HS-SPME in packaging materials, Artykuły naukowe, SPME i HS-SPME
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//-->Journal of Chromatography A, 985 (2003) 247–257www.elsevier.com / locate / chromaDirect quantitation of volatile organic compounds in packagingmaterials by headspace solid-phase microextraction–gaschromatography–mass spectrometry´˜´Oscar Ezquerro, Begona Pons, Marıa Teresa Tena*˜Department of Chemistry,University of La Rioja,C/Madre de Dios51,E-26006Logrono(La Rioja),SpainAbstractThe quantification of volatile organic compounds (VOCs) in flexible multilayer packaging materials using headspacesolid-phase microextraction–gas chromatography–mass spectrometry (HS-SPME–GC–MS) was studied. The analytesinclude 22 compounds such as aldehydes, ketones, carboxylic acids and hydrocarbons formed by thermooxidativedegradation of polyethylene during the extrusion coating process in the manufacture of the packaging, and many of them areinvolved in the unpleasant and undesirable odour of these materials. External standard calibration using a solution of theanalytes in an appropriate solvent was the first approach studied. Aqueous solutions of the analytes provided lowreproducibility and the reduction of aldehydes to alcohols under the HS-SPME conditions. Hexadecane was chosen as thesolvent since its polarity is similar to that of polyethylene and its volatility is lower than that of the analytes. However,hexadecane should be added to the sample before the analysis as it modifies the absorption capacity of the fibre. A 75-mmCarboxen–poly(dimethylsiloxane) fibre was used to extract the VOCs from the headspace above the packaging in a 15-mlsealed vial at 1008Cafter 5 min of preincubation. The influence of the extraction time on the amount extracted was studiedfor a standard solution of the analytes in hexadecane, together with the influence of the volume of the standard solution andthe amount of the sample placed in the vial. Standard addition and multiple HS-SPME were also studied as calibrationmethods and the results obtained in the quantitative analysis of a packaging material were compared.2002 Elsevier Science B.V. All rights reserved.Keywords:Packaging materials; Headspace analysis; Volatile organic compounds1. IntroductionPolyethylene is a polymer widely used as apackaging material due to its properties (strength,low cost, flexibility, inert character, stability, easyprocessing and chemical resistance). The packagedproducts are mainly foods, as well as medicines,cosmetic products and farming products. Frequently,*Corresponding author. Tel.:134-941-299-627;fax:134-941-299-621.E-mail address:maria-teresa.tena@dq.unirioja.es(M.T. Tena).the packaging materials are composed of severallayers of different materials, i.e. cellulose–poly-ethylene–aluminium–polyethylene. In order toproduce these multilayer packaging materials it isnecessary to deposit the melted polymer on a solidsurface such as cellulose or aluminium; this processis called extrusion-coating process.The combination in the extruder of high tempera-tures, frequent extreme mechanical stresses and thepresence of oxygen causes the degradation of poly-mers [1,2]. The mechanism of thermooxidative deg-radation highlights the presence of alkyl radicals that0021-9673 / 02 / $ – see front matter2002 Elsevier Science B.V. All rights reserved.PII: S0021-9673( 02 )01829-0248´O.Ezquerro et al./J.Chromatogr.A985 (2003) 247–257combined with oxygen produce alkoxy and peroxyradicals [2,3], and the combinations of these radicalsproduces volatile organic compounds (VOCs) suchas hydrocarbons, alcohols, aldehydes, ketones andcarboxylic acids [4–6]. The VOCs formed duringextrusion-coating process can migrate to the materi-als contained in the packaging and change theirorganoleptic properties imparting undesirable odoursand flavours. The factors that determine the migra-tion are mainly the temperature, the contact time, theequilibrium constant, the concentration, the solubilityand the diffusion coefficient [7]. It is necessary toidentify and quantify the VOCs formed in order toestablish whether they can be toxic or modify thequality of the products.The purge and trap technique is usually reportedas the method to determinate VOCs in polymers[1,4,5,8–11]. Bravo and Hotchkiss [3] reported apurge and trap method in which the trap was cooledin liquid nitrogen and VOCs were extracted bywashing with ultrapure Freon-113. Ligon