Quantitative Detection of Benzene in Toluene- and Xylene-RichAtmospheres Using High-Kinetic-Energy Ion Mobility Spectrometry(IMS)Jens Langejuergen,* Maria Allers, Jens Oermann, Ansgar Kirk, and Stefan Zimmermann

Institute of Electrical Engineering and Measurement Technology, Leibniz University Hannover, Appelstr. 9a, 30167 Hannover,Germany

ABSTRACT: One major drawback of ion mobility spectrom-etry (IMS) is the dependence of the response to a certainanalyte on the concentration of water or the presence of othercompounds in the sample gas. Especially for low proton affineanalytes, e.g., benzene, which often exists in mixtures withother volatile organic compounds, such as toluene and xylene(BTX), a time-consuming preseparation is necessary. In thiswork, we investigate BTX mixtures using a compact IMSoperated at decreased pressure (20 mbar) and high kinetic ionenergies (HiKE-IMS). The reduced electric field in both thereaction tube and the drift tube can be independentlyincreased up to 120 Td. Under these conditions, the watercluster distribution of reactant ions is shifted toward smallerclusters independent of the water content in the sample gas. Thus, benzene can be ionized via proton transfer from H3O

+

reactant ions. Also, a formation of benzene ions via charge transfer from NO+ is possible. Furthermore, the time for interactionbetween ions and neutrals of different analytes is limited to such an extent that a simultaneous quantification of benzene, toluene,and xylene is possible from low ppbv up to several ppmv concentrations. The mobility resolution of the presented HiKE-IMSvaries from R = 65 at high field (90 Td) to R = 73 at lower field (40 Td) in the drift tube, which is sufficient to separate theanalyzed compounds. The detection limit for benzene is 29 ppbv (2 s of averaging) with 3700 ppmv water, 12.4 ppmv toluene,and 9 ppmv xylene present in the sample gas. Furthermore, a less-moisture-dependent benzene measurement with a detectionlimit of 32 ppbv with ca. 21 000 ppmv (90% relative humidity (RH) at 20 °C) water present in the sample gas is possibleevaluating the signal from benzene ions formed via charge transfer.

Benzene is a widely used aromatic chemical compound thatis classified by the International Agency for Research on

Cancer (IARC) as carcinogenic to humans (Group 1).1 It is anatural constituent of crude oil and currently is mainly used as aprecursor to benzene derivatives such as styrene and phenol.The current European legal exposure limit for benzene (2004/37/EG) is 1 ppmv. The threshold limit value (TLV) providedby the U.S. National Institute for Occupational Safety andHealth (NIOSH) for a time-weighted average is 500 ppbv.However, the recommended airborne exposure limit is usuallylower than the threshold exposure limit. Thus, a benzeneconcentration of <100 ppbv should be accurately and regularlyquantified. Benzene is often found in mixtures together withtoluene and xylene in comparable or even higher concen-trations; these three are collectively referred to as BTX.Therefore, a sensor system suitable for monitoring benzene inambient air must be able to quantitatively measure benzenedown to low-ppbv levels in the presence of varying amounts oftoluene, xylene, and water, as well as other volatile organiccompounds (VOCs) and compounds such as N2, O2, and CO2.Multiple methods for a measurement of BTX are described inthe literature, varying from preconcentration via thermal

desorption systems and analysis with a gas chromatography(GC) system coupled with a flame ionization detection (FID)device2 or a mass spectrometer (MS), to a direct detection withproton transfer reaction mass spectrometry (PTR-MS).3

Furthermore, ion mobility spectrometry (IMS) devices withGC preseparation and a continuous atmospheric pressurechemical ionization (APCI) or direct optical ultravioletionization (UV)4 are used, as well as IMS-MS5 and IMS withpulsed ionization sources.6 Also, the absorption of mid-infraredradiation emitted by quantum cascade lasers (QCLs) in hollowwave guides7 and Bragg grating sensors8 were investigated.However, in order to simultaneously analyze gas mixturescontaining BTX with sufficient limits of detection for benzene,all described methods either necessitate preconcentration(QCL, GC-MS), preseparation (IMS), or a comparably highinstrumental effort (PTR-MS). Measurement systems based onthe spectral absorption do not provide sufficient sensitivity,while more-sensitive systems based on the ionization at

