Introduction

Choosing an appropriate sampling (preparation) technique that provides representative samples of analytes plays a key role in accurate and reliable assessment of the content and the concentration of volatile and semivolatile organic compounds present in various environmental components and products resulting from the generation of human labour. The sample taken should reflect the actual state of the object. Therefore, the selected sampling (preparation) technique should be characterized by such features as:

  • Being easy to conduct activities and operations in situ

  • Possibility of determining the analytes in the medium in a given range of concentrations with the required precision and accuracy

  • Low unit cost (so that research can be conducted on a large scale)

However, other aspects concerning operator safety and the environmental impact of analytical methods should be considered. For example, during the 1990s, side effects of analytical methodologies developed to analyse different kinds of samples (including environmental samples that generate a large amount of chemical waste) resulted in a great environmental and human impact. In some cases, chemicals employed for analysis were even more toxic than the substances being determined. Taking into account current public concern with environmental matters, environmental analytical studies and the consequent use of toxic reagents and solvents have increased to a point at which they have become unsuitable for continued usage without an environmentally friendly perspective [1].

In recent years there has been rapid growth of interest in subjects related to clean analytical chemistry or environmentally friendly analytical methods, including solventless sample preparation techniques. The scientific references found in the Science Citation Index (SCI) database relating to green analytical chemistry (GAC) [2] and the literature on this topic have grown since the 1990s. This change in the rate of publications on GAC methods is related to the increasing concern of the scientific community about the environmental impact of their actions [1]. Strong interest in this approach is associated with both ecotoxicological and economic aspects. As a result of the application of solventless sample preparation techniques for primary laboratory practice, the emission of toxic solvents into the environment is avoided—as is the use of high purity solvents, which are expensive [3].

In this paper, state of the art solventless sample preparation techniques based on solid- and vapour-phase extraction are reviewed. Direct extraction methods of analytes in different areas are included.

Sorptive extraction techniques

Sorptive extraction techniques are based on the distribution equilibria between the sample matrix and sorptive materials. Analytes are extracted from the matrix into the non-miscible extracting phase. In contrast to adsorption techniques (e.g. solid-phase extraction, SPE), where the analytes are bound to active sites on the surface of the adsorbent materials, the total volume of the extraction phase is important. Sorptive materials (or sorbents) are a group of polymeric materials with a glass transition temperature (T g) below the temperature at which the material is used during the sampling, storage and desorption processes. At temperatures above their T g, polymeric materials no longer behave as solid materials but assume a gum-like, or even liquid-like, state with properties similar to those of organic solvents (e.g. diffusion and distribution constants). Sorbents are, in principle, homogeneous, non-porous materials in which analytes can actually dissolve. The analytes do not, therefore, undergo real (temporary) bonding with the material but are retained by dissolution. Extraction of analytes depends on the partitioning coefficient of solutes between the phases. The octanol–water distribution coefficient (K o/w) can be used as an indication of how well a given analyte will be extracted [4].

Four sorptive extraction techniques can be distinguished. The first, open-tubular trapping (OTT), is the oldest technique and employs a (thick film) capillary gas chromatography (GC) column for sampling. The second technique, solid-phase microextraction (SPME), is based on the use of a polydimethylsiloxane (PDMS)-coated fibre which, when not in use, is protected by being withdrawn into the needle of a syringe-like device. The third technique is stir bar sorptive extraction (SBSE) which is based on the static extraction of liquid samples with a sorbent-coated stir bar [5]. The fourth sorptive technique, gum-phase extraction (GPE), is based on a bed packed with sorbent material. In this contribution, the state of the art in sorptive sampling and thermal desorption is reviewed.

Open-tubular trapping (OTT)

A capillary microextraction technique which employed an open-tubular fused silica capillary column as an extraction device was first developed in 1986 as open-tubular trapping (OTT) [6]. In OTT, ambient air, solution, or solution headspace is sampled by passing a gas or liquid through the open capillary. In OTT, sorption is carried out as an equilibrium process and the amount of analyte retained by the stationary phase and the equilibrium is directly related to its concentration in the sample solution. The analytes are desorbed either with a small amount of solvent or by thermal desorption and are usually used in combination with GC. The sample is forced to flow through the capillary and analytes reach the trapping medium, coated onto the walls, by diffusion. The thermal stability of GC stationary phases allows the collected analytes to be thermally desorbed from a trap after sampling. These analytes can be desorbed directly onto a GC column for analysis, avoiding dilution of the sample with a solvent. Sample cross-contamination and possible degradation are minimized because intermediate sample handling steps are eliminated. A schematic illustration of OTT sampling is presented in Fig. 1 [7]. Open-tubular traps have been successfully employed for a range of gaseous samples [8, 9], plant volatiles [10–12] and environmental air samples [13]. Dudek at al. [14] used a piece of a commercial capillary GC column coated with PDMS in OTT for the sampling and enrichment of select volatile non-polar organic compounds from a workplace atmosphere (a woodworking shop). Results obtained by means of OTT confirm that this method is suitable for sampling organic pollutants from air.

