static and
dynamic. The procedures may involve a certain number of extraction
cycle(s), the extraction cell being flushed with a pre-determined volume
of fresh solvent and then purged with nitrogen gas (N2) in order to recover
all of the extraction solvent and analyte.
In specific applications, further clean-up is usually required for some
target analytes. Sometimes, the clean-up step can be done in situ, by adding
sorbent materials or a desiccant (e.g., sodium sulfate) directly to the extraction cell. When the in situ clean-up procedure is not strictly required, after
performing ASE, it is possible to use a typical sorbent to produce the
cleanest extracts for target samples from the initial extract.[118] Other
clean-up steps coupled with the ASE technique and their details can be
found in the literature.[84,91,119–121]
Preliminary ASE, with non-polar solvents to eliminate the hydrophobic
compounds prior to the extraction of the analytes of interest, represents an
alternative approach. There are some situations when elevated temperatures and pressures are not enough to dissolve analytes from a complex
matrix. In such a case, modifiers (e.g., sodium dodecyl sulfate) can be
added to the extraction solvent.[122]
The application of ASE as a sample preparation technique for the
analysis of matrix components in food and biological samples was already
reviewed in 2005.[114] Since then, many other applications of this
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technique for the determination of organic analytes from different
matrices have been published.[123–127]
Subcritical Water Extraction (SWE)
In the last few years there has been an interest in the use of water as the
solvent for pressurized liquid extraction as this can reduce or eliminate the
use of organic solvents.[128] This technique usually adopts water in the
condensed phase between 100 C and the critical point, and it is generally
referred to as superheated water extraction (SHWE). It has also been called
subcritical water extraction (SWE), hot water extraction (HWE), pressurized hot water extraction (PHWE), or high temperature water extraction
(HTWE). SHWE is cleaner, faster, cheaper, and more environmentally
friendly than conventional methods.
Water as a solvent is unique due to its high level of hydrogen-bonding,
giving it a high boiling point and high dielectric constant and polarity. As
the temperature of water is increased (under pressure), the polarity
decreases and, therefore, extraction becomes more selective. At
100–374 C it can act as a medium=non-polar solvent.[129] The useful temperatures and pressures of water for SWE are lower than the critical point,
in contrast to super-critical fluid extraction (SFE) with carbon dioxide. A
review of the SWE technique, including several applications was given by
Smith in 2006[115] and more recently in 2008.[129] Most applications to date
are for solid samples, such as soil, and include the determination of
selected polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenyls
(PCBs), and pesticides.
Supercritical Fluid Extraction
Supercritical fluid extraction (SFE) is a technique that became popular
during the 1980s.[130] Generally speaking, supercritical fluids (SFs) are
gases with high density above their critical temperature and pressure that
exhibit simultaneously properties associated with both gases and
liquids.[131–133] Thus, like gases, they are compressible, but they also display
solvencies similar to those of the liquids.
As the name suggests, supercritical fluid extraction (SFE) employs
supercritical fluids for extraction purposes in place of the organic solvents
of conventional extraction. Any increase in temperature at constant pressure reduces the solvent power of a supercritical fluid, but it also leads to an
increase of the diffusion rate, which tends to lower the minimum required
extraction time. Compared to conventional extractants, supercritical fluids
have low viscosity and have diffusion rates that are higher by a factor of 10
to 100, both of which contribute to reduce the extraction times. Moreover,
analyte melting points and solubility in the SF are important properties to
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Trace Analysis by Chromatographic Methods
1189
consider.[132] With supercritical CO2 and N2O,, which are gases under normal conditions, the extractant is separated by reducing the pressure to
atmospheric levels, leading to simultaneous concentration of the extract.
Supercritical CO2 is the most frequently used extractant for SFE. It has
recoverable characteristics and the ability to solubilize lipophilic substances.[134,135] It has the advantage of being chemically inert. Its critical
temperature is low, so it is acknowledged as a valuable chemical for the
extraction of thermolabile analytes such as steroids and fragrances.[136]
Other advantages of CO2 as an extractant include high purity and low cost.
