JOURNAL OF MASS SPECTROMETRYJ. Mass Spectrom. 2002; 37: 1200–1204Published online 10 September 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jms.367

A new external EI/CI source–ion trap system devotedto the study of ion-molecule reactions†

Giuseppe Pace,1 Roberta Seraglia,1 Fulvio Cacace,2 Giulia de Petris2 and Pietro Traldi1∗

1 CNR, Istituto di Scienze e Tecnologie Molecolari, Corso Stati Uniti 4, I 35100 Padova, Italy2 Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Universita ‘La Sapienza’, Piazzale Aldo Moro 5, I 00185 Roma,Italy

Received 23 May 2002; Accepted 27 July 2002

A new instrumental arrangement dedicated to the study of ion-molecule reactions and consists of by anelectron impact/chemical ionization source mounted 420 mm away from an ion trap is described. It hasbeen designed and developed to exclude diffusion into the ion trap of the neutral reactants from the EI/CIsource and so to avoid undesired side-reactions. Its instrumental parameterization is described and somepreliminary results are illustrated that show promise for the successful application of the system to thestudy of ion-molecule reactions. Copyright 2002 John Wiley & Sons, Ltd.

KEYWORDS: chemical ionization; ion-molecule reactions; ion trap

INTRODUCTION

From the early stages of its development, the ion traphas proved to be a valid and simple tool for the studyof ion-molecule reactions.1 Selection and confinement in alimited volume of the reactant ion and the study of itsreactivity with neutrals can easily be achieved by the iontrap. The evaluation of the reaction yield as a function ofthe reaction time, the molar ratio of the reactants and, tosome extent, the temperature of the reacting ions (achievedby changing their qz value), makes this device particularlyinteresting. Furthermore, the structural characterization ofthe ions produced by collisional experiments makes the iontrap an attractive and effective reaction environment.

As examples of the wide applications of the ion trap inthe field of ion-molecule reactions, one can cite: the studiesof Einhor et al.2 on the reactions of alkene molecular ions,generated by charge exchange with CSCž

2 , with differentalkenes; the reaction of dimethyl ether ions with diols,3

showing the occurrence of an interesting methyne additionprocess; the studies of Brodbelt et al.4 on the use ofdimethyl ether as a selective CI reagent for mono- anddi-substituted oxyaromatic compounds; and the researchesof Nourse and Cooks5 on the evaluation of the protonaffinities (PA) of different compounds by producing andmass-selecting proton-bound dimers and measuring theirdissociation kinetics.

A drawback of the ion trap as a tool for the study ofion-molecule reactions is the possible co-existence, inside thetrap, of the neutral reactant of interest and of the neutral

ŁCorrespondence to: Pietro Traldi, CNR, Istituto di Scienze eTecnologia Molecolari, Corso Stati Uniti 4, I 35100 Padova, Italy.E-mail: pietro.traldi@adr.pd.cnr.it†Paper presented at the 20th Informal Meeting on MassSpectrometry, Fiera di Primiero, Italy, 12–16 May 2002.

precursors employed for ion generation. This could lead,in principle, to undesired side reactions, a problem morerelevant when the reactant ions derive, in turn, from anion-molecule reaction. To overcome this problem the use ofpulsed valves, by which is possible temporally to separate theintroduction of the neutrals employed for the ion generation(either by EI or CI) from the introduction of the reactant ofinterest, has been proposed.2 In general this approach workseffectively, but requires a careful and time-consuming set-upof the instrumental parameters, in particular the sample sizeand the pulse duration, because of the different diffusioncoefficients of different species. These considerations aredone, on the basis of previous experiences, by some of us(P.T. and R.S.) with a pulsed Iota One Valve (General Valve,NJ, USA) mounted on a Finnigan ITMS instrument.

Alternatively, coupling of the ion trap with an externalEI/CI source can be considered. Commercial analyticalinstruments of this configuration are nowadays available,6

and some home-made devices have been recently describedin the literature.7 However, in these cases the EI/CI sourcesare mounted very close to the ion trap and, consequently,the diffusion into the trap of the neutrals employed to obtainthe reactant ions cannot be excluded.

