Solutions for Online Coupling of Extraction and ... - ACS Publications

Chapter 8

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Solutions for Online Coupling of Extraction and Chromatography in the Analysis of Food and Agricultural Samples Tuulia Hyötyläinen Department of Chemistry, Univeristy of Helsinki, P.O. Box 55, F I N 00014, University of Helsinki, Helsinki, Finland (email: [email protected])

This overview is focused on the on-line coupling of pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), supercritical fluid extraction (SFE) and sonication-assisted extraction (SAE) with liquid and gas chromatography for the analysis of solid agricultural and food samples. In addition, head-space techniques and direct thermal extraction are discussed.

Introduction Even with the emergence of advanced techniques of separation and identification, it is rarely possible to analyze food and agricultural samples without manipulation. Particularly solid and semi-solid samples typically require complex, multistep sample preparation procedures. During recent years, much effort has been invested in the development of separation techniques, especially chromatography. However, the role of sample pretreatment is too often neglected in the method development. Often, the sample preparation is not at the same high

© 2006 American Chemical Society

In Modern Extraction Techniques; Turner, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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110 level as the sophisticated separation methods. Many of the sample preparation methods currently in use have been around for decades, with essentially no modification over the years. The main problems with traditional sample preparation schemes are that they are too time-consuming, tedious and they even introduce serious errors to the analytical procedure. In the sample preparation of semi- and nonvolatile compounds, solvent extraction is typically used for extracting the analytes of interest from a sample matrix. For volatile analytes, head-space or thermal extraction are good alternatives to solvent-based techniques. Several novel extraction systems that utilize elevated temperatures of pressures in the extraction have been developed particularly for solvent-based extraction methods. These new methods typically are much faster and often more selective than older methods and consume smaller amounts of organic solvents and reagents. Commercially available systems with the ability to heat and pressurize liquids include pressurized liquid extraction (PLE), microwave-assisted extraction ( M A E ) and supercritical fluid extraction (SFE). Also sonication-assisted extraction (SAE) has given promising results. Most of the extraction methods are used off-line, meaning that extraction and analysis are done separately. Many of the systems could nevertheless be integrated as an on-line system, where the whole analytical procedure takes place in a closed, usually automated system. In this way, many of the problems associated with the traditional approaches could be avoided. Additional benefits are the increased sensitivity and reliability because sample clean-up in an on-line system tends to be more effective. It should be noted, however, that on-line procedures are not always the best choice for the analytical task in question. For the preparation of an appropriate number of samples, the methods have to be selected not only on the basis of the expected performance of the analytical system, but also according to general requirements such as number and mass of samples, laboratory equipment and the experience of the analytical staff. A n off-line procedure is a good choice, when the number of samples is small, because then there is usually no need for an automated method and no incentive for the time-consuming development of such a method, and conventional methods will suffice. Setting up an automated method, either at-line or on-line, can be worth the effort i f the number of (similar) samples to be analyzed is large. In both off-line and at-line methods, optimization of individual steps of the sample pretreatment is quite flexible, in contrast to on-line methods, where compromises often have to be done in this aspect. Conversely, the sensitivity of off-line and at-line procedures is usually lower than in on-line methods because only a part of the sample is injected to the chromatographic system. On-line systems are beneficial when the amount of sample is limited, or very high sensitivity is required (1-6). In most cases, experienced personnel is required even though the use of an automated on-line


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instrument is quite straightforward, for the method development and eventual trouble-shooting. In the case of labile or reactive sample components, on-line system can be clearly advantageous over the off-line and at-line methods. In an on-line system, where extraction and analysis take place in the same, closed system, the risk of decomposition is minimal.

Approaches For On-Line Coupling of Extraction with Chromatography The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction can be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. In on-line analysis sample sizes are frequently small, often 4-1000 mg of the solid sample is sufficient for trace level analysis. The reason for this is that in on-line system, the whole sample extract reaches the analytical column, thereby increasing the sensitivity. As very small sample amounts are not always desirable with solid samples, careful homogenization of the sample is crucial in order to obtain representative sub-samples.

Solvent or Fluid Based Extraction In the field of food and agricultural samples, relatively few on-line systems based on solvent extraction have been developed. SFE, P L E and S A E have been combined on-line with L C or G C (7-20). In theory, it would also be possible to use M A E in on-line combination, but so far, only one application for environmental analysis has been reported (27).