The principal disadvantage of CO2 is a relatively low polarity. However,
its solvent power with respect to polar analytes can be improved by
adding polar modifiers (or a mixture of them) such as methanol and
n-hexane,[137] ethanol,[138] aqueous acetonitrile,[139] or dichloromethane.[140]
Extraction with supercritical CO2 has been used for separating a wide
variety of analytes, including pesticides from food,[141] vegetables,[142]
aquaculture and marine environmental samples,[143] vitamins from tablet
matrices,[144] PCBs from fish muscle,[145] sediments,[146,147] and powdered
full-fat milk.[148]
Solid-Phase Microextraction (SPME)
Solid phase microextraction (SPME) is a simple, rapid, sensitive, and
solvent-free sample preparation technique in which analytes in either air
or water matrices are extracted into the polymeric coating of a fiber.[19]
It was originally developed by Arthur and Pawliszyn in 1990.[149] Subsequently a number of books have been written on the technique.[150–152]
The mechanism of SPME is based on the partitioning equilibrium of the
analytes between the sample or the headspace above the sample, respectively, and a fused silica fiber coated with a suitable stationary phase. The
amount of analyte extracted by the fiber is proportional to the initial analyte concentration in the sample and depends on the type of fiber. After
sampling, the fiber can be thermally desorbed directly into the injector
of a gas chromatograph. SPME combines sampling, analyte enrichment,
matrix separation, and sample introduction within one step.[153] Since its
development, this innovative technique has found widespread use in
environmental analysis. It has, for example, been applied in the determination of volatile organic compounds,[154] biologically active substances,[155] phenols,[156] pesticides,[157] polyaromatic hydrocarbons, and
polychlorinated biphenyls[158,159] in water. In a technical note, the application of the SPME hyphenated with a temperature-programmed desorption (TPD) for the analysis of chemicals with wide-ranging volatilities
without causing their thermal degradation is presented.[19] Degradation
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R.-I. Olariu et al.
is, however, a problem often encountered in the analysis of active
pharmaceutical ingredients. Shepard et al. described a complex preparation alternative for the analysis of L-ascorbic acid, a compound which
can be degraded in the solid phase under the influence of moisture.[160,161]
SPME fibers have also been used as air sampling devices for volatile
organic compounds in ambient and workplace air. The results obtained with
SPME were in good agreement with traditional sampling methods.[162,163]
In a detailed review on SPME technique, fibers with different thickness
and polarities are presented.[164] Many examples of SPME applications,
including direct immersion into a liquid sample and headspace sampling,
are presented in a review of food analysis.[165] However, direct immersion
of SPME into some natural matrices can be difficult and the fiber can be
damaged or some analytes can adsorb irreversibly to the fiber, changing
its properties and making it unusable for more than one sample. The
use of SPME-LC for the analysis of pesticides was reviewed in 2005.[166]
In 2007, the most suitable sample preparation techniques for organic compounds in air and water matrices were also reviewed.[167]
The main advantages of the SPME technique compared to solvent
extraction include the reduction in solvent use and the sensitivity for polar
and non-polar analytes in a wide range of matrices, when SPME is combined
with both GC and LC. The main disadvantage of the SPME is the low storage
stability of the samples, due to uncontrolled losses of analytes by adsorption
on the walls of the vials or by evaporation from the loaded fiber.
A recent development of the SPME technique is the new superelastic
fiber type. This is a metal alloy with elastic properties that can be coated
with polydimethylsiloxane-divinylbenzene (PDMS=DVB), carboxen=PDMS
and DVB=carboxen-PDMS as well as PDMS. This alternative improves the
robustness and overcomes the problems of the breaking of fibers due to
misalignment with injection ports or in viscous matrices.[168] Two relatively
new reviews include discussion of recent developments that may have significant implications for automation, such as superelastic fiber assemblies
and internally cooled fiber-SPME.[169,170] These reviews also describe the
recent developments of solid-phase microextraction technology applied
to food, environmental, and bioanalytical chemistry.
Stir Bar Sorptive Extraction (SBSE)
Stir bar sorptive extraction (SBSE) technique is a sample preparation
tool based on sorptive extraction of interest analytes that can later be
removed by thermal desorption in the gas chromatographic injection port.
