effect of the humidity on analysis of aromatic compounds with planar differential ion mobility spectrometry

1、effect of the humidity on analysis of aromatic compounds with planar differential ion mobility spectrometryion mobility spectrometry (faims), is a rapidly advanc-ing technology that is both sensitive and fast, operatesat atmospheric pressure, and provides a unique type ofselectivity, which is orthog

2、onal to most of other separa-tion techniques 7, 8. in contrast to the conventionaltime of flight ion mobility spectrometry (tof-ims), inwhich the separation of ions is based on specific coef-ficients of ion mobility in a uniform electric field, dmsseparates ions based on a nonlinear dependence of th

3、emobility coefficient on the electric field strength.the dependence of the ion mobility coefficient on theelectric field can be explained by the reversible cluster forma-tion model, which describes field dependent cluster forma-tions that lead to variation of the average ion cluster crosssection 9.

4、the functional principles of the dms are de-scribed elsewhere 10, 11.initially, in the most of dms/faims applications thegaseous samples were analysed. over the last years,however, a number of applications for dms/faims-ms coupling 8, 12, as well as stand-alone dms/faims for direct liquid sample ana

5、lysis 13, 14, werepresented. in case of the direct measurements of theliquid samples the increased level of the humidity inthe dms can be expected. in case of the polar com-pounds of high proton affinity the increased humiditydoes not lead to the reduction in the sensitivity and caneven results in i

6、ncreased separation power of the dms14.recently it was demonstrated that addition of the appropri-ate amount of volatile organic compounds (modifiers) to thetransport gas can significantly enhance the resolving power ofplanar dms 12, 1517. for curved geometries the introduc-tion of a solvent vapor o

7、f high concentration is problematic,since the focusing effect causes a dramatic decrease in ionsignalorevenacompletesignalloss.18,19.theintensityofthe ion signal in the spectrometers with planar electrodesgeometry is not as affected by solvent vapors due to the lackof ion focusing effect. typically

8、polar modifiers (e.g. 2-propanol, acetone, ethylacetate, acetonitrile, water vaporsetc.) were utilized for improvement of the spectrometer re-solving power. it is assumed that polar modifiers may inducethe cluster/declustermechanism,whichprovidesincreasingofthe ion separation in comparison with the

9、measurements withpure nitrogen. the separation efficiency is dependent on thestrength of the interaction between the analyte and themodifier.in this paper, we demonstrate that utilization of polarmodifiers (e.g. 2-propanol and water vapor) which areusually applied for the improvement of separation a

10、bilityof dms based systems does not improve the separation ofaromatic compounds. moreover, the peak area of themodel compounds decreases with the increase of themodifier concentration.experimentalexperimental setupthe principle scheme of the experimental setup used in thisstudy is shown in fig. 1. t

11、he overall nitrogen flow enteringthe dms was prepared by mixing the main flow of purenitrogen, controlled by a mass flow controller (mfc,pneutronics, vso-gc), with an additional nitrogen flow con-taining the modifier (water vapors or 2-propanol). the mainflow of the pure nitrogen was dried over mole

12、cular sieves(4 ?, typ 514, roth). the homemade vapor generator (vg)includedatemperaturecontrolledsaturatedvaporsourceandaflow of nitrogen. the nitrogen flow containing the moisturewas controlled by a mass flow controller (gfc17, 050 ml min ?1 n 2 , aalborg, usa). the overall nitrogen flow,controlled

13、 by a solid state flow meter (restek 6000, restek,uk) located on the exhaust of dms, was kept constant at270mlmin ?1 .thepressurewasmonitoredusingthepressuresensor from the dms, which is built on the input to theanalyzer. the humidity was measured with mdm-300 ad-vanced dew-point hydrometer (michell

14、 instruments, uk).the minimum humidity level reached with the current setupwas 6.5 ppm v .chemicalsto check the influence of the modifier concentration on thecompensation voltage and signal area seven compounds oflow polarity were selected. six of these compounds, namelybenzene (applichem, 99+ %), t

15、oluene (j.t. baker, 99.9 %),ethylbenzene (fluka, 99+ %), p-xylene (fluka, 99+ %), 1,2,4-trimethylbenzene (aldrich, 98 %), and naphthalene (sigma-aldrich, 99 %) represent aromatic compounds. n-hexane(sigma, 97+ %) was taken to compare the behavior of thearomatic and aliphatic compounds in the presenc