112 In solvent-based extraction methods, on-line coupling requires some modifications to the off-line extraction techniques. First of all, the volume of the extract must be kept small in on-line coupling. Typically, it should be less than 1-2 milliliters, or the extract must be concentrated before the transfer to chromatography. Moreover, the solvent (or fluid) must be compatible with the following chromatographic system. In coupling with G C this means that the solvent must be sufficiently volatile and preferably nonpolar. In coupling with L C the extracts should preferably be in a solvent of weak eluent strength in the L C (7). In on-line coupling of extraction with large-volume G C , the volumes of extracts transferred are typically up to milliliters, and special interfaces are required for the transfer. The most common techniques are based on largevolume G C and on-line L C - G C . Several interfaces have been implemented, including on-column, loop-type and vaporizer interfaces, which are almost exclusively used in on-line methods today. The situation is more straightforward in L C because rather large volumes can be transferred to an L C column without problems. If necessary, the coupling can be done via a short solid-phase extraction column, or the extract can be diluted with suitable solvent to reduce the solvent strength of the extract before the transfer.

Microwave-assisted extraction Microwave-assisted extraction (MAE) is widely recognized as a versatile extraction technique especially for solid samples. It has also been used widely in digestion of food and agricultural samples, but it has also been utilized in the extraction of organic compounds (22-26). In several comparative studies, M A E has proven to be equal or superior to Soxhlet extraction, P L E and SFE (22,23). In M A E , organic solvent and the sample are subjected to radiation from a magnetron. The solvent (or the sample) must be dielectric in character. The main advantages of M A E are the fast rate of extraction due to the quick heating and elevated temperatures, but also the ease of instrument operation. Almost all M A E applications involve off-line procedures, and on-line systems have not been utilized in food and agricultural analyses. Only a few approaches involving an on-line system have been published, for example in the determination of organophosphorus compounds in air particulates (27). In this M A E - G C system, an interface based on solid-phase trapping was used. Methanol was as the extraction solvent, and before the solid-phase trap the extract was diluted with water to enable efficient trapping to a polymeric sorbent. The sorbent was then dried with a nitrogen flow and the trapped analytes were eluted with organic solvent to the G C equipped with a P T V injector. A similar system should be applicable to the analysis of other types of solid samples as well.


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Sonication-assisted extraction In sonication assisted extraction (SAE), acoustic vibrations with frequencies above 20 kHz are applied to the sample. The vibrations cause cavitation in the liquid; that is, bubbles with a negative pressure are formed. When the cavitation bubbles collapse, shock waves are generated, which enhance the removal of analytes from the matrix surface. Moreover, implosion of cavities creates microenvironments of high temperatures and pressures. Traditionally, S A E is performed in static mode. In on-line combinations, the extraction is dynamic (27,28). In principle, SAE can be coupled on-line with both L C and G C . However, no S A E - L C application has been reported and only one S A E - G C application for the analysis of organophosphates in air particulates (27,28). In this application, the extract was transferred directly to the P T V injector of the G C . Direct transfer was possible by using a miniaturized extraction vessel, a very short extraction time (3 min) and a moderate flow rate (200 nl/min) in the extraction.

Pressurized liquid and hot water extraction In pressurized liquid extraction (PLE), rapid extraction is performed with small volumes of conventional solvents by using high temperatures (up to 200°C) and high pressures (up to 20 000 kPa) to maintain the solvent in a liquid state. The extraction can also be done with a purely aqueous phase, a technique often referred to as pressurized hot water extraction (PHWE) or subcritical water extraction. Temperatures of up to 325°C have been used in P H W E (29-38). In principle, it is possible to combine P L E on-line with chromatography but so far, no applications have been published with organic solvents. On the other hand, several applications involving on-line coupling of P H W E with either L C or G C have been reported (29-31,35-39). The coupling of P H W E with L C is relatively simple because the solubility of the analytes in water decreases dramatically when the water is cooled to ambient temperature. Trapping of the extract in a solid-phase (SP) material is thus easy. In P L E , the extract is usually already in a relatively large volume of an organic solvent, and trapping is not easily achieved. Direct transfer is not possible either, because the volume of the extract tends to be too large. The connection of P L E to chromatography might be done in a similar manner to that described for on-line M A E - G C , i.e. by diluting the organic extract with water before trapping. In on-line coupling, P H W E is performed in dynamic mode, i.e., the water is continuously flushed through the extraction vessel. After the extraction, the water is cooled and the extracted analytes are collected either on a solid-phase trap (for L C or GC) or a membrane extraction unit (for GC) (35-39). P H W E - L C