Fig. 1
figure 1

Schematic illustration of OTT sampling. A sampling is started; B analytes are sorbed into the coating; C analytes start to break through; D the sorbent is in equilibrium with the sample; E the trap is desorbed in backflush mode

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Open-tubular trapping can be an attractive alternative to traditional techniques for the enrichment of aqueous samples [5, 15]. Several OTT approaches, involving off-line [16] or on-line [17, 18] coupled with GC, have been described.

Solid-phase microextraction (SPME)

SPME was introduced by Pawliszyn in the early 1990s [19]. It is a solvent-free sample preparation technique that uses a fused silica fibre coated with an appropriate stationary phase attached to a modified microsyringe (Fig. 2) [20, 21]. In SPME, partitioning of analytes between the stationary phase on a fibre and the sample takes place until equilibrium is achieved. Maximum sensitivity is obtained at the equilibrium point; however, it is not necessary to reach this point and the extractions can instead be performed for a defined period of time [22]. The extraction temperature, time and sample agitation must be optimized for each application and operating conditions must be consistent [23]. SPME can be applied to different types of samples, using two different approaches. For relatively pure liquids, extraction is performed by dipping the SPME fibre directly into the sample. For solid matrices and wastewater samples, headspace (HS) SPME is preferred, because it results in faster equilibration and higher selectivity [20]. After the coated fibre has been exposed to the sample for a given period, it is inserted into the injection port of a GC system in order to realize the analyte. In GC, this is achieved by thermal desorption, whereas in HPLC it is accomplished by dissolution and further injection with the elution solvent [24, 25].

Fig. 2
figure 2

Schematic diagram of SPME

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SPME has several advantages compared to other extraction methods. It gives quantifiable results from very low concentrations of analytes and avoids the losses that can occur during extraction, concentration and clean-up steps in traditional sample procedures [26]. On the other hand, one of the main drawbacks of this technique is its limited range of stationary phases which are commercially available, only roughly covering the scale of polarity. Some of the commercially available fibres and their applications are presented in Table 1.

Table 1 Recommended application fields for different SPME fibres [27]
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Recent trends in SPME are focused on solving these problems by:

  • Studying new coatings with higher extraction efficiencies, selectivity and stability [28, 29]

  • Development of new devices to improve the extraction process [30]

  • Studying novel calibration processes [31, 32]

  • Development of derivatization strategies [33, 34]

In order to effectively couple the extraction efficiency of SPME with the detection capability of ion mobility spectrometry (IMS), Liu et al. [35] developed a new prototype SPME-IMS system as a robust, simple, rapid, energy-saving fieldable approach for on-site analysis of analytes in various matrices.

Meanwhile, SMPE has routinely been used in combination with GC and GC-MS and successfully applied to a wide variety of compounds, especially for the extraction of volatile and semivolatile organic compounds from environmental, biological and food samples.

In-needle solid-phase microextraction techniques (in-needle SPME)

In-needle extraction techniques were developed to overcome fibre-related drawbacks such as fragility, low sorption capacity, and bleeding from thick-film coatings. For this purpose, the extraction phase is fixed inside a needle instead of the surface of the SPME fibre [26]. The main advantage of this resolution is that it overcomes the mechanical stability problems of fibre SPME.

In-needle extraction techniques can be divided into methods with:

  • Extraction coatings (which use a coating as an internal extraction phase immobilized in the needle)

  • Extraction fillings (which use a sorbent packing material as an extraction phase)

Independent of the type of extraction materials, in-needle extraction techniques can be used in two modes:

  • Static mode (in which analytes are transferred by diffusion through needles)

  • Dynamic mode (in which analytes are transferred actively by pumping or under the gravitational flow of the sample phase through needles)

Inside needle capillary adsorption trap (INCAT) and needle trap devices (NTDs)

In 1997, McComb et al. designed a novel method of solventless extraction based on a combination of the SPME and purge and trap (PT) methods. In this technique, a hollow needle with either a short length of GC capillary column placed inside it, or an internal coating of carbon, is used as the preconcentration device. This approach is called inside needle capillary adsorption trap (INCAT) for the analysis of benzene, toluene, ethylbenzene and xylenes (BTEX) in air [36]. Sampling may be performed in ambient air, on the solution, or the solution headspace, by passing gas or liquid through the device, either actively with a syringe, or passively via diffusion. The trapped analytes are recovered by using direct thermal desorption, by placing the needle into the heated GC injection port [37].

The main advantages of the INCAT device lie in the simple methodology and easiness and rapidity of the analyses. Compared to SPME, the in-needle sampling device has been recognized as a robust and efficient sample preparation method [38]. The drawbacks involve the fact that the collected samples cannot be particularly large, and the desorption temperature is limited by that of the gas chromatographic injection port [38].

To enable high efficiency and repeatability of adsorption and desorption of trace quantities of BTEX from a water matrix, Kubinec et al. developed the INCAT device. A new arrangement of the fully internal volume needle capillary adsorption trap device, with Porapak Q as a sorbent material and wet alumina as a source of desorptive water vapour flow, was used for the analysis of BTEX in drinking and waste water samples [39]. To counter the disadvantages of the INCAT method, a new on-column injection system facilitating the use of large diameter INCAT devices was developed by Hrivňák et al. [40].