16、e of modi-fiers. water was prepared by treating of deionized water withpurelab ultra (elga). to compare the behavior of the com-pounds of low and high polarity in the presence of humidity,2-hexanone (sigma-aldrich, 98 %) was taken as a represen-tative compound of high polarity. the molecular weights

17、, theoutletn 2inlet n 2inlet mfc msr dms hs mfmsampleinlet mfc vg fig. 1 the principle scheme of the experimental setup: differential ionmobility spectrometer (dms), humidity sensor (hs), mass flowcontroller (mfc), mass flow meter (mfm), molecular sieves reservoir(msr), vapor generator (vg)68 int. j

18、. ion mobil. spec. (2015) 18:6775proton affinities, and the vapor pressures of the model com-pounds are summarized in table 1.the samples for the measurements were prepared as fol-lows: an analyte sample volume of 1 ml was transferred into a20 ml vial under nitrogen atmosphere. the vial was closedwi

19、th the screw cap equipped with butyl/ptfe septa (s/n100032, bgb, germany) and equilibrated for a 1 h at 20 c.each sample was a mixture of 0.5 l of the headspace overthe analyteand4.5lofthenitrogen(99.999%,airliquide,germany). 0.5 l of the sample was introduced using 5 lsyringe (sge, australia) by ma

20、nual injection through asilicone/ptfe septum (vwr, germany).dmsthe differential ion mobility spectrometer (svac-v,63 ni185 mbq, scionex corp., usa) settings were as follows:sensor temperature =80 c, number of steps =100, step dura-tion =10 ms, step settle time =3 ms, steps to blank =1. themeasuremen

21、ts were analysed in the positive (positive ions)mode at rf-voltage of 1000 v (20 kv cm ?1 ) and nitrogen(99.999%,airliquide,germany)flowrateof270mlmin ?1 ,otherwise noted. for the compounds demonstrating a signifi-cant shift on the cv scale (2-hexanone) the compensatingvoltage range was set from ?43

22、 to +5 v. for other compoundsthe range from?10to+5v was set in order toachieve a betterresolution.for each sample, five single measurements were recordedusing sionex expert software (version 2.4.0). for the deter-mination of peak parameters (centre, area, fwhm) the mea-sured data were analysed by th

23、e fityk (version 0.9.4) program20. the peaks were fitted with gaussian functions using thelevenberg-marquardt algorithm.it should be noted that the analyte signal positions on thecompensation voltage scale are very sensitive to even minorpressure differences. to enable the comparison of spectraobtai

24、ned under different experimental conditions the methoddescribed by nazarov et al. 21 was used. this methodproposes utilization of e/n scaling in townsend units (td).in our study, utilization of this method has minimized but notcompletely eliminated the differences between the measure-ments at differ

25、ent pressures. that is why the data presented inthis manuscript were recorded at the narrow pressure gapbetween 14.40 and 14.54 psi.results and discussioneffects of the humidity on the reactive ion positive (rip)therelationshipbetweentheripcompensationvoltage(cv),the rip area, the rip fwhm, and the

26、humidity are demon-strated in fig. 2. the shift of the rip toward more negativecv with the increase of the humidity was observed within thehumidity range of 20 to 550 ppm v . this shift is in a goodagreement with the reversible cluster formation model. how-ever, for humidities lower than 20 ppm v th

27、e opposite relation-ship between cvand humidity was observed (see fig. 2, top).additionally, a substantial increase of the rip area was ob-served within the same humidity range.the observed effects can be explained by the followingmodel. in the nitrogen of very low humidity (few ppm v andless)thewat

28、erhasaminorinfluenceonthepositiveionchargecarriers.inthishumidityrange,thepositiveionchargecarriersare represented by ions of a relatively low lifetime(n 2 (h 2 o) n + , no(h 2 o) n + , n=0,1; etc. 22) and only aminor part of the positive charges are represented by h 3 o + .this results in a very we

29、ak rip signal. with increase of watervapor concentration the amount of the water clusters in-creases, increasing with it the role of relatively long leavingwaterclustersasthechargecarriers.thisleadstotheobservedincrease of the rip area (see fig. 2, top and fig. 3). in thisrange of humidity (up to 20

30、 ppm v ), however, the watertable 1 list of compounds used in the current study with the corresponding molecular weights, proton affinities, and vapor pressurescompound mw g mol ?1 proton affinity kj mol ?1 vapor pressure at 20 c hpabenzene 78.11 744.8750.4 2830 99.5 31toluene 92.14 782.4784.0 2830