For these reasons we designed and developed aninstrumental arrangement consisting of a differentiallypumped EI/CI source mounted 420 mm away from theion trap entrance cap, and we report here the outline ofthe instrument, its operative parameterization and somepreliminary results.

EXPERIMENTAL

A Thermofinnigan LCQ system, modified as described belowwas employed. It was linked, by a stainless steel bellows(30 mm i.d.) with suitable flanges, to a stainless steel ‘T’

Copyright 2002 John Wiley & Sons, Ltd.

A new EI/CI–ion trap system 1201

housing containing a Hewlett-Packard EI/CI source (takenfrom a HP5998A instrument) and bakeable to 150 °C. Theion source housing was pumped by an Edwards EXT 255turbo pump (250 l s�1) and an Edwards RV12 rotary vanepump, and the pressure in this region was monitored by anEdwards Active Ion Gauge controller head.

A Pfeiffer TMH064 turbodrag pump (pumping speed60 l s�1) was mounted in the foreline pumping line (in thecapillary/skimmer region of the LCQ system).

The power-supply for the EI/CI source was designedand realised by Alessandro Vianello sas, and its electricalschemes are available upon request.

The ‘T’ housing was fitted with an inlet system that couldbe heated to 200 °C that was suitable for the injection of liquidand gaseous samples, and had originally been mounted ona Finnigan MAT 90 system. A home-made inlet system forgaseous and liquid samples was mounted in parallel to theHe entrance line in the ion trap housing. The gases used wereresearch grade products with a stated purity of 99.95 mol%,obtained from commercial sources and used without furtherpurification.

Methyl chloride, chloroform, methanol and acetone wereHPLC grade samples (purity ½ 99.5%) were purchased fromSigma (Milan, Italy).

RESULTS AND DISCUSSION

The aim of the present work was to obtain an instrument con-figuration optimized for the study of ion-molecule reactions,and able to exclude, or significantly reduce, the presence inthe reaction environment of the neutrals employed for thegeneration of the reactant ion. To this end the use of the iontrap with an external ion source seemed particularly inter-esting, but neither the commercially available instruments6

nor a recently suggested instrumental arrangement7 satisfyall the above requirements: indeed, in both cases the ionsource is close to the ion trap entrance cap and consequentlydiffusion of neutral species from the source into the trap cantake place. It is worth-noting, in this context, the instrumentalconfigurations proposed by Morand et al.8 and Kofel et al.9

(EI–CI source/Q/IT, EI–CI source/Q/IT/Q respectively); ahigher selectivity of the reactant ions can be obtained by boththese approaches, and differential pumping can exclude thediffusion of neutrals from the source into the trap.

Our aim was the development of a less complex system,in which the above objectives are achieved using an instru-mental set-up that is as simple as possible. Considering thation trap systems designed for external ion injection are com-mercially available nowadays, we focused attention on themand, in particular, a Thermofinnigan LCQ system was consid-ered of interest. It is usually employed for ESI/APCI MS, andconsists of an entrance capillary, a system of lenses and twooctapoles that focalize the ion beam inside the trap. Pulsingthe first octapole allows one to pulse the ion beam and, conse-quently, to control the quantity of ions injected into the trap.

The entrance flange with the heated capillary and thefirst series of lenses were removed. A Hewlett-PackardEI/CI source, originally mounted on a HP5998A instrumentavailable in the laboratory, was employed. The source power

supply was designed and built to achieve the analogic controlof the source that was mounted in a ‘T’ vacuum housing, in itsturn mounted on a movable plane. The source housing wasconnected to the vacuum system described below. The twocomponents, i.e. the LCQ and the ion source were connectedby a stainless steel bellows with suitable flanges (ISO forLCQ side, Conflat for the ‘T’ side).

The overall view of the system is outlined in Fig. 1.The most relevant feature of this configuration is the largeseparation between the ion source and ion trap, located as faras 420 mm away. At this distance, with an adequate pumpingsystems, the diffusion of the neutrals from the source to theion trap is significantly depressed.

