Camel_2001

8/13/2019 Camel_2001

http://slidepdf.com/reader/full/camel2001 1/12

8/13/2019 Camel_2001


changes over the lifetime of the cylinder), the use of anadditional pump is preferred despite its higher initial capitalinvestment.

The SFE may be carried out in either static or dynamic mode.The pressure in the system is maintained by means of a restrictor(either fixed or variable, the latter making the pressureindependent of the flow rate).9 At the end of the restrictor, the

fluid is depressurized and the extracted analytes are trapped inan organic solvent or on a solid phase filled cartridge (fromwhich the analytes are later eluted with a small volume of organic solvent). The trapping conditions (nature of the solventor the solid trap, and the temperature) are of prime importanceto ensure efficient collection of the extracted compounds, andtherefore efficient recoveries at the end. The solid trapcollection device offers both a higher selectivity (by the choiceof both the nature of the solid phase and the elution solvent) and

a higher sensitivity (a highly concentrated extract is obtained aselution is performed with very small organic solvent volumes,typically 2–5 mL).

Due to the numerous parameters affecting the extractionefficiencies, SFE affords a high degree of selectivity. However,on the other hand, this makes the optimization quite tedious anddifficult in practice. Table 1 summarizes the main factors thatinfluence the results of extraction. The parameters to considerare linked to the extraction parameters inside the cell, to thenature of the solutes or to the nature of the matrix.10–12 Theimportant parameters in SFE are both the pressure andtemperature inside the cell. A pressure increase leads to a higherfluid density, thus increasing the solubility. The inverse isobserved with the temperature; however, increasing the tem-

perature may enhance the solubility of volatile analytes. Inaddition, higher temperatures may be required to overcome

Fig. 1 Comparison of the operating procedures for Soxhlet and sonicationrecommended by the EPA methods for extracting PCBs from soils(according to EPA methods 3540C2 and 3550B3).

Table 1 Parameters influencing the recent techniques and optimization strategy

Technique Parameter Main effect Optimization strategy

SFE Nature of the solutes Apolar Solubilization CO2

Moderately polar to polar Solubilization CO2 + organic solvent; other fluidsIonic Solubilization CO2 + reagent: ion pairing, complexation,

derivatization Extraction parameters Pressure Solubilization Increase pressure

Temperature Desorption; diffusion Increase temperatureTime Extent of extraction Increase extraction time

Nature of the matrix Large particles Low extraction rate; low recoveries Grinding the matrixActive sites Solute adsorption Addition of a modifierWater content Water entrainment; low recoveries Drying/drying agent addition

PFE Nature of the solutes Apolar to polar Solubilization Solvent or solvent mixture Extraction parameters Temperature Solubility; desorption; diffusion Increase temperature

Time Extent of extraction Increase extraction time Nature of the matrix Large particles Low extraction rate; low recoveries Grinding the matrix

Water content Low recoveries Drying/drying agent additionMAE Nature of the solutes Apolar to polar Solubilization Solvent or solvent mixture that usually

absorbs microwaves Extraction parameters Temperature Solubility; desorption; diffusion Increase temperature

Time Extent of extraction Increase extraction time Nature of the matrix Large particles Low extraction rate; low recoveries Grinding the matrix

Water content Overheating; low reproducibility Drying/drying agent addition

Fig. 2 Principle of an SFE system and influencing parameters.

Analyst , 2001, 126, 1182–1193 1183

8/13/2019 Camel_2001


solute–matrix interactions, as observed for the extraction of polycyclic aromatic hydrocarbons (PAHs) and polychlorinateddibenzo- p-dioxins from environmental matrices.13

With regard to the nature of the compounds to be extracted,polarity is the characteristic to be taken into account. Pure CO2

efficiently extracts non-polar to low polarity compounds. Forpolar solutes, a modifier is added to enhance the extraction. Forvery polar and ionic compounds, the modifier may be acomplexing agent, an ion-pair reagent or a derivatization

reagent.12,14,15 As an example, the addition of tetrabutyl-ammonium enabled the extraction of anionic surfactants fromsewage sludge to be performed.16

The addition of the modifier directly to the matrix (prior tothe extraction) may help in disrupting the analyte–matrixinteractions; however, it requires that a static extraction beperformed first, to avoid sweeping the modifier out of the cell.In cases where the analytes do not readily derivatize, theaddition of a derivatization reagent may still be useful as it canreact with the active sites of the matrix, thus enhancing theextraction, as has already been observed during the extraction of PAHs from urban dust.17

The users of SFE must be aware of the fact that the additionof a modifier to CO2 presents severe drawbacks, and so it should

be avoided or minimized whenever possible. The presence of the modifier changes the values of the critical pressure andtemperature, so that too high a modifier content may result in atemperature lower than the critical value, resulting in asubcritical state, with higher viscosity and lower diffusioncoefficients than the supercritical state; in this case, thetechnique is commonly called enhanced-fluidity liquid extrac-tion (EFLE).18 In addition, as the modifier enhances thesolvating power of the fluid, it reduces the extraction selectivityas more matrix materials or non-target analytes are co-extracted. Finally, the modifier condenses upon depressuriza-tion, which may result in elution of the retained compoundswhen a solid trap is used as the collection device, since then itmay act somewhat like a chromatographic device.

The nature of the matrix (water content, percentage of organic carbon, humic/fulvic materials, etc.) and its physicalcharacteristics (such as porosity or particle size) are of primeimportance for the success of an extraction,9 as with otherextraction techniques. Grinding the matrix is recommended, tolimit the diffusion step inside the matrix and to increase thesurface area, which increases the rate of extraction when it islimited by matrix effects. Also, the addition of a drying agent(such as sodium sulfate) may prevent the plugging of therestrictor by ice in the presence of humid matrices. Caution mustalso be taken when filling the vessel to ensure a homogeneousbed of material (to prevent channelling) and to take into accountpossible swelling of the matrix (such as polymers) uponintroduction of supercritical CO2.19 In addition, very fine

particles may be swept out of the cell by the fluid and result inplugging and mechanical transfer problems. Finally, a sorbentmay be added in the cell to retain matrix material and increasethe selectivity of the extraction. For instance, recent results haveshown that the addition of silica gel and Florisil favours the SFEof organochlorinated pesticides and chloramphenicol, re-spectively, from whole eggs.20,21

Since the early use of supercritical fluids for extraction in themid-1980s, numerous applications of the technique have beenreported.7,22–24 A large field has been covered: environmentalmatrices, plants, foods and fats, and polymers. Similarly,several classes of compounds have been investigated: PAHs andpolychlorobiphenyls (PCBs) (probably the most studied com-pounds), phenols, pesticides, organometallic compounds, lip-

ids, flavour and volatile compounds, natural products, additives,etc. In addition, SFE has been adopted by the US EnvironmentalProtection Agency (EPA) as a reference method for extractingPAHs,25 PCBs26 and total recoverable petroleum hydro-carbons27 from solid environmental matrices.