and George[12] used a direct thermal desorption technique, andVillberg et al. [5] proposed a technique that uses asolid adsorbent (Tenax GR) and a thermal desorptiondevice. Gas chromatography–mass spectrometrywith simultaneous sniffing [4–6] or odours panels[13] has also been reported for the identification ofoff-odour compounds.In this work, the determination of VOCs has beencarried out by headspace solid-phase microextrac-tion–gas chromatography–mass spectrometry (HS-SPME–GC–MS). SPME [14] is a technique thatallows direct analysis of the volatile compounds insolid samples, thus avoiding the use of solvents.HS-SPME variables such as the type of fibre, theincubation temperature, the pre-incubation time, thesize of vial and the extraction time were previouslystudied to identify the optimal analysis conditions[15].Usually, quantitative analysis by SPME does notrequire any treatment of the samples. The calibrationis carried out using external standards of exactlyknown concentration, or by standard addition toavoid the matrix effect. These procedures are easyfor liquid samples, but are complex or impossible toapply to solid samples since there are no certificatedreference materials for most of analytes in these solidsamples at different ranges of concentration.There are few applications to quantify VOCs insolid samples by SPME. The analysis of solidsamples by SPME has been reported using somesolid–liquid extraction techniques such as: lixiviationwith solvents [16], extraction using ultrasound [17],microwave-assisted extraction [18], or pressurisedsolvents extraction [19] before the SPME, and thequantitative determination by SPME is carried out inthe liquid extract. Besides, the direct analysis of solidsamples by SPME has been carried out by suspend-ing the soil in a solvent (usually water) in sealedvials, and by SPME performed in the vial headspace[20–24]. However, there are no described directSPME quantitative methods for other kinds of solidsamples such as polymers.Multiple extraction allows calculation of the totalarea count of the analytes that corresponds to anexhaustive extraction, and, in this way, the matrixeffect is avoided. The procedure involves samplingrepeatedly the same sample at equal time intervals toobtain the exponential decay of the concentration ofanalytes. Some applications of this technique havebeen reported for headspace [25] and for SPME [26].In this article, the quantitative analysis of VOCs ina multilayer packaging sample with an odour prob-lem was carried out by three different methods:external standard calibration, standard addition andmultiple headspace solid-phase microextraction.2. Experimental2.1.SampleThe sample was a flexible packaging materialconsisting of a layer of cellulose, a layer of poly-ethylene, a layer of aluminium, and another layer ofpolyethylene, and was provided by Tobepal (Log-˜rono, Spain).2.2.ChemicalsThe following chemicals were used to preparestandard solutions: pentanoic acid ($99.0%), butanal($97.0%), pentanal ($98%), 2,4-pentanedione($99.5%), 3-methylbutanal ($98%), cyclohexanone($99.5%), hexanal ($98%), heptanal ($95%), 3-heptanone ($99.5%), 2-ethylhexanal ($97%), oc-´O.Ezquerro et al./J.Chromatogr.A985 (2003) 247–257249tanal ($98%), nonanal (|97%), decanal (|97%),undecanal (|97%), and dodecanal (|97%) fromFluka, hexanoic acid (199.5%), decane (199%),undecane (199%), and dodecane (199%) fromAldrich, acetone (99.8%) and toluene (99.8%) fromCarlo Erba, and acetic acid (80%) from Panreac.Hexadecane ($98%) from Fluka was used as sol-vent.Stock solutions of pure compounds were made inhexadecane, and dilutions from 25 ng / ml to40mg/ ml in hexadecane were used in the differentstudies. Stock solutions of pure compounds werealso made in methanol, and dilutions of 270–1700ng / ml in water were used.and then equilibrated with a 75-mm Carboxen–poly-(dimethylsiloxane) fibre immersed in the headspacefor 60 min. The VOCs were thermally desorpted inthe injector port of the chromatograph for 15 minand transferred to the chromatograph column wherethey were separated. Finally, the VOCs were taken tothe mass spectrometer for their identification andquantification.2.5.Chromatographic conditionsThe GC–MS was equipped with a CP5860 wall-coated open tubular (WCOT) fused-silica column(30 m30.25 mm I.D. with a 0.25-mm CP-SIL8 CBlow-bleed / MS phase, Varian). An initial oven tem-perature of 358Cfor 5 min was used, followed by anincrease