An in-needle trap device was also developed by Wang et al. for analysis of volatile organic compounds (VOCs) in gaseous samples [41]. Construction of this simple and integrated sampling/extraction/sample introduction device was optimized. A novel in-needle extraction device was also developed by Saito et al. [42] for the analysis of several organic solvents commonly used in a typical chemical laboratory. The specially designed needle was packed with porous beads made from polymeric material that showed excellent performance for the extraction and a suitable thermal stability for typical analysis in GC. In 2008, a needle trap device with Carbopack X as a sorbent material for sampling, preconcentration and injection of BTEX into gas chromatograph was developed by Jurdáková et al. [43]. A schematic diagram of a needle trap extraction (NTE) device is shown in Fig. 3 [21].

Fig. 3
figure 3

Schematic diagram of an NTE device

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Solid-phase dynamic extraction (SPDE)

Solid-phase dynamic extraction (SPDE), also known as “the magic needle”, is a further development of SPME. It was first described by Lipinski [44] for the analysis of pesticides in water samples. SPDE works on the same principle as SPME, but it is a dynamic process where the headspace of the sample is repeatedly pumped through a hollow needle attached to a gas-tight syringe. The extraction phase (e.g. PDMS/Carboxen) is on the inside of the needle, as opposed to SPME where it is on the outside of a fibre. Also, the needle is much longer than an SPME fibre. The advantage of SPDE over SPME is the increased volume of sorption material. Therefore, sensitivity is better and competition effects, which may be an issue with SPME, are largely eliminated. Desorption is carried out directly in the GC injector, similar to SPME [45]. A schematic diagram of SPDE is shown in Fig. 4 [21].

Fig. 4
figure 4

Schematic diagram of SPDE

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There is a fully automated commercially available system for the analysis of liquid called in-solution solid-phase dynamic extraction (IS-SPDE) [46].

Stir bar sorptive extraction (SBSE)

Another solid-phase alternative to using organic solvents is stir bar sorptive extraction (SBSE), which is based on the interaction of analytes with a coating of PDMS deposited on a glass stirrer bar (Twister). It was introduced in 1999 by Baltussen et al. to overcome the limited extraction capacity of SMPE fibres [47, 48].

Stir bar sorptive extraction applies stir bars, varying in length from 1 to 4 cm, coated with a relatively thick layer of PDMS (0.3–1 mm). Using a thicker polymeric layer than that employed in SPME results in a high enrichment factor. Sampling is performed until breakthrough or, for even higher sensitivity, until all analytes are in equilibrium with the sorbent. This technique can be applied for gaseous and liquid samples, although for the latter drying is required, which induces a loss of volatile compounds [49].

The applicability of SBSE can be evaluated by using the octanol–water distribution coefficient (K o/w) as an indicator of how well, if at all, a given solute can be extracted with SBSE [50]. Sandra [51] reported that a high enrichment factor could be achieved for analytes even with octanol–water distribution coefficients higher than 100 (log K o/w > 5). Typically, solutes should have relatively high log K o/w values for SPME (less than ca. 3).

After a certain stirring time, the stir bar can be desorbed with a small volume of a suitable solvent, but for volatile and semivolatile compounds, on-line thermal desorption provides an approach that avoids using organic solvents.

Currently only PDMS coating is commercially available, making the technique most suited to non-polar analytes from aqueous media. However, this technique can be used for more polar compounds by using derivatization. To improve the recovery of more polar analytes, a “dual-phase twister”, which combines both absorption and adsorption, has been described for SBSE [52]. A schematic diagram of a dual-phase stir bar for SBSE is shown in Fig. 5 [53].

Fig. 5
figure 5

Schematic diagram of a dual-phase stir bar for SBSE

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The large amount of sorbent causes some problems. The extraction time is longer, as it takes more time to reach equilibrium. Moreover, desorption takes more time and it may be necessary to reconcentrate the sample band by using cold-trapping during the transfer. The clear benefit of SBSE over SPME is better sensitivity, because the absolute amount of analytes transferred into the chromatographic system is higher. In addition, SBSE can also be used for extraction of relatively polar analytes [54, 55].

Popp et al. [56, 57] employed silicone materials (in the form of rods and tubes) for enrichment of organic compounds, similar to the commercialized Twister. These materials are inexpensive, flexible and very robust, and have successfully been applied for direct extraction of semivolatile compounds in water [58].

Gum-phase extraction (GPE)

Gum-phase extraction (GPE) is another technique that employs polymeric sorbents (e.g. PDMS) filled as a bed in a column, most commonly in the form of particles (typically 300 μl). This technique, which resembles SPE, was first reported by Baltussen et al. in 1997 [48, 59].In principle, GPE can be used for both liquid and gaseous samples. Dynamic sampling has been employed with both gaseous and liquid (aqueous) samples, but the usefulness of the GPE technique for the enrichment of trace compounds from water is limited since the packed tube needs to be dried prior to desorption. For gaseous samples, GPE has the same advantages as OTT and SPME in terms of inertness and thermal desorption characteristics [60].