114 coupling with a SP-trap is more straightforward than P H W E - G C coupling, as the extract from the SP-trap can be directly eluted to the L C column by a suitable eluent (7,38,39). The apparatus closely resembles column switching L C systems. In P H W E - G C coupling using a SP-trap, the trap must first be dried with gas flow, after which the analytes are eluted with a suitable organic solvent (55). In P H W E - G C applying a membrane unit as interface, the extract can be directly transferred from the membrane extractor to the chromatograph (36,37). The membrane unit is based on microporous membrane liquid-liquid extraction (MMLLE). In P H W E , degradation of the analytes can occur due to harsh conditions during the extraction, and this must be considered when choosing the extraction conditions. For relatively polar analytes, such as caffeine, chlorophenols and anilines, quantitative extraction can be obtained already at 100°C, whereas for less polar compounds, such as selected pesticides, PAHs or PCBs, 250 - 300 °C should be used (16,29-32, 34-37). Very high temperatures should be used with caution, however, because excessively high temperatures may cause degradation of analytes prone to hydrolytic attack (33). Moreover, the selectivity of the extraction decreases at high temperatures because of the larger amounts of unwanted low-polar matrix species that are coextracted. P H W E - L C systems have been utilized, for example, to determine N methylcarbamates in food. P H W E was done at 75 °C and a combination of static and dynamic extraction was used. In this system, the effluent from the chromatograph was coupled with a flow injection manifold where the N methylcarbamates were hydrolyzed and then derivatizatized by reaction with a fluorogenic reagent (o-phthalaldehyde). The derivatized analytes were driven to a fluorimeter equipped with a flow-cell allowing monitoring (31). For many pesticides, G C separation is more appropriate than L C in terms of separation efficiency and sensitivity. A system utilizing P H W E - M M L L E - G C has been developed for the determination of pesticides from grapes (Figure 1 A ) (16). A similar system has also been used for the determination of organic pollutants from soil and sediment samples (35-37). In this system, during the dynamic P H W extraction (120 °C, 40 minutes) the aqueous effluent flowed through a membrane extraction unit, and L L E took place through the organic liquid in the pores of the membrane. The M M L L E organic extract was obtained in a small volume and could easily be transferred directly to G C . In addition, the M M L L E was selective, as also size exclusion takes place due to the small pores of the membrane (40). Therefore, the extract was clean from high molar mass compounds that could disturb the G C analysis. Figure I B shows P H W E M M L L E - G C - M S determination of pesticides from grapes. The only sample pretreatment required prior to analysis was crushing and weighing of the samples. The LOQs of the method were in the range of 0.3-1.8 ng/kg, using total ion monitoring in the M S detection.



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Figure 1. (A) PHWE-MMLLE-GC-MS apparatus and (B) determination of pesticides in grapes by the system.

Supercritical fluid extraction Supercritical fluid extraction (SFE) has several distinct physical properties, and it is regarded as a promising alternative technique to conventional solvent extraction because it is fast, selective and environmentally friendly (41-44). SFE exploits the gas-like and liquid-like properties of a supercritical fluid, typically carbon dioxide. With addition of a suitable organic modifier to C 0 it is possible to extract a range of compounds of different polarity. It should be noted that in SFE, water in samples can cause problems in the repeatability of the extraction. Therefore, samples containing large amounts of water, drying can be required before the extraction. In some cases it is also possible to add asuitable drying material directly to the extraction cell, i f the analytes are not adsorbed onto it (15). Fat removal can often be combined with the extraction step, also by adding a suitable adsorbent material (e.g. Florisil) directly to the extraction cell (41). SFE is well suited for on-line coupling with chromatography since C 0 becomes gaseous upon depressurization and is easily eliminated from the analytical system. The solvating properties of the fluid are also decreased substantially during the depressurization. Typically, dynamic extraction combined with a sufficient static period is used in on-line coupling. In the field of food and agricultural analyses, several SFE-GC (11,12,17,19,20,44) and SFE-LC applications have been published (58-80\ and also a few SFE-SFC (13,14) applications have been reported. In SFE-GC coupling, the main factor to be considered is the flow rate of the fluid, which increases upon depressurization by a factor of about 400. The interfaces used in SFE-GC are typically based on a split/splitless injector, an oncolumn injector or a programmable temperature vaporizer (45-57). In principle, 2

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116 the interfacing is straightforward: the SFE restrictor is placed directly in the G C injector where the supercritical C 0 is allowed to expand to gas. The expansion cools the injector and the analytes in the extract are trapped in the cold injector while the gaseous C 0 is directed away, either via a split exit or through the G C column. The first approach is faster and more suitable. With pure C 0 the interfacing is relatively easy, but the situation is more complex i f modifiers are applied in the SFE. The effect of the modifier on the on-line system depends on the nature and amount of the modifier. Examples of on-line coupled SFE-GC include determination of pesticides in honey (72), identification of volatile and thiolcarbamate herbicides in soil matrices (77), in the determination of free fatty acids and some other flavor compounds of Swiss cheese (10), in the analysis of aroma compounds sorbed by plastic packaging material (77), in the analysis of semivolatile compounds in wood and bark (72) and in the analysis of semivolatile compounds in plants (44). On-line SFE-GC allows also other interesting features of SFE, such as on-line fractionation, to be utilized in the analysis. For example, in SFE of Thymus mastichina L., two on-line separation vessels were used to obtain the less and the medium volatile fractions at two different temperatures and pressures (50°C, 150 bar and 25°C, 50 bar) and the extracts were collected directly to a glass liner of P T V packed with Tenax T A adsorbent to collect the volatile fraction (18). The first and second fractions were recovered by solvent elution, while the third fraction, which was retained in the glass liner, was introduced into a G C using thermal desorbtion. The on-line coupled SFE-GC is an especially useful for analyzing volatile compounds, and the trapping can be easily enhanced by using cryogenic focusing for retaining volatile constituents (19). 2