SBSE is a dynamic variation of SPME in which a spinning glass-covered
magnetic bar (coated with a thick layer of polydimethylsiloxane) is used
to sorb. This technique was developed in 1999 using stir bars coated with
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Trace Analysis by Chromatographic Methods
1191
50–300 ml of polydimethylsiloxane (PDMS).[171] The advantage of SBSE is a
higher enrichment factor combined with the application range and extraction mechanism of SPME.[172] Transfer of the analyte from the bar is
achieved either by GC thermal desorption or by elution with an LC solvent.
As with SPME, the stir bar can also be used to sample the volatiles and
semi-volatiles in the headspace above the sample. It can be used for liquid
or semi-solid complex matrices and, therefore, has potential for many
applications in coffee brew analysis[173], in the determination of polycyclic
aromatic hydrocarbons (PAH) in aqueous samples,[174] and for the determination of pesticide residues in honey.[175]
There are several articles that compare SBSE with other extraction
techniques on different target analytes. Steam distillation extraction,[176]
membrane assisted solvent extraction (MASE),[177] and SPME[178] represent some alternative extraction techniques used in different studies, in
which the authors concluded that SBSE is more sensitive and affords
improved reproducibility and less artifact formation. However, despite
good sensitivity, SBSE extraction was not suitable for the analysis of some
polar pesticides in food.[179] David et al. gave examples of food analysis
by using the SBSE technique and described the analysis of solid samples
after an initial extraction with a water-miscible solvent.[180]
Because of the PDMS coating, SBSE is most suitable for the analysis of
non-polar analytes from aqueous media. To a lesser extent, it can be
concluded that this technique can be used for more polar compounds after
a proper derivatization step. Bicchi et al. describe how to improve the
recovery of more polar analytes in SBSE and HSSE (HeadSpace Sorptive
Extraction) techniques.[173] Applications of SBSE in analysis are increasing,
but due to the limitations of the PDMS phase, they are still currently
limited to non-polar or semi-polar analytes.[181]
Hollow Fiber Membrane Extraction
The demand for automation in analytical liquid-liquid extraction
(LLE) combined with organic solvent reduction or elimination has led to
the recent development of liquid-phase microextraction (LPME) based
on disposable hollow fibers.[182] This approach has been reviewed by
Rasmussen and Pedersen-Bjergaard.[183] This review focuses on basic
extraction principles, technical set-up, recovery, enrichment, extraction
speed, selectivity, applications, and future trends in hollow fiber-based
LPME. In this technique, analytes of interest are extracted from aqueous
samples, through a thin layer of organic solvent immobilized within the
pores of a porous hollow fiber, into an acceptor solution inside the
lumen of the hollow fiber. Subsequently, the acceptor solution is directly
subjected to a final analysis by capillary gas chromatography (CGC),
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R.-I. Olariu et al.
high-performance liquid chromatography (HPLC), capillary electrophoresis (CE), or mass spectrometry (MS) without any further effort.
Hollow fiber-based LPME may provide high analyte pre-concentration
and excellent sample clean-up, and it has a broad application potential
within areas such as drug analysis and environmental monitoring. A simple
and easy-to-use extraction procedure has been applied for the extraction of
2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in urine and
spiked plasma samples, using a short piece of narrow capillary-like
microporous hollow-fiber (HF) membrane as an extraction device.[184]
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Enrichment Techniques Driven by Gas-Liquid or Gas-Solid
Equilibriums
The rapidly developing trend of chromatographic analysis foresees the
use of the substance to be analyzed as one of the phases of the heterogeneous (usually) gas-liquid system. The analytical and physico-chemical
characteristics of the liquid phase (and sometimes solid) are determined
by analyzing the gaseous phase, into which some part of the components
of the liquid material to be analyzed is partitioning with the establishment
of an equilibrium. The name of the method, ‘‘analysis of equilibrium
vapor’’ or ‘‘vapor-phase analysis’’ (head-space analysis, HSA) is derived from
this equilibrium condition.[185] However, it must be noted that the thermodynamic equilibrium between the phases is not required in all the practical
applications. Accordingly, there are possible methods of determination that
assume only a certain degree of equilibration. A distinctive feature of the
HSA technique is that the chemical information contained in the gas phase
is used to determine the nature and composition of the condensed phase
with which it is in contact. The development of HSA methods opens up
wide possibilities for determining trace contaminants in the atmosphere,
in other gaseous media, and in other substances. The features of HSA make
it a very effective technique. Two groups of methods of head-space analysis
can be distinguished, namely, 1) static methods: when the equilibrium
between the gas and the condensed phases is reached within a closed system, and 2) dynamic methods. In the latter, the contact between the phases
occurs in an open system in which a flow of gas is passed through a layer of
liquid sample or a granulated solid phase, and the volatile analytes (or their
derivatives) are purged out of the system.