In GPE, the analytes are dynamically trapped on the sorptive preconcentration trap, an approach called breakthrough sampling. In breakthrough sampling, the analytes will be retained in the packed bed and, consequently, the concentration of analyte in the sample will decrease through the bed. Initially the analyte concentration in the outgoing sample phase will be zero and sampling is usually stopped when the first analyte of interest starts to elute from the trap. Desorption of the trapped analytes can be performed with liquid or by heating (the thermal desorption approach is preferred, because it ensures higher sensitivity).

The performance of a cartridge filled with 100% PDMS particles was compared to the performance of adsorbents like Tenax TA and Carbotrap 300 for the sampling of volatile analytes by Baltussen et al. [61]. Dynamic sampling on PDMS and Tenax was examined for sampling of the volatile solutes emitted by living plants [62].

Equilibrium gum-phase extraction (EGPE)

A new concept for sorptive sample enrichment is that of equilibrium gum-phase extraction (EGPE). This technique is very similar to GPE, but instead of employing breakthrough sampling, the PDMS sorbent in EGPE is completely saturated to equilibrium so that the maximum amount of all analytes is sorbed [63]. Because of the nature of the sorption mechanism (basically dissolution) all analytes partition independently into the sorbent and displacement effects do not occur. This is an advantage over adsorption materials. Additionally, this theory allows for the calculation of enrichment factors from literature retention index data [64]. EGPE can be applied to aqueous and gaseous samples. It has proven to be very successful for the enrichment of volatile compounds [60].

Closed-loop stripping analysis (CLSA)

Closed-loop stripping analysis (CLSA) is a quantitative method used for extraction and detection of VOCs in water. It was introduced in 1973 by Grob [65]. A new technique for isolating analytes from the water to the gas phase involves eluting analytes from the water with a continuous stream of inert gas which is then directed to a bed of solid sorbent, where the compounds are trapped. After removal of analytes the inert gas is recycled back into the vessel with aqueous sample and purges the next batch of analytes on solid sorbent. The analytes are released from the sorbent by elution with a solvent or by thermal desorption [66].

Closed-loop stripping analysis is routinely used to monitor the quality of river water for a broad range of volatile compounds [67]. However, lower volatility compounds are not likely to be analysed with CLSA, which can severely limit its application. Moderately and highly polar ionizable organics are poorly purged or not recovered [68].

Sorption tubes

In sorbent-based methods, such as sorption tubes, analytes are extracted from air by adsorption onto the sorbent surface. The sampling of analytes from the gas phase into the sorption tube might occur using the dynamic or passive methods [69].

In the case of the active sampling method, air is passed through tubes packed with appropriate sorbents. Air flow through the trap is forced by the appropriate mechanical devices (i.e. aspirators) or pumps [70]. Unlike active sampling techniques, passive sampling techniques do not require forced air flow through the bed of sorbent. In this case, the movement of molecules is effected by diffusion according to Fick’s first law and, therefore, no additional devices for collecting air samples or measuring their volume are necessary [71].

For any type of sampling exercise, it is possible to make a choice of the type of sorbent, the method of sampling (pumped or diffusive), the method of liberation of trapped analyte (solvent or thermal extraction) and the method of analysis. Typically, the same sorbents are used in diffusive samplers as in pumped sorbent tubes. On the other hand, the most common sorbents used for sampling with solvent desorption are only rarely encountered where thermal desorption is required. This is because their high surface activity can lead to sample degradation at the high temperatures required for desorption [72].

The tubes used to hold the sorbent for thermal desorption are made of stainless steel or precision-bore borosilicate glass, with precise dimensions to ensure a leak-free connection to the desorption units [73].

A wide variety of organic and inorganic sorbents are available for collection of ambient VOCs [74, 75]. Selection of suitable adsorbents is very critical as it depends on the sample matrix and on the compound to be collected. A useful review of sorbents for thermal desorption has been published [76, 77].

There are some criteria that should be taken into account during the selection of sorbents:

  • Breakthrough of the analytes has to be avoided [78]

  • Contamination of sorbents must be avoided before and after sampling

  • High adsorption capacity in relation to the fortified analytes

  • Hydrophobic nature of the sorbent to reduce to a minimum the process of simultaneous adsorption of water vapour

  • High thermal stability, such that thermal desorption can be performed at sufficiently high temperature

  • Affinity for a particular group of compounds, allowing a degree of selectivity for enrichment.

The arrangement of the sorbents is such that the least volatile compounds are trapped on the weakest sorbent at the front end of the tube, and successively more volatile compounds are trapped by increasingly strong sorbents further down the tube. Desorption then takes place in the reverse direction, as with single-bed tubes. A schematic diagram of a multi-sorbent tube for dynamic sampling of VOCs is shown in Fig. 6 [79].

Fig. 6
figure 6

Multi-sorbent tube for dynamic sampling of VOCs

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In the literature, many papers can be found describing different thermal desorption (TD) coupled to GC-MS methods development for a wide range of VOCs analysis [80, 81].

Sorbent tubes have formed the basis of the US Government’s agency methods (NIOSH 2549, EPA TO-17) [82, 83].