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Also supercritical fluid chromatograph (SFC) has been combined with SFE (13,42). For example, SFE-SFC has been developed for the characterization of compounds in desert botanical species (Dalea spinosa) (13). A slightly more complex combination utilizing SFE-SFC-GC has been developed for the determination of volatile compounds in raw and baked Baltic herring (Clupea harengus membras) (14). In this study, the analytes were extracted with supercritical C 0 (45°C, 10 MPa) and the volatiles and coeluted lipids were separated on-line using SFC, and the volatile fraction was introduced directly into a G C . In SFE-LC, the most common interface is based on solid-phase trapping (2,15,44,58-63,66-68,70-80), although a few other types of interfaces such as impactor interface (69) open-tubular trapping (64) and the sample loop interface (65) have been developed as well. Direct trapping into a conventional packed L C column is not possible because of the high back pressure that analytical columns create. Because of the back pressure, the fluid cannot be efficiently decompressed and thus it will retain (partially) its solvation properties and efficient trapping will not be achieved, especially if modifiers are used in the 2


117 fluid. However, monolithic columns, which have a very low back-pressure, can be used for direct trapping of the extract (75). Both R P L C and N P L C have been used with SFE. Solid-phase trapping is done using a multiport valve at the interface. During dynamic extraction, the analytes are trapped into die solid-phase, which is typically composed by C -modified silica or porous graphitic carbon (60,74,79). The analytes can be eluted from the trap directly to the analytical column, or a separate system can be used to remove gas from the trap before the transfer. The first approach may cause problems, because most liquid mobile phases are not miscible with gaseous C 0 , which will be present in the SP-trap after the trapping. For a conventional L C system, the mobile phase should be gas-free for optimal pump and detector performance. The depressurized C 0 can be removed from the trap before elution by flushing the column with a suitable solvent of weak eluent strength before the transfer (74,75), or by using an extra, pressurized outlet (79). Also pressurization of the outlet of the L C detector has been tested (60).


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Figure 2. (A) SFE-LC- UV system utilizing a monolithic column bothfor trapping and analysis and (B) determination of lycopene in tomatoes with the apparatus. When a monolithic column is used for SFE-LC (Figure 2 A), the coupling is much more simple (75). Monolithic columns have much lower low back pressure than packed L C columns, and CO2 is efficiently depressurized in them. In addition, monolithic columns can tolerate complete drying of the packing material without problems with the performance. In the on-line system, the extract is directly trapped in the beginning of the column, and after the extraction is completed, the L C analysis can be started. SFE-LC applications used in the food and agricultural analyses include, for example, determination of lycopene in various fruits and foods and



118 determination of pesticides in food products (75, 58-80). A n on-line SFE-LC system based on a monolithic column has been developed to determine lycopene in various food matrices, such as fruits and tomato products (75). Lycopene, which is an antioxidative compound, is challenging to determine with conventional methods, because of its reactivity towards atmospheric conditions. In the on-line system, lycopene could be determined reliably, as the extraction and analysis were carried out on-line in a closed system. Lycopene, which is an unonpolar analyte, was trapped efficiently in the beginning of the monolithic column during the extraction cycle, and the L C analysis started immediately after the extraction was completed. There was no signs of deteoration of the performance of the monolithic column. The whole analytical procedure took less than 25 minutes (Figure 2B).

Dynamic Head-Space Extraction and Thermal Desorption G C If the analytes of interest are volatile or semivolatile, solvent extraction is not always necessary, and head-space techniques (HS) can be applied for the analysis, typically utilizing G C as the final analytical step. HS analysis can be defined as a vapor-phase extraction, involving first the partitioning of analytes between a non-volatile liquid or solid phase and the vapor phase above the liquid or solid. The vapor phase is then transferred further and either analysed as vapor or (ad)sorbed to an (ad)sorbent. The head-space techniques have been widely utilized in the analysis of volatiles, such as fragrances and aroma compounds, in various food and agricultural samples (81-84). The dynamic head-space (DHS), or purge-and-trap technique, is easily coupled on-line with G C . In an on-line system, desorption of trapped analytes for subsequent analysis is usually performed using on-line automated thermal desorption (ATD) devices. DHS involves the passing of carrier gas through a liquid or a solid sample, followed by trapping of the volatile analytes on a sorbent or a cold-trap and desorption onto a G C . The sampling may be conducted at ambient or slightly elevated temperatures, but below the degradation temperature region for the material. Because the process requires several minutes to complete, the purged analytes need to be focused by cryofocusing on a column or trapping onto a solid phase adsorbent bed placed in-line with the G C . The trapped compounds are then subsequently desorbed to a G C column. In modern DHS instrumentation the two-stage thermal desorption process consists of tube desorption onto a Peltiercooled trap, followed by trap desorption of the heated trap. It is essential that the thermal desorption is very rapid in order to get sharp initial bands to the G C . Dynamic thermal desorption (DTD), which closely resembles the D H S technique, is also commonly used in combination with G C in the analysis of (semi)volatile compounds in both solid and liquid matrices (85-88). In D T D ,