2.2 Pressurized fluid extraction

A new technique appeared around 5 years ago. It is calledpressurized fluid extraction (PFE), accelerated solvent extrac-tion (ASE™, which is a Dionex trade mark), pressurized liquidextraction (PLE), pressurized solvent extraction (PSE) orenhanced solvent extraction (ESE) and it partly derives fromSFE. However, in the former case, the extractant is maintainedin its liquid state.28–30 In order to achieve elevated temperatures,

pressure is applied inside the extraction cell. In this way,temperatures around 100–200 °C may be attained with classicalorganic solvents. In fact, at such high temperatures andpressures, the solvent may be considered as being in asubcritical state, with advantageous mass transfer properties.Indeed, this technique affords the ability to perform fast,efficient extractions due to the use of elevated temperature, asthe decrease in solvent viscosity helps to disrupt the solute–matrix interactions and increases the diffusion coefficients. Inaddition, the high temperature favours the solubilization of thecompounds by the solvent due to a change in their distributioncoefficients. Finally, the pressure favours the penetration of thesolvent into the matrix, which again favours the extraction.Consequently, this very recent technique is of growing interest,

and numerous commercial systems have been sold.The scheme of a typical PFE unit is given in Fig. 3. As for

SFE, a high pressure system is used (up to 100–150 bar).Performing an extraction requires several successive steps.Firstly, the cell is filled with the liquid solvent before attainingthe desired temperature and pressure; then a static extraction isperformed (5–10 min is most often sufficient) and the pressureis further released and the extract collected in glass vials. Toensure that all the extract reaches the collection vials, the cell isrinsed with fresh solvent; finally, to avoid any losses or memoryeffects, the cell is purged with an inert gas. The typical volumecollected depends on the cell size. Volumes between 10 and 100mL may be required, and hence evaporation steps are needed toconcentrate the final extract.

In such a system, the pressure is of minor importance, itsmain effect being to maintain the solvent in its liquid state. Thisreduces the number of parameters that need to be optimized toachieve efficient extractions (see Table 1), thereby reducing thetime devoted to the development of extraction procedures. Asan example, temperatures around 100–150 °C were found toincrease the extraction recoveries of herbicides from soilsamples, while little effect of pressure was observed for drysoils.31 However, in the case of moisturized soils, increasing thepressure from 500 to 1500 psi was beneficial, probably due tobetter dissolution of the pesticides. In addition, for wet soils,non-polar solvents were found to give incomplete recoveries of hydrocarbons, and a mixture of dichloromethane–acetone (1+1,v/v) at 175 °C was found to be optimal.32,33

Fig. 3 Principle of a PFE system and influencing parameters.

1184 Analyst , 2001, 126, 1182–1193

8/13/2019 Camel_2001


PFE has been successfully applied to the same types of matrices and compounds as SFE.30 The development of PFEprocedures is very attractive, as the same solvent as recom-mended in the official Soxhlet or sonication methods may betested, and the procedure is expected to give efficient extrac-tions. This probably explains the great interest in this techniquetoday. In particular, PFE has been recognized as an officialmethod by the EPA,34 and the method has enabled the efficientscreening of soils to be performed for selected semivolatile

organic priority pollutants.35

However, reality is not that simple. In particular, withpolymeric matrices, it has been demonstrated that a differentsolvent should be used, as that used with classical Soxhlet is aswelling agent for the matrix, leading to partial solubilization of the polymer under high pressure, with subsequent pluggingwhen the pressure drops.36

In addition, as PFE uses organic solvents as the extractants, itoffers only limited selectivity. As a consequence, it usuallyrequires a clean-up of the extract obtained. For instance, furthertreatment with sulfuric acid and Florisil was necessary for thedetermination of PAHs in smoked food samples (fish and meattissues) to remove co-extracted lipids;37 similarly, the determi-nation of PCBs in fish extracts by PFE required a treatment with

sulfuric acid to destroy lipids before gas chromatographicanalysis.32

2.3 Microwave-assisted extraction

Microwave energy is a non-ionizing radiation (frequency,300–300 000 MHz) that causes molecular motion by migrationof ions and rotation of dipoles. Dipole rotation refers to thealignment, due to the electric field, of molecules that have eitherpermanent or induced dipole moments in both the solvent andsamples. As the field intensity decreases, thermal disorder isrestored which results in thermal energy being released. At 2450MHz (the frequency used in commercial systems), the align-ment of the molecules followed by return to disorder occurs 4.9

3 109 times per second, resulting in rapid heating. However, theabsorption of microwave energy and its release as heat arestrongly dependent on the relative permittivity and the dipolarmoment of the medium. The greater the relative permittivity,the more thermal energy released, and the more rapid theheating for a given frequency.

Even though microwaves have been used for several years inanalytical laboratories to mineralize samples,38,39 their use toenhance extraction is very recent. Preliminary studies per-formed in the late 1980s using domestic ovens showed the greatpotential of microwaves for extraction.40,41 However, theirextended use in laboratories began around 5 years ago, with thecommercialization of several instruments dedicated to extrac-tions.

Microwave-assisted extraction (MAE) uses microwave ra-diation as the source of heating of the solvent–samplemixture.42–44 Due to the particular effects of microwaves onmatter (namely dipole rotation and ionic conductance), heatingwith microwaves is instantaneous and occurs in the heart of thesample, leading to very fast extractions. Most of the time, theextraction solvent is chosen to absorb microwaves. Alter-natively (for thermolabile compounds), the microwaves may beabsorbed only by the matrix, resulting in heating of the sampleand release of the solutes into the cold solvent.

The results obtained so far have concluded that microwaveradiation causes no degradation of the extracted compounds,unless too high a temperature arises in the vessel.45,46

At the same time, a specific effect of microwaves on plant

material has been found.42 They interact selectively with thefree water molecules present in the gland and vascular systems,leading to rapid heating and temperature increase, followed byrupture of the walls and release of the essential oils into thesolvent. Similar mechanisms are suspected in soils and

sediments, where strong, localized heating should lead to anincrease in pressure and subsequent destruction of the matrixmacrostructure.47,48

The application of microwave energy to the samples may beperformed using two technologies: either closed vessels (undercontrolled pressure and temperature), or open vessels (atatmospheric pressure).49,50 These two technologies are com-monly named pressurized MAE (PMAE) or focused MAE(FMAE), respectively. Both systems are shown in Fig. 4.