Hryniuk et al. [84] demonstrated the potential of using a combination of TD tubes and selected ion flow tube mass spectrometry (SIFT-MS) for breath analysis, an approach that may find utility in a clinical setting which does not allow on-line analysis of breath.

Field air sampling with sorbent tubes and multidimensional GC-MS/olfactometry for simultaneous chemical and sensory analysis of livestock odorants was used to develop an odour characterization method for specific livestock odorants and develop a quantitative method for the key odorous compounds responsible for livestock odour emissions [85].

Vapour-phase extraction

The first application that mentioned the concept of headspace (HS) sampling was the “aerometric method” for rapid determination of alcohol in water and bodily fluids [86]. The terms “headspace” and “headspace analysis” were first used in 1960 by Stahl et al. [87], while the first communication in which HS sampling was combined with GC analysis was by Bovijn et al. [88].

A common feature of headspace analysis methods is the use of partition law which states that, at given conditions of pressure and temperature, the ratio of the component concentrations in the liquid phase (L) and gas phase (G) at thermodynamic equilibrium is constant. This ratio is called partition coefficient [89]. Headspace analysis as a method of preparing samples for proper analysis (performed by any method) involves the transfer of the analytes from the original sample—condensed matter, mostly liquid—to the gas phase, which is to be analysed. Therefore, with the analytical procedure, combining HS with an appropriate separation technique, it is possible to obtain information on the composition of the original sample (liquid or solid) based on analysis of the gas phase remaining in equilibrium with it. Traditionally HS sampling operates either in static (S-HS) or dynamic mode (D-HS).

Static headspace (S-HS)

Static headspace (S-HS) procedures for the analysis of VOCs in aqueous or other matrices have been used extensively as a means of determining analytes without interferences from the sample matrix. In S-HS, which relies on volatilization to separate analytes from a sample matrix, important factors are related to diffusion and surface area. For accurate quantitative analysis, the temperature/pressure conditions of a sample vessel are critical. In this resolution, the headspace is sampled directly with a microsyringe or by filling a loop. Such determinations require large Henry’s law constants and therefore are applicable to volatile compounds [90].

The S-HS method eliminates many steps of error-prone and time-consuming manual sample preparation procedures and allows for the introduction of a gaseous sample into an analysis system. On the other hand, the most significant drawback of S-HS is its lack of sensitivity. Generation of a gaseous sample is an equilibrium process that limits the amounts of a specific analyte available for analysis within the practical restraints of time and temperature. Additionally, the injection size is a bottleneck since most GC systems can handle injections of only a few cubic centimetres [91].

Headspace analysis could run under equilibrium or non-equilibrium conditions. The latter may be carried out in two cases [92] when:

  • The time needed for equilibration is too long for the intended purpose, control or for routine measurements

  • The sample is heat sensitive, and it might be damaged in the course of full equilibration.

Dynamic headspace (D-HS)

One of the first applications of dynamic headspace (D-HS) was due to Herout, who collected the volatile fractions of Viol odorata, Lycaste macrobulbum and Hyacinthus orientalis through an Apiezon trap [93].

The dynamic headspace method is a solventless, highly reproducible, automated extraction procedure for volatiles from almost any matrix for quantitative and qualitative determinations, which extends the headspace method and uses concentrator technology to achieve far more sensitive detection limits. In the dynamic headspace technique, the equilibrium between the phases is continually altered. D-HS is generally based on two main approaches:

  • The purge and trap (PT) approach, which is based on bubbling through the sample (liquid or solid) with an inert gas (usually helium or nitrogen). The volatile fraction is accumulated from the gaseous flow stream stripped through the matrix onto a trapping medium: cold trap, a sorbent, an adsorbent or specific reagent or sorbent for a given class of compounds [94]. This step can be carried out in an open or closed loop [95]. In the open-loop configuration, the non-trapped molecules are eliminated. In the closed-loop method, the gaseous phase flows through the sample and the trap in a closed circuit [96].

  • The dynamic approach, where analytes are sampled from the gaseous flow stream passed over the sample [97].

The sampled volatiles are generally recovered either by solvent elution or (more often) by thermal desorption on-line or off-line to the GC.

One of the main problems of dynamic headspace is the adsorption of water on the trap. Water can be a major source of trouble if the sample contains it in high amounts (beverage, aqueous sample, foods). In the case of adsorption of the analytes onto a solid sorbent, a certain percentage of water will also be retained. Water subsequently released during desorption may clog up the cold trap or the cryofocusing trap at the head of the column. Therefore, efforts have been made to develop a sorbent with low water affinity. But even in the case of the hydrophobic sorbents, the trapping of water can cause problems when the relative humidity of the sample is above 90% [98]. Too much water entering the system can also damage the MS detector [99] and induce a modification of the spectrum, rendering identification difficult [100]. Therefore it may be necessary to introduce some solutions to avoid the presence of water in the analytical system (Table 2).

Table 2 Examples of solutions avoiding the presence of water in the analytical system [101–107]
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Massolo et al. [108] described the optimization of the main instrumental parameters of a home-made purge and trap GC system for the simultaneous determination of chlorofluorocarbons (CFCs) in seawater samples. In order to concentrate high volumes of water for trace analyses and stable carbon isotope measurements of volatile halogenated organic compounds in seawater a purge and trap continuous flow system was developed [109].