in the temperature at a rate of 108C/ min to2308C.A 0.8-mm I.D. insert was used, and thecarrier gas was helium (99.996%), at a rate of1 ml / min. The injector was maintained at 2808C,with a 1:20 split ratio at the initial time, followed bya 1:50 split ratio at 0.5 min. The mass spectrometerwas scanned fromm/z 40 to 230 at one cycle persecond, the fragmentation was made by electronicimpact, the ion trap temperature was 2008Cand theelectron multiplier voltage was 1550 V.2.3.Instruments and materialsA Varian 3900 gas chromatograph with a VarianSaturn 2100T MS detector was used. The SPME wasperformed manually with a SPME holder fromSupelco, together with a hot plate from Corning. Theassignment of each chromatographic peak was de-termined using a GC–MS mass spectral library (USNational Institute of Standards and Technology,NIST).2.4.Sampling procedureThe sampling procedure depended on the quantifi-cation method.In the external calibration method, 1.0 ml ofhexadecane and 120 cm2of flexible multilayerpackaging material were placed in a 15-ml sealedvial with a screw top.In the standard addition method, 120 cm2offlexible multilayer packaging material were placed ina 15-ml sealed vial, and two standard additions of1.0 ml of hexadecane solution with different con-centrations of VOCs were performed.In the multiple HS-SPME method, 4.0 cm2offlexible multilayer packaging material were placed ina 15-ml sealed vial and sampled five times at equaltime intervals (60 min). The calibration was madeusing 10mlof a VOC standard solution in hexade-cane sampled in the same way.The SPME conditions were the same for all thecalibration methods. The samples were incubated at1008Cfor 5 min to speed up the volatile compounds,3. Results and discussion3.1.Selection of a solvent for the standardsolutionsWater and hexadecane were tested as solvents forstandard solutions. Water could not be used assolvent since it provided a low reproducibility and areduction of the aldehydes to alcohols was observed.Hexadecane is a long-chain non-polar solvent with ahigh boiling point (283–2868C),a volatility lowerthan the analytes and a polarity similar to that of thepolyethylene matrix. Consequently, hexadecane wasselected as solvent.The analytes studied were acetone, acetic acid,butanal, 3-methylbutanal, pentanal, toluene, 2,4-pen-tanedione, hexanal, 3-heptanone, pentanoic acid,cyclohexanone, heptanal, hexanoic acid, 2-ethylhex-anal, decane, octanal, undecane, nonanal, dodecane,decanal, undecanal, and dodecanal.250´O.Ezquerro et al./J.Chromatogr.A985 (2003) 247–2573.2.Optimisation of HS-SPME variablesSome of the HS-SPME variables, such as the typeof fibre, the incubation temperature, the extractiontime, the pre-incubation time or the size of the vialhad been already studied for the packaging material[15]. The following studies complete the optimi-sation of the HS-SPME variables.3.2.1.Extraction time with VOC standard solutionin hexadecaneThe amount of analyte extracted was modified byincreasing the extraction time (the exposition time ofthe fibre to the headspace gas) until the equilibriumtime was reached.A 1-ml aliquot of hexadecane solution was placedin a 15-ml sealed vial; the concentration of VOCs inthe solution ranged from 27 ng / ml (pentanoic acid)to 4mg/ ml (cyclohexanone). The extraction timevaried from 1 to 90 min, and triplicate extractionswere performed. The relative areas of the chromato-graphic peaks versus the extraction time are shownin Fig. 1.The influence of the extraction time depends onthe compound. The signals of the smaller com-pounds, such as acetone, 3-methylbutanal, butanal,cyclohexanone, or 2-ethylhexanal, decreased afterreaching a peak at 10–20 min, by increasing theextraction time, whereas the signal increased for thevolatile compounds with an increased number ofcarbon atoms, such as nonanal, decanal, or undecan-al. This suggests that semi-volatile compounds dis-place to the most volatile compounds from the fibrewhen the extraction time is higher.An extraction time of 60 min was selected forfurther experiments because the variation of thesignals between 60 and 90 min was small, within thestandard deviation, for most of the analytes.3.2.2.Solution volume with VOC standard solutionin hexadecaneThe amount of analyte extracted increased byincreasing the VOCs solution volume until reaching avalue at which the amount extracted remained ap-proximately constant.The volume varied from 50 