Whereas in open vessels the temperature is limited by theboiling point of the solvent at atmospheric pressure, in closedvessels the temperature may be elevated by simply applying thecorrect pressure.51 The latter system seems most suitable in thecase of volatile compounds. However, with closed vessels, afterextraction one needs to wait for the temperature to decreasebefore opening the vessel, increasing the overall extraction time(by around 20 min). With regard to the extraction efficiencies,both systems were shown to have similar performances for theextraction of PAHs from soils.52,53

The closed vessel technology is quite similar to the PFEtechnology, as the solvent is heated and pressurized in bothsystems. The main difference is in the means of heating, eithermicrowave energy or conventional oven heating. Consequently,

as for PFE, the number of influential parameters is reduced, thusmaking the application of this technique quite simple in practice(see Table 1).49,50

The nature of the solvent is obviously of prime importance inMAE. As with other techniques, the solvent (or solvent mixture)should efficiently solubilize the analytes of interest withoutsignificantly extracting matrix material (i.e. the extractionshould be as selective as possible to avoid further clean-up). Inaddition, it should be able to displace the solute moleculesadsorbed onto matrix active sites in order to ensure efficientextractions. Finally, the microwave-absorbing properties of thesolvent are of great importance as sufficient heating is required(to allow efficient desorption and solubilization, thus efficientextraction). Most of the time, the chosen solvent should absorb

the microwaves without leading to strong heating so as to avoiddegradation of the compounds. Thus, it is common practice touse a binary mixture (e.g. hexane–acetone, 1+1), with only onesolvent absorbing microwaves.54–56 However, in some cases,polar solvents (such as water or alcohols) may give efficientextractions.57–59 Alternatively, apolar solvents may be used if

Fig. 4 Principle of MAE systems.

Analyst , 2001, 126, 1182–1193 1185

8/13/2019 Camel_2001


the matrix absorbs microwaves or if an additional microwave-absorbing material (such as Weflon®) is added.60,61

Other important parameters are the applied power, thetemperature and the extraction time (the latter being dependenton the number of simultaneous extractions performed56).Sufficient heating is usually required to enable efficientextractions to be performed, as shown for triazines andphenols.62,63 However, too high a temperature may lead tosolute degradation as has been observed previously.46,52,64

As with other techniques, the nature of the matrix is also animportant factor for the success of the extraction. In particular,the water content needs to be carefully controlled to avoidexcessive heating and to allow reproducible results. As anexample, 30% water was found to be the optimum for extractingPAHs from soils and sediments.65 Therefore, drying the matrixbefore the extraction or adding a drying agent, with subsequentaddition of the required water content, may be advisable. Also,the strength of the analyte–matrix interactions may inducematrix effects and require a change in the extraction conditionsfrom one matrix to another, as has been observed for pollutantsfrom soil.66

MAE, especially using closed vessels, has been successfullyused for several applications, most of them environ-

mental.43,49,52 An official EPA method for PMAE is currentlyunder standardization.67

3 Comparative performances of SFE, PFE andMAE

Very few studies have compared the recent techniques with theclassical Soxhlet or sonication extraction methods for particularapplications. Based on the reported results and on thespecifications of each technique, the comparative performancesof the three recent techniques are discussed with regard toextraction efficiency and susceptibility to matrix effects,selectivity, level of automation and simplicity of the operating

procedure.

3.1 Efficiency and susceptibility to matrix effects

In terms of extraction efficiencies, once correctly optimized fora given solute–matrix couple, all techniques are comparable, asrecently discussed for PAHs, PCBs and pesticides from soilsand sediments.68

For the same class of compounds, different results may beobtained when several matrices have to be extracted. Inparticular, SFE has been claimed to be highly matrix dependent,and the extraction conditions should most of the time beoptimized for each new matrix. A possible explanation for thisapparent strong matrix dependence may lie in the use of SFE

conditions that are as selective as possible (in order to limit theco-extraction of matrix material). This implies the minimizationof modifier content as well as a mild extraction temperature. Asa consequence, when matrices with stronger adsorption of theanalytes have to be extracted, the SFE conditions have to be re-optimized, with increased modifier content and/or temperature.Therefore, as already noted by Smith,8 established SFE methodsmust have an excess of extraction capability (by using a strongerextractant or a longer time than required for an ideal sample) toensure robustness.

The other extraction techniques (Soxhlet, sonication, PFEand MAE) do not seem to be as matrix dependent as SFE as theyinitially use less selective conditions (due to the use of organicsolvents), thereby overcoming part of the matrix effects. Yet,

the nature of the matrix may still strongly influence theextraction efficiency. For example, the extraction of a fungicide(hexaconazole) from two different aged (52 weeks) soilsshowed that PMAE under the same conditions leads to lowerrecoveries for the soil with the highest carbon content due to

stronger solute–matrix interactions.69 Therefore, in this case,more extreme extraction conditions should be used to overcomesuch interactions and achieve efficient extractions with thissoil.

As already discussed, matrix effects due to strong adsorptionof the solutes onto the matrix are particularly crucial forenvironmental matrices. As an example, the extraction of PAHsfrom fly ash samples was highly dependent on the matrix,whatever the technique (Soxhlet, SFE or PFE). As shown in Fig.

5, satisfactory recoveries were observed from a lignite coal flyash (with a low carbon content), while under the same extractionconditions insufficient recoveries were obtained from a bitumi-nous coal fly ash (with a high carbon content and a much largersurface area) due to the stronger solute–matrix interactions thatrequire more stringent extraction conditions, as well as thepossible decomposition of the PAHs onto fly ash.70,71 The ashsamples had been initially spiked with a mixture containing thePAHs, and left to equilibrate during 2 weeks. It was suspectedthat, during this time, losses of the volatile PAHs (such asnaphthalene) occurred due to volatilization.

In addition to the organic content, the particle size of thematrix has been shown to influence the extraction efficiency (asobserved during the PFE of PCBs from a harbour sediment72),

and grinding of the sample before the extraction is highlyrecommended.