High concentration capacity headspace techniques (HCC-HS)

Interest in HS technique concurred with the introduction of an additional approach: high concentration capacity headspace techniques (HCC-HS). HCC-HS techniques are based on either the static or dynamic accumulation of volatile(s) on polymers operating in sorption and/or adsorption modes, or more seldom, on solvents [97]. HCC-HS techniques are as simple, fast, easy to automate, and reliable as S-HS, and they show analyte concentration factors comparable to those of D-HS [110].

Headspace solid-phase microextraction (HS-SPME)

The first HCC-HS technique to appear was HS-SPME, introduced by Zhang and Pawliszyn in 1993 as an extension of SPME [111]. They advanced a theory for SPME applied to HS sampling [112] and showed that analyte recovery from headspace by fibre depends on two closely related but distinct equilibria: the matrix/headspace equilibrium and the headspace/polymeric fibre coating equilibrium. HS-SPME has been shown to be a successful bridge between static (S-HS) and dynamic (D-HS) headspace being as simple, reproducible and easy to automate as S-HS, and as sensitive and as selective as D-HS.

During HS extraction there are three phases involved: the condensed phase, its headspace and the SPME polymer. The last phase (SPME fibre) forces compounds out of the matrix (liquid or solid) into the headspace and then into the fibre. The vapour phase should be in equilibrium with the matrix sample to effect headspace sampling of volatiles. The equilibrium of the extraction is reached when the concentration of the analyte is homogenous within each of the three phases [113]. In HS-SPME, a higher temperature may result in less deposition onto the fibre as volatile compounds again favour the vapour phase. This can be a useful tool for selective analysis, as the fibre will favour lower volatility analytes than direct headspace [114].

Headspace solid-phase microextraction has the potential to extract a wide range of organic compounds, volatile or semivolatile, from various matrices, both in their liquid and solid phase. In terms of precision, linearity and sensitivity, HS-SPME equals the HS method.

A new automated HS-SPME sampling device was developed, with the capability of heating the sample matrix and simultaneously cooling the fibre coating. The device was evaluated for the quantitative extraction of polycyclic aromatic hydrocarbons (PAHs) from solid matrices [115]. A simple device consisting of a closed headspace vial equipped with an integral cutting was used for the collection, homogenisation, and HS-SPME sampling. It has been applied for the microscale sampling of volatile monoterpene hydrocarbons from conifer needles [116].

Headspace sorptive extraction (HHSE)

Headspace sorptive extraction (HHSE) was introduced in 2000 by Bicchi et al. [117] and Tienpont et al. [118] as a variant of static headspace analysis with the use of stir bar sorptive extraction (SBSE) to sample the headspace of a sample. This is very similar to HS-SPME, but a coated stir bar is held in the headspace in equilibrium or not with the matrix, for a fixed time, in place of the fibre [119]. After sampling, the stir bar is placed in a glass tube and transferred to a thermo-desorption system where the analytes are thermally recovered and analysed by GC or GC-MS.

The HSSE, compared to other techniques, has high concentration capability, mainly due to the high volume of PDMS. Additionally, PDMS twisters can be applied in both S-HS and D-HS modes for trace analysis and passive sampling.

On the other hand, the main drawbacks of HSSE are the need for dedicated and expensive instrumentations and the lack of “polar” polymer coating for stir bars, to improve HSSE’s effectiveness with medium to high polarity compounds [97].

The combination of HS-SBSE and TD-GC-MS was used for accurate and precise simultaneous determination of mercury and tin organometallic species at the low concentration levels found in many environmental samples. Additionally, the applicability of this method has been proven with a wide range of different samples [120]. Bicchi et al. evaluated the performance of dual-phase twisters for HSSE sampling by analysing the headspace composition of two matrices, i.e. coffee and dried sage leaves [121].

Solid-phase aroma concentrate extraction (SPACE)

Solid-phase aroma concentrate extraction (SPACE) is a modified version of the SPME technique for headspace analysis, with an increased area of the adsorbent to allow more sensitive analysis of volatiles analytes. This new method was introduced by Ishikawa et al. [122]. The SPACE rod used in the technique is fabricated from stainless steel coated with an adsorbent mixture (mainly of graphite carbon). The SPACE rod is fixed on the head of a closed flask, where it adsorbs the aroma for a given time. Next, the rod is thermally desorbed on-line to the GC or GC-MS system [97]. The SPACE rod collects the analytes with good reproducibility, with the exception of highly polar compounds. The SPACE method proved to have superior capabilities with high concentrations, and it produced a well-balanced chromatogram. This technique has been shown to be successful with roasted coffee beans and other plant matrices [123].