Received: September 11, 2014Accepted: October 31, 2014Published: October 31, 2014

Article

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© 2014 American Chemical Society 11841 dx.doi.org/10.1021/ac5034243 | Anal. Chem. 2014, 86, 11841−11846

This is an open access article published under an ACS AuthorChoice License, which permitscopying and redistribution of the article or any adaptations for non-commercial purposes.

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atmospheric pressure suffer from competing ion−moleculereactions. Solely PTR-MS and SIFT-MS (selected ion flowtube) enable a sensitive real-time measurement, but asophisticated high-vacuum system is always necessary, makingthe design of compact systems difficult. Anyhow, we recentlyintroduced a high-kinetic-energy ion mobility spectrometer(HiKE-IMS), which is operated at a pressure of 20 mbar andreduced electric fields up to 120 Td. (The unit Td represents atownsend; 1 Td = 10−17 V cm2.) As described in the followingsection, this enables simultaneous ionization and detection,even of low proton affine compounds, in a humid environ-ment.9 In this context, the reduced field is a measure of theions’ average kinetic energy, which can be gained byacceleration through the electric field between two ion-neutralcollisions. The reduced field in HiKE-IMS is more than 1 orderof magnitude higher than the reduced field in conventional drifttube IMS, which is <10 Td. For a 2-s-averaged signal, a limit ofdetection (LOD) of 7 ppbv toluene, independent from themoisture of the sample (up to 5400 ppmv of water) gas, waspresented. Furthermore, HiKE-IMS enables an observation ofadditional orthogonal parameters related to an increase inkinetic ion energy such as fragmentation and field-dependention mobility, which may help to separate compounds that havesimilar ion mobility under low field conditions. In this work, weshow that it is possible to quantify traces of benzene in air withdifferent concentrations of toluene, xylene, and water withoutpreconcentration or preseparation using HiKE-IMS.

■ THEORETICAL BACKGROUNDIn IMS, ions are separated and identified based on theircompound specific ion mobility (K). In a drift-tube IMSprocess, the reduced ion mobility (K0) can be calculated usingthe length of the drift tube (L), the time needed for ions totraverse the drift tube (τ), and the applied voltage (U), as wellas the pressure (p) and temperature (T) of the drift gas:

τ=

××

⎜ ⎟⎛⎝

⎞⎠K

LU

pT

273 K1013 mbar0

2

Even though the mobility is constant under definedconditions, it may vary with the changing water content ofthe drift gas or kinetic energy of the ions. Thus, values of themobility must be compared with care. A comprehensiveintroduction into ion mobility is given by Eiceman et al.10

and Revercomb and Mason.11

In APCI, reactant ions are initially generated by ionization ofthe main constituents of the sample gas, which is usually air.Because of the residual water that is present, even in dry andclean air, the positive reactant ions are mainly H3O

+, NO+, andO2

+, which can form water clusters with several neutral watermolecules. It is known that comparably large amounts of NO+

reactant ions are typically generated in a corona dischargeionization.12 However, with increasing water concentration,larger water clusters are formed and NO+(H2O)n andO2

+(H2O)n lose their charge in a ligand-switching reactionwith water. The resulting reactant ion water cluster distributionis dependent on the kinetic ion energy and the waterconcentration.13 Positive analyte ions are generated either bycharge transfer, association, proton transfer, or ligand switchingreactions with these positive reactant ions. The reactionconstant (k) for such a gas-phase reaction is typically in theorder of the collision rate, resulting in a very efficient and fastionization.14