Static Headspace Technique
The classical HSA methods (including their automated variations) are
based on sampling the volatile analytes from an enclosed space in a static
system using a gas-tight syringe. Analytes of interest are equilibrated in a
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Trace Analysis by Chromatographic Methods
1193
closed vial at a specified temperature and pressure. Use of a gas-tight
syringe (or autosampling system) is one of the common techniques used
to transfer the headspace sample into the gas chromatograph.
HSA is considered to be a technique directly connected with a gas chromatographic analytical instrument, but it can be used with practically any
analytical technique. In fact, the first applications of headspace sampling
were not in GC.[185] The main disadvantage of static headspace extraction
is the lack of preconcentration. Consequently, this technique is not fully
suitable to the analysis of trace and ultra-trace constituents, unless the loss
(or the lack of gain) in sensitivity during extraction is compensated by the
detection technique.
A brief overview of headspace analysis techniques and the underlying
theory has been provided by Snow and Slack in 2002.[186] The paper
includes examples of applications in environmental, clinical, forensic,
biological, food, flavor, and pharmaceutical analysis.
Headspace-single-drop microextraction (SDME) is a variation on static
headspace. Volatile components are trapped on a single drop of solvent
that is suspended from the tip of a syringe in the headspace above the
sample. Practical difficulties with the technique include a limited choice
of solvents due to viscosity requirements, and further work is needed to
prove the reproducibility of this technique.[187]
Most of the papers related to HS-SDME deal with the determination of
trace polycyclic aromatic hydrocarbons in environmental samples,[188] the
analysis of carbonyl compounds in biological and oil samples after derivatization with 2,4,6-trichlorophenylhydrazine (TCPH),[189] and the analysis
of volatile halocarbons.[190] Lambropoulou et al. published an analytical
application of HS-SDME, which was adopted as an isolation and trace
enrichment step prior to the analysis of organic pollutants (pesticides,
polycyclic aromatic hydrocarbons, polychlorinated compounds, organotin
compounds, and phenolic derivatives, aromatic amines, phthalates, etc.)
by gas and liquid chromatography.[191]
Headspace-solid phase microextraction (HS-SPME) is another variation
on static headspace that traps the volatile components onto a SPME fiber
held above the sample. Such mode of extraction is based on the equilibriums between three phases: sample matrix, vapor phase, and fiber. Interestingly, in conventional headspace analysis raising the temperature
increases the amount of analyte in the vapor phase and hence gives
improved sensitivity. In contrast, with HS-SPME a higher temperature
may result in less deposition onto the fiber as volatile components again
favor the vapor phase. In some cases, the peculiar features of HS-SPME
can make a definite advantage: in studying the aroma profile of cocoa products, the fiber was proved to be more favorable for the enrichment of
lower volatility compounds than the direct headspace.[192] HS-SPME was
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also used for the determination of pesticide residues in fruits and
vegetables[193] and for the analysis of trihalomethanes in water samples.[194]
Dynamic Headspace Technique
Dynamic headspace is basically a development of the static headspace
so that complete removal of the volatile analyte from the HS is
accomplished. In principle, the volatile analytes are stripped out of the
solution by a carrier gas and then subjected to a pre-concentration step
by trapping them on various media. It usually follows a thermal desorption
into a gas chromatograph. Such an approach involves, on the one hand,
the purging of the analytes and, on the other hand, their trapping and is
generally called purge-and-trap injection (PTI).
The major drawback of the dynamic headspace is the limitation to the
analysis of relatively volatile compounds with boiling points below 200 C.