to 1500mland theconcentration of VOCs in the solution ranged from460 ng / ml (dodecanal) to 770 ng / ml (2,4-pen-tanedione). The solution was placed in a 15-mlsealed vial and sampled as described in the Ex-perimental section. Triplicate extractions were per-formed. The relative areas of the chromatographicpeaks versus the solution volume are shown in Fig.2.Most of the VOCs showed a plateau from 500 to1000mlonwards, whereas the compounds with ahigher molecular mass showed a peak using 300ml.A solution volume of 1000mlwas selected forfurther experiments.3.2.3.Packaging amountThe influence of the packaging amount was alsostudied. The sample amount ranged from 30 to 200cm2. The samples were bent in order to introducethem into a 15-ml sealed vial and expose themaximum polyethylene surface in the headspace.Triplicate extractions were performed. The relativeareas of chromatographic peaks versus the packagingsurface are shown in Fig. 3.The signals increased by increasing the packagingamount from 30 to 120 cm2, most of the analytesreached a peak at 120 cm2, and then the signalsdecreased with increasing packaging amount due toproblems introducing this amount of packaging inthe vial with enough free surface for the masstransport. A packaging amount of 120 cm2wasselected for further experiments. The thickness of thepackaging material was 85mm,therefore the packag-ing amount for 120 cm2was 1.02 ml, which isapproximately equal to the optimised volume ofhexadecane.3.3.Linearity study with VOC solutions inhexadecaneAfter optimisation of the HS-SPME variables, alinearity study was carried out. A 1-ml sample ofVOC standard solution in hexadecane was placed ina 15-ml sealed vial and processed as described in theExperimental section.Table 1 shows the ranges of the VOC concen-trations studied, the linear ranges, the limits ofdetection (LODs), the correlation coefficients (R) andthe relative standard deviations (RSDs) found. Alinear behaviour was observed when the concen-trations were low, together with a curved trend at´O.Ezquerro et al./J.Chromatogr.A985 (2003) 247–257251Fig. 1. Influence of the extraction time on the HS-SPME of VOCs from hexadecane solutions. For HS-SPME and GC–MS conditions, seethe text. 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//-->Journal of Chromatography A, 985 (2003) 247–257www.elsevier.com / locate / chromaDirect quantitation of volatile organic compounds in packagingmaterials by headspace solid-phase microextraction–gaschromatography–mass spectrometry´˜´Oscar Ezquerro, Begona Pons, Marıa Teresa Tena*˜Department of Chemistry,University of La Rioja,C/Madre de Dios51,E-26006Logrono(La Rioja),SpainAbstractThe quantification of volatile organic compounds (VOCs) in flexible multilayer packaging materials using headspacesolid-phase microextraction–gas chromatography–mass spectrometry (HS-SPME–GC–MS) was studied. The analytesinclude 22 compounds such as aldehydes, ketones, carboxylic acids and hydrocarbons formed by thermooxidativedegradation of polyethylene during the extrusion coating process in the manufacture of the packaging, and many of them areinvolved in the unpleasant and undesirable odour of these materials. External standard calibration using a solution of theanalytes in an appropriate solvent was the first approach studied. Aqueous solutions of the analytes provided lowreproducibility and the reduction of aldehydes to alcohols under the HS-SPME conditions. Hexadecane was chosen as thesolvent since its polarity is similar to that of polyethylene and its volatility is lower than that of the analytes. However,hexadecane should be added to the sample before the analysis as it modifies the absorption capacity of the fibre. A 75-mmCarboxen–poly(dimethylsiloxane) fibre was used to extract the VOCs from the headspace above the packaging in a 15-mlsealed vial at 1008Cafter 5 min of preincubation. The influence of the extraction time on the amount extracted was studiedfor a standard solution of the analytes in hexadecane, together with the influence of the volume of the standard solution andthe amount of the sample placed in the vial. Standard addition and multiple HS-SPME were also studied as calibrationmethods and the results obtained in the quantitative analysis of a packaging material were compared.2002 Elsevier Science B.V. All rights reserved.Keywords:Packaging materials; Headspace analysis; Volatile organic compounds1. IntroductionPolyethylene