Benzene (B) can be ionized via proton transfer from H3O+

and charge transfer with NO+ and O2+, as well as by association

of NO+, resulting in BH+, B+, and BNO+ product ions.Especially, the generation of BH+ is extremely moisture-dependent. Toluene (T) and xylene (X) can also be ionized byH+(H2O)2 clusters; therefore, the moisture dependence is lesspronounced here. However, when BTX are present in anatmospheric-pressure chemical ionization (APCI) source, theamount of benzene ions will drastically decrease, because ofcompeting ionization processes. Toluene and xylene havehigher proton affinity and lower ionization energy, compared tobenzene (values are given in Table 1). This is also true for

ultraviolet (UV) ionization at atmospheric pressure, because ofthe high frequency of ion−molecule collisions. Thus, aquantitative measurement of benzene in ambient air withIMS is not possible without preseparation, because of moisturedependence and competing ion−molecule reactions.However, the equilibrium water cluster distribution can be

shifted toward a greater abundance of H3O+ with increasing

kinetic energy of the reactant ions.15 This is possible byincreasing the quantity E/N, the so-called “reduced electricfield”, which is defined as the electric field (E) divided by theconcentration of neutral molecules (N). At E/N values of >100Td, a significant amount of H3O

+ can be present, even in moistgases. The presented HiKE-IMS is operated at 20 mbar andusing electric fields up to 60 V/mm, resulting in E/N values ofup to 120 Td. In order to provide a water cluster distributionthat is relatively independent of the water content of theanalyzed sample gas, an additional and constant amount ofwater is introduced into the reaction tube. Thus, thecomparably much lower amount of water introduced with thesample gas can be neglected.The reaction time in HiKE-IMS is dependent on the E/N

value in the IMS reaction tube, where both reactant ions andneutral analyte molecules are present. At the maximum E/Nvalue, the reaction time is on the order of 100 μs. Therefore,the relative abundances of different ion species do not reflectthe thermal distribution. Even at reaction rates k on the orderof the collision rate (kc), the effect of competing ionizationprocesses of analytes present in the ppmv range is significantlysuppressed.

■ EXPERIMENTAL SETUPThe basic setup of the HiKE-IMS has recently been published.9

The main components are a reaction tube and a drift tube,which are separated by an ion gate consisting of three metallicgrids. The experimental setup is shown in Figure 1, and thedefault operating parameters are given in Table 2. The lengthand diameter of the drift tube have been slightly modified from

Table 1. List of Proton Affinities and Ionization Energies

name formula proton affinitya (kJ/mol) ionization energyb (eV)

water H2O 691 12.621nitric oxide NO 531.8 9.2642oxygen O2 421 12.0697benzene C6H6 750.4 9.2437toluene C7H8 784.0 8.828p-xylene C8H10 794.4 8.44m-xylene C8H10 812.1 8.55o-xylene C8H10 796.0 8.56

aData taken from ref 16. bData taken from ref 17.

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the previously published setup in order to increase the mobilityresolution and the sensitivity. The body of the IMS iscomposed of polyether ether ketone (PEEK), the electrodesfrom stainless steel. Polytetrafluoroethylene (PTFE) foil is usedfor electrical insulation of the gate electrodes and sealing. Thereaction tube and drift tube are installed in a temperature-controlled housing. The HiKE-IMS is evacuated via a smallmembrane pump (Pfeiffer Vacuum, Model MVP 003-2). Theresulting pressure is monitored via a capacitive pressure gauge(Pfeiffer Vacuum, Model CMR 362). If pressure fluctuationsoccur, the drift-tube voltage (UDT) and the reaction-tubevoltage (URT) are automatically adjusted in order to achieve aconstant reduced reaction field (ERT/N) and drift field (EDT/N) during data acquisition. Reduced fields ERT/N and EDT/Ncan be independently increased up to 120 Td. Coronadischarge (corona needle APCI, Agilent) is used for primaryionization. A constant amount of water vapor is added close tothe corona needle in order to promote the generation of H3O