Finally, the water content of the matrix may be crucial for thesuccess of the extraction, especially with SFE and MAE. Withthe former, a high content may prevent the extraction of non-polar compounds and also result in plugging of the restrictor.10

With the latter, water will cause strong heating of the matrix,with possible degradation of the compounds. Therefore, the

Fig. 5 Comparison of the performances for extracting PAHs from fly ashsamples:70,71 (a) lignite coal fly ash (surface area, 0.453 m2 g21; carboncontent, 0.02 wt.%); (b) bituminous coal fly ash (surface area, 2.859 m2 g21;carbon content, 15.42 wt.%). N, Naphthalene; F, fluorene; Ph, phenan-threne; Py, pyrene; BP, benzo(a)pyrene; IP, indeno(1,2,3-cd )pyrene.

1186 Analyst , 2001, 126, 1182–1193

8/13/2019 Camel_2001


water content of the matrix must be carefully controlled. Insome cases, it may also be advisable to add a drying agent to thematrix. PFE affords the advantage of being much less prone towater influence. However, a recent study showed that betterrecoveries of chloroacetanilide and heterocyclic herbicideswere obtained from moisturized soils as compared to dry soilsusing PFE with acetone.31

In order to take into account the matrix effects, a generalstrategy for optimizing the extraction conditions, whatever the

technique, is given in Fig. 6. First, extracting spiked samples isrecommended to determine the minimum conditions to ensureefficient extraction, as well as to check that the extracted solutesare quantitatively collected (as losses may occur in SFE upondepressurization, or in MAE during the filtration step). Then,certified matrices should be extracted as they may require moredrastic conditions (especially higher temperatures) to overcomesolute–matrix interactions. Finally, the optimized conditionsmay be used to extract real samples.

3.2 Selectivity

SFE appears undoubtedly to be the most potentially selectiveextraction technique.9 This is particularly true when using pureCO2 as the extractant. The more modifier added to the CO2, thelower the selectivity of the extraction. To illustrate the higherselectivity of SFE as compared to PFE (as well as MAE andtraditional techniques as they also use liquid organic solventssimilar to PFE), the values of the limits of detection for selectedpesticides in tomatoes are presented in Table 2.73 The highervalues obtained by PFE are due to the greater matrixinterference that hinders pesticide determination by gas chro-matography. In addition, despite further clean-up after PFE, thefinal extract may not be as clean as that obtained after SFE asobserved for PCBs from several environmental matrices.72

Another recent study reported that pure CO2 removed only 8%of a bulk organic PAH-contaminated soil, while the use of solvents in Soxhlet and PFE led to 20–30% matrix material

being extracted.74

In addition, SFE offers the potential of performing selectiveextractions by correctly adjusting the solvent power by varyingthe pressure, temperature and modifier content, as recentlydemonstrated for the fractionation of the phenolic content of grape seeds.75 However, a compromise must be found betweenselectivity and susceptibility to matrix effects (i.e. robustness)of SFE methods as already discussed.

As MAE and PFE use organic solvents, they offer a lowerselectivity than SFE. To minimize the extraction of matrix

material, the nature of the solvent should be correctly chosenand moderate temperatures are recommended. However, forstrongly adsorbed compounds, elevated temperatures are re-quired to achieve efficient extraction of the analytes.

3.3 Level of automation

With the present commercial systems, SFE and PFE systems aremore automated than MAE, as the latter requires manualoperations (such as loading the solvent into the cell, andespecially filtration or centrifugation to separate the extractfrom the matrix). On the other hand, PMAE systems offer theopportunity to perform as many as 14 (or 50 for a given system)extractions simultaneously, while SFE and PFE systems

commercially available today allow only one or two extractionsat a time. However, to avoid overpressure in the cells, thesamples to be simultaneously extracted should have similarcharacteristics (to ensure that the temperature and pressurerecorded in one cell are similar to those in the others), such asthe matrix nature (i.e. carbon or water content) and theextraction solvent. In addition, in the case of FMAE, operatingunder atmospheric pressure offers the possibility to add reagents(such as sulfuric acid) to the medium after extraction in order toperform microwave-assisted purification in the same vessel.76

3.4 Simplicity of the operating procedure

To illustrate the different steps that are involved when

performing SFE, PFE or MAE, the extraction of PCBs from

Fig. 6 General strategy for optimizing extraction conditions for SFE, PFE and MAE.

Analyst , 2001, 126, 1182–1193 1187

8/13/2019 Camel_2001


soils has been chosen, as EPA methods have been reported forall three techniques for this application. All the steps, from thepretreatment of the matrix to the generation of the final extract,are indicated in Fig. 7. From this practical application of therecent techniques, several comments can be made: (i) pretreat-ment of the matrix (especially grinding and removal of water) iscrucial for the success of extractions, whatever technique isconsidered; (ii) the sample mass that can be extracted variesfrom a few grams in the case of SFE up to 20 or 30 g for MAE

and PFE, respectively; (iii) MAE is the sole technique requiringthe manual addition of the solvent in the cell prior to the

beginning of the extraction; (iv) whereas SFE uses supercriticalcarbon dioxide as an extractant, PFE and MAE use a mixture of hexane–acetone as solvent, which is the same solvent mixture asrecommended in the EPA methods of classical extractiontechniques (namely Soxhlet and sonication) as shown in Fig. 1;(v) MAE requires a filtration step before collecting the finalextract, which may lead to losses or contamination; (vi) withregard to the final volume of the extract, SFE offers the lowestvalues as already noted;77 however, if the ratio of the extract

volume (mL) to the sample mass that has been extracted (g) isconsidered, which can be regarded as a dilution factor, then weobtain the approximate ranges: 0.64–3.2 for SFE, 1.5–5 for PFEand 2–40 for PMAE; thus, the extent of dilution of the extractobtained by PFE and MAE depends on the sample size, andunder some conditions it may be close to that obtained by SFE;(vii) in PFE and MAE, the extracted PCBs are collected in theextraction solvent, which is hexane–acetone, while in SFE theyare eluted from the Florisil trap by n-heptane; the nature of thesolvent is important for further steps, such as concentration orgas chromatographic analysis; (viii) the time devoted to thewhole SFE procedure is nearly 1 h, against around 30–45 minfor PFE and MAE.

