Moisture manegement for the succeful analysis of polymers wi, Artykuły naukowe, Polimery i ich analiza
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//-->Sensors and Actuators B 69�½2000.372–378www.elsevier.nlrlocatersensorbMoisture management for a successful analysis of polymers withchemical sensor systemsFrank Wellea,), Alexandra Mauera, Erwin-Martin Keilb, Michael SlamacaFraunhofer Institute for Process Engineering and Packaging, Giggenhauser Straße 35, Freising 85354, Germanyb¨Perkin Elmer Verkauf und SerÕice, Rengoldshauser Straße 11, Uberlingen 88662, GermanycPerkin Elmer Food and BeÕerage Business Unit, Gotzinger Straße 56, Munich 81371, GermanyReceived 27 September 1999; accepted 22 February 2000AbstractThis study focuses on the moisture impact on headspace analysis and its implications to the use of chemical sensor systems with aspecial focus on polymer applications. The results showed clearly that different moisture contents of the polymer samples influence theheadspace composition of the samples and change the sensor discrimination. However, the influence of the headspace composition is notonly a sensor problem since it influences also the results of gas chromatographic headspace measurements as well as it could influencethe human sensory perception. For the quality assurance with a QMB-based sensor system, a pragmatic solution was presented. Thepolymers, HDPE and PET, could be analyzed directly in the presence of unknown moisture contents after a calibration that took care ofthe water-influenced vector. The polymer, PA, required a sample treatment prior to analysis.q2000 Elsevier Science S.A. All rightsreserved.Keywords:Moisture management; Polymers; Chemical sensor systems1. IntroductionSince a few years ago, chemical sensor systems havebeen on the market to assess the quality of polymer-madeproducts. For example, the automobile industry uses sensorsystems to assess the sensory influence of componentsmade of polymers like poly olefines, poly amide or otherpolymers. Chemical sensors are also suitable for the qual-ity control of polymeric materials for pharmaceutical orfood packaging. This kind of quality assurance is possiblebecause of the fact that the content of volatile compoundsin polymers describes a characteristic feature of the prod-uct. In practice, three sensor types are commercially avail-able: sensor systems based on metal oxide sensors�½MOX.,conducting polymers�½CPs.or quartz microbalances�½QMB.. Usually, all sensors are combined with a headspacesampling unit.Chemical sensor systems are capable of detecting awide range of molecules in the headspace because theyconsist of an array of sensor elements with a different)Corresponding author. Tel.:q49-8161-491-724; fax:q49-8161-491-777.E-mail address:fw@ivv.fhg.de�½F. Welle..selectivity to each volatile component. However, most ofthe sensors usually recognize not only volatile compoundswhich are part of the intrinsic product feature, but they arealso influenced by environmental parameters like moisturew1x. In consequence, sensors based on CPsw2xas well asMOX systems are strongly influenced by moisture. Suchsystems require a moisture management system to preventan incorrect analysis of the polymer samples. QMB-basedsystems usually show a smaller influence of moistureaccording to the low molecular weight of water. However,even in the case of a QMB-based sensor, the influence ofwater on the sensor signals cannot be neglected. This couldhave an impact on sensor application on a quality assur-ance of polymer products.The starting point to control the influence of humidityon sensor systems, which were applied in practice, ismeasuring the moisture content of each sample or tomoisturize the carrier gas of the sensor system. But mea-suring the moisture content and correlation of the resultswith standard samples of known moisture content raisesthe calibration expense of the measurements significantly,which is in most cases unsuitable for routine applications.Moisturizing the carrier gas only records the influence onthe sensor system itself by masking the humidity influence0925-4005r00r$ - see front matterq2000 Elsevier Science S.A. All rights reserved.PII: S 0 9 2 5 - 4 0 0 5�½0 0.0 0 4 9 0 - 1F. Welle et al.r Sensors and Actuators B 69 (2000) 372–378373on the sensor system with large amounts of moisture. Thelatter is not able to assess the influence of moisture on theheadspace composition of the polymer samplesw3,4x.This paper focuses on the humidity impact on headspaceanalysis and its implications to the use of chemical sensorsystems with a