+-based reactant ions. Furthermore, the water content in thereaction tube is mainly governed by this intentionallyintroduced amount of water and the influence of sample gasmoisture is decreased. The sample gas is directly introduced

through a PEEK capillary into the reaction tube. In order tokeep the drift tube free of any contaminants, a constant flow ofdry and clean air is used as drift gas. This drift gas flows throughthe drift tube and the reaction tube and mixes within thereaction tube with the sample gas. To inject ions into the drifttube, the center gate electrode is set to the mean potentialbetween the first and last gate electrode. To close this ion gate,the potential of the center electrode is increased to 12 V abovethe potential of the first electrode. The gate opening time is 6μs. A longer injection leads to an increase in signal intensity,but also to a decrease in mobility resolution. A fast currentamplifier (developed in-house, 150 kHz, gain = 3 × 108 V/A) isconnected to the detector. Spectra are recorded and averagedusing in-house developed data acquisition hardware andsoftware. All voltages can be inverted in order to analyzepositive and negative ions. The experimental setup includingthe reaction tube, drift tube, heating, water reservoir, amplifier,data acquisition, pressure gauge, gate switch, and membranepump fits into conventional 19 in. housing with six RU (rackunits) and a height of 26.7 cm. The repetition rate can be set to500 Hz; thus, 1000 spectra can be acquired and averaged within2 s. All shown spectra and all given detection limits refer to aspectrum that has averaged 1000 times.For sample gas preparation, two permeation ovens (VICI,

Model 150) are used. The carrier and dilution gas is providedby a zero-gas generator (JAG, Model ZA350S). The moistureof the sample gas can be increased by adding a defined amountof clean air through a gas-washing bottle filled with water(Fluka, GC headspace tested). Benzene (Fluka, analyticalstandard), toluene (Sigma−Aldrich, Chromasolv, HPLC,99.9%) and different isomers of xylene (Fluka, analyticalstandard) are used as analytes. The analyte concentration of thesample gas is calculated from the weight loss of a permeationtube.

■ EXPERIMENTAL RESULTSIn this section, the ion mobility spectrum of benzene at highkinetic energy is shown and the relative abundances of differentbenzene peaks, as a function of ERT/N, are given. Furthermore,the benzene detection limits in dry (<1 ppmv) and moistsample gas (3700 ppmv) are calculated. Spectra of toluene andxylene are shown, and the influence of these analytes on thebenzene signal is analyzed. Furthermore, the spectra of differentxylene isomers are shown.

Figure 1. Experimental setup of the high-kinetic-energy ion mobility spectrometry (HiKE-IMS) system.

Table 2. Default HiKE-IMS Operating Parameters

parameter value

temperature 35−35.5 °Cpressure 19−20 mbardrift tube length 145 mmreaction tube length 108 mminner electrode diameter 21 mmdrift tube voltage, UDT 650−7400 Vreaction tube voltage, URT 1500−6050 Vreduced drift field, EDT/N 10−110 Tdreduced reaction field, ERT/N 30−120 Tdinjection time 6 μsgate voltage, UGate 12 Vcorona voltage, UCorona 1000 VU(t) voltage shutter grid (open) UDT + 1/2UGate

U(t) voltage shutter grid (closed) UDT + 1/2UGate + 12 Vaperture voltage, UAperture 20 Vdrift gas flow 5 sccm dry clean air (<1 ppm water)sample gas flow 6.5 sccm via capillary flow restrictionreaction chamber additive water 1 sccm

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A separation of the benzene peaks from the reactant ion peak(RIP) is possible at EDT/N ≥ 90 Td. As described in previouswork,9 because of the decrease in average water cluster size inthe drift tube with increasing EDT/N, the RIP mobility increasesto such an extent that it can be separated from the benzenepeaks. The mobility resolution (drift time divided by the fullwidth at half-maximum) of the analyte peaks under theseconditions is R ≈ 65. The shape of the RIP under theseconditions is asymmetric, rather than Gaussian, possibly due tothe presence of smaller, faster water clusters. The mobilityresolution can be further increased up to R = 73 if a value ofEDT/N = 40 Td is used. This is in accordance with theoreticalwork18 that describes the optimal IMS resolution as a functionof the drift field. However, under these conditions, the firstbenzene peak and the RIP merge. Thus, a value of EDT/N = 90Td is chosen in this work as the best tradeoff.In Figure 2, the positive ion mobility spectrum of 4.3 ppmv

benzene in dry sample gas is shown at different ERT/N values.