Headspace solid-phase dynamic extraction (HS-SPDE)

The headspace solid-phase dynamic extraction (HS-SPDE) technique was developed by the use of polypyrrole (PPy) sorbent, electropolymerized inside the surface of a needle, as a possible alternative to SPME. In HS-SPDE, analytes are accumulated in the polymer coating of the inner needle wall by pulling in and pushing out a fixed volume of HS to be sampled, through the gas-tight syringe for an appropriate number of times within a fixed time. Because the vapour phase flowing over the accumulating phase layer is continuously renewed, HS-SPDE is a D-HS approach [124]. The trapped analytes are recovered by thermal desorption, employed to transfer the extracted analytes into the GS injection port, and analysed by GC or GC-MS.

A few HS-SPDE applications have been reported in the literature [125]. They are fully automated systems for the analysis of headspace samples (HS-SPDE) [126, 127].

More recently, other approaches based on the same principles known as inside needle dynamic extraction (INDEX) [128] and in-tube extraction (ITEX) [129] have been introduced.

Multiple headspace extraction (MHE)

In 1977, Kolb and Pospisil presented a technique called discontinuous gas extraction [130]. This method was later renamed as multiple headspace extraction (MHE) [131]. This technique involves performing several extractions from a single sample. In this way, the concentration of the analyte decays exponentially and the total peak area corresponding to an exhaustive extraction of the analyte can be calculated as the sum of the areas of each individual extraction. Therefore, the matrix effect is completely removed. Because the MHE procedure follows a logarithmic function, it is not required that the extractions are carried out until all of the analyte is removed from the sample matrix. Instead, the logarithms of the various area values from the consecutive analyses are plotted versus the number of analyses in a linear scale and the total area value is obtained by regression calculation from the areas obtained in only a few extraction steps [132].

Multi HS-SPME has the same aim as MHE. The amount of analyte extracted by the fibre is proportional to the initial amount, and it can be proven that the peak area decays exponentially with the number of extractions. The total peak area can be estimated by performing a few (three or four) successive extractions by HS-SPME [133]. In multi HS-SPME, the relationship between the peak area and the amount of analyte in the fibre coating must be linear over the whole range being studied. Additionally, the distribution constants of the analyte between the fibre and the sample and the volume of the three phases must be constant during all of the extraction steps. The next condition which must be fulfilled to carry out a proper multiple HS-SPME is that equilibrium of the analyte in the three-phase system must be established [134, 135].

Thermal desorption (TD)/thermal extraction (TE)

For environmental reasons and cost, there is great interest in reducing the impact of wet chemical handling in laboratories. This favours heat extraction techniques [91]. Thermal desorption (extraction) is a widely used technique for extracting and isolating semivolatile compounds from various matrices. Almost any sample containing volatile organic compounds can be analysed by using some variation of this technique. This method is well established in environmental analysis, food analysis and forensic science. Thermal extraction provides an attractive alternative to solvent extraction (SE). In the process of TD, heat and inert gas flow (usually helium) are used to extract analytes retained in a sample matrix or on a sorbent bed. A temperature is needed that is high enough to allow desorption of the analytes from the matrix but also low enough to avoid degradation of the sample matrix itself. The analytes are desorbed into the gas stream and are ultimately transferred to the analyzer. Although compounds can be transferred directly from the original sample (sorbent bed) to the analyzer in one thermal desorption step, this simple, single-stage approach has limited practical application [136, 137]. The elution volume required for complete extraction of a typical 100-mg to 1-g sample is too large, giving poor analytical resolution and relatively low sensitivity. TD in its most simple single-stage form is of limited application for packed column chromatography and cannot be used at all for capillary column GC [138]. For this reason, most thermal desorbers are two-stage, i.e. they contain a focusing mechanism (capillary cryofocusing or cold adsorbent trapping) for concentrating analytes desorbed from the matrix (sorbent bed) before releasing them into the analytical system in as small a volume of vapour as possible. Both procedures do produce excellent, capillary-compatible chromatography, but capillary cryofocusing is quite costly in terms of liquid cryogen consumption. Moreover, the volatility range of capillary cryofocusing devices is limited. More importantly, such systems are prone to blocking with ice during the desorption of humid samples. This procedure is also prone to sample degradation from condensed oxygen. Any blockage or restriction of the desorption gas flow has a significant impact on the efficiency of the process. Thermal desorption as a method of releasing organic compounds from the sorbent bed, or fixed directly from the solid sample matrix, offers the following advantages, in comparison with conventional solvent extraction [139, 140]:

  • Typically 1,000 times more sensitivity

  • Minimal sample preparation (eliminates the problem of contamination)

  • A smaller sample amount is required for the analysis

  • No analytical interference from solvent artefacts

  • Time efficiency

  • Greater than 90% desorption efficiency

  • Selective focusing/extraction

  • Environmentally friendly (no solvent disposal)

  • Cost effectiveness

  • Eliminates problem associated with accurate dosing and repetition of the injection of liquid extracts

  • Eliminates the appearance of the solvent peak in the chromatogram, the components of which may mask the analytes

  • Eliminates difficulties associated with the choice of suitable solvents for the extraction of analytes especially when analytes differ significantly in polarity

  • The method is fully automated.