31、29.1 32ethylbenzene 106.17 788.0789.9 29,30 9.5 33p-xylene 106.17 785.4794.4 2830 8.7 341,2,4-trimethylbenzene 120.19 837 35 2.3 36naphthalene 128.17 800.0802.9 30 0.08 37n-hexane 86.18 672.4 38 160 392-hexanone 100.16 840 40 3.6 41water 18.02 690 h + (h 2 o) 42 23.3 43808 h + (h 2 o) 2 42int. j. io

32、n mobil. spec. (2015) 18:6775 69concentration is not high enough for the formation of a sig-nificant number of water clusters containing multiple watermolecules.therefore,mostofthe water clustersare represent-ed by h 3 o + (h + (h 2 o) n , n=1). as a result, no negative ripcompensation voltage shift

33、, typical for the water clusters withhigher water content, is observed. the further increase of thewater concentration results in an increase of the average watercluster size leading to the shift of the rip signal toward thenegative cv. the shift of the rip toward the negative cvvalues can be explai

34、ned by the reversible cluster formationmodel, which describes field dependent cluster formationsthat lead to variation of the average ion cluster cross section9. this model demonstrates that water cluster size and thecluster mobility are dependent on the concentration of theclustering particles, clu

35、ster temperature, and complex forma-tion energy. with an increase of electric field strength, theeffective cluster temperature rises, resulting in a rapiddeclustering due to a high collision rate at atmospheric pres-sure.hence,theaverageclustersizeisreduced.areductioninthe average cluster size may i

36、ncrease ion mobility significant-ly. under a low electric field, a higher water concentration inthe gas phase leads to an increase in the average cluster size.an increase in the cluster cross-section, which correlates withthe humidity, results in reduced cluster mobility and a corre-sponding increas

37、e in frequency of cluster to carrier gas colli-sions. as a result, the difference between mobility of the ionsin the low and in the high electric fields at a higher humidityincreases, leading to increase of the signal shift on the cvscale.anotherfeatureshowninfig.2(bottom)istherelationshipbetween th

38、e rip fwhm and humidity. strong reduction ofthe peak width is observed when humidity is increased to20 ppm v . this effect takes place in the same humidity rangein which the increase of the rip area and the rip shift toward0.60.81-1.3-1.2rip area (relative)cv td00.20.4-1.6-1.5-1.40 100 200 300 400 5

39、00humidity ppmvrip cv rip area relative5-1.3-1.2fwhm tdcv td00.050.1-1.6-1.5-1.40 100 200 300 400 500humidity ppmvrip cv rip fwhmfig. 2 top: the relationshipbetween the rip compensationvoltage (cv), the rip area, andthe humidity. bottom: therelationship between the ripcompensation voltage,

40、 the ripfwhm, and humidity70 int. j. ion mobil. spec. (2015) 18:6775more positive cv values were observed. therefore, it can beassumed that all these effects have the same nature. thereduction of the rip width can be explained in context ofthe proposed model described above. at humidities lowerthan

41、20 ppm v the positive charges are distributed betweendifferent charge-carriers, and as a result the peak of the ripisrepresentedbythesuperpositionofindividualchargecarrierpeaks. this leads to the peak broadening (see waveforms athumidities of 6.5 and 9 ppm, fig. 3, insert). with the increasein humid

42、ity the fraction of the positive charge carriers repre-sented by the protonated water clusters grows. as a result, thepeak represented by the protonated water clusters dominatesoverthe peaks ofother charge carriers, decreasingthe fwhmof the rip peak. the further increase in humidity (20550 ppm v ) r

43、esults in the increase of the average cluster sizewith no change in the ion core nature. in this humidity range,the fwhm of the rip peak is almost unchanged but the shiftof the rip peak toward more negative cv values is observed.the dms spectra at humidities of 6.5, 9.0, 12.4, 15.3, 176,538 ppm v ar

44、e presented in the fig. 3.effects of the humidity on the compensation voltage and peakarea of model compoundstheeffectofhumidityontof-imssignaliswellinvestigated23, 24. the influence of moisture on the ion-peak compen-sation voltage, measured with the dms with the63 ni-ioniza-tion source, was previo

45、usly investigated on example of organ-ophosphorus compounds by eiceman et al. 25 and later bykrylov et al. 9. significant compensation voltage shift wasobserved for all tested compounds at humidities higher than50 ppm.in contrast to the polar compounds analyzed in previousstudies, hexane and all aro