Besides the RIP, there are up to four different peaks visible inthe spectrum. As described in earlier work,9 the signal intensity(peak height) of these peaks depends on ERT/N, because offragmentation, declustering, different possible ionization path-ways, and an increase in the amount of charge injected into thedrift tube with increasing ERT/N. At a drift time of 209 μs, apeak left of the RIP is visible, which is also present in thespectrum of dry and clean air. Thus, we expect it to be NO+.The maximum intensity of the RIP is 26 nA at ERT/N =120 Td.As described in the Theoretical Background section, there

are three different ionization mechanisms possible for benzene,which lead to three different analyte ions. In the givenspectrum, three peaks with different ion mobilities (see Table 3,presented later in this work) are visible, which correlate to thepresence of benzene in the sample gas. We expect these peaksto be B+, BH+, and BNO+. This assumption seems sound if therelative signal intensity of these peaks (signal intensity dividedby the RIP intensity) is evaluated for different ERT/N values, asshown in Figure 3. At ERT/N < 50 Td, no significant benzenepeaks are observed. With increasing ERT/N, the amount of NO+

within the reaction tube increases and BNO+ and B+ aregenerated. The maximum relative intensity for BNO+ isobserved at ERT/N = 70 Td. With further increases in ERT/N,BNO+ dissociates and the relative intensity decreases until noBNO+ is observed above 110 Td. Above ERT/N = 100 Td, the

average size of water clusters within the reaction tube isdecreased to such an extent that a significant amount of H3O

+

is present and benzene can be ionized via proton transfer,leading to BH+ ions. With increasing fraction of H3O

+, the BH+

peak also increases and becomes the most prominent peak inthe spectrum above 105 Td. A similar behavior has beendescribed9 for toluene. However, toluene can be also ionized byH+(H2O)2. Thus, an increase in abundance of the protonatedtoluene ion is observed at lower ERT/N. The ion mobility ofBH+ is significantly lower than the mobility of B+. We expectthis not to be caused by the mass shift of 1 u but rather by theincrease in dipole moment by protonation, possibly resulting inan increase in effective collision cross section. However, theexact nature of this observation is not clear.The limit of detection (LOD), which is the concentration at

which the signal intensity is three times higher than thestandard deviation of the noise, can be calculated from theresponse curve displayed in Figure 4. The most sensitive

detection of benzene with a LOD of 8 ppbv (2 s of averaging,2.5 pA standard deviation of the noise, 0.925 pA/ppbv slope) indry air (<1 ppmv water) is possible when evaluating theintensity of the BH+ peak. However, even when increasing thewater content of the sample gas to 3700 ppmv, the sensitivityonly changes by ca. 30% resulting in a LOD of 12 ppbv.Evaluating the B+ peak, which is less water-dependent, because

Figure 2. Positive ion mobility spectrum of 4.3 ppmv benzene in cleanand dry air at different ERT/N values (75−120 Td) and at EDT/N = 90Td.

Figure 3. Relative signal intensities for the different peaks observed inthe spectrum of 4.3 ppmv benzene in clean and dry air (<1 ppmvwater) at different ERT/N values from 30 Td to 120 Td. Peakintensities are normalized to the RIP intensity.

Figure 4. Response curve of the BH+ peak with increasing benzeneconcentration in dry and moist sample gas (ERT/N = 120 Td, EDT/N =90 Td).