special focus on polymer applications.Different polymer samples were spiked with solvents asmodel contaminants and were examined with a specialemphasis on the dependency of the overall headspacecomposition from any moisture content. The aim of theseattempts was to find practical solutions for a successfulmoisture management system. Up to now, the still opendiscussion about a moisture influence prevented the accep-tance of sensor systems as quick quality control tools.panol,n-hexane,methyl ethyl ketone�½MEK., ethyl acetateand toluene.The following procedure was used for sorption of themodel compounds in the polymers. The polymer was putin a glass jar. The sorption mixture was placed into abeaker in the glass jar so that the contamination of thepolymer with the sorption mixture ensued only over thegas phase. The screw cap of the jar was isolated from thecontaminants using an aluminum foil in order to avoid asorption of the chemicals in the plastic screw cap. Thepolymer stands at 508C in a drying chamber for 3 weeks inthe case of PA and HDPE and 7 days in the case of PET.The successful contamination was detected by headspaceGC.2.4. Standard solutions in PEG 4002. ExperimentalCommercially available polymers were spiked withsmall amounts of chemical substances�½model contami-nants.of different polarity and volatility. The influence ofthe moisture content of the polymers was investigated byheadspace gas chromatography�½GC.rflameionisation de-tector�½FID.and by a QMB-based sensor system. Polyethy-lene glycol with an approximate molecular weight of 400 gmoly1�½PEG 400.was used in this study as a liquid modelpolymer. The liquid nature of PEG 400 allowed the appli-cation of standard solutions of model substances and en-abled an exact dosage of water to the high molecularmatrix.2.1. Test samplesThe following polymers were chosen for the measure-ments:PETPA 6HDPEPEG 400EASTMAN 9921 WBASF UltramidRigitex 580 25AMerck eurolabPEG 400 was used in this study as a liquid modelpolymer. A standard solution of the six model compoundswas prepared in PEG 400 with a concentration of 167 ppmof each compound. This standard solution was split up intoequal parts. Subsequently, water was added to the standardsolutions with concentrations of 0.1%, 0.25%, 0.5%, 1%,5%, 7.5% and 10% of water. In addition, standards ofwater in PEG 400 were prepared as reference sampleswithout contaminants. The influence on the headspacecomposition of these standard solutions was analyzed byheadspace GC as well as with a QMB system.2.5. Moisture contamination of polymer samplesSamples of the polymers, HDPE, PA and PET, wereprepared with different moisture contents by following theprocedure. An amount of 1.0 g of the polymer�½neat orwith model contaminants.was given into a 22-mlheadspace vial. In each headspace vial was placed an insertfilled with 100mlwater, the vial was sealed immediately,and stored for 3 days at different temperatures�½238C, 408Cand 608C.. After the storage time, the inserts were re-moved and the vials resealed. According to the differentstorage temperatures, the polymer samples contained dif-ferent amounts of moisture. Control groups of polymersamples without water contact were treated, stored andresealed in the same way.2.6. Headspace GCrFID measurementsGas chromatograph: Perkin Elmer AutoSystem XL,combination of DB VRX 30 m–0.32 mm i.d.–1.8mmfilmthickness and DB 624 30 m–0.32 mm i.d.–1.8mmfilmthickness, temperature program: 408C�½4 min., heat rate108C miny1, 2408C�½10 min., pressure: 50 kPa helium,split: 10 ml miny1. Headspace autosampler: Perkin ElmerHS 40 XL, oven temperature: 808C, needle temperature:1008C, transfer line: 1008C, thermostatization time: 902.2. Pre-drying of the polymersThe polymers, HDPE and PA, were pre-dried for aminimum time of 15 h at 608C. PET was dried for 15 h at1708C in a drying chamber. PEG 400 was degassed in awater bath at 808C under vacuum�½40 mbar.for a mini-mum of 2 h.2.3. Sorption of the test samples with model contaminantsForty grams of the polymers, PET, PA and HDPE, wasinserted in a 200-ml glass jar and contaminated withoutdirect contact at 508C in a drying chamber with 4 ml of amixture of the following model substances: acetone, 1-pro-374F. Welle et al.r Sensors and Actuators B 69 (2000) 372–378min, pressurizing time: 3 min, injection time: 0.04 min,withdrawal time: 1 min. The FID signals in the gaschromatograms were integrated.Retention times of the model contaminants under theapplied GC conditions: acetone, 6.5 min; 1-propanol, 8.2min;n-hexane,8.9 min; MEK, 9.3 min; ethyl acetate, 9.7min; and toluene, 