So, from this particular application, it can be shown that PFE

and MAE offer the advantages of using liquid organic solvents(mostly the same as Soxhlet and sonication) and of a reducedtime devoted to the overall procedure. In contrast, SFE uses aspecific extractant, and the procedure is somewhat longer. Atthe same time, it must be pointed out that, once the extract hasbeen collected after PFE or MAE, it frequently containsinterferents that have been co-extracted, so that further clean-upis required, which increases the overall sample preparation time

Table 2 Limits of detection (LODs/ng g21) of pesticides from tomatoes73

obtained for SFEa and PFEb

Pesticide SFE PFE Pesticide SFE PFE

Trifluralin 0.3 0.5 Carbofuran 3 4Pendimethalin 0.4 0.7 Phosalone 5 8Diazinon 1 1 Chlorpyrifos 1 9Ethion 0.6 2 Atrazine 2 13Malathion 4 2 Iprodione 6 28Parathion 1 3 Carbaryl 3 35Methoxychlor 2 3 Lindane 32 82Hexachlorobenzene 1 3 Azinphos-methyl 29 130Parathion-methyl 1 3

a SFE conditions: 2 g tomato + 2 g fibrous cellulose powder; CO2, 50 °C,350 bar; 2 min equilibration time; 20.3 min dynamic time (2 mL min21);C18 silica trapping, elution with 1.2 mL acetone. b PFE conditions: 2 gtomato + 2 g fibrous cellulose powder; acetonitrile, 60 °C, 2000 psi; 5 minpreheating; 2 min static + 100% flush volume + 1 min N2 purge; addition of NaCl to the collected extract; collection of the supernatant; addition of Na2SO4; concentration.

Fig. 7 Comparison of the operating procedures for SFE, PFE and MAE recommended by the EPA methods for extracting PCBs from soils (according toEPA methods 3562,26 3545A34 and 354667).

1188 Analyst , 2001, 126, 1182–1193

8/13/2019 Camel_2001


before analysis. When using SFE, especially with CO2 as thesole extractant, the collected extract is often ready for analysis.As a consequence, in terms of the time required for samplepreparation, it can be considered that, whatever recent techniqueis used, approximately 1 h is sufficient to obtain a final extractthat can be directly analysed.

To better illustrate the extraction performance of eachtechnique in terms of extraction conditions, time and furthertreatment required, comparisons are given for several applica-

tions in Tables 3–7. From these results, it can be observed thatextraction efficiencies are on the whole similar, and thatreduced time and solvent consumption are achieved with therecent techniques. In addition, PFE and MAE offer theadvantage of using solvents similar to those used with thetraditional methods, with the major drawback of usuallyrequiring further filtration, clean-up and concentration steps.

4 The right technique for the right application

Choosing the right technique for the right application requires aconsideration of the features of the matrix and of the analytes.

SFE is very suitable for extracting thermolabile compounds

as it allows the possibility to perform fast extractions atmoderate temperatures (around 40 °C). In addition, when pure

CO2 is used as the extractant, this technique remains the mostselective for most applications, except for the extraction of lowpolarity compounds from samples that contain a high lipidcontent. In this case, further clean-up will be required due to thesimultaneous extraction of lipids and solutes, unless anappropriate solid phase is added to the sample (for example,alumina allowed the retention of lipids during the SFE of drugresidues in poultry feed, eggs and muscle tissue84). However, insuch an application, the other techniques will also require a

clean-up step due to the co-extraction of lipids, as observed forPFE of PCBs from fish tissues.32

The main drawbacks of SFE are the difficulties in extractingpolar compounds (this requires the addition of an appropriatemodifier to the fluid) and the different operating conditions thatmay be required between spiked and real samples as alreadydiscussed.77 This may lead to a tedious optimization of SFEmethods, as several parameters need to be considered with thistechnique. Consequently, the current use of SFE in laboratorieswill require the promulgation of official methods (at the presenttime, only a few have been published by the US EPA forenvironmental applications25–27).

As they require less parameters to be optimized, and as theyuse liquid organic solvents as the extractant, PFE and MAE are

more easily optimized than SFE and, for the same reasons,experimental conditions developed for spiked samples are more

Table 3 Comparison of technique performances for the extraction of PAHs

Application Technique Solvent Conditions TimeFurther treatmentafter extraction Extraction resultsa Reference

PAHs/certifiedmarinesediment(NIST 1941a)

Soxhlet (EPAmethod)

Hexane–acetone,1+1

10 g sediment;300 mL solvent

18 h Concentration 50.3–161% 64

Sonication (EPAmethod)

CH2Cl2–acetone,1+1

30 g sediment; 33 100 mLsolvent

3 3 3 min Concentration 20.6–213%

SFE CO2 + 10%CH3OH

5 g sediment;copper filings;450 bar, 120°C; 1–1.5 mLmin21

collection:CH2Cl2

60 min dynamic None 31.7–171%

PMAE Hexane–acetone,1+1

10 g sediment; 30mL solvent; 115°C, 72 psi

10 min + 20 mincooling

Supernatantcollected;concentration;centrifugation;concentration

26–97.5%

PAHs/nativecontaminatedsoil

Soxhlet CH2Cl2 10 g soil + 10 gNa2SO4; 150mL solvent

24 h Concentration Total: 1623 mg kg21 53

SFE CO2 + 20%

CH3OH

1 g soil; 250 kg

cm22; 70 °C; 2mL min21

collection:CH2Cl2

5 min static + 30

min dynamic

Concentration Total: 1544 mg kg21

PFE CH2Cl2–acetone,1+1

7 g soil; 100 °C;2000 psi

5 min preheating +5 min static

Concentration Total: 1537 mg kg21

PMAE Acetone 2 g soil; 40 mLsolvent; 120 °C

20 min + cooling Filtration;concentration

Total: 1578 mg kg21

FMAE CH2Cl2 2 g soil; 70 mLsolvent

20 min Filtration;concentration

Total: 1492 mg kg21

PAHs/ contaminatedsoil

Soxhlet CH2Cl2 10 g soil + 30 gNa2SO4; 100mL solvent

6 h + cooling None Total: 297.4 mg kg21 78,79

SFE CO2 + 20%CH3OH

1 g soil; 250 kgcm22; 70 °C; 1mL min21

collection:CH2Cl2

5 min static + 60min dynamic

None Total: 458.0 mg kg21


20 min + cooling Filtration Total: 422.9 mg kg21

a Concentration values obtained (mg kg21) or recoveries versus certified values (%).