Also, this technique is expensive and requires a fully dedicated and
complex instrument.[186]
Several experimental designs have been developed as trap devices. The
needle capillary adsorption trap device, described by McComb et al., is
based on a combination of SPME and purge and trap methods.[195] Only
limited applications of such SPME devices in dynamic headspace analysis
have been published, such as the determination of volatile compounds
from aqueous[196] or gaseous samples.[197]
A recent development in headspace analysis is solid phase dynamic
extraction, also called headspace-solid phase dynamic extraction
(HS-SPDE). In this technique the headspace is repeatedly drawn up into
a syringe through a coated needle which traps the analytes. The latter
are then desorbed directly into a GC. A variety of sorbents is available
including divinylbenzene, carboxen, carbowax, polyacrylate, and mixed
phases. This technique has been applied for the trace determination of
volatile or semi-volatile analytes in aqueous matrices[198] or in food
matrices.[199]
Solid Phase Extraction (SPE)
This technique is used for the selective separation and concentration of
analytes from liquid or gas samples and often is used to clean up and concentrate liquid extracts. Therefore, SPE could have been included in Paragraph 5.2 as well. SPE is a technique referring to a non-equilibrium
exhaustive removal of analytes (semi-volatiles and non-volatiles) from a
liquid sample by retention on a solid phase (sorbent), and to the subsequent elution of selected analytes from the solid phase by an appropriate
solvent.[65] The desorption of the analytes from the solid phase can also
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Trace Analysis by Chromatographic Methods
1195
occur under thermal treatment. The efficient use of this technique requires
optimization of the sorption and desorption processes.
Extraction by SPE is based on the distribution of the analytes between
the sample and a solid phase, which is usually contained in a cartridge. Separation of target analytes from complex matrices may be a sum of several
synergic effects such as differing polarities, differences in molecular size,
and differences with respect to ion-exchange capacity.
Solid-phase extraction (SPE) has several advantages over liquid-liquid
extraction (LLE). SPE can be accomplished more rapidly, requires less
solvent, and provides more highly concentrated extracts. It is possible to
choose among a range of adsorbents that use different mechanisms for
the extraction=retention of analytes. The different adsorbents that are
applicable in the SPE technique can be classified according to their nature
as nonpolar (e.g., those carrying octadecyl, octyl, butyl, cyclohexyl, phenyl,
amino and diol groups), polar (e.g., cyano, Kieselguhr, silica gel, Florisil,
aluminum oxide), anion exchangers (e.g., primary amine, secondary
amine, quaternary ammonium salt), and cation exchangers (e.g., carboxylic acid, sulfonic acid). The most common adsorbents for solid-phase
extraction are based on silica gel, the surface of which has been modified
in some way.[200]
Octadecyl surface solid-phase (C18) is used for the reversed-phase
extraction of nonpolar substances from aqueous solutions. Typical applications include the extraction of organochlorine pesticides,[201] organophosphorus pesticides,[202] chlorinated hydrocarbons,[203] PAHs,[204] phenols
and chlorophenols,[205] and antibiotics.[206] Octyl surface solid-phase
(C8) is used for extracting substances of medium polarity. Substances that
bind irreversibly to C18 phases can often be concentrated and re-eluted
successfully with C8 phases.[200]
Unmodified silica gel, aluminum oxide, and Florisil, usually called normal phase materials, separate sample constituents into fractions of comparable polarity. They are often utilized to separate and concentrate
pesticides,[207] PCBs,[208] polychlorodibenzodioxins and dibenzofurans
(PCDD=PCDFs),[209] and for the simultaneous determination of bisphenol
A, triclosan, and tetrabromobisphenol A[210] from biological, agricultural,
and environmental samples.
Normal phases are also used to extract polar sample constituents, such
as amines, alcohols, phenols, dyes, medicaments, or vitamins.[211,212]
Apart from modified silica gel, the most frequently used solid-phase
adsorbents are activated charcoal and resins (XAD). Activated charcoal is
a universal adsorbent for concentrating trace organic materials from aqueous solutions and air. XAD resins are also commonly employed for extracting organic trace constituents, such as persistent organic pollutants from
the atmosphere.[213]
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R.-I. Olariu et al.