is a polymer widely used as apackaging material due to its properties (strength,low cost, flexibility, inert character, stability, easyprocessing and chemical resistance). The packagedproducts are mainly foods, as well as medicines,cosmetic products and farming products. Frequently,*Corresponding author. Tel.:134-941-299-627;fax:134-941-299-621.E-mail address:maria-teresa.tena@dq.unirioja.es(M.T. Tena).the packaging materials are composed of severallayers of different materials, i.e. cellulose–poly-ethylene–aluminium–polyethylene. In order toproduce these multilayer packaging materials it isnecessary to deposit the melted polymer on a solidsurface such as cellulose or aluminium; this processis called extrusion-coating process.The combination in the extruder of high tempera-tures, frequent extreme mechanical stresses and thepresence of oxygen causes the degradation of poly-mers [1,2]. The mechanism of thermooxidative deg-radation highlights the presence of alkyl radicals that0021-9673 / 02 / $ – see front matter2002 Elsevier Science B.V. All rights reserved.PII: S0021-9673( 02 )01829-0248´O.Ezquerro et al./J.Chromatogr.A985 (2003) 247–257combined with oxygen produce alkoxy and peroxyradicals [2,3], and the combinations of these radicalsproduces volatile organic compounds (VOCs) suchas hydrocarbons, alcohols, aldehydes, ketones andcarboxylic acids [4–6]. The VOCs formed duringextrusion-coating process can migrate to the materi-als contained in the packaging and change theirorganoleptic properties imparting undesirable odoursand flavours. The factors that determine the migra-tion are mainly the temperature, the contact time, theequilibrium constant, the concentration, the solubilityand the diffusion coefficient [7]. It is necessary toidentify and quantify the VOCs formed in order toestablish whether they can be toxic or modify thequality of the products.The purge and trap technique is usually reportedas the method to determinate VOCs in polymers[1,4,5,8–11]. Bravo and Hotchkiss [3] reported apurge and trap method in which the trap was cooledin liquid nitrogen and VOCs were extracted bywashing with ultrapure Freon-113. Ligon and George[12] used a direct thermal desorption technique, andVillberg et al. [5] proposed a technique that uses asolid adsorbent (Tenax GR) and a thermal desorptiondevice. Gas chromatography–mass spectrometrywith simultaneous sniffing [4–6] or odours panels[13] has also been reported for the identification ofoff-odour compounds.In this work, the determination of VOCs has beencarried out by headspace solid-phase microextrac-tion–gas chromatography–mass spectrometry (HS-SPME–GC–MS). SPME [14] is a technique thatallows direct analysis of the volatile compounds insolid samples, thus avoiding the use of solvents.HS-SPME variables such as the type of fibre, theincubation temperature, the pre-incubation time, thesize of vial and the extraction time were previouslystudied to identify the optimal analysis conditions[15].Usually, quantitative analysis by SPME does notrequire any treatment of the samples. The calibrationis carried out using external standards of exactlyknown concentration, or by standard addition toavoid the matrix effect. These procedures are easyfor liquid samples, but are complex or impossible toapply to solid samples since there are no certificatedreference materials for most of analytes in these solidsamples at different ranges of concentration.There are few applications to quantify VOCs insolid samples by SPME. The analysis of solidsamples by SPME has been reported using somesolid–liquid extraction techniques such as: lixiviationwith solvents [16], extraction using ultrasound [17],microwave-assisted extraction [18], or pressurisedsolvents extraction [19] before the SPME, and thequantitative determination by SPME is carried out inthe liquid extract. Besides, the direct analysis of solidsamples by SPME has been carried out by suspend-ing the soil in a solvent (usually water) in sealedvials, and by SPME performed in the vial headspace[20–24]. However, there are no described directSPME quantitative methods for other kinds of solidsamples such as polymers.Multiple extraction allows calculation of the totalarea count of the analytes that corresponds to anexhaustive extraction, and, in this way, the matrixeffect is avoided. The procedure involves samplingrepeatedly the same sample at equal time intervals toobtain the exponential decay