Analyst , 2001, 126, 1182–1193 1189

8/13/2019 Camel_2001


rapidly adapted to real samples (e.g. by simply increasing thetemperature or the extraction time). The nature of the extractantneeds to be chosen to match the polarity of the solutes and tolimit the co-extraction of matrix material, while the extractiontemperature should allow a high and selective solubilization anddesorption of the analytes, along with rapid mass transfer.Therefore, PFE and MAE can be successfully applied to alltypes of solutes and solid matrices, with the limitation thatexperimental conditions are chosen to avoid possible thermal

degradation of the analytes.Of prime interest is the very recent use of PFE with

subcritical water as the extractant.85 This fluid has beensuccessfully used for several applications and is a promisingalternative to the well-known supercritical carbon dioxide. Bysimply increasing the temperature at constant pressure, therelative permittivity of water may be reduced, so that analyteswith a wide range of polarities may be extracted. Thus, at hightemperatures (around 200–300 °C), water acts as a low polaritysolvent, allowing the selective extraction of non-polar analytes,such as PCBs, from soils and sediments.86–88 In addition, class-selective extractions may be achieved by simply raising thetemperature.89 However, even though subcritical water showsgreat potential as a ‘clean’ solvent, it leads to dilution of the

analytes in the liquid extract (thus requiring concentration stepsfor some applications), and more matrix material is extracted as

compared to pure CO2 extraction.74 In addition, at such hightemperatures, degradation of the solutes may occur.90

5 Future trends

As legislation will tend to restrict or even ban the use of manycommon solvents, recent extraction techniques will in the futureundoubtedly supersede the traditional methods, as the former

considerably reduce the solvent volumes required, along with areduction in the time devoted to the extraction step (see Table8).

Yet, future trends will depend on the commercially availableapparatus. Table 9 summarizes the characteristics of thesystems available today for the recent techniques. Indeed, whenwe look back over the past 10 years, numerous changes haveoccurred in the commercial systems available for the recenttechniques devoted to solid sample extraction. About 10 yearsago, when SFE appeared promising and a viable alternative toSoxhlet extraction, several manufacturers proposed their ownapparatus, the most famous being Hewlett-Packard (Agilent),Dionex, Isco and Suprex. Then, due to the emergence of MAEand PFE, as well as problems inherent in SFE as previously

discussed, the number of types of apparatus commerciallyavailable was reduced as indicated in Table 9. This reflects the

Table 4 Comparison of technique performances for the extraction of PCBs

Application Technique Solvent Conditions Time Further treatmentafter extraction

Extraction resultsa Reference

PCBs/certifiedindustrial soil(CRM 481)

Soxhlet Hexane–acetone,25+75

0.2 g soil + 2 gNa2SO4; 200mL solvent

24 h + cooling Concentration;centrifugation;florisil clean-up;elution withhexane;concentration

81–100 80

PFE Hexane–acetone,25+75

0.2 g soil + 2 gNa2SO4; 100°C; 1800 psi

10 min Concentration;florisil clean-up;elution withhexane;concentration

89.2–133


0.2 g soil + 2 gNa2SO4; 15 mLsolvent; 21 psi

40 min Filtration;concentration;florisil clean-up;elution withhexane;concentration

94.2–111

PCBs/certifiedsoil CRM910-050

Soxhlet n-Hexane 5–20 g soil; 75mL solvent

12 h AgNO3 and silicagel clean-up;concentration

97 61

PFE n-Heptane 2 g soil; 120 °C;120 MPa

10 min Concentration 98

PMAE n-Heptane 2 g soil + 2 gNa2SO4;Weflon® disc;15 mL solvent;150 °C

15 min + cooling Centrifugation;concentration

111

PCBs/certifiedharboursedimentCRM 536

SFE (EPAmethod)

CO2 1 g sediment + 7 gNa2SO4 + 2 gCu powder; 80°C, 305 bar 1mL min21;

florisil trapping


None 79–141 72

PFE (EPAmethod)

Hexane–acetone,1+1

1.5 g sediment +1.5 g Na2SO4;

100 °C

5 min preheating +5 min static

Concentration;sulfuric acid;impregnatedsilica clean-up;elution with

hexane;concentration;Cu powderaddition

109–211

a Recoveries versus certified values (%).

1190 Analyst , 2001, 126, 1182–1193

8/13/2019 Camel_2001


disinterest of several laboratories towards this technique, and itwill in the future additionally hinder the routine development of this technique.

In the case of PFE, there is so far only one commercial systemavailable devoted to this technique, from Dionex. However, itshould be noted that the new SFE extractor commercialized byIsco is presented as a dual mode system, operating in either SFEor PFE modes. This shows the growing interest of analyststowards PFE for the extraction of solid matrices.

For MAE, the open systems initially commercialized byProlabo (Soxwave 100 and 3.6) were sold at the end of 1999 toCEM Corporation, but they have not been commercially

available since (only cells or pieces for repair can be obtainedfrom CEM). As a consequence, the interest in FMAE shoulddecrease in the future, unless other systems become available.Thus, today, there are only two PMAE systems commerciallyavailable: Mars-X from CEM and Ethos SEL from Milestone.Both systems offer possible stirring of the samples duringextraction (which was not the case with previous systems), aswell as the use of additives to allow heating when using apolarsolvents.

However, due to the high investment cost of these techniques(especially SFE and PFE as shown in Table 8), economicconsiderations will in practice also influence the choice of the

Table 5 Comparison of technique performances for the extraction of pesticides

Application Technique Solvent Conditions Time Further treatmentafter extraction

Extraction resultsa Reference

Hexaconazole/ weatheredsoils (fresh +52 week aged). Soil 1:sandy loamsoil; soil 2:

sandy claysoil

Soxhlet CH3CN–H2O, 1+1 40 g soil; 80 mLsolvent

6 h Partitioning:CH2Cl2;reduced todryness; dilution1 mL CH2Cl2;silica clean-up;dryness; dilution

in hexane–acetone, 1+4

Fresh soil 1: 0.140;aged soil 1: 0.070;fresh soil 2: 0.140;aged soil 2: 0.080

69

SFE CO2 + 20%CH3OH

4 g soil; 245 bar,55 °C; 2 mLmin21

collection: 5 mLCH3OH


Cyano-bondedsilica clean-up;elution withCH3OH;C18silica clean-up; elution withCH3OH;concentration

Fresh soil 1: 0.074;aged soil 1: 0.034;fresh soil 2: 0.119;aged soil 2: 0.072

PFE Acetone 5 g soil + sand;2000 psi, 100°C

5 minequilibration +10 min

Concentration Fresh soil 1: 0.124;aged soil 1: 0.093;fresh soil 2: 0.136;aged soil 2: 0.070


15 min + 30 mincooling

Filtration; C18

silica clean-up;

elution withCH3OH;concentration

Fresh soil 1: 0.08;aged soil 1: 0.035;

fresh soil 2: 0.134;aged soil 2: 0.073

HCH isomers/ pollutedlandfill soil

Soxhlet Hexane–acetone,75+25

0.2 g soil + 2 gNa2SO4; 200mL solvent

8–24 h + cooling Concentration;centrifugation;Florisil clean-up; elution withhexane;concentration

a-HCH: 370; b-HCH:820

81

PFE Hexane–acetone,1+1

0.2 g soil + 2 gNa2SO4; 120°C; 1700 psi

16 min Concentration;Florisil clean-up; elution withhexane;concentration

a-HCH: 340; b-HCH:800


0.2 g soil + 2 gNa2SO4; 10 mLsolvent; 21 psi

10 min Filtration;concentration;Florisil clean-

up; elution withhexane;concentration

a-HCH: 370; b-HCH:829

a Concentration values obtained (mg kg21).