119 heat and a flow of inert gas are used to extract (semi)volatile organics retained in a sample matrix or on a sorbent bed. Similar adsorbent traps as in DHS are also used in D T D , and accordingly most samples require cryofocusing. It is also possible to perform the thermal desorption directly in the injector of a GC. In such a system, the solid sample is loaded into the cold injector, the carrier gas flow is temporarily halted, then the injector is rapidly heated to the desired temperature, the carrier gas is resumed and the thermally extracted components are swept onto the column. It should be noted that the D T D technique is appropriate only i f 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 thermal desorption tube is representative of the sample as a whole. DHS applications have been developed, for example, for the determination of aroma-active compounds in bamboo shoots (83), styrene in yoghurt (84) and volatile acids in tobacco, tea, and coffee (88), volatile compounds of strawberries (89) and odor-active compounds of hams (90). The applications of D T D - G C include, for example, in the determination of volatile components of Lavandula luisieri (85), in the analysis of volatile components of oak wood (87) and volatiles in various solid-food products such as spices and herbs (black pepper, oregano, basil, garlic), coffee, roasted peanuts, candy and mushrooms (82). A n example of a DHS application is the determination of aroma-active compounds in bambuu shoots. In this study, compounds such as p-cresol, methional, 2-heptanol, acetic acid, (£,Z)-2,6-nonadienal, linalool, phenyl acetaldehyde, were extracted from the bambuu shoot samples and analysed by G C . The required sample amount was 10 g, and the extraction temperature was 60°C, using a 30 min extraction time. The stripped analytes were first trapped into a cooled adsorbent tube ( V O C A R B 3000, at 0 °C), and then thermally desorbed to G C . In D T D , the sample amount required for the analysis is typically smaller than in solid head-space (SHS). In the determination volatile components such as camphor, 1,8-cineole and 2,3,5,5-tetramethyl-4-methylene2-cyclopenten-l-one, from Lavandula luisieri, only 10-20 mg of (dry) plant sample was required for the analysis. The volatiles were desorbed from the sample under a helium flow and then cryofocused on a Tenax T A trap at -30 °C. The trap was then quickly heated and the desorbed volatiles were transferred directly to the G C column through a heated fiised-silica line (85).

Conclusions The number of on-line methods using solvent/fluid extraction developed for the analysis of solid food and agricultural samples is still small. The solventless



120 extraction methods (DHS and DTD) are, on the other hand, relatively widely utilized in the analysis of volatile analytes. However, their applicability is limited to relatively volatile, thermally stable analytes. Most on-line methods utilizing solvent or fluid-based extraction methods provide quantitative recoveries for the analytes and samples investigated. It is impossible to state categorically, which method is best suited for a certain type of sample, as several factors should be taken into consideration in the comparison. O f the different extraction methods used for on-line combinations, SFE is the most selective method and the selectivity can be adjusted by changing the pressure and temperature of the fluid. It is best suited for relatively nonpolar or volatile analytes, and it can be easily coupled on-line with L C , G C and SFC. SFE generally produces clean extracts with so little residual organic solvent modifier that additional concentration may not be necessary before chromatographic analysis. On the other hand, SFE requires modifiers for efficient extraction of more polar analytes, which complicates the on-line coupling. In addition, since methods using SFE are heavily matrix-dependent, separate method development is required, which requires experienced operators. For polar and medium polar compounds, P H W E , D M A E and D S A E suit better than SFE. On-line coupling of these techniques is possible with both L C and G C . P H W E is easily combined with (RP)LC, while D M A E and S A E can be easily coupled with R P L C i f water is used in the extraction. With the use of organic solvents, coupling with G C is often more simple. The drawback of P H W E is the rather harsh conditions during extraction, which can lead to degradation of thermally labile analytes. D M A E and D S A E are less selective than SFE and P H W E and they suit to a large range of analytes. O f these techniques, D S A E seems to be faster and as the choice of solvent is less critical than in D M A E , it is more flexible in on-line coupling. On the other hand, D M A E offers a particularly intriguing feature for analysis of labile food flavor compounds, such as the extraction of plant or animal tissue in a microwavetransparent solvent. SFE-GC system is the most widely applied on-line technique for the analysis of solid samples and it is well suited also for routine based applications. Also SFE-LC, P H W E - L C and D S A E - G C systems are relatively easily accomplished in an instrumental point of view even though more work will be required before these techniques are suitable for routine analyses. More complex systems, such as SFE-LC-GC, can be applied for research in special, very demanding applications, and due to their complexity, this type of systems are not suitable for routine laboratories. The main benefit of the on-line coupling of an extraction to a chromatographic technique is that the whole analysis can be performed in a closed system. The main advantages are the improved sensitivity, minimal sample contamination and the possibility for a totally automated analytical