Thermal extraction is not without its limitations, however. Not all types of substrates are suitable for high temperature desorption. The use of TE is therefore complicated by the potential for carry-over, transfer loss, molecular rearrangement, fragmentation or breakdown of more thermally labile analytes at higher extraction temperatures and matrix effects, leading to quantification inaccuracies [141]. Another drawback is sample consumption in a single analysis, although modern TD equipment incorporates design modifications to allow re-collection of split samples in a fresh tube [142].

TD was originally developed as an off-line sampling method with preconcentration of workplace atmosphere by pumping air through a solid adsorbent material [143].

Direct thermal desorption (DTD)

Volatile or semivolatile analytes from liquid or solid samples can also be released by direct thermal desorption (DTD). In the direct thermal desorption technique, a small amount of homogeneous sample is placed directly in a thermal desorption unit. DTD permits the analysis of samples without any prior solvent extraction or other time-consuming sample preparation. Depending on the nature of the materials being tested, samples may be either weighed into empty TD tubes or tube liners for direct desorption.

Direct desorption of analytes from a sample weighed straight into empty desorption tubes or appropriate tube liners is a cost-effective sampling procedure. Sample clean-up, analyte extraction and sample introduction are combined into one automated operation. In addition, since the instrument does not contain a heated transfer line or switching valves it is possible to transfer compounds with a high molecular weight.

Conditions for DTD are [144]:

  • High surface area solid materials

  • Unrestricted flow of gas through the sample tube

  • Sample should be placed well within the heated zone of the thermal desorber

  • Molecules should be desorbed intact from the matrix.

DTD is appropriate only if the desired extraction takes place at a temperature below the decomposition point of other materials in the sample matrix and the relatively small sample size that can be measured in a TD tube is representative of the sample as a whole [144].

In 1987, Chen et al. developed a direct sample introduction and thermal desorption GC-MS technique for the analysis of volatile constituents in Chinese medicinal herbs [145]. It offered several distinct advantages: minimal sample preparation, small sample size and short analysis time. Meanwhile, commercial direct sample introduction and thermal desorption devices have become available. This technique is widely applied for the analysis of volatile compounds in plant materials [146–148].

Short-path thermal desorption (SPTD)

Short-path TD, patented by Scientific Instrument Services, Inc. (Ringoes, NJ, USA), is a TD system that sits directly on top of the GC injection port. As a result of the short path of sample flow, these systems eliminate transfer lines, which are contaminated by samples, and optimize the delivery of samples to the GC injector via the shortest path possible [149]. SPTD provides maximum sensitivity by minimizing artefacts, losses and carry-over effects [150].

Temperature-programmed desorption (TPD)

A commercial direct thermodesorption system with a programmable temperature cooled injection system (CIS) and GC-MS for identification has been introduced, which is suitable for the analysis of packed adsorbent tubes and direct analysis of solids and liquids. Gas samples are prepared for analysis by being passed through a desorption tube containing an appropriate adsorbent. All other sample types, placed directly in an empty tube without further preparation, are inserted directly into the desorption chamber. After purging with the carrier gas and heating to the desired temperature, the analytes are transferred into the CIS for cryofocusing. After complete desorption, the CIS liner is then heated to the desired temperature to allow transfer of the trapped analytes to the analytical column [151].

Advantages of direct desorption of volatile trace compounds by TD-CIS-GC-MS are [91]:

  • Universal applicability of GC-MS to all sample matrices (gaseous, liquid or solid)

  • Solvent-free analysis of complex matrices

  • Wide boiling point range of analytes (usually C2 to C40, but up to C100 in a modern multimode injection system)

  • Complete transfer of high-boiling analytes

  • Lower detection limits through large volume injection

  • Allowance for large concentration ranges through the use of split, splitless or solvent venting modes

  • Avoidance of cross-contamination

  • Preparation of standards and samples by spiking solutions onto the desorption tube

  • Autosampler capability

  • High desorption flow allowing fast analysis times

Application of solventless sample preparation techniques for the analysis of environmental contaminants in different matrices

It should be noted that there is no universal sample technique suitable for all types of sample. Sample preparation is dependent on the nature of the sample’s analytes, matrix, final separation method and the type of information which is sought. For this reason, a number of different sample preparation (extraction) techniques exist, each suited to a different analyte and matrix type. Table 3 lists the application of solventless sample preparation techniques for the analysis of contaminants in the different matrices coupled with TD followed by GC, sorted by analyte.

Table 3 Application of solventless sample preparation techniques coupled with TD followed by GC
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Concluding remarks and future trends

Research trends in solventless sample preparation techniques based on solid- and vapour-phase extraction for the analysis of environmental contaminants in different matrices are focused on studying new resolutions for higher extraction efficiencies, selectivity and stability. The development of new devices to improve the sampling process and the study of novel calibration processes are considered. Trends in instrumentation indicate focus on improved automation and ruggedness, field portability and novel selectivity for specific applications. Recent trends focus on the minimization of the use of organic solvents in sample preparation, automation, and speeding up sample preparation procedures. This means that the application of solventless sample preparation for different fields continues to increase.

Table 4 summarizes the main advantages and disadvantages of the techniques discussed, including the conditions used and/or parameters adjusted to optimize the sample extraction step.

Table 4 Advantages, disadvantages and conditions used to optimize solventless extraction techniques
Full size table