Table 6 Comparison of technique performances for the extraction of phenols

Application Technique Solvent Conditions Time

Furthertreatmentafterextraction

Extractionresultsa Reference

Phenols/cokerysoil

Sonication (EPAmethod)

CH2Cl2 2.5 g soil; 3 3 10 mLsolvent

3 3 3 min None 1.7–8.3 82

SFE CO2 1 g soil + Celite + 0.1 mL

CH3OH; 90 °C; C18silica trapping

10 min static + 15

min dynamic

None 2.1–15.1


1 g soil; 130 °C 5 min equilibration +10 min + cooling

None 3.7–16.8

a Concentration values obtained (mg kg21).

Analyst , 2001, 126, 1182–1193 1191

8/13/2019 Camel_2001


8/13/2019 Camel_2001


3 EPA Method 3550B, Ultrasonic Extraction, Test Methods for Evaluating Solid Waste, EPA, Washington DC, December 1996.

4 C. F. Poole and S. K. Poole, Anal. Commun., 1996, 33, 11H.5 R. E. Majors, LC-GC Int., 1996, 9, 638.6 M.-L. Riekkola and P. Manninen, Trends Anal. Chem., 1993, 12,

108.

7 S. B. Hawthorne, Anal. Chem., 1990, 62, 633A.8 R. M. Smith, J. Chromatogr., A, 1999, 856, 83.9 J. M. Levy, LC-GC Eur., 2000, 13, 174.

10 M. E. P. McNally, Anal. Chem., 1995, 67, 308A.

11 M. E. P. McNally, J. AOAC Int., 1996, 79, 380.12 L. T. Taylor, Anal. Chem., 1995, 67, 364A.13 J. J. Langenfeld, S. B. Hawthorne, D. J. Miller and J. Pawliszyn, Anal.

Chem., 1995, 67, 1727.

14 M. D. Luque de Castro and M. T. Tena, Trends Anal. Chem., 1996,15, 32.

15 P. R. Eckard, G. L. Long, L. T. Taylor and G. C. Slack, J.Chromatogr. Sci., 1998, 36, 547.

16 J. A. Field, D. J. Miller, T. M. Field, S. B. Hawthorne and W. Giger, Anal. Chem., 1992, 64, 3161.

17 J. W. Hills and H. H. Hill, J. Chromatogr. Sci., 1993, 31, 6.18 T. S. Reighard and S. V. Olesik, Anal. Chem., 1996, 68, 3612.19 H. J. Vandenburg, A. A. Clifford, K. D. Bartle, J. Carroll, I. Newton,

L. M. Garden, J. R. Dean and C. T. Costley, Analyst , 1997, 122,101R.

20 W. Fiddler, J. W. Pensabene, R. A. Gates and D. J. Donoghue, J. Agric. Food Chem., 1999, 47, 206.

21 J. W. Pensabene, W. Fiddler and D. J. Donoghue, J. AOAC Int., 1999,82, 1334.

22 V. Camel, A. Tambuté and M. Caude, J. Chromatogr., 1993, 642,263.

23 V. Camel, Trends Anal. Chem., 1997, 16, 351.24 R. M. Smith, LC-GC Int., 1996, 9, 8.25 EPA Method 3561, Supercritical Fluid Extraction of Polynuclear

Aromatic Hydrocarbons, Test Methods for Evaluating Solid Waste,EPA, Washington DC, December 1996.

26 EPA Method 3562, Supercritical Fluid Extraction of PolychlorinatedBiphenyls (PCBs) and Organochlorinated Pesticides, Test Methods for Evaluating Solid Waste, EPA, Washington DC, January 1998.

27 EPA Method 3560, Supercritical Fluid Extraction of Total Recover-able Petroleum Hydrocarbons, Test Methods for Evaluating Solid Waste, EPA, Washington DC, December 1996.

28 B. E. Richter, J. L. Ezzell, D. Felix, K. A. Roberts and D. W. Later,

Am. Lab., 1995, 27, 24.29 B. E. Richter, B. A. Jones, J. L. Ezzell, N. L. Porter, N. Avdalovic and

C. Pohl, Anal. Chem., 1996, 68, 1033.

30 R. C. Richter, LC-GC , 1999, 17, S22.31 Y. Zhu, K. Yanagihara, F. Guo and Q. X. Li, J. Agric. Food Chem.,

2000, 48, 4097.

32 R. C. Richter and L. Covino, LC-GC , 2000, 18, 1068.33 R. C. Richter and L. Covino, J. Chromatogr., A, 2000, 874, 217.34 EPA Method 3545A, Pressurized Fluid Extraction, Test Methods for

Evaluating Solid Waste, EPA, Washington DC, January 1998.

35 J. A. Fisher, M. J. Scarlett and A. D. Stott, Environ. Sci. Technol.,1997, 31, 1120.

36 X. Lou, H.-G. Janssen and C. A. Cramers, Anal. Chem., 1997, 69,1598.

37 G. Wang, A. S. Lee, M. Lewis, B. Kamath and R. K. Archer, J. Agric.Food Chem., 1999, 47, 1062.

38 F. E. Smith and E. A. Arsenault, Talanta, 1996, 43, 1207.39 R. C. Richter, D. Link and H. M. Kingston, Anal. Chem., 2001, 73,

30A.

40 K. Ganzler, A. Salgo and K. Valko, J. Chromatogr., 1986, 371,299.

41 K. Ganzler and A. Salgo, Z. Lebensm.-Unters.-Forsch, 1987, 184,274.

42 J. R. J. Paré, J. M. R. Bélanger and S. S. Stafford,Trends Anal. Chem.,1994, 13, 176.

43 C. S. Eskilsson and E. Björklund, J. Chromatogr., A, 2000, 902,227.

44 R. E. Majors, LC-GC Int., 1995, 8, 128.45 V. Lopez-Avila, R. Young and W. F. Beckert, J. AOAC Int., 1998, 81,

462.