14.3 min.2.7. QMB sensor measurementsPerkin Elmer Chemosensory System QMB 6 with HS40 XL autosampler, oven temperature: 808C, transfer line:808C, sensor temperature: 808C, thermostatization time: 60min, pressurizing time: 1:30 min, zero level: 0:30 min,building up: 2:30 min, signal level: 0:30 min, purge: 15min.3. Results and discussion3.1. PEG 400 as model polymerIn this study, PEG 400 was used as a high molecularmatrix for headspace and sensor measurements. The liquidnature of PEG 400 enabled the preparation of standardswith a defined concentration of the model contaminants.Also, the water influence on the headspace compositioncan be easily investigated using these standard solutionswhich were spiked with different water amounts from0.1%�½vrv.up to 10%�½vrv..The influence of water on the headspace composition ofPEG 400 measured by GCrFID is summarized in Table 1.The results are given in area counts of the FID detector asmean value of two independent experiments. Control ex-periments with a lower equilibration temperature of 608Cshow the same relative headspace composition, indicatingthat the equilibrium between the headspace and the PEGmatrix had been reached at the applied temperature of808C for 90 min.The relative influence on the headspace composition ofeach model contaminant was calculated in percent of thedry reference sample by a comparison of the area counts ofthe GCrFID and the area counts of reference sampleswithout water�½Fig. 1.. Between 0% and 1% water, areduction of the model contaminants was measured for allsamples and model contaminants. This is probably due tothe change of the molecular structure of the pre-dried PEG400.The relative amount of each model compound on thesum of area counts indicated the relative headspace com-position over the PEG 400 matrix�½Fig. 2.. With anincreasing polarity of the analyte, the relative amount inthe headspace composition was lower. This indicated thatpolar substances play a minor role on the headspacecomposition of polar polymers. Changing the amount ofwater in the PEG 400 matrix, the share of the polarsubstances�½1-propanol, acetone, ethyl acetate and MEK.remained nearly unchanged. In comparison, the relativeamount of non-polar model contaminants liken-hexaneand toluene has changed significantly by an increasingwater amount in the PEG matrix. Thus, regardless of anydirect influence of water on the sensor, classes of onesample will drift within the graphic feature space onlybecause of increasing moisture. Moisturizing of the carriergas presents no solution for the observed influence on theheadspace composition. On the other hand, if only polarcontaminants should be detected in a polar polymer matrix,Fig. 1. Influence of the moisture content�½relative area counts, 0%waters100.of PEG 400 spiked with six solvents.Table 1Influence of water on the headspace composition of model polymer PEG 400 spiked with six volatile compoundsWater content�½vrv.Without0.1%0.25%0.5%1%5%10%Area counts of analyteAcetone1-Propanol125,854120,17591,12094,197115,134120,859131,05750,59850,93137,91939,84747,95046,23351,296n-Hexane381,711288,220240,219219,304292,359307,991301,760MEK144,665141,394106,684108,291135,324145,907166,864Ethyl acetate128,018124,48396,58696,098120,715132,239151,091Toluene224,580224,633168,778176,001220,735269,063357,656Data obtained by HS-GCrFID.F. Welle et al.r Sensors and Actuators B 69 (2000) 372–378375Fig. 2. Influence of moisture on the relative headspace composition ofPEG 400 spiked with six solvents.the absolute amount, i.e. area counts, increased by increas-ing moisture content, but the relative headspace composi-tion remained nearly similar. Therefore, in the case ofQMB sensors, a normalization of the signal intensitymarked no influence of moisture on the sensor graphicfeature space.3.2. High-density polyethylene HDPEFirst, the influence of water on the headspace composi-tion of pre-treated and non-treated HDPE was investigatedby GCrFID�½Table 2.. Only the non-polar compounds,n-hexaneand toluene, could be detected in the headspace.This indicated that the other compounds are either notabsorbed at sample pretreatment conditions, or that thesecompounds were not released into the headspace. In ab-sence of a water treatment, the headspace concentrationwas higher.The sensor measurements of the HDPE samples wereused to firstly determine a description plane by a fewcalibrated classes. Additionally, test classes of knowncomposition were projected into this calibration plane�½Fig.3a.. The sensor measurements revealed an influence of themoisture content and an influence