The activity of these solids must be accurately adjusted to ensure
reproducible results and well-defined fractions, and sample solutions must
always be carefully dried prior to extraction. One of the drawbacks of SPE is
that the packing must be uniform to avoid poor efficiency. The sample
matrix can also affect the ability of the adsorbent to extract the analyte
due to competition for retention. The limits of many traditional adsorbents
in terms of selectivity and insufficient retention of very polar compounds
can also be a problem.[214]
A recently developed variant of SPE is the extraction with synthetic molecularly imprinted polymers (MIPs). Thus, retention of analytes on the
adsorbent is due to shape recognition in the cavities or imprints. Due to
the nature of their selectivity, synthetic MIPs can often be used for a number
of matrices, even though the interaction may be different.[215] They can be
heated and are stable in both organic solvents and strong acids and bases.
However, custom-made products must be prepared for each analyte. Moreover, stringent cleaning of the MIP, prior to trace analysis, is necessary to
remove the analyte, which had been used as a template. MIPs have been used
as selective adsorbents for a range of analytes and matrices.[216]
As already discussed, SPE is a useful clean-up technique for trace analysis
but, usually, it is not employed alone for sample preparation. SPE would rather
follow an initial extraction step. Selective adsorbents in SPE are a useful tool for
both selective and sensitive analysis of trace contaminants in complex matrices.
CONCLUSIONS
Detection methods are becoming more specific and sensitive. In trace
analysis, sample preparation is particularly important as it can account
for a significant amount of the variability of a particular method. Thus, sample preparation is a critical step in the overall process of obtaining reliable
and accurate data, especially in the trace analysis of nonvolatile and
semi-volatile organic compounds. Considering the large number of articles
published in the last decade, it can be easily concluded that trends in sampling preparation are focused on the minimization of the use of organic
solvents in, automation of, and speeding up of the sample preparation procedure. Some guiding principles can be given depending on the purpose of
the analysis, the available amount of sample, the required sensitivity, and
the type of sample matrix and analytes to be investigated.
For liquid and solid samples, SPME and SBSE combined with gas chromatographic analysis are good options when the target analytes are nonpolar and volatile. The two extraction methods can also be used for analyzing
volatile compounds from solid samples, but only when combined with the
dynamic head space technique.
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1197
For polar analytes, SPE and LPME are suitable. The big advantage of
miniaturized SPE extraction is the minimum amount of sample and organic
solvent required, but sensitivity is lower. However, SPE techniques are exhaustive extraction and purification methods, and are suitable for quantitative
analysis as well. MAE, also, can be used for the extraction of liquid samples.
For the analysis of volatile compounds, head-space extraction combined with SPME or SBSE are simple and direct options. However, for
semi-volatile compounds more exhaustive methods are probably needed.
Nowadays, ASE is gradually replacing Soxhlet extraction. ASE is a good
method for the extraction of tightly bound compounds in samples such as
sediment and soil. The disadvantage of the ASE extracts is that they typically contain a large amount of undesirable matrix components. Accordingly, a clean-up step of the extract (e.g., by SPE) is always needed.
For samples, in which the analytes are not very tightly bound to the
matrix, for example, food, biological tissues, plant, and atmospheric aerosol particles, MAE and USE represent good extractions methods.
The trend of sample preparation is towards automated systems that can
be integrated with the final separation step. Presently, a few automated
methods are available but only for liquid samples. For solid and semisolid
samples, fully automated systems are not available for routine analyses,
although several interesting systems have been reported for special applications. For complex matrices, such as biological, environmental, and food
samples, a combination of different extraction techniques is often
required. However, the objective of any chromatographic method should
be to achieve the required performance (e.g., sensitivity, accuracy and precision) in as few steps as possible. Further development is needed to make
such systems applicable to large-scale analysis.
ACKNOWLEDGMENTS
R.I. Olariu wants to acknowledge the financial support for research
in the field under PN-II-ID PCE 2007 program, project No. 405,
CNCSIS-UEFISCUS-ANCS. C. Arsene gratefully acknowledges financial
support for research from the European Commission, under the project
MERG-CT-2007, No. 203934 (ICAARUS).
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