46 V. Lopez-Avila, J. Benedicto and K. M. Bauer, J. AOAC Int., 1998,81, 1224.

47 O. F. X. Donard, B. Lalere, F. Martin and R. Lobinski, Anal. Chem.,1995, 67, 4250.

48 J. Szpunar, V. O. Schmitt, O. F. X. Donard and R. Lobinski, Trends

Anal. Chem., 1996, 15, 181.

49 V. Camel, Trends Anal. Chem., 2000, 19, 229.50 M. Letellier and H. Budzinski, Analusis, 1999, 27, 259.51 B. W. Renoe, Am. Lab., 1994, 34.52 V. Lopez-Avila and J. Benedicto, Trends Anal. Chem., 1996, 15,

334.

53 N. Saim, J. R. Dean, M. d. P. Abdullah and Z. Zakaria, J.

Chromatogr., A, 1997, 791, 361.

54 V. Lopez-Avila, R. Young, R. Kim and W. F. Beckert, J.

Chromatogr. Sci., 1995, 33, 481.

55 K. K. Chee, M. K. Wong and H. K. Lee, J. Chromatogr., A, 1996,

723, 259.56 E. V. Blanco, P. L. Mahia, S. M. Lorenzo, D. P. Rodriguez and E. F.

Fernandez, Fresenius’ J. Anal. Chem., 2000, 366, 283.

57 S. J. Stout, A. R. daCunha, G. L. Picard and M. M. Safarpour, J.

Agric. Food Chem., 1996, 44, 3548.

58 S. J. Stout, A. R. daCunha and D. G. Allardice, Anal. Chem., 1996, 68,653.

59 G. Xiong, B. Tang, X. He, M. Zao, Z. Zhang and Z. Zhang, Talanta,1999, 48, 333.

60 K. Hummert, W. Vetter and B. Luckas, Chromatographia, 1996, 42,300.

61 R.-A. Düring and St. Gäth, Fresenius’ J. Anal. Chem., 2000, 368,684.

62 R. Hoogerbrugge, C. Molins and R. A. Baumann, Anal. Chim. Acta,

1997, 348, 247.

63 M. P. Llompart, R. A. Lorenzo, R. Cela and J. R. J. Paré, Analyst ,

1997, 122, 133.64 V. Lopez-Avila, R. Young and N. Teplitsky, J. AOAC Int., 1996, 79,

142.

65 H. Budzinski, M. Letellier, P. Garrigues and K. Le Menach, J.

Chromatogr., A, 1999, 837, 187.

66 V. Lopez-Avila, R. Young and W. F. Beckert, Anal. Chem., 1994, 66,1097.

67 EPA Method 3546, Microwave Extraction, Test Methods for

Evaluating Solid Waste, pre-release version, EPA, Washington DC,September 1999.

68 J. R. Dean and G. Xiong, Trends Anal. Chem., 2000, 19, 553.

69 S. P. Frost, J. R. Dean, K. P. Evans, K. Harradine, C. Cary and M. H.I. Comber, Analyst , 1997, 122, 895.

70 D. V. Kenny and S. V. Olesik, J. Chromatogr. Sci., 1998, 36, 59.71 D. V. Kenny and S. V. Olesik, J. Chromatogr. Sci., 1998, 36, 66.72 E. Björklund, S. Bowadt, T. Nilsson and L. Mathiasson, J.

Chromatogr., A, 1999, 836, 285.73 S. J. Lehotay and C.-H. Lee, J. Chromatogr., A, 1997, 785, 313.74 S. B. Hawthorne, C. B. Grabanski, E. Martin and D. J. Miller, J.

Chromatogr., A, 2000, 892, 421.

75 R. Murga, R. Ruiz, S. Beltran and J. L. Cabezas, J. Agric. Food

Chem., 2000, 48, 3408.

76 H. Budzinski, M. Letellier, S. Thompson, K. LeMenach and P.

Garrigues, Fresenius’ J. Anal. Chem., 2000, 367, 165.

77 M. D. Luque de Castro and M. M. Jimenez-Carmona, Trends Anal.

Chem., 2000, 19, 223.

78 I. J. Barnabas, J. R. Dean, W. R. Tomlinson and S. P. Owen, Anal.

Chem., 1995, 67, 2064.

79 I. J. Barnabas, J. R. Dean, I. A. Fowlis and S. P. Owen, Analyst , 1995,120, 1897.

80 O. Zuloaga, N. Etxebarria, L. A. Fernandez and J. M. Madariaga,Trends Anal. Chem., 1998, 17, 642.

81 O. Zuloaga, N. Etxebarria, L. A. Fernandez and J. M. Madariaga,Fresenius’ J. Anal. Chem., 2000, 367, 733.

82 M. P. Llompart, R. A. Lorenzo, R. Cela, K. Li, J. M. R. Bélanger andJ. R. J. Paré, J. Chromatogr., A, 1997, 774, 243.

83 H. J. Vandenburg, A. A. Clifford, K. D. Bartle, J. Carroll and I. D.Newton, Analyst , 1999, 124, 397.

84 D. K. Matabudul, N. T. Crosby and S. Sumar, Analyst , 1999, 124,499.

85 M. D. Luque de Castro and M. M. Jimenez-Carmona, Trends Anal.

Chem., 1998, 17, 441.

86 K. Hartonen, K. Inkala, M. Kangas and M.-L. Riekkola, J.

Chromatogr., A, 1997, 786, 219.

87 Y. Yang, S. Bowadt, S. B. Hawthorne and D. J. Miller, Anal. Chem.,1995, 67, 4571.

88 S. B. Hawthorne, C. B. Grabanski, K. J. Hageman and D. J. Miller, J.

Chromatogr., A, 1998, 814, 151.

89 S. B. Hawthorne, Y. Yang and D. J. Miller, Anal. Chem., 1994, 66,2912.

90 K. J. Hageman, L. Mazeas, C. B. Grabanski, D. J. Miller and S. B.

Hawthorne, Anal. Chem., 1996, 68, 2892.

Analyst , 2001, 126, 1182–1193 1193

Date post:	04-Jun-2018
Category:	Documents
Upload:	sunny-arora
View:	216 times
Download:	0 times

Camel_2001

Documents