of the solvent mix, butboth influences could not be clearly distinguished becausethe corresponding vectors showed different directions. Theclasses with an increased moisture content showed a gradi-ent as well as the classes with an increased solvent content.Furthermore, it could be observed that the classes HDPEaa,HDPEab,HDPEba,and HDPEbbpresented thecorners of a parallelogram. The first letter indicates themoisture level —a no water,bmaximal water, and thesecond letter indicates the solvent content —ano solvent,—bsolvent pre-treated. Thus, the influence of moisturecould be covered up by a merge of the classes with thesame solvent content and subsequent change of the projec-tion�½Fig. 3b.. The influence of the moisture content isnow perpendicular to the discrimination plane and couldnot be recognized any more. Indeed, the control by anindependent test sample that contains a not calibrated levelof moisture was recognized correctly as a solvent-con-taminated HDPE.Table 2Results on the influence of the headspace composition of HDPE, PET and PA spiked with six volatile compounds and stored for 3 days at 238C, 408C or608C in the presence of waterArea counts of analyteAcetoneWithout H2OHDPE, 238CHDPE, 408CHDPE, 608CHDPE, 238CHDPE, 408CHDPE, 608CPET, 238CPET, 408CPET, 608CPET, 238CPET, 408CPET, 608CPA, 238CPA, 408CPA, 608CPA, 238CPA, 408CPA, 608Cn.d.n.d.n.d.n.d.n.d.n.d.63,01194,226218,07452,47664,674159,1692435366629,96875,285173,959226,1391-Propanoln.d.n.d.n.d.n.d.n.d.n.d.4,239,955a4,460,810a5,264,028a3,263,635a2,587,386a3,229,553a468,742620,8192,699,321a4,127,556a4,852,575a4,809,800an-Hexane3794250222959989971515n.d.n.d.n.d.n.d.n.d.n.d.667713355227,40887,87317,568MEKn.d.n.d.n.d.n.d.n.d.n.d.1,717,9722,130,6423,097,664a1,395,3411,559,4842,593,696a27,498367,282249,6361,132,4632,510,5022,764,118Ethyl acetaten.d.n.d.n.d.n.d.n.d.n.d.808,8931,020,0161,857,819767,712964,3531,644,97312,28316,07197,767491,0881,293,1821,551,097Toluene2,913,173a2,899,804a2,948,718a2,826,021a2,887,339a2,934,868a4,285,327a4,372,503a4,899,495a4,339,444a4,413,955a4,932,942a146,932173,8761,251,5013,915,853a4,816,374a5,139,707aWith H2OWithout H2OWith H2OWithout H2OWith H2OData obtained by HS-GCrFID.n.d., not detectable.aResponse)1000 mV and therefore not correctly integrated.376F. Welle et al.r Sensors and Actuators B 69 (2000) 372–378measured by HS-GCrFID. The results were compared todry, but solvent-treated, PET�½Table 2.. The headspace ofthe solvent-pre-treated PET contained five components:acetone, 1-propanol, MEK, ethyl acetate and toluene. Onlyn-hexaneas the most non-polar component in the solventmix could not be found in the headspace. It was observedfor all components in the headspace that the concentrationFig. 3.�½a.Graphic feature space of HDPE. Measurements of calibratedclasses�½q.spanned the discrimination plane whereas independent testsamples�½`.of known composition were projected into this plane.aa—dry, without solvent;ba— moisturized, without solvent;ab— dry,contaminated by solvents;bb— moisturized, contaminated by solvents.Vectoraa–badirects to an increased moisture; vectoraa–abdirects to anincreased solvent concentration.�½b.Graphic feature space of HDPE.View perpendicular to the moisture vector.aa–ba,samples withoutsolvent, but different contents of moisture.ab–bb,samples contaminatedby the same solvent concentration, but different contents of moisture.Control by independent test samples�½`.of unknown moisture, butcontaminated by solvents.3.3. Poly(ethylene terephthalate) PETThe analysis of PET samples started with GCrFIDinvestigations about the moisture influence on the head-space composition prior to any investigations with a QMBsensor. Pre-dried PET was contaminated with a solventmix and moisturized as described above, and subsequentlyFig. 4.�½a.Graphic feature space of PET. Measurements of calibratedclasses�½q.spanned the discrimination plane whereas independent testsamples�½`.of known composition were projected into this plane.aa—dry, without solvent;ba— moisturized, without solvent;ab— dry,contaminated by solvents;bb— moisturized, contaminated by solvents.Vectoraa–badirects to an increased moisture; vectoraa–abdirects to anincreased solvent concentration.�½b.Graphic feature space of PET. Viewperpendicular to the moisture vector.aa–ba,samples without solvent, butdifferent contents of moisture.ab–bb,samples contaminated by the samesolvent concentration, but different contents of moisture. Control byindependent test samples�½`.of unknown moisture, but contaminated bysolvents.
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