46、matic compounds except of naph-thalene demonstrate only a minor change of the peak positiononthe cvscale withtheincreaseinhumidity(seefig.4,top).the most likely reason for the humidity independent cv ofthese compounds isa weakinteraction withthe water clusters.due to the weak analyte-ion to water in

47、teraction, the analyte-ion size is almost independent of the humidity and no signif-icant dependence of the analyte peak cvon the humidity canbe observed.in the case of naphthalene, the increase in humidity withintherangeof820ppm v resultsinaslightpeakshifttowardthemore negative cv values. the follo

48、wing increase of thehumidity leads to the peak shift toward more positive cvvalues. unlike hexane and non-polar aromatic compoundsthe polar 2-hexanone demonstrates a dependence of the cvon humidity typical for the polar compounds. due to thestrong interaction of 2-hexanone with water clusters, witht

49、he increase of humidity, the difference in the ion size duringthe low and high field portions increases. this results in thepeak shift towards more negative cv values.all of the tested non-polar compounds demonstrate a de-crease of the signal area with an increase of the humidity. theeffect of the w

50、ater vapor concentration on the signal intensityof the aromatic compounds is not sufficiently investigated forthe dms-based systems. on the other hand, this effect wasdescribed for the proton-transfer-reaction mass spectrometry(ptr-ms). warneke et al. have observed that the increasedconcentration of

51、 the water leads to a significant reduction ofthe benzene and toluene signals 26. most of the other com-pounds investigated in this study demonstrate no dependenceof the signal on the water vapor concentration. the unusual0.22-2 -1.5 -1 -0.5 0intensity vcv td6.5 ppm 9.0 ppm 12.

52、4 ppm 15.3 ppm 176 ppm 538 ppm40.10.1070.114-2 -1 0intensity vintensity vcv td6.5 ppm 9.0 ppm 176 ppmfig.3 dmsspectraathumiditiesof 6.5, 9.0, 12.4, 15.3, 176,538 ppm v . insert: the comparisonofwaveformsatlow(left axis, 6.5and 9.0 ppm v ) and high (rightaxis, 176 ppm v ) humiditiesint. j.

53、ion mobil. spec. (2015) 18:6775 71sensitivity of benzene and toluene to moisture was explainedby their low proton affinity. at the low water vapor concen-tration the mainproton carrier ish + (h 2 o), which has a protonaffinity of 690 kj mol ?1 . all of the tested compounds, includ-ing benzene and to

54、luene, have a significantly higher protonaffinity (see table 1) and can be efficiently ionized. at higherwater vapor concentration the main proton carrier was foundto be h + (h 2 o) 2 , which has a proton affinity of 808 kj mol ?1 .due to the lower proton affinity the ionization of the benzeneand to

55、luene is problematic.the results achieved in this study are in a good agreementwith the observation described above. all measured com-pounds can be divided into three groups according to theproton affinity (see table 1). hexane and benzene, whichhave the lowest proton affinities, demonstrate the str

56、ongestdecreaseofthe signalareawithincrease ofhumidity.the nextgroup is represented by four compounds: toluene, ethylben-zene, p-xylene, and naphthalene. these compounds haveproton affinity in the range from 782 to 803 kj mol ?1 . thesmallest loss of signal area was demonstrated by tmb, whichhas the

57、highest proton affinity among the analyzed com-pounds. it is remarkable that tmb signal area decreases by87 % at water vapor concentration of 540 ppm v . however, inour previous publication we demonstrated that 2-hexanone,which has a proton affinity comparable with the tmb(840kjmol ?1 ),canbeanalyze

58、dwithnosignificantdeviationbenzene toluene et-benzene p-xylene tmb naphthalene hexane 2-hexanone11.2normalized area a. u.00.20 100 200 300 400 500normalized area a. u.humidity ppmvbenzene toluene et-benzene p-xylene tmb naphthalene hexane-0.6-0.5-0.4-0.3-0.2-0.1cv td-0.8-0.7-0.60 100 200 30

59、0 400 500humidity ppmvfig. 4 top: the relationshipbetween the compensationvoltage (cv) of the modelcompounds and humidity.bottom: the relationship betweenthe normalized peak area of themodel compounds and humidity72 int. j. ion mobil. spec. (2015) 18:6775ofthepeakareawithinthehumidityrangeof13007000ppm v14. we assume that the reason for this observation is theprobable formation of the 2-hexanone/water cluster