The vacuum system and the introduction lines of theinstrument are schematized in Fig. 2. The original LCQvacuum system has been modified only by adding in theforeline pumping line (in the capillary/skimmer region)a turbo pump with a pumping speed of 60 l s�1. Forthe EI/CI ion source housing we employed a separatevacuum line including an Edwards EXT turbo pump andan Edwards RV12 rotary vane pump. The pressure in theion source region is monitored by an Edwards Active IonGauge controller (1 of Fig. 2). Two introduction systems forliquid and gaseous samples are mounted on the instrument.The first one, (originally mounted on a Finnigan MAT 90instrument) is connected to the ion source housing (see Fig. 2)and the second, home-made, is mounted in parallel to theHe gas entrance into the ion trap housing. It is emphasizedthat the ion trap operates effectively with a He pressure ofthe order of 1 ð 10�5 Torr (1 Torr D 133.3 Pa). The He gasdiffuses in the system, leading to a final pressure of about8 ð 10�6 Torr in the source region.

In order to evaluate roughly the diffusion rate of thevapour from the EI/CI source to the ion trap region, weintroduced different amounts of chloroform into the sourceand measured the pressure by ion gauges 1, in the source, and2, in the ion trap (see Fig. 2). The results, illustrated in Fig. 3,show that the ion trap pressure remains constant when thepressure in the ion source is in the range 8 ð 10�6 –10�3 Torr.For higher values, a slight pressure increase is observed inthe trap, indicating that some vapour diffusion takes place.In the same figure the plot of ion intensity versus the ion-source pressure is also reported, showing a good linearityof ion transmission. It is emphasized that analogous resultswere achieved without the turbo pump added in the forelinepumping line.

This point was more accurately investigated by injectingdifferent amounts of acetonitrile into the ion source operatingunder EI conditions. It has been reported that introductionof acetonitrile in an ion trap, at the total pressure (He CCH3CN) of 3ð10�5 Torr, leads to the formation of the MHC

species of m/z 42 but two other ions are detectable at m/z40 (CH2CNC) and m/z 54 (C3H4NC�.10 – 12 The former ion isgenerated by the loss of H2 from the MHC ion, while the lat-ter one originates from the reaction of CH2CNC with neutralCH3CN, through the loss of HCN. Hence, formation of theions at m/z 42, 41 and 54 provides a valid tool to evaluatethe occurrence of ion-molecule reactions in the trap. Thespectra obtained with CH3CN pressures in the source from

Copyright 2002 John Wiley & Sons, Ltd. J. Mass Spectrom. 2002; 37: 1200–1204

1202 G. Pace et al.

Figure 1. Scheme of the external EI/CI source–ion trap system.

Figure 2. Scheme of the vacuum system and sample introduction lines.

Ion

trap

pre

ssur

e

105

106

107

108

Ion

inte

nsity

Ion source pressure (mbar)

10−6

10−5

10−4

10−6 10−5 10−4 10−3 10−2

Ion intensity

Ion trap pressure(mbar)

Figure 3. Plots of ion trap pressure (mbar) and ion intensity(arbitrary units) versus ion source pressure (mbar) obtained byintroducing different amounts of CHCl3 into the EI/CI source.

5 ð 10�6 to 5 ð 10�4 Torr were always identical, showing theproduction in the EI source and storage inside the trap ofonly the MCž species of m/z 41 (see, as an example Fig. 4a,obtained with a pressure of 4 ð 10�5 Torr). When the pres-sure in the EI source was increased up to 3 ð 10�4 Torr thespectrum changed. MHC and [M-H]C ions became detectableat m/z 42 and 40, respectively (see Fig. 4b) proving thatsome CH3CN vapours reached the ion trap, reacting withthe CH3CNCž ions through the processes discussed above.Finally, the spectrum obtained at a low ion source pressure(3 ð 10�5 Torr) when introducing neutral CH3CN inside thetrap up to a pressure of 1.2 ð 10�5 Torr (in presence of Heat a partial pressure of 1.0 ð 10�5 Torr) is reported in Fig. 4c.This is a good example of the effectiveness of an ion trapin performing ion-molecule reactions: after a reaction timeof 20 ms the molecular ion of acetonitrile (m/z 41) totallydisappears and the only ions detectable are C2H2NC (m/z40), C2H4NC (m/z 42) and C3H4NC (m/z 54).