121 system. The main benefit of the on-line analysis of solid samples is that due to the high sensitivity, the amount of sample required is small, and the methods are consequently well suited for trace level determinations and to applications where the amount of sample available is small. Unfortunately, the great potential of these on-line techniques is not reflected in present-day practice. One explanation for this may be that new techniques are not readily accepted, especially as the potential economic advantages of a new technique have to be balanced against the considerable costs of validation and standardization. The limited commercial availability of instruments also plays a role. Moreover, on-line coupling of an extraction system to a chromatographic instrument requires generally some adaptation and optimization as well as special knowledge of the underlying principles. The time invested in optimization of the conditions is rapidly repaid in more efficient sample throughput, better reproducibility and improved sensitivity.

References 1. Hyötyläinen, T.; Riekkola, M . - L . Anal. Bioanal. Chem. 2004, 378, 19621981. 2. Shimmo M ; Adler, H . ; Hyötyläinen T.; Hartonen, K . ; Kulmala M .; Riekkola M . - L . Atmos. Environ. 2002, 36,2985-2995. 3. Batlle, R.; Carlsson, H . ; Holmgren, E , Colmsö, A . ; Crescenzi, C. J. Chromatogr. 2001, 963, 73-82 4. Wang Z.; Ashraf-Khorassani M .; Taylor L . T. Anal. Chem. 2003, 75, 39793985. 5. Johansen, H . R.; Becher, G.; Greibrokk, T. Anal. Chem. 1994, 66, 4068-4073. 6. Lou, X .; Janssen, H.-G.; Cramers, C. A . J. Chromatogr. A 1996, 750, 215226. 7. L i B,, Yang Y , GanY., Eaton C D , He P, Jones A D (2000) J Chromatogr A 873, 175-184 8. Modey, W.K.; Mulholland, D. A .; Mahomed, H . ; Raynor, M . W . J. Microcol. Sep. 1996, 8, 67-74 9. Johansen, H . R.; Becher, G.; Greibrokk, T. Anal. Chem. 1994, 66, 4068-4073. 10. Herrera M .C.; Prados-Rosales RC.; Luque-Garcia J. L.;, Luque de Castro M . D. Anal.Chim. Acta 2002, 463, 189-197 11. Nielsen, T. J.; Jaegerstad, M . Special Publication - Royal Society of Chemistry 1995, 162, 59-64. 12. Rissato, S.R.; Galhiane, M.S.; Knoll, F.R.N.; Apon, B . M . J Chromatogr A 2004, 1048, 153-159. 13. Taylor, S.L.; King, J. W.; Snyder, J.M. J. Microcol.Sep. 1994, 6, 467-74