of the concentration ofanalytes. Some applications of this technique havebeen reported for headspace [25] and for SPME [26].In this article, the quantitative analysis of VOCs ina multilayer packaging sample with an odour prob-lem was carried out by three different methods:external standard calibration, standard addition andmultiple headspace solid-phase microextraction.2. Experimental2.1.SampleThe sample was a flexible packaging materialconsisting of a layer of cellulose, a layer of poly-ethylene, a layer of aluminium, and another layer ofpolyethylene, and was provided by Tobepal (Log-˜rono, Spain).2.2.ChemicalsThe following chemicals were used to preparestandard solutions: pentanoic acid ($99.0%), butanal($97.0%), pentanal ($98%), 2,4-pentanedione($99.5%), 3-methylbutanal ($98%), cyclohexanone($99.5%), hexanal ($98%), heptanal ($95%), 3-heptanone ($99.5%), 2-ethylhexanal ($97%), oc-´O.Ezquerro et al./J.Chromatogr.A985 (2003) 247–257249tanal ($98%), nonanal (|97%), decanal (|97%),undecanal (|97%), and dodecanal (|97%) fromFluka, hexanoic acid (199.5%), decane (199%),undecane (199%), and dodecane (199%) fromAldrich, acetone (99.8%) and toluene (99.8%) fromCarlo Erba, and acetic acid (80%) from Panreac.Hexadecane ($98%) from Fluka was used as sol-vent.Stock solutions of pure compounds were made inhexadecane, and dilutions from 25 ng / ml to40mg/ ml in hexadecane were used in the differentstudies. Stock solutions of pure compounds werealso made in methanol, and dilutions of 270–1700ng / ml in water were used.and then equilibrated with a 75-mm Carboxen–poly-(dimethylsiloxane) fibre immersed in the headspacefor 60 min. The VOCs were thermally desorpted inthe injector port of the chromatograph for 15 minand transferred to the chromatograph column wherethey were separated. Finally, the VOCs were taken tothe mass spectrometer for their identification andquantification.2.5.Chromatographic conditionsThe GC–MS was equipped with a CP5860 wall-coated open tubular (WCOT) fused-silica column(30 m30.25 mm I.D. with a 0.25-mm CP-SIL8 CBlow-bleed / MS phase, Varian). An initial oven tem-perature of 358Cfor 5 min was used, followed by anincrease in the temperature at a rate of 108C/ min to2308C.A 0.8-mm I.D. insert was used, and thecarrier gas was helium (99.996%), at a rate of1 ml / min. The injector was maintained at 2808C,with a 1:20 split ratio at the initial time, followed bya 1:50 split ratio at 0.5 min. The mass spectrometerwas scanned fromm/z 40 to 230 at one cycle persecond, the fragmentation was made by electronicimpact, the ion trap temperature was 2008Cand theelectron multiplier voltage was 1550 V.2.3.Instruments and materialsA Varian 3900 gas chromatograph with a VarianSaturn 2100T MS detector was used. The SPME wasperformed manually with a SPME holder fromSupelco, together with a hot plate from Corning. Theassignment of each chromatographic peak was de-termined using a GC–MS mass spectral library (USNational Institute of Standards and Technology,NIST).2.4.Sampling procedureThe sampling procedure depended on the quantifi-cation method.In the external calibration method, 1.0 ml ofhexadecane and 120 cm2of flexible multilayerpackaging material were placed in a 15-ml sealedvial with a screw top.In the standard addition method, 120 cm2offlexible multilayer packaging material were placed ina 15-ml sealed vial, and two standard additions of1.0 ml of hexadecane solution with different con-centrations of VOCs were performed.In the multiple HS-SPME method, 4.0 cm2offlexible multilayer packaging material were placed ina 15-ml sealed vial and sampled five times at equaltime intervals (60 min). The calibration was madeusing 10mlof a VOC standard solution in hexade-cane sampled in the same way.The SPME conditions were the same for all thecalibration methods. The samples were incubated at1008Cfor 5 min to speed up the volatile compounds,3. Results and discussion3.1.Selection of a solvent for the standardsolutionsWater and hexadecane were tested as solvents forstandard solutions. Water could not be used assolvent since it provided a low reproducibility and areduction of the aldehydes to alcohols was observed.Hexadecane is a long-chain non-polar solvent with ahigh boiling point (283–2868C),a volatility lowerthan the analytes and a polarity similar to that of thepolyethylene matrix. Consequently, hexadecane wasselected as solvent.The analytes studied were acetone, acetic acid,butanal, 