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of the different ionization mechanism, the detection is lesssensitive and the LOD in dry sample gas is 25 ppbv. However,an increase of the water content of the sample gas up to >21000 ppmv (90% relative humidity at 20 °C) only leads to adecrease in B+ intensity by 27%, which enables a fast and directand almost moisture-independent quantification of benzenewith a LOD of 32 ppbv, even in very moist environments.Besides the influence of the water content of the sample gas,

masking effects that occur when mixtures of differentcompounds are analyzed are a major problem when usingIMS without preseparation. In Figure 5, the spectra of benzene

(850 ppbv) in dry air with different toluene (T) concentrationsfrom 0 ppbv to 2.1 ppmv are shown. Two additional peaks, T+

and TH+, related to the toluene concentration are visible whichcan be separated from the benzene peaks. The LOD for tolueneevaluating the TH+ peak is 4 ppbv in dry and 5 ppbv in moistsample gas (3700 ppmv water). A detection of TH+ with theused parameters is more sensitive than BH+, because anionization via H3O

+ and H+(H2O)2 is possible. At the givenbenzene concentration, the increase in toluene concentrationhas no significant effect on the benzene signal. Nevertheless, aminor decrease in NO+ and RIP can be observed becausecharge is transferred from these ions to the toluene ions.However, for higher toluene concentrations, two effects with

opposing influence on the benzene signal are observed thatmust be considered for a quantitative measurement of benzenein toluene-rich atmospheres. First, in the ion mobility spectrumof toluene, a small but constant percentage (2%) of ions with asimilar mobility as the benzene ions is always present, as well asa slight baseline offset between the benzene and the toluenepeaks. We expect these ions to be benzene fragments oftoluene. The influence of a change in ERT/N on these ions issimilar to the influence of such a change on the toluene signal.Thus, we do not expect this effect to be caused bycontamination of the toluene analyte or the washing of residualbenzene from surfaces by toluene. Especially, when hightoluene concentrations are present, this leads to an offset in thebenzene signal, which must be corrected. Thus, 2% of thetoluene signal are considered as an offset in the benzene signaland are subtracted. On the other hand, the presence of severalppmv of toluene leads to a decrease in available reactant ionsand increases the probability for competing ionizationprocesses. Thus, the response for benzene in the presence ofhigh toluene concentrations is attenuated by several percent.

However, the detection limit for benzene with 12.4 ppmvtoluene and 3700 ppmv water in the sample gas is 16 ppbv.In Figure 6, the influence of increasing the toluene

concentration from 0 ppbv to 12.4 ppmv on a constant benzene

concentration (850 ppbv) is shown. The BH+ signal is corrected

for a baseline elevation caused by the increasing tolueneconcentration. A decrease of the BH+ peak by only 25% at atoluene concentration of 12.4 ppmv is observed. The TH+

signal is almost linear with a toluene concentration up to ca. 6ppmv. Above 6 ppmv beginning signal saturation can beobserved due to the limited number of reactant ions.In Figure 7, the positive ion mobility spectrum of a mixture

of benzene (3.3 ppmv), toluene (3.2 ppmv), and o-xylene (1.95

ppmv) is shown. Two peaks that we expect to be positivelycharged o-xylene (X+) and protonated o-xylene (XH+) can beobserved that correlate to the o-xylene concentration. Theresponse of these peaks to a shift in ERT/N is comparable to theresponse of the toluene peaks. Even though the two o-xylenepeaks are not baseline-separated, all compounds in this mixturecan be clearly distinguished. All three xylene isomers have beeninvestigated separately and show only minor influence on thesignal intensity of toluene and benzene. At high xyleneconcentrations, a baseline elevation (3% of the XH+ intensityat the BH+ mobility to 6% of the XH+ intensity at the TH+

mobility) between the benzene peaks and the xylene peaks is

Figure 5. Positive ion mobility spectrum of benzene (850 ppbv) anddifferent toluene concentration from 0 ppmv to 2.1 ppmv in dry (<1ppmv water) sample gas (ERT/N = 120 Td, EDT/N = 90 Td).