Copyright 2002 John Wiley & Sons, Ltd. J. Mass Spectrom. 2002; 37: 1200–1204

A new EI/CI–ion trap system 1203

41

41

42

54

(a)

(b)

(c)

Source pressure = 4 × 10−5 Torr

Source pressure = 3 × 10−4 Torr

Source pressure = 3 × 10−5 Torr

Ion trap pressure = 1.2 × 10−5 Torr

42

40

40

20

50

100

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

20

50

100

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

20

50

100

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Figure 4. Mass spectra obtained by: (a) injecting CH3CN inthe EI/CI source at a pressure of 4 ð 10�5 Torr; (b) injectingCH3CN in the EI/CI source at a pressure of 3 ð 10�4 Torr;(c) injecting CH3CN in both the EI/CI source at a pressure of3 ð 10�5 Torr and the ion trap (pressure of 1.2 ð 10�5 Torr).

Two different sets of experiments were made to optimizethe ion transmission and to find the ion trap parametersmost suitable to study ion-molecule reactions and to recordcollisionally activated decomposition spectra. In the first casethe ion beam intensity was monitored by varying the threeion source fields, i.e. those generated by the repeller, theion focus and the draw out potentials. The optimization ofthe octapoles and the intermediate lens potentials was car-ried out by the automatic programme of the spectrometer.A further check by manual variation of these parametersconfirmed that the programme properly evaluates them.Passing to the control of the ion-molecule reaction param-eters, it must be emphasized that the following temporalsequence occurs in the described experimental set-up: theion injection time (which, when the automatic gain controlis switched off, can be varied in the range from 0.20 msto 10 s); the actual reaction time (i.e. the time allowed forion-molecule collisions, with collision energy D 0); and thescanning time. The ion injection time is a parameter tobe kept constant and as low as possible in kinetic exper-iments. During this time the ion quantity inside the trapincreases and the reaction rate is consequently affected in anunpredictable way. The reaction time can be varied in the10�6 –100-s range and represents the most relevant parameterin ion-molecule kinetic studies. If kept sufficiently long, it

makes negligible the contributions to the injection and thescanning times.

The reaction of protonated acetonitrile produced in theEI source with neutral acetone injected into the trap wasstudied by varying the reaction time at constant injectionand scanning times. As apparent from the data illustrated inthe plot of Fig. 5, even at zero reaction time a small amountof protonated acetone is produced, probably during the ioninjection time. The intensity of protonated acetone rapidlyincreases with the reaction time up to 60 ms. At longertimes another ion is detected at m/z 117, i.e. the proton-bound dimer of acetone, originating from the clusteringof protonated acetone with the corresponding neutralmolecule. Increasing the reaction time the amount of thisspecies steadily increases and, after 100 ms, it predominates,becoming to the most abundant ion in the spectrum.

Finally, we performed some methylation experiments,producing (CH3�2ClC ions in the CI source and injectingthem into the trap. When methanol was introduced into thetrap, the spectrum shown in Fig. 6a was obtained. Togetherwith the reactant ion, i.e. 35Cl(CH3�C

2 of m/z 65, a newand abundant ionic species was detected at m/z 45, mostlikely produced by methylation of methanol followed byfast H2 loss (see reaction (i) of Scheme 1). The product ionis particularly stable and this can explain its ease of itsformation. In order to verify the above reaction sequence,traces of acetone were injected into the trap in the presence ofmethanol and of the (CH3�2ClC ions. The spectrum obtainedis reported in Fig. 6b. Again, the most abundant ion isdetected at m/z 45, but a further ionic species is presentat m/z 73, corresponding to methylated acetone (process(ii) of Scheme 1). It is of interest that protonated acetoneis also detectable at m/z 59. Based on the proton affinityvalues of the species involved,13 namely 180.3 kcal mol�1

(CH3OH), 189.3 kcal mol�1 (CH3OCH3� and 194.1 kcal mol�1

(CH3COCH3�, and according to Scheme 1, formation ofprotonated acetone can be reasonably traced to the reaction(iii)—of neutral acetone with the protonated dimethyl ether,invoked as the intermediate of reaction (i).

0 10 20 30 40 50 60 70 80 90 100 110 1200

10

20

30

40

50

60

70

80

90

100 m/z 42m/z 59m/z 117

Rel

ativ

e ab

unda

nce

Reaction time, ms

Figure 5. Plots of relative abundances of the ions at m/z 42,59 and 117 versus time, obtained by injection of the MHC ionsof CH3CN into the ion trap containing traces of acetone(injection time: 2 ms).

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1204 G. Pace et al.

30 35 40 45 50 55 60

m/z m/z

65 70 75 80 85 30

50

100

50

100 45

65

45

65

59

73

35 40 45 50 55 60 65 70 75 80 85

Rel

.Ab.

%

Rel

.Ab.

%

(a) (b)

Figure 6. Spectra obtained by injection of (CH3�2ClC ions into the ion trap containing traces of methanol (a) and methanol Cacetone (b).

CH3OH (CH3)2Cl+ [H3C O

H

+CH3] CH3Cl

-H2

H3C O CH2

+

m/z 45

+ +(i)

m/z 73CH3 C

O

CH3 + (CH3)2Cl+ CH3 C CH3

+O

CH3

+ CH3Cl(ii)

CH3 C

O

CH3+ CH3 CH3O

H

+CH3 C CH3

+O

H

+ CH3 O CH3(iii)

Scheme 1

In conclusion, the preliminary tests reported show thatthe new instrumental configuration described can representan effective tool for the study of the reactivity of selectedionic species with the neutrals of interest, preventing, or atleast significantly reducing, interfering reactions due to thediffusion into the ion trap of undesired neutral species fromthe ion source. Of course, further developments are neededto optimize the operative conditions (e.g. a new inlet systems,accurate measurement of the partial pressures of the sample

vapours inside the trap and the EI/CI source). Work in thisdirection is in progress.

REFERENCES1. Vedel F, Vedel M, Brodbelt JS. In Practical Aspects of Ion Trap

Mass Spectrometry, March RE, Tood JFJ (Eds). CRC Press: BocaRaton; 1995; 343–402.

2. Einhorn J, Kenttamaa HI, Cooks RG. J. Am. Soc. Mass Spectrom.1991; 2: 305.

3. Eichmann ES, Alvarez E, Brodbelt J. J. Am. Soc. Mass Spectrom.1992; 3: 535.

4. Brodbelt J, Lion C-C, Donovan T. Anal. Chem. 1991; 63:1205.

5. Nourse BD, Cooks RG. Int. J. Mass Spectrom. Ion Processes 1991;106: 249.

6. ThermoFinnigan (San Jose, CA, USA), technical literature.7. Mathurin JC, Faye T, Brunot A, Tabet JC, Walsh G, Fuche C.

Anal. Chem. 2000; 72: 5055.8. Morand KL, Horning SR, Cooks RG. Int. J. Mass Spectrom. Ion

Processes 1991; 105: 13.9. Kofel P, Reinhard H, Schlunegger UP. Org. Mass Spectrom. 1991;

26: 463.10. Wincel H, Wlodek S, Bohme DK. Int. J. Mass Spectrom. Ion

Processes 1988; 84: 69.11. Wincel H, Kokkens RH, Nibbering NMM. Int. J. Mass Spectrom.

Ion Processes 1990; 96: 321.12. Moneti G, Pieraccini G, Favretto D, Traldi P. J. Mass Spectrom.

1999; 34: 1354.13. Lias SG, Bartmess JE, Liebman JF, Holmes JL, Levin RD,

Mallard WG. J. Phys. Chem. Ref. Data 1988; 17: Suppl. 1.

Copyright 2002 John Wiley & Sons, Ltd. J. Mass Spectrom. 2002; 37: 1200–1204