3-methylbutanal, pentanal, toluene, 2,4-pen-tanedione, hexanal, 3-heptanone, pentanoic acid,cyclohexanone, heptanal, hexanoic acid, 2-ethylhex-anal, decane, octanal, undecane, nonanal, dodecane,decanal, undecanal, and dodecanal.250´O.Ezquerro et al./J.Chromatogr.A985 (2003) 247–2573.2.Optimisation of HS-SPME variablesSome of the HS-SPME variables, such as the typeof fibre, the incubation temperature, the extractiontime, the pre-incubation time or the size of the vialhad been already studied for the packaging material[15]. The following studies complete the optimi-sation of the HS-SPME variables.3.2.1.Extraction time with VOC standard solutionin hexadecaneThe amount of analyte extracted was modified byincreasing the extraction time (the exposition time ofthe fibre to the headspace gas) until the equilibriumtime was reached.A 1-ml aliquot of hexadecane solution was placedin a 15-ml sealed vial; the concentration of VOCs inthe solution ranged from 27 ng / ml (pentanoic acid)to 4mg/ ml (cyclohexanone). The extraction timevaried from 1 to 90 min, and triplicate extractionswere performed. The relative areas of the chromato-graphic peaks versus the extraction time are shownin Fig. 1.The influence of the extraction time depends onthe compound. The signals of the smaller com-pounds, such as acetone, 3-methylbutanal, butanal,cyclohexanone, or 2-ethylhexanal, decreased afterreaching a peak at 10–20 min, by increasing theextraction time, whereas the signal increased for thevolatile compounds with an increased number ofcarbon atoms, such as nonanal, decanal, or undecan-al. This suggests that semi-volatile compounds dis-place to the most volatile compounds from the fibrewhen the extraction time is higher.An extraction time of 60 min was selected forfurther experiments because the variation of thesignals between 60 and 90 min was small, within thestandard deviation, for most of the analytes.3.2.2.Solution volume with VOC standard solutionin hexadecaneThe amount of analyte extracted increased byincreasing the VOCs solution volume until reaching avalue at which the amount extracted remained ap-proximately constant.The volume varied from 50 to 1500mland theconcentration of VOCs in the solution ranged from460 ng / ml (dodecanal) to 770 ng / ml (2,4-pen-tanedione). The solution was placed in a 15-mlsealed vial and sampled as described in the Ex-perimental section. Triplicate extractions were per-formed. The relative areas of the chromatographicpeaks versus the solution volume are shown in Fig.2.Most of the VOCs showed a plateau from 500 to1000mlonwards, whereas the compounds with ahigher molecular mass showed a peak using 300ml.A solution volume of 1000mlwas selected forfurther experiments.3.2.3.Packaging amountThe influence of the packaging amount was alsostudied. The sample amount ranged from 30 to 200cm2. The samples were bent in order to introducethem into a 15-ml sealed vial and expose themaximum polyethylene surface in the headspace.Triplicate extractions were performed. The relativeareas of chromatographic peaks versus the packagingsurface are shown in Fig. 3.The signals increased by increasing the packagingamount from 30 to 120 cm2, most of the analytesreached a peak at 120 cm2, and then the signalsdecreased with increasing packaging amount due toproblems introducing this amount of packaging inthe vial with enough free surface for the masstransport. A packaging amount of 120 cm2wasselected for further experiments. The thickness of thepackaging material was 85mm,therefore the packag-ing amount for 120 cm2was 1.02 ml, which isapproximately equal to the optimised volume ofhexadecane.3.3.Linearity study with VOC solutions inhexadecaneAfter optimisation of the HS-SPME variables, alinearity study was carried out. A 1-ml sample ofVOC standard solution in hexadecane was placed ina 15-ml sealed vial and processed as described in theExperimental section.Table 1 shows the ranges of the VOC concen-trations studied, the linear ranges, the limits ofdetection (LODs), the correlation coefficients (R) andthe relative standard deviations (RSDs) found. Alinear behaviour was observed when the concen-trations were low, together with a curved trend at´O.Ezquerro et al./J.Chromatogr.A985 (2003) 247–257251Fig. 1. Influence of the extraction time on the HS-SPME of VOCs from hexadecane solutions. For HS-SPME and GC–MS conditions, seethe text. 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