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Available online at www.sciencedirect.com

Sensors and Actuators B 132 (2008) 90–98

Fabrication and characterisation of microporous activatedcarbon-based pre-concentrators for benzene vapours

F. Blanco a, X. Vilanova a, V. Fierro b, A. Celzard c, P. Ivanov d, E. Llobet a,N. Canellas a, J.L. Ramırez a, X. Correig a,∗

a Departament d’Enginyeria Electronica, Electrica i Automatica, Universitat Rovira i Virgili, Avda dels Paısos Catalans, 26, 43007 Tarragona, Spainb Laboratoire de Chimie du Solide Mineral, UMR CNRS 7555, Nancy-Universite, BP 239, 54506 Vandoeuvre-les-Nancy cedex, France

c Laboratoire de Chimie du Solide Mineral, UMR CNRS 7555, Nancy-Universite, ENSTIB, 27 rue du Merle Blanc, BP 1041, 88051 Epinal cedex 9, Franced CSIC, CNM, IMB, E-08193 Bellaterra, Spain

Received 17 September 2007; received in revised form 28 December 2007; accepted 6 January 2008Available online 17 January 2008

bstract

During the last years, the use of gas pre-concentrators coupled to gas sensors became more important due to the difficulty of fabricating gasensors with sensitivities in the range of tens of ppb (or lower). Among the different adsorbents used in pre-concentrators, carbons are the mostsual, since they are very common materials in chemistry, and because their adsorption performances are very high. In the present work, a familyf microporous activated carbons prepared from chemical activation of Kraft lignin was used as adsorbents, and compared with a commercialaterial. The carbonaceous adsorbents were sprinkled onto self-heated alumina substrates using a binding layer deposited by screen-printing. Their

haracterisation was performed by means of a GC–MS with a 6-way valve injection system. Concentration factors higher than 1000 for benzene

ave been achieved with some particular adsorbents. In the present work, the microporous activated carbons and Carbopack have been adheredver the self-heated surface of pre-concentrators. The performances obtained for all the microporous activated carbons are much higher than thosebtained with the commercial carbon black Carbopack X, so these lab-made materials are very promising adsorbents for micro pre-concentrators,hich could be used in conjunction with sensors in gas detection systems.2008 Elsevier B.V. All rights reserved.

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eywords: Pre-concentrators; Carbon-based adsorbents; Semiconductor gas se

. Introduction

Among the hazardous volatile organic compounds (VOCs)hat can be found in the atmosphere as a result of industrialctivity and transport, one of the most dangerous is benzene,hich is known to be carcinogenic at repeated low-concentration

ppb) exposures [1]. In the EU, the exposure level for ben-ene is 1.6 ppb, while the ones for toluene and xylene are 70nd 200 ppb, respectively [2]. Exposure to VOCs is likely toccur in the petrochemical industry, in petrol stations, in carepair facilities, when building roads, etc. Additionally, benzene

s a possible contaminant found in CO2 used in the beveragendustries that should be monitored on-line [3]. In this context,here is a need for systems that could continuously monitor

∗ Corresponding author. Tel.: +34 977559623; fax: +34 977559605.E-mail address: xavier.correig@urv.cat (X. Correig).

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925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2008.01.016

enzene concentration (and some other VOCs) at ppb levels.here are also many other situations in which the on-line mea-urement of toxic vapours at ppb or tens of ppb would be ofigh interest. Solid state microsensors have been successfullypplied for measuring gases and gas mixtures concentrations4].

Semiconductor gas sensors (also known as chemoresistors)re a good alternative to other detectors, given their low cost,cceptable sensitivity (ppm level), easiness of control and com-atibility (to some extent) with microelectronics technology [5].heir main drawbacks are low selectivity and response drift. Innvironmental applications, it is necessary to selectively mea-ure one or a few species at concentrations that range betweennits and tens of ppb. Therefore, during the last years, research

n semiconductor sensors has focussed on new materials withigher surface areas such as nanostructured [6] materials oraving nanometric grain size [7], which resulted in improvedensitivity and selectivity. Despite these efforts, it is not likely

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F. Blanco et al. / Sensors an

hat sensitivity and selectivity could reach the values neededo meet the specifications of VOC monitoring, in the short or

id-term. Under these conditions, adding a concentration stagepre-concentrator) and a separation step can not be avoided,espectively allowing a sensitivity enhancement of two or threerders of magnitude and enabling the identification of the com-onents within a gas mixture (using a chromatographic device,or example). Silicon micromachining technologies allowiniaturising these concentration and separation components,aking them compatible with sensors. The result is a portable

normally hybrid) microsystem for the analysis of gas-phasenalytes.

Terry and co-workers were the group that pioneered thispproach in the 1970s [8]. Many other teams subsequentlyntered this field of research (see the review in [9]) and reportedifferent systems based on chromatographic columns coupledo pre-concentrator systems. Pre-concentrators consist of bothn adsorbing material confined in a reduced volume and a sys-em able to heat this material. The adsorbing material trapsnalytes when kept at a moderate temperature (e.g., room tem-erature), and desorbs them when suitably heated. Because thedsorption time is higher than the desorption one, concentrat-ng analytes is possible. Currently used discrete concentratorsonsist of stainless steel (or Pyrex) tubes filled with an adsorb-ng material (usually, commercially available activated carbonranules), a resistance for heating the system up to 300 ◦C, andthermo-couple for controlling the heating cycles. Concentra-

ors are connected either to a separation column (in case thatomponent separation is judged necessary) [10] or directly to aensor chamber [11].

As seen from the review of the open literature, the materialssed as adsorbents are, in most cases, commercially available.hey belong to three different types: (a) powders of inorganicxides; (b) polymeric materials; and (c) materials based on car-on particles. As an example of the first type material, Ueno etl. [12] developed a microfluidic system for the selective detec-ion of benzene, toluene and xylene (BTX) based on a siliconre-concentrator, and used mesoporous silica powder (SBA-15)s an adsorbent. However, the lower detection limit was in thepm range only. Concerning the application of polymers, Kimnd Mitra [13] designed a micromachined silicon concentra-or and used OV17 as the adsorbing material. More recently,he same author has reported a technique for immobilising solidhase extraction fibres in poly-dimethyl-siloxane (PDMS) chan-els [14]. Basile [15] reported the use of solid phase microxtraction fibres as an adsorbing material in a BTX concen-rator, the sensitivity of which reached the range of tens of ppb,nd Voiculescu et al. [16] used HCSA2 polymer injected with aiezoinkjet dispensing unit. Finally, the so-called MicroChem-ab was an integrated micro-GC including a pre-concentratorased on microporous hydrophobic sol–gel coating or a polymeroating [17].

Nevertheless, the reported performances of the afore-

entioned devices are still lower than those obtained with

ommercial carbon particles (Tenax, Carbopack, Carboxen,arbosieve, etc. [18]), that are more effective materials. Dr.homas’s group [19], from Sandia National Laboratories,

if4S

uators B 132 (2008) 90–98 91

esigned an analytical system for the measurement of xyleneoncentration (lower detection limit of 60 ppb) that integrates

carbon-based concentrator fabricated over a thin mem-rane coupled to an array of four chemiresistors based ononducting polymers. Zellers’s group, from Michigan Uni-ersity, has introduced recently the first generation hybridEMS gas chromatograph, a complete gas analysis system

hat includes a silicon-based multi-stage concentrator basedn commercial carbons Carbopack B and X and Carboxen20,21]. The concentration factors reached values near 100022].

Generally speaking, the higher is the surface area of theonsidered adsorbent, the higher are the corresponding adsorp-ion performances. This is the reason why activated carbonshould be of great interest in the fabrication of concentrators,specially for benzene detection and quantification. Such mate-ials may indeed possess very narrow pores, called microporeshaving widths lower than 2 nm), that are particularly suitableor the strong retention of small molecules in the gas phase.

oreover, benzene adsorption at room temperature is a wayf measuring micropore volumes [23]. Activated carbons arerepared from various carbonaceous precursors (mineral formsf carbon like lignite or anthracite, polymers, or biomass likeood or any other lignocellulosic matter) by heat-treatmentnder various conditions. Either the precursor is pyrolysed andxidised with suitable gases (generally steam or CO2), or its impregnated with dehydrating agents (like strong acids orases, or metallic salts) and heat-treated. The first procedure isalled “physical activation”, and the second one is known aschemical activation” [24]. Activation is thus the treatment byhich the porosity of a carbonaceous precursor is both open

nd developed, resulting in sometimes extremely high surfacereas (up to 3000 m2/g). However, the adsorption performancesre even more closely related to the pore-size distribution thano the surface area. One major advantage of activated carbons,s compared with other porous materials, is that their poreexture may be finely tuned for designing optimised adsor-ents. Activated carbons are then expected to lead to higherformances in concentrators, higher than those reported withommercial carbons like Carbopack which are, strictly speak-ng, carbon blacks (a kind of soot), evidently having muchider pores and hence a much lower surface area (see next

ection).The present article is organised as follows. In Section 2,

he preparation and the main characteristics of lignin-derivedctivated carbon adsorbents are detailed, and the fabricationf the pre-concentrators is presented in Section 3. These pre-oncentrators might be used coupled with a semiconductorensor matrix in order to produce a system capable of detect-ng selectively benzene with a resolution of tens of ppb. Such aetection device is needed for the quality control of industrialO2 used for the carbonated beverages industries. Following

he regulation [3], the concentration of benzene contamination

n CO2 must be kept below 20 ppb. The experimental set-upor pre-concentrators characterisation is described in Section. The main results and discussion are finally developed inection 5.

92 F. Blanco et al. / Sensors and Act

Table 1Main characteristics and amounts of the active carbons present in the fabricatedpre-concentrators

Carbonaceous adsorbent Type A Type B Type C Carbopack XSynthesis temperature (◦C) 700 700 770 –Activating agent KOH KOH NaOH –Weight ratio MOH/lignin 1/1 5/1 3.3/1 –Surface area (m2/g) 1300 1800 2300 250Gurvitch volume (cm3/g) 0.55 1.13 1.65 0.48Micropore volume (cm3/g) 0.54 0.74 1.13 ∼0Pore diameter (nm) 0.62a 2.08a 3.37a ∼7b

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a Average pore diameters.b Pore diameter at the maximum of the micropore-size distribution.

. Preparation and characterisation of microporousctivated carbons

Three activated carbons were prepared from Kraft ligninsupplied by LignoTech Iberica) according to the procedurextensively detailed in the paper of Fierro et al. [25]. Two kindsf alkaline hydroxides (NaOH and KOH) were used at dif-erent temperatures (700 and 770 ◦C) in various proportions;he experimental conditions are given in Table 1. Very briefly,he synthesis of the activated carbons was achieved as follows.he commercial Kraft lignin was first thoroughly de-ashed byn extensive washing with sulphuric acid, followed by wash-ng with distilled water and drying. The material thus obtainedas then physically mixed with granules of alkaline hydrox-

de, and the blend was introduced into a nickel crucible whichas installed inside an electric oven. The latter was flushed withitrogen (flow rate of 200 mL/min) and heated at 5 ◦C/min upo the final temperature, which was held for 1 h before coolingown to room temperature under nitrogen flow. The materialas subsequently recovered, washed with distilled water, dried,

nd characterised.The pore texture parameters (surface area and pore volumes)

f these activated carbons were determined from the correspond-ng nitrogen adsorption–desorption isotherms obtained at 77 K

ith an automatic instrument (ASAP 2020, Micromeritics); the

urves are given in Fig. 1. For that purpose, samples were out-assed, and their adsorption data at relative pressures rangingrom 10−5 to 0.99 were analysed. The micropore volume, corre-

ig. 1. Nitrogen adsorption–desorption isotherms at 77 K on the activated car-ons prepared from the chemical activation of Kraft lignin.

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uators B 132 (2008) 90–98

ponding to pores narrower than 2 nm, was calculated accordingo the Dubinin–Radushkevitch method [26] and refs. therein.he total pore volume, sometimes referred to as the so-calledurvitch volume, was defined as the volume of liquid nitro-en corresponding to the amount adsorbed at a relative pressure/p0 = 0.99 [27]; the Gurvitch volume is assumed to be the sumicro + mesopores volumes. The relevant pore texture param-

ters are reported in Table 1, and those of Carbopack X (afterruk et al. [28] and Li et al. [29] and refs. therein) are given

or comparison. It can be clearly concluded that the 3 lab-madectivated carbons are essentially microporous, with high surfacereas, while Carbopack X, which can be seen as a packing aon-porous spherical particles, has a low surface and is a purelyesoporous material (i.e., the only pores correspond to the voids

etween the carbon spherules).Given the shape of the adsorption–desorption isotherms, a

ew comments should be made. The type A activated carbonxhibits an adsorption curve characteristic of the so-called typemorphology [30]. This material is therefore almost purelyicroporous. The very sharp slope at the very beginning of the

urve immediately followed by a horizontal plateau indicateshat the pore-size distribution is very narrow. Type A adsorbents thus characterised by narrow micropores of similar widths (seeor example Perrin et al. and refs. therein [31]). The adsorbedmount at a relative pressure p/p0 < 0.2 corresponds to the micro-ore volumes: types B and C materials thus present slightlyigher micropore volume than type A, but the correspondingider knees of the isotherms (now sloppy up to relative pres-

ures near 0.5) also indicate a widening of the micropore sizeistribution. Since its desorption branch exhibits a very smallysteresis, the activated carbon of type B is poorly mesoporous.owever, type C exhibits a very sloppy isotherm with a largeysteresis, indicating that the mesoporosity is now well devel-ped. In summary, A has identical narrow micropores only, Bs microporous but the distribution of pore sizes is broad, and

has an even broader distribution of sizes since mesopores areresent. All these features, summarised by the changes of aver-ge pore size given in Table 1, may explain the various adsorptionapacities of benzene and the related concentration factors (seeelow).

. Fabrication of pre-concentrators

The usual pre-concentrators based on activated carbons con-ist of quartz or stainless steel tubes with a heater, filled with thedsorbents. These latter materials are very light and can movenside the tube due to the gas stream dragging; it is thus neces-ary to include fibreglass or filters as stoppers, in order to avoidhe obstruction of the pipes. This problem is particularly impor-ant for integrated micro-concentrators based on silicon, due toheir reduced dimensions and because introducing the activatedarbons inside the tubes is uneasy.

Due to the aforementioned facts, another strategy was consid-

red, consisting in the deposition of the active material over a flatelf-heated surface (or membrane). The main advantage is obvi-usly that the adsorbent cannot move inside the pre-concentratornd therefore cannot stop the air flowing inside the overall sys-

F. Blanco et al. / Sensors and Act

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ig. 2. Heated alumina substrates used for the fabrication of pre-concentrators.

em. As a deposition method, screen-printing techniques coulde used, since they are very well known and because a higheproducibility can be achieved with them. Moreover, the usef thin membranes (micro-hot-plates) could reduce the poweronsumption and allow an accurate control of the temperatureycles and the integration with other subsystems, like a sensoratrix. As a main drawback of this approach, there is the loss of

as concentrating efficiency due to the 2-D nature of the device,nd also the reduction of active surface of the carbons lying overhe substrate.

The substrate used consisted of a 1 cm × 1 cm sheet of alu-ina with a Pt heater and a Pt resistance used for temperature

ontrol (see Fig. 2). Over the substrate, a layer of non-corrosivedhesive sealant Tempflex (Loctite 5145) was applied by screen-rinting, capable to withstand the temperatures needed foresorption (up to 250 ◦C) without releasing organic volatiles.he average thickness of Loctite layers was about 70 �m and

he active area (alumina area covered by adsorbent) was 16 mm2.he carbon particles were then sprinkled over the substrate. The

atter was carefully weighed before and after deposition of thearbonaceous adsorbents, in order to know the total amounteposited on it.

As adsorbent layers, the three lab-made activated carbonseferenced in the previous section were used, together with Car-opack X (supplied by Supelco) [18] for the sake of comparison.our concentrators were prepared for each adsorbing material.he amount of deposited carbon strongly depended on the sizef the grains. The grains of activated carbons of types A and Cere very small, so the surface substrate was coated by a thin

ayer of carbon powder, hence corresponding to small depositedmounts (between 300 and 500 �g). In the case of activated car-on B and Carbopack X, the grains were much bigger, and theeposited quantities ranged from 1 to 2.5 mg.

In Fig. 3, front and cross-section views of pre-concentratorsabricated with lab-made activated carbons and with Carbopack

are presented. It can be seen that, in all cases, a large amountf material has been deposited, completely coating the surfacef the substrate (Carbopack X and type B activated carboneading to much thicker layers). In the cross-sections shown

n Fig. 3, the Loctite layer between the carbon grains and thelumina substrate can be observed. No degradation of the activeayer after characterisation of the pre-concentrators could bebserved.

uators B 132 (2008) 90–98 93

. Pre-concentrator characterisation setup

The performance of a system sensor including a pre-oncentrator depends not only on the characteristics of the latterconcentration factor), but also on the test chambers’ volumes ofre-concentrator and sensors, and on the total volume of pipesetween them. The higher are the volumes, the higher is theilution, leading to a less concentrated analyte. Nevertheless,he flow that carries the volatiles from the pre-concentrator tohe sensor must be calculated according to the volumes of theest chambers and to the sensor response velocity. In addition,he response of the sensors is often not known and is stronglyffected by the temperature of the gas stream; it is thus uneasy toeparate the response due to the concentrated analyte from thenterference due to the temperature changes during desorptionycles. In most of the published literature, the characterisation ofhe pre-concentrators is performed in the whole detection device,ncluding the gas sensors. This way of doing has the advantagef taking into account all the factors and subsystems involvedn the device, but it is not advisable if we need to characterizenly the pre-concentrator alone.

In the present case, the performances of the carbonaceousdsorbents and the behaviour of the pre-concentrators needed toe evaluated, so a gas chromatograph coupled to a mass spec-rometer (GC–MS) system with a 6-way valve injection systemas used. The latter system is very reliable, allowing to deter-ine accurately the amount of benzene desorbed by the adsor-

ents and injected into the GC–MS (see sub-section (d)) below).he whole characterization set-up is described in Fig. 4. Picturef the pre-concentrator test chamber, fabricated in Teflon, is pre-ented in Fig. 5. Its volume is very small (about 180 �L), in ordero avoid analyte dilution during the desorption phase.

The characterisation procedure was the following:

(a) Purge of the pre-concentratorIn this preliminary phase, any contaminant adsorbed in

the pre-concentrator was eliminated. The 6-way valve wasset in the configuration 1–2, 3–4 and 5–6, allowing the flush-ing with He (or N2) of the pre-concentrator, which washeated for desorption.

b) Concentration phaseThe 6-way valve was set in the same configuration as

in (a), and the manual 3-way valve was switched to thecalibrated gas bottle (150 ppb of C6H6 with balance ofCO2), allowing the benzene to be adsorbed into the pre-concentrator during a period ranging from 5 to 90 min atroom temperature. The gas flow rate used in this step wasset to different values, from 100 to 400 mL/min.

(c) Flushing before desorptionWith the 6-way valve in the same configuration, the man-

ual valve was switched to He in order to clean the pipes.d) Desorption from the pre-concentrator

The 6-way valve was configured as 1–6, 3–2 and 5–4,allowing the injection of the pre-concentrator chamber con-tent to the GC–MS. In this cycle, the pre-concentratorwas heated during 30 s at 250 ◦C by means of a voltage

94 F. Blanco et al. / Sensors and Actuators B 132 (2008) 90–98

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pulse, and injected into the GC–MS. This is the highesttemperature that can bear the Templex. The benzene iscompletely desorbed from the pre-concentrator. The des-orption is excellent, provided that we are working with aflat pre-concentrator. We do not need greater and/or larger

desorption pulses, as in ref. [18]

After the pre-concentrator desorption, the area under theeak m/z = 78 was integrated, in order to calculate the amount

tiat

arbons of (a) type A; (b) type B; and (c) type C; and (d) Carbopack X carbon

f formerly adsorbed benzene, assuming that this peak isharacteristic of this aromatic compound. Before and afterny desorption cycle, a bottle target was performed. It con-isted in measuring the concentration of benzene inside thealibrated bottle benzene used for pre-concentrator adsorp-

ion. For that purpose, the content of the bottle was injectednto the GC–MS, under the same conditions consideredbove for the pre-concentrator desorption. The concentra-ion factor CF of the pre-concentrator was thus calculated

F. Blanco et al. / Sensors and Actuators B 132 (2008) 90–98 95

Fig. 4. Pre-concentrator characterisation set-up based on a GC–MS system.

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ABbefore + ABafter

here ADes is the area under the peak m/z = 78 after a pre-oncentrator desorption, and ABbefore and ABafter are the areasf the same peak for the bottle target measurements before andfter desorption, respectively.

. Results and discussion

The benzene concentration factor (CF) was evaluated forll the pre-concentrators, using different flow rates (100, 200nd 400 mL/min) and different adsorption times (from 5 to0 min), according to the aforementioned procedure. The chro-atographic peaks corresponding to benzene desorption of a

re-concentrator of type A for different adsorption times andas flow rates are presented in Fig. 6 . The peak area iseen to increase with adsorption time, indicating that no pre-oncentration saturation was reached.

In Fig. 7, the dependence of the CF on the desorption flow rateor each type of pre-concentrator is shown for different adsorp-ion times. The presented data are the average values and the

tandard deviations of the CF corresponding to each kind ofre-concentrator.

Fig. 7a evidences that, using type A carbon, a CF as highs 600 can be achieved with a 200 mL/min flow rate and

d

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ig. 6. Benzene desorption chromatographic peaks for (a) different adsorptionimes and (b) different flow rates.

0 min of adsorption time. Changing the flow rate to 100 and00 mL/min both resulted in slightly lower CF. However, theF was expected to increase with the flow rate, due to the cor-

espondingly higher amount of analyte passing throughout there-concentrator. An optimal flow rate means that a competi-ion between two antagonistic effects should exist. A calculationsing the micropore volume and the deposited amount of carbongiven in Table 1 evidenced that no saturation of the microp-

res by adsorbed (condensed) benzene occurred, even at theighest flow rate, 60 min after the beginning of the experiment.he phenomenon leading to a decrease of CF while the flow

ate is increased probably originates from the time required forhe benzene to evacuate the pore network of the carbon. At00 mL/min, more benzene was adsorbed, in particular in theeepest parts of the pore structure, thus requiring more timeo diffuse out of the carbon grains. Consequently, at identicalnite (small) desorption times, the amount of molecules releasedthus related to the concentration factor) is not proportionalo the benzene uptake. Consequently, it can be assumed that00 mL/min is excessive, so measurements were only performedetween 100 and 200 mL/min for all the other pre-concentratorypes.

It can be observed from Fig. 7 that the standard deviation ofhe pre-concentrator response of each type is in general mod-rate, except in the case of Carbopack X, that is high. A visualnspection of the four latter pre-concentrators fabricated revealshat the deposited carbon layer is rather irregular, supporting the

ifferences found in their response.

As can be seen from Fig. 7b, for type B pre-concentrator, theest CF (1150) was achieved using a desorption time of 90 minnd a 200 mL/min flow rate. For the two investigated flow rates,

96 F. Blanco et al. / Sensors and Act

Fig. 7. Concentration factors (CF) for different adsorption times and flow ratesfor pre-concentrators based on (a) type A; (b) type B; (c) type C activatedcarbons; and (d) Carbopack X carbon black.

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Carbonaceous adsorbent CF (at 200 mL/min and 60 min)

Type A 600Type B 800Type C 240Carbopack X 150

uators B 132 (2008) 90–98

good linearity between CF and adsorption time was observed.ype C (Fig. 7c) gave lower CF than in the previous cases, andtendency to saturation. The same kind of results is shown inig. 7d for Carbopack X, although concentration factors wereven lower. The CF’s found for the lab-made activated carbonsre among the best ever found in the open literature [22], and areuch higher that those obtained with the commercial Carbopack.It may also be concluded from Fig. 7 that after 90 min of

dsorption, the break-through volume, defined as the volumef gas that must pass throughout the pre-concentrator in ordero saturate the adsorbent with benzene, could not be reached.his result indicates that all the total adsorption capacity of thectivated carbons is not used. Nevertheless, the adsorption timeas not further increased for determining this factor because,

rom a practical point of view, it makes no sense using adsorp-ion times greater than 90 min. Indeed, according to the possiblepplication of the pre-concentrators, the total measurement cyclehould not be longer than 60 min. Consequently, adsorptionimes greater than 45 min have no interest.

The gas stream in the test chamber flows parallel to the pre-oncentrator, so does not enhance the introduction of gases intohe pre-concentrator and therefore adsorption. We hope that with

different test chamber design, in which the gas flows per-endicularly to the pre-concentrator, we could achieve betteroncentration factors.

It should be emphasised that the concentration factors pre-ented in Fig. 7 are directly related to the amount of adsorbentresent in the pre-concentrator. Now, as already explained inection 3, the amount of deposited activated carbon was stronglyependent on the size of the grains. Dividing the concentrationactor by the mass of deposited adsorbent (mg) gave a normalisedF (mg−1), reported in Table 2. Considering such values, theest result was found for type A material, far higher than theata corresponding to types B and C and even higher than thosef Carbopack X. Such results cannot be correlated to the sur-ace areas of the corresponding adsorbents, since the highesterformances are those of the activated carbon having the lesseveloped surface. Nevertheless, it should be recalled here thathe adsorption properties are more closely related to the poreolume and to the pore-size distribution of the considered mate-ial, and that the surface area is just an average property thatoes not account for the details of the pore texture. Therefore,wo materials having very different pore sizes may have simi-

ar surface areas, provided that their pore volumes are also veryifferent. Activated carbon A was the most microporous one,ith the highest number of very narrow micropores, thus hav-

ng the highest selectivity for benzene adsorption. Consequently,

Average deposited amount (mg) Normalised CF (mg−1)

0.3 20001.3 6150.4 6002.4 62

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F. Blanco et al. / Sensors an

t is not surprising that such a material can desorb the highestmount of benzene in a given time, since it is the one havingrapped the highest amount. As far as the other materials areoncerned, the lower is the micropore volume having the opti-al width, so the higher is the mesopore volume (useless for

enzene adsorption), the lower is the corresponding normalisedF. Because of that, the Carbopack X carbon black is definitely

he material presenting the lowest performances when used asn adsorbent for pre-concentrators.

In the introduction, the authors claimed the interest of are-concentrator that could be used in an analytical systemesigned for the detection of benzene present as traces (20 ppb)n CO2 used for carbonated beverages industries [3]. In suchn application, higher levels of methane (about 30 ppm) cane present in the CO2, in addition to the benzene concentra-ion (150 ppb). In this case, at such a low partial pressure,

ethane is not expected to interfere much with benzene adsorp-ion in the pre-concentrator. Indeed, even if the adsorption of

ethane and benzene occurs in the same kind of micropores,ethane is supercritical at room temperature so the correspond-

ng adsorbed amount is very small as compared to that of benzenet the same pressure [31]. This statement may be justified bybserving experimentally that the desorption of benzene remainsnchanged when compared to the situation in which methane isot present.

. Conclusion

In this paper, the properties of various carbonaceous materialssed as adsorbents for pre-concentrators (three lab-made acti-ated carbons and a commercial carbon black), were tested andompared. The adsorbents have been deposited over self-heatedlumina substrates. The adsorption–desorption cycles have beenrogrammed applying voltage pulses to the heater. A GC–MSith a 6-way valve injection system have been used in order to

haracterise the pre-concentrators.It was clearly shown that lab-made activated carbons, at

onstant mass, are those leading to the highest relative con-entration factors. These values are among the best foundn the literature. Such performances could not be correlatedo the corresponding surface areas, but to the volume of the

icropores having the relevant width. Such a finding is espe-ially important for preparing highly efficient pre-concentrators.ince each compound whose presence has to be detected anduantified possesses its own molecular diameter and affinityor the carbon surface, adsorbents having the highest volumef pores of suitable size are required. Activated carbons arearticularly appropriate for that purpose, since their pore tex-ure characteristics are easily optimised through the judicioushoice of precursor, type of activation and synthesis parame-ers. In this work, the chemical activation of lignin with KOHnder definite conditions, leading to almost purely microporousdsorbents with a very narrow micropore size distribution, was

videnced to be a very good option. In the near future, theseaterials will be deposited over a micro-hot-plate, in order to

et a full integration of pre-concentrators within the detectionnit.

[

uators B 132 (2008) 90–98 97

cknowledgements

This work has been financially supported by the Span-sh Ministry of Education and Science (project MICODEGASEC2006-03671) and Carburos Metalicos S.A. This researchas made possible in part by financial support from ALFArogram (project LIGNOCARB-ALFA II 0412 FA FI).

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iographies

ernando Blanco graduated in electronic engineering from the Technologicalnstitute of Durango, Mexico in 2002. Since 2003 he has been a PhD studentn the Electronic Engineering Department at the Universitat Rovira i VirgiliTarragona, Spain). His work focuses on the design of micro-concentrators foras sensing microsystems.

avier Vilanova graduated in telecommunication engineering from the Univer-

itat Politecnica de Catalunya (UPC), (Barcelona, Spain) in 1991, and receivedis PhD in 1998 from the same university. He is currently an associate profes-or in the Electronic Engineering Department at the Universitat Rovira i VirgiliTarragona, Spain). His main areas of interest are in semiconductor chemicalensors modelling and simulation.

TicI

uators B 132 (2008) 90–98

anessa Fierro graduated in Chemistry in 1994 at the University of ZaragozaSpain), and obtained her PhD in Chemical Engineering in 1998 from the sameniversity. After working several years as a contractual researcher in France (IFPSolaize, IRC – Lyon) than at the Universitat Rovira i Virgili (Tarragona, Spain),

he is now a full-time researcher of the National Centre for Scientific ResearchCNRS) since 2006. Her present research deals with the preparation, charac-erisation and application of nanoporous adsorbents, and especially activatedarbons.

lain Celzard graduated in Chemical Physics in 1992 and received his PhDn Materials Science in 1995 at the University Henri Poincare (Nancy, France).e is presently full-time professor at the School of Wood Science and Timberngineering (ENSTIB) (Epinal, France). His scientific interests deal with dis-rdered carbons and related materials, ranging from carbon-based compositeso adsorbents like activated carbons, with application in catalysis, depollution,nergy and gas storage.

eter Ivanov graduated in telecommunications from the Technical Universityf Sofia (Bulgaria) in and received his PhD in 2004 from the Universitat Rovira iirgili (Tarragona, Spain). His thesis has been focused on the design, fabricationnd characterisation of screen-printed gas sensors and micro-concentrators foras sensing microsystems. He is currently a researcher in the Gas Sensors Groupt the National Microelectronic Centre (Barcelona, Spain).

duard Llobet was graduated in telecommunication engineering from theniversitat Polit‘ecnica de Catalunya (UPC), (Barcelona, Spain) in 1991, and

eceived his PhD in 1997 from the same university. During 1998, he was aisiting fellow at the School of Engineering, University of Warwick (UK).e is currently an associate professor in the Electronic Engineering Depart-ent at the Universitat Rovira i Virgili (Tarragona, Spain). His main areas

f interest are in the fabrication, and modelling, of semiconductor chemi-al sensors and in the application of intelligent systems to complex odournalysis.

icolau Canellas graduated in telecommunication engineering from the Uni-ersitat Politecnica de Catalunya (UPC, Barcelona, Spain), and received hishD in 2005 from the Universitat Rovira i Virgili. He is currently an associaterofessor in the Electronic Engineering Department at the Universitat Rovira iirgili (Tarragona, Spain). His main areas of interest are the modelling and sim-lation of microelectronic and smart systems, and signal processing for patternecognition systems.

ose Luıs Ramırez graduated in telecommunication engineering from theniversitat Politecnica de Catalunya (UPC), (Barcelona, Spain) in 1994, and

eceived his PhD in 2003 from the same university. He is currently anssociate professor in the Electronic Engineering Department at the Univer-itat Rovira i Virgili (Tarragona, Spain). His main areas of interest are inhe modelling and simulation of microelectronic systems, specially chemical

icrosystems.

avier Correig graduated in telecommunication engineering from the Univer-itat Politecnica de Catalunya (UPC), (Barcelona, Spain) in 1984, and received

echnology in the Electronic Engineering Department at the Universitat RoviraVirgili (Tarragona, Spain). His research interests include heterojunction semi-onductor devices and solid-state gas sensors. Dr. Correig is a member of thenstitute of Electrical and Electronic Engineers.