122 14. Aro, T.; Brede, C.; Manninen, P.; Kallio, H . J. Agric. Food Chem. 2002, 50, 1970-1975. 15. Pól, J; Ranta-Aho, O.; Hyötyläinen, T.; Riekkola, M . - L . J. Chromatogr. A, 2004, 1052, 52-58. 16. Lüthje, K . , T. Hyötyläinen, T.; Rautiainen-Rämä, M .; Riekkola, M . - L . Analyst, 2005, 130, 52-58. 17. Yarita, T.; A . Nomura, A . ; Y . Horimoto, Y.; Gonda, S., J. Chromatogr. 1996, 750, 175-181. 18. Blanch, G.P.; Caja M . M . ; del Castillo M . L . R ; Santa-María G.; Herraiz, M. J. Chromatogr. Sci. 1999, 37, 407. 19. Smith, R . M . ; Burford, M . D . J. Chromatogr. 1992, 600, 175. 20. Modey, W. K . ; Mulholland, D . A . ; Mahomed, H . ; Raynor, M . W. J. Microcol. Sep. 1996, 8, 67-74. 21. Ericsson, M .; Colmsjö, A Anal. Chem 2003, 75, 1713-1717. 22. Dean, J. R.; Xiong, G . Trends Anal. Chem. 2000, 19, 553-564. 23. Pallaroni, L . ; von Hoist, C.; Sparr Eskilsson, C.; Björklund, E . Anal. Bioanal. Chem. 2902, 374, 161-166. 24. Cairo, N .; Garcia, I.; Ignacio, M .-C.; Llombart, M .; Yebra, M . C.; Mouteira, A . Anal. Bioanal. Chem. 2002, 374, 547-553. 25. Luque-Gracia, J.L.; Luque de Castro, M . D . Trends in Anal. Cxhem. 2003, 22, 90-98. 26. Diagne, R. G.; Foster, G . D.; Khan, S. U. J. Agrig. Food Chem. 2002, 50, 3522-3528. 27. Sanchez, C.; Ericsson, M .; Carlsson, H . ; Colmsjö, A . ; Dyremark E . J. Chromatogr. A 2002, 957, 227-234. 28. Sanchez, C .; Ericsson, M. ; Carlsson, H .; Colmsjö, A . J. Chromatogr. A 2003, 995, 103-110. 29. L i, B.; Yang, Y .; Gan,Y.; Eaton, C . D.; He, P.; Jones, A . D . J. Chromatogr. A 2000, 873, 175-184. 30. Yang, Y . ; L i , B . Anal. Chem. 1999, 71, 1491-1495. 31. Herrera, M . C , ; Prados-Rosales, R.C.; Luque-Garcfa, J.L.; Luque de Castro, M . D. Anal.Chim. Acta, 2002, 463, 189-197. 32. Smith, R. M. J. Chromatogr. A 2002, 975, 31-46. 33. Crescenzi, C.; D i Gorcia, A . ; Nazzarri, M .; Samperi, R.Anal. Chem., 2000, 72, 3050-3055. 34. Richter, P.; Sepúlvedaa, B . ; Olivaa, R.; Calderóna K . ; Segue, R. J. Chromatogr. A, 2003, 994, 169-177. 35. Hyötyläinen T.; Andersson, T.; Hartonen, K . ; Kuosmanen, K . ; Riekkola, M . L . Anal. Chem. 2000, 72, 3070-3076. 36. Kuosmanen K , Hyötyläinen T.; Hartonen, K . ; J-ÅJönsson; Riekkola, M . L . Anal. Bioanal. Chem. 2003, 375, 389-399.


123 37. 38.


39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60.

Kuosmanen, K . ; Hyötyläinen, T.; Hartonen, K ; Riekkola, M . L . J Chromatogr. A 2002, 943, 113-122. Li B, Yang Y , GanY., Eaton C D , He P, Jones A D (2000) J Chromatogr. A 2000, 873, 175-184. Yang, Y.; Li, B . Anal. Chem. 1999, 71, 1491-1495. Lüthje, K . ; Hyötyläinen, T.; Hartonen, K . ; Riekkola, M . - L . J. Chromatogr. A. 2004, 1025, 41-49. Pihlström, T.; Isaac, G.; Waldebäck, M.; Österdahl, B . G.; Markides, K. E . Analyst, 2002, 127, 554-559. Sato, K . ; Irok, Y.; Sagara, H . ; Itokawa, H . J. Chromatogr. 1997, 346, 81-86. Fuoco, R.; Ceccarini, A . ; Onor, M.; Lottici, S. Anal Chim Acta , 1997, 346, 81-86 Shimmo, M.; Saarnio, K . ; Aalto, P.; Hartonen, K . ; Hyötyläinen, T.; Kulmala, M . ; Riekkola, M . - L . J. Atmos. Chem. 2004, 47, 223-241. Hawthorne, S. B . ; Miller, D. J.; Krieger, M. S. J. High Resol. Chromatogr. 1989, 12, 714-720. Hawthorne, S. B . ; Miller, D . J.; Langenfeld, J. J. J. Chromatogr. Sci. 1990, 28, 2-8. Hawthorne, S. B . ; Miller, D. J.; Krieger, M. S. J. Chromatogr. Sci. 1989, 27, 347-354. Hawthorne, S. B . ; Miller, D. J.; Krieger, M. S. Fresenius Zeitsch Anal. Chem. 1988, 330, 211-215. Hawthorne, S. B.; Krieger M. S.; Miller, D . J. Anal. Chem. 1988, 60, 472-7 Hawthorne, S. B.; Miller, D . J. J. Chromatogr. A 1987, 403, 63-76 Burford, M . D . ; Hawthorne, S. B . ; Miller, D . J. J. Chromatogr. A 1994, 685, 95-111. Burford, M . D . ; Hawthorne, S. B . ; Miller, D. J. J. Chromatogr. A 1994, 685, 79-94. Slack, G.C.; McNair, H . M . ; Hawthorne, S. B . ; Miller, D.J. J. High Resol Chromatogr. 1993, 16, 473-478. Hawthorne, S. B . ; Miller, D.J.; Krieger, M. S. J. High Resol Chromatogr. 1989, 12, 714-720. Ashraf-Khorassani, M.; Taylor, L . T. J. Chromatogr. Sci. 2000, 38, 477-482 Blanc, G . P.; Reglero, G.; Heraiz, M, J. Agric. Food Chem. 1995, 43, 12511258. Modey, W . K . ; Mulholland, D . A . ; Mahomed, H . ; Raynor, M. W. J. Microcol. Sep. 1996, 8, 61-14. Gmuer, W.; Bosset, J. W.; Plattner, E. J Chromatogr A 1987, 388, 335-349. van Bavel, B . ; Jaeremo, M.; Karlsson, L.; Lindström, G . Anal. Chem. 1996, 68, 1279-83. Johansen, H . R.; Becher, G.; Greibrokk, T. Anal. Chem. 1994, 66, 40684073.



124 61. Sato, K . ; Sasaki, S. S..; Goda, Y.; Yamada, T.; Nunomura, O.; Ishikawa, K . ; Maitani, T. J. Agric. Food Chem 1999, 47, 4665-4668 62. Unger, K . K . ; Roumeliotis, P. J. Chromatogr. A 1983, 282, 519-526 63. Nair, J. B.; Huber, J. W. LC-GC 1988, 6, 1071-1073 64. Stone, M . A . ; Taylor, L . J. Chromatogr. A 2001, 931, 53-65 65. Ashraf-Khorassani, M.; Barzegar, M . ; Yamin, Y . J. High Resolut. Chromatogr. 1995, 18, 472-476 66. Salleh, S. H . ; Saito, Y.; Kiso, Y.; Jinno K . Anal. Chim. Acta 2001, 433, 207215. 67. Johansen, HR.; Becher, G.; Greibrokk, T. Anal. Chem. 1994, 66, 4068-4073. 68. L i u M . H . ; Kapila S.; Nam K . S. J. Chromatogr. A 1993, 639, 151-157. 69. Cortes, H.J.; Green, L . S.; Campbell, R. M. Anal. Chem 1991,. 63, 27192724. 70. Mougin, C.; Dubroca, J.; Barriuso, E. J. High Resolut. Chromatogr. 1996, 19, 700-702. 71. Batlle, R.; Carlsson, H . ; Holmgren, E . ; Colmsjö, A . ; Crescenzi, C. J. Chromatogr. A 2001, 963, 73-82. 72. Ramsey, ED.; Minty B, Rees, A . T . Anal. Commun. 1997, 34, 261-264. 73. L i u M H.; Kapila S.; Nam S.; Elseewi A . A . J. Chromatogr. 1993, 639, 151. 74. Ashraf-Khorassani M .; Nazem N .; Taylor, L.T. J. Chromatogr. A 2003, 995, 227-232. 75. Wan, Z.; Ashraf-Khorassani, M.; Taylor, L. T. Anal. Chem. 2003, 75,39793985. 76. Isci F.; Haerdi W . Chromatographia 1995, 41, 238-242. 77. Hirata Y, Okamoto Y J. Microcol. Sep. 1989, 1, 46. 78. Wenclawiak, B W . ; Hees, T.; Zöller, C.E.; Kabus H . P. Fres. J. Anal. Chem. 1997, 35, 471-474. 79. Shimmo, M.; Anttila, P.; Hartonen, K . ; Hyötyläinen, T.; Paatero, J.; Kulmala, M.; Riekkola, M . - L . J. Chromatogr. A 2004, 1022, 151-159. 80. Pol, J.; Wenclawiak, B . W .Anal.Chem. 2003, 75, 1430-1435. 81. Butrym, E.,L C - G C17 9S(1999), S19. 82. Wilkes J. G ; Conte E . D ; K i m Y.; Holcomb M.; Sutherland J. B.; Miller, D . W. J. Chromatogr. A 2000, 880, 3-33. 83. Fu, S.-G.; Yoon, Y.; Bazemore, R.; J. Agric. Food Chem. 2002, 50,549-554. 84. Nerin, C.; Rubio, C.; Cacho, J.; Salafranca, J. Food Add Contam 1998, 15, 346-354. 85. Sanz, J.; Soria, A . C.; Garcia-Vallejo, M. C. J. Chromatogr. A 2004, 1024, 139-146. 86. H . G . Wahl, A . Hoffmann, H . - U . Häring and H . M . Liebich. J. Chromatogr. A 1999, 847, 1-7.


125


87. Perez-Coello, M. S.; Sanz, J.; Cabezudo, M. D . J. Chromatogr. A 1997, 778, 427-434. 88. Clark, T. J.; Bunch, J. E . J Chromatogr Sci 1997, 35, 206-208. 89. Hakala, M. A . ; Lapveteläinen, A . T.; Kallio, H . P.; J. Agric. Food Chem. 2002, 50, 1133-1142. 90. Ventanas A . I.; Garcia J.C.; J. Agric. Food Chem. 2002, 50, 1996-2000.


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