Figure 6. Signal intensity of BH+ and TH+ at constant benzeneconcentration (850 ppbv) and increasing toluene concentration (0ppbv to 12.4 ppmv) in dry (<1 ppmv water) sample gas (ERT/N = 120Td, EDT/N = 90 Td).

Figure 7. Positive ion mobility spectrum of benzene (3.3 ppmv),toluene (3.2 ppmv), and o-xylene (1.95 ppmv) in dry (<1 ppmv water)sample gas (ERT/N = 120 Td, EDT/N = 90 Td).

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observed, which correlates to the xylene concentration. For aquantitative measurement of benzene in a xylene-richatmosphere, this offset must be considered. Similar to thetoluene signal, we expect this elevation to be caused by benzeneor toluene fragments of xylene. Furthermore, at high xyleneconcentrations, the response for benzene is attenuated byseveral percent. At 9 ppmv of o-xylene, the offset-corrected BH

+

signal is 55% of the BH+ signal without o-xylene. However, theresulting sensitivity for benzene in toluene- and xylene-richatmospheres is significantly higher than that observed whenusing conventional IMS without GC preseparation. If tolueneand xylene are the only VOCs present in comparably highconcentrations in the sample gas, the influence of these analyteson the measured benzene signal can be calculated and aquantitative measurement of all compounds in this mixture ispossible. The ion mobility of all shown peaks is given in Table3. It is noteworthy that we observed slightly different mobilitiesfor the three xylene isomers (X+ peak).

■ CONCLUSIONA direct quantitative analysis of benzene, toluene, and xylene(BTX) mixtures even in moist sample gases is possible withoutany preconcentration and preseparation. The limit of detection(2 s of averaging) for benzene in a sample gas containing 3700ppmv water, 12.4 ppmv toluene, and 9 ppmv xylene is 29 ppbv.The mobility resolution in the presented HiKE-IMS varies fromR = 60 at EDT/N = 90 Td to R = 74 at EDT/N = 40 Td, which issufficient to separate all three compounds and identify up tothree different peaks for each compound resulting fromdifferent ionization pathways. At high ERT/N values (>100Td), even in moist sample gases, a significant amount of H3O

+

is present and protonated ions of all three compounds wereobserved. Ions generated by charge transfer via NO+ are alsopresent in all spectra above ERT/N = 50 Td. The ion mobility ofthe protonated ions is significantly lower than the mobility ofthe ions generated via charge transfer. We expect this to becaused by an increase in dipole moment by protonation. Oneindication for this is that the difference in mobility between B+

and BH+ is higher than for T+ and TH+ or X+ and XH+, becausebenzene is nonpolar while toluene and xylene are moderatelypolar. An increase in toluene concentration from 0 ppbv to 12.4ppmv leads to a decrease in BH+ signal of only 25%. Theinfluence of 9 ppmv xylene is more pronounced (45%).Furthermore, a fragmentation of toluene and xylene with a

resulting baseline elevation affecting the benzene signal must beconsidered for benzene quantification.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +49 511-762-4680. E-mail: Langejuergen@geml.uni-hannover.de.NotesThe authors declare no competing financial interest.

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Table 3. Reduced Ion Mobility in Dry (<1 ppmv Water)Clean Air at 35 °C and EDT/N = 90 Td

ion reduced ion mobility, K0a (cm2/(V s))

NO+ 2.87RIP 2.45B+ 2.27BH+ 2.20BNO+ 1.97T+ 2.1TH+ 2.06p-X+ 1.97m-X+ 1.95o-X+ 1.96p/m/o-XH+ 1.92

aValues listed have a deviation of ±0.005 cm2/(V s).

Analytical Chemistry Article

dx.doi.org/10.1021/ac5034243 | Anal. Chem